SUMMARY

        This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents to which the parties refer before you decide how to vote with respect to the proposals. In addition, the parties incorporate by reference important business and financial information about CUB and PacWest into this document. For a description of this information, please see the section entitled "Incorporation of Certain Documents by Reference." You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled "Where You Can Find More Information" in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

The Merger and the Merger Agreement (pages 44 and 80)

        The terms and conditions of the merger are contained in the merger agreement, which is attached to this document as Appendix A. The parties encourage you to read the merger agreement carefully, as it is the legal document that governs the merger.

        Under the terms of the merger agreement, CUB will merge with and into PacWest, with PacWest being the surviving corporation.

Merger Consideration (page 44)

        In the merger, each CUB common share owned by a CUB shareholder, other than excluded shares and dissenting shares, will be converted into the right to receive (i) the cash consideration, and (ii) a number of shares of PacWest common stock equal to the exchange ratio, subject to adjustment as set forth in the merger agreement and as further described in the section entitled "The Merger Agreement—Termination of the Merger Agreement." For each fractional share that would otherwise be issued, PacWest will pay cash in an amount equal to the fraction of a share of PacWest common stock which the holder would otherwise be entitled to receive multiplied by the volume-weighted average price of PacWest common stock as quoted on the NASDAQ over the determination period. No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

Recommendation of the CUB Board of Directors (page 51)

        After careful consideration, the CUB board of directors unanimously recommends that CUB shareholders vote "FOR" the merger proposal and "FOR" the adjournment proposal.

        For a more complete description of CUB's reasons for the merger and the recommendations of the CUB board of directors, please see the section entitled "The Merger—Recommendation of the CUB Board of Directors and Reasons for the Merger."

Opinion of CUB's Financial Advisor (page 53)

        On April 5, 2017, the CUB board of directors received an opinion from Keefe, Bruyette & Woods, Inc., referred to as KBW, its financial advisor, to the effect that, as of such date and subject to the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by KBW as set forth in its opinion, the merger consideration in the proposed merger was fair, from a financial point of view, to the holders of CUB common shares. Subsequent changes in the operations and prospects of CUB or PacWest, general market and economic conditions and other factors that may be beyond the control of CUB or PacWest, and on which KBW's opinion was based, may significantly alter the value of CUB or PacWest or the prices of PacWest common stock or CUB common shares by the time the merger is completed. Because CUB does not anticipate asking KBW to update its opinion, the opinion will not address the fairness of the merger consideration from

a financial point of view at the time the merger is completed or as of any other date other than the date of such opinion.

        The description of the opinion set forth herein is qualified in its entirety by reference to the full text of the opinion, which is attached as Appendix D to this document and is incorporated herein by reference, and describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by KBW in preparing the opinion.

        KBW's opinion speaks only as of the date of the opinion. The opinion was for the information of, and was directed to, the CUB board of directors (in its capacity as such) in connection with its consideration of the financial terms of the merger. The opinion addressed only the fairness, from a financial point of view, as of the date of the opinion, of the merger consideration in the merger to the holders of CUB common shares. It did not address, among other things as set forth in KBW's opinion, the underlying business decision of CUB to engage in the merger or enter into the merger agreement or constitute a recommendation to the CUB board of directors in connection with the merger, and it does not constitute a recommendation to any holder of CUB common shares or any shareholder of any other entity as to how to vote in connection with the merger or any other matter, nor does it constitute a recommendation regarding whether or not any such shareholder should enter into a voting, shareholders' or affiliates' agreement with respect to the merger or exercise any dissenters' or appraisal rights that may be available to such shareholder.

        For a description of the opinion that CUB received from its financial advisor, please refer to the section entitled "The Merger—Opinion of CUB's Financial Advisor."

CUB Special Meeting (page 35)

        The special meeting will be held at 8:30 a.m., local time, on August 24, 2017, at City Club Los Angeles—Santa Monica Room, 555 South Flower Street, 51st Floor, Los Angeles, California 90071. At the special meeting, holders of CUB common shares will be asked to approve the merger proposal and the adjournment proposal.

        The CUB board of directors has fixed the close of business on June 30, 2017 as the record date for determining the holders of CUB common shares entitled to receive notice of and to vote at the special meeting. As of the close of business on the record date, there were 17,831,131 CUB common shares outstanding and entitled to vote at the special meeting held by 594 shareholders of record. Each CUB common share entitles the holder to one vote on each proposal to be considered at the special meeting.

        As of the close of business on the record date, directors and executive officers of CUB and their affiliates owned and were entitled to vote 1,473,883 CUB common shares, representing approximately 8.27% of the CUB common shares outstanding on that date. As of the close of business on the record date, PacWest beneficially held no CUB common shares.

        In connection with the merger agreement, each of the directors of CUB, in his or her capacity as a shareholder of CUB, has entered into a voting agreement with PacWest in which such director has agreed to vote all CUB common shares that he or she owns and has the power to vote in favor of the merger proposal and any other matter that is required to be approved by the shareholders of CUB to facilitate the transactions contemplated by the merger agreement. The directors also agreed to vote against any proposal made in opposition to the approval of the principal terms of the merger agreement or in competition with the merger agreement and against any acquisition proposal. As of the close of business on the record date, CUB's directors beneficially owned, in the aggregate, 1,095,309 CUB common shares, allowing them to exercise approximately 6.14% of the voting power of CUB common shares (which does not include shares issuable upon the exercise of stock options or

upon the vesting of CUB RSUs that were not outstanding as of the close of business on the record date).

        Approval of the merger proposal requires the affirmative vote of a majority of the outstanding shares entitled to vote on such proposal. Approval of the adjournment proposal requires a vote that satisfies two criteria: (i) the affirmative vote of a majority of the shares represented and voting at a duly held meeting at which a quorum is present, and (ii) the shares voting affirmatively must also constitute at least a majority of the required quorum.

Interests of CUB Directors and Executive Officers in the Merger (page 68)

        In considering the recommendation of the CUB board of directors with respect to the merger, CUB shareholders should be aware that certain directors and executive officers of CUB have interests in the merger that may be different from, or in addition to, the interests of CUB shareholders generally. The CUB board of directors was aware of these interests and considered them, among other matters, in approving the merger agreement and the transactions contemplated thereby and in making its recommendation that CUB shareholders vote to approve the CUB merger proposal. These interests include:

•

CUB executive officers have equity awards that, under the merger agreement, will accelerate and become vested upon completion of the merger and will be settled for the merger consideration;



•

CUB non-employee directors have CUB restricted shares that, depending on the closing date of the merger, may accelerate and become vested upon completion of the merger;



•

David I. Rainer, Chairman and Chief Executive Officer of CUB, K. Brian Horton, President and a director of CUB, and Karen A. Schoenbaum, Executive Vice President and Chief Financial Officer of CUB, have entered into consulting agreements with PacWest that become effective as of, and are conditioned upon, the closing of the merger;



•

CUB executive officers are participants in plans and party to agreements that provide for severance payments and other benefits following a change in control of CUB or upon termination of employment within 24 months following a change in control of CUB; and



•

CUB directors and officers are entitled to continued indemnification and insurance coverage under the merger agreement.

        For a more complete description of the interests of CUB's directors and executive officers in the merger, see "The Merger—Interests of CUB Directors and Executive Officers in the Merger."

Treatment of CUB Stock Options (page 44)

        At the effective time, each outstanding CUB stock option, whether vested or unvested, will be cancelled and will entitle the holder thereof to receive an amount in cash equal to the product of (i) the total number of CUB common shares subject to such option and (ii) the excess, if any, of (A) an amount equal to (1) $12.00 in cash plus (2) the product of the PacWest average closing price and 0.5308 over (B) the exercise price per CUB common share underlying such option, less any applicable taxes required to be withheld with respect to such payment.

Treatment of CUB Restricted Shares and CUB RSUs (page 45)

        At the effective time, any vesting conditions applicable to each outstanding CUB restricted share and CUB RSU will automatically accelerate in full and such CUB restricted shares and CUB RSUs will be converted into, and become exchanged for, the merger consideration, less any applicable taxes required to be withheld with respect to such vesting.

Board of Directors and Officers of PacWest After the Merger (page 81)

        The directors and officers of PacWest immediately prior to the effective time will be the directors and officers of the surviving corporation until the earlier of their resignation or removal or until their respective successors are duly appointed and qualified.

Regulatory Approvals Required for the Merger (page 73)

        PacWest and CUB have each agreed to use reasonable best efforts to obtain all regulatory approvals required to complete the merger and the bank merger and the other transactions contemplated by the merger agreement. Regulatory approvals are required from the Federal Reserve Board, referred to as the FRB, the Federal Deposit Insurance Corporation, referred to as the FDIC, and the California Department of Business Oversight, referred to as the CDBO. As of the date of this proxy statement/prospectus, PacWest and CUB have submitted applications and notifications to obtain the required regulatory approvals. Approval from the CDBO was received on June 27, 2017. There can be no assurances that such approvals will be received on a timely basis, or as to the ability of PacWest or CUB to obtain the approvals on satisfactory terms or the absence of litigation challenging such approvals. The regulatory approvals to which completion of the merger and bank merger are subject are described in more detail under the section entitled "The Merger—Regulatory Approvals Required for the Merger."

Conditions to the Merger (page 94)

        The respective obligation of each party to effect the merger is subject to the satisfaction or written waiver at or prior to the effective time of each of the following conditions:

•

CUB having obtained the CUB shareholder approval;



•

no governmental authority of competent jurisdiction having enacted, issued, promulgated, enforced or entered any law (whether temporary, preliminary or permanent) that is in effect and restrains, enjoins or otherwise prohibits consummation of the merger, the bank merger or the other transactions contemplated by the merger agreement; and



•

the S-4 registration statement having become effective under the Securities Act, no stop order suspending the effectiveness of the S-4 registration statement, of which this document is a part, having been issued, and no proceedings for that purpose having been initiated or been threatened, by the SEC.

        CUB's obligation to effect the merger is also subject to the fulfillment or waiver of the following conditions:

•

the accuracy of the representations and warranties of PacWest set forth in the merger agreement, subject to the materiality standards set forth in the merger agreement, as of the date of the merger agreement and as of the closing date of the merger as though made at and as of the closing date (except that representations and warranties that by their terms speak as of the date of the merger agreement or some other date need only be true and correct as of such date), and CUB's receipt of a certificate signed on behalf of PacWest by an executive officer of PacWest, dated as of the closing date, to such effect;



•

performance by PacWest in all material respects of all obligations required to be performed by it under the merger agreement at or prior to the effective time, and receipt by CUB of a certificate signed by an executive officer of PacWest, dated as of the closing date, to such effect;



•

all consents, registrations, approvals, permits and authorizations required to be obtained prior to the effective time by either party or any of its respective subsidiaries from the FRB, the FDIC and the CDBO which are necessary to consummate the merger and the bank merger, and any

other consents, registrations, approvals, permits and authorizations from or with any governmental authority the failure of which to be obtained is reasonably likely to have, individually or in the aggregate, a material adverse effect on CUB or PacWest (measured on a scale relative to CUB), having been made or obtained (as the case may be) and remaining in full force and effect and all statutory waiting periods in respect thereof having expired;

•

since April 5, 2017, no event having occurred or circumstance arisen that, individually or taken together with all other facts, circumstances or events, has had or is reasonably likely to have a material adverse effect with respect to PacWest; and



•

receipt by CUB of the opinion of its counsel, dated the closing date, to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify for U.S. federal income tax purposes as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

        PacWest's obligation to complete the merger is also subject to the satisfaction or waiver of the following conditions:

•

the accuracy of the representations and warranties of CUB set forth in the merger agreement, subject to the materiality standards set forth in the merger agreement, as of the date of the merger agreement and as of the closing date of the merger as though made at and as of the closing date (except that representations and warranties that by their terms speak as of the date of the merger agreement or some other date need only be true and correct as of such date), and PacWest's receipt of a certificate signed on behalf of CUB by an executive officer of CUB, dated as of the closing date, to such effect;



•

performance by CUB in all material respects of all obligations required to be performed by it under the merger agreement at or prior to the effective time, and receipt by PacWest of a certificate signed by an executive officer of CUB, dated as of the closing date, to such effect;



•

all consents, registrations, approvals, permits and authorizations required to be obtained prior to the effective time by either party or any of its respective subsidiaries from the FRB, the FDIC and the CDBO which are necessary to consummate the merger and the bank merger, and any other consents, registrations, approvals, permits and authorizations from or with any governmental authority the failure of which to be obtained is reasonably likely to have, individually or in the aggregate, a material adverse effect on CUB or PacWest (measured on a scale relative to CUB), having been made or obtained (as the case may be) and remaining in full force and effect and all statutory waiting periods in respect thereof having expired, and none of such consents, registrations, approvals, permits and authorizations containing any materially burdensome regulatory condition;



•

as of the last business day of the month reflected in the closing financial statements, (i) the adjusted CUB shareholders' equity (defined below) will not be less than $338,000,000 and (ii) CUB's allowance for loan losses will not be less than $19,000,000, each as determined in accordance with generally accepted accounting principles in the United States, referred to as GAAP; "adjusted CUB shareholders' equity" means the consolidated equity of CUB as set forth in the closing financial statements, minus any unrealized gains or plus any unrealized losses in CUB's securities portfolio due to mark-to-market adjustments, and adding the sum of (i) all amounts paid or accrued in connection with any actions taken in connection with the purchase of CUB's outstanding shares of Series A preferred stock, referred to as the Series A preferred stock, from the U.S. Treasury Department, (ii) all amounts paid or accrued in connection with certain actions taken to conform certain policies and practices to those of PacWest to the extent that such actions were not necessary to bring CUB into conformity with GAAP or any applicable law of any governmental authority, (iii) all fees and expenses of all attorneys, accountants,

investment bankers and other advisors and agents for CUB for services rendered solely in connection with the transactions contemplated by the merger agreement paid by CUB prior to the effective time and which do not exceed in the aggregate $10,000,000 (exclusive of reasonable costs incurred or advanced by such advisors), (iv) attorneys' fees arising directly and exclusively from any actions, claims, suits or hearings brought by the Company's shareholders with respect to the merger agreement or the transactions contemplated thereby, and (v) any amounts payable by CUB in connection with the cancellation of CUB's equity awards pursuant to the merger agreement, and any costs associated with the termination of employee benefit plans or programs (inclusive of any severance compensation paid or to be paid as provided in the merger agreement) or any retention bonuses paid in accordance with the merger agreement, and any costs incurred in connection with the termination of CUB's contracts pursuant to the merger agreement;

•

since April 5, 2017, no event having occurred or circumstance arisen that, individually or taken together with all other facts, has had or is reasonably likely to have a material adverse effect with respect to CUB;



•

holders of not more than 10% of outstanding CUB common shares having duly exercised their dissenters' rights under Chapter 13 of the CGCL;



•

the purchase of CUB's Series A preferred stock from the U.S. Treasury Department having been consummated; and



•

receipt by PacWest of the opinion of its counsel, dated the closing date, to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify for U.S. federal income tax purposes as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

        For more information, please see the section entitled "The Merger Agreement—Conditions to Consummation of the Merger."

Acquisition Proposals (page 93)

        Under the terms of the merger agreement, CUB has agreed not to, directly or indirectly:

•

initiate, solicit, encourage or otherwise facilitate any inquiries or the making of any proposal or offer with respect to any acquisition proposal (as defined in the section entitled "The Merger Agreement—Acquisition Proposals"); or



•

engage in any negotiations concerning, or provide any confidential information or data to, or have any discussions with any person relating to an acquisition proposal or otherwise facilitate any effort or attempt to make or implement an acquisition proposal.

        However, the above restriction would not prevent CUB or its board of directors from:

•

complying with Rule 14d-9 and 14d-2 under the Exchange Act;



•

at any time before, but not after the CUB shareholder approval is obtained, providing information in response to a request therefor by a person who has made an unsolicited bona fide written acquisition proposal if CUB receives from the person so requesting such information, an executed confidentiality agreement on terms not less restrictive to the other party than those contained in the confidentiality agreement between PacWest and CUB; or



•

engaging in any negotiations or discussions with any person who has made an unsolicited bona fide written acquisition proposal;

        only if, however, in the cases referred to in the second and third bullet points above, the CUB board of directors determines in good faith (after consultation with outside legal counsel) that the failure to take such action would reasonably be expected to violate the directors' fiduciary duties under applicable law.

        Further, the merger agreement provides that, subject to certain exceptions, the CUB board of directors and each committee thereof will not:

•

except as expressly permitted by the merger agreement, withhold, withdraw, qualify or modify (or publicly propose or resolve to withhold, withdraw, qualify or modify), in a manner adverse to PacWest, the recommendation of its board of directors that its shareholders approve the principal terms of the merger agreement; or



•

cause or permit CUB to enter into any letter of intent, memorandum of understanding, agreement in principle, acquisition agreement, merger agreement or other agreement (other than a confidentiality agreement referred to above) relating to any acquisition proposal.

        For more information, please see the section entitled "The Merger Agreement—Acquisition Proposals."

Termination of the Merger Agreement (page 96)

        The merger agreement may be terminated and the merger may be abandoned:

•

at any time prior to the effective time, whether before or after the CUB shareholder approval, by action of the board of directors of either party, in the event that both parties mutually consent in writing to terminate the merger agreement;



•

at any time prior to the effective time, whether before or after the CUB shareholder approval, by action of the board of directors of either party, in the event that the merger is not consummated by February 28, 2018 (which may be extended to April 30, 2018 by either party if all conditions to closing have been met except for the receipt of the required regulatory approvals), which date is referred to as the end date, except to the extent that the failure of the merger to be consummated results from the knowing action or inaction of the party seeking to terminate, which action or inaction is in violation of its obligations under the merger agreement;



•

at any time prior to the effective time, whether before or after the CUB shareholder approval, by action of the board of directors of either party if:



•

the approval of any governmental authority required for consummation of the merger, the bank merger or the other transactions contemplated by the merger agreement has been denied by final and nonappealable action of such governmental authority, or an application therefor has been permanently withdrawn by mutual agreement of the parties at the request or suggestion of a governmental authority, or



•

the CUB shareholder approval is not obtained at the duly convened special meeting;



•

at any time prior to the effective time, whether before or after the CUB shareholder approval, by action of either party's board of directors if there has been a breach of any representation, warranty, covenant or agreement made by the other party, such that if continuing on the closing date of the merger, the condition as to the accuracy of the representations and warranties or the compliance with covenants by the other party would not be satisfied and such breach or condition is not curable or, if curable, is not cured within 30 calendar days after written notice thereof is given by the terminating party (or such shorter period as remaining prior to the end date); provided, that the terminating party is not then in material breach of any representation, warranty, covenant or agreement;

•

by action of PacWest's board of directors at any time prior to the CUB shareholder approval, in the event:



•

CUB has breached in any material respect the prohibitions in the merger agreement relating to acquisition proposals;



•

the CUB board of directors has effected a change of recommendation;



•

at any time after the end of 10 business days following receipt of an acquisition proposal, the CUB board of directors has failed to reaffirm its recommendation that the CUB shareholders approve the principal terms of the merger agreement as promptly as practicable (but in any event within five business days) after receipt of any written request to do so by PacWest; or



•

a tender offer or exchange offer for outstanding CUB common shares has been publicly disclosed (other than by PacWest or an affiliate of PacWest) and the CUB board of directors recommends that CUB shareholders tender their shares in such tender or exchange offer or, within 10 business days after the commencement of such tender or exchange offer, the CUB board of directors fails to recommend unequivocally against acceptance of such offer; or



•

by action of CUB's board of directors, by written notice to PacWest on the business day immediately following the determination date, effective as of three business days following the date of such written notice, in the event that:



•

the PacWest average closing price is less than $44.84 per share (with a proportionate adjustment for any stock dividend, reclassification, recapitalization, split-up, combination, exchange of shares or similar transaction between the date of the merger agreement and the determination date); and



•

the quotient of the PacWest average closing price divided by $52.75 is less than (i) the quotient of the average price of the KBW Index during the determination period divided by the index ratio, minus (ii) 0.15.

If CUB elects to terminate the merger agreement and provides written notice thereof to PacWest, then within two business days following PacWest's receipt of such notice, PacWest may elect to reinstate the merger and related transactions, and adjust the exchange ratio to equal the lesser of (i) the quotient of $23.80 divided by the PacWest average closing price and (ii) (a) the quotient of $23.80 divided by the PacWest average closing price (b) multiplied by the index ratio. Alternatively, PacWest may elect not to adjust the exchange ratio, and, in lieu thereof, may increase the cash consideration per CUB common share such that the value of the total consideration received for each share is the same as if the exchange ratio had been adjusted as described above.

        For more information, please see the section entitled "The Merger Agreement—Termination of the Merger Agreement."

Termination Fee (page 98)

        CUB must pay PacWest a termination fee of $26.5 million in the following circumstances:

•

PacWest terminates the merger agreement because CUB (i) has breached in any material respect its obligations relating to acquisition proposals under the merger agreement, (ii) the CUB board of directors has changed its recommendation to shareholders, (iii) any time after the end of 10 business days following receipt of an acquisition proposal, the CUB board of directors has failed to reaffirm its recommendation to shareholders after receipt of a written request from PacWest,

or (iv) a tender offer or exchange offer for outstanding CUB common shares has been publicly disclosed (other than by PacWest or an affiliate of PacWest) and the CUB board of directors recommends that the CUB shareholders tender their shares or, within 10 business days after the commencement of such tender or exchange offer, the CUB board of directors fails to recommend unequivocally against acceptance of such an offer; or

•

(i) (a) an acquisition proposal has been made to CUB or its shareholders generally or (b) any person has publicly announced an intention to make an acquisition proposal with respect to CUB; (ii) thereafter the merger agreement is terminated by either party because (A) the merger was not consummated on or before the end date and the CUB shareholder approval has not been obtained or (B) the CUB shareholder approval was not obtained at the special meeting, and (iii) within 12 months after the termination of the merger agreement, CUB enters into a definitive agreement with respect to or consummates an acquisition proposal (except that for purposes of the foregoing, references to "15%" in the definition of the term "acquisition proposal" will instead refer to "50%").

        For more information, please see the section entitled "The Merger Agreement—Termination Fee."

Voting Agreements (page 98)

        Each of the directors of CUB, in his or her capacity as a shareholder of CUB, has entered into a voting agreement with PacWest in which such director has agreed to vote all CUB common shares that he or she owns and has the power to vote in favor of the merger proposal and any other matter that is required to be approved by the shareholders of CUB to facilitate the transactions contemplated by the merger agreement. The directors also agreed to vote against any proposal made in opposition to the approval of the principal terms of the merger agreement or in competition with the merger agreement and against any acquisition proposal. As of the close of business on the record date, CUB's directors beneficially owned, in the aggregate, 1,095,309 CUB common shares, allowing them to exercise approximately 6.14% of the voting power of CUB common shares (which does not include shares issuable upon the exercise of stock options or upon the vesting of CUB RSUs that were not outstanding as of the close of business on the record date).

        The voting agreements terminate in certain circumstances, including in the event that the merger agreement is terminated in accordance with its terms.

        For more information, please see the section entitled "The Merger Agreement—Voting Agreements."

Material U.S. Federal Income Tax Consequences of the Merger (page 101)

        The merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code. A CUB shareholder who receives PacWest common stock and cash (other than cash received in lieu of a fractional share of PacWest common stock) in exchange for CUB common shares pursuant to the merger, will generally recognize gain (but not loss) in an amount equal to the lesser of (1) the amount of gain realized (i.e., the excess of the sum of the amount of cash and the fair market value of the PacWest common stock received pursuant to the merger over such holder's adjusted tax basis in the CUB common shares surrendered in the exchange) and (2) the amount of cash received pursuant to the merger (excluding any cash received in lieu of a fractional share of PacWest common stock). It is a condition to the completion of the merger that PacWest and CUB receive written opinions from their respective counsel to the effect that the merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

        Tax matters are complicated and the tax consequences of the merger to each CUB shareholder may depend on such shareholder's particular facts and circumstances. CUB shareholders are urged to

consult their tax advisors to understand fully the tax consequences to them of the merger. For more information, please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger."

Litigation Related to the Merger (page 100)

        Two putative shareholder class action lawsuits, referred to collectively as the merger litigation, have been filed in connection with the merger agreement. The first action, Parshall v. CU Bancorp et al., referred to as the Parshall complaint, Case No. 2:17-cv-04303, was filed on June 9, 2017 in the United States District Court for the Central District of California. The Parshall complaint alleges that the members of the CUB board of directors violated Section 14(a) of the Securities Exchange Act of 1934, referred to as the Exchange Act, and Section 20(a) of the Exchange Act, based upon alleged omissions and misrepresentations in the S-4 registration statement filed with the U.S. Securities and Exchange Commission on May 26, 2017, referred to as the initial registration statement, by approving the proposed merger for inadequate consideration and by entering into the merger agreement containing preclusive deal protection devices. The plaintiff in this action seeks injunctive relief prohibiting consummation of the merger, rescission and damages in the event the merger is consummated, an accounting of damages suffered by the plaintiff and the putative class, attorneys' fees and costs and other and further relief.

        The second action, Klein v. CU Bancorp et al., referred to as the Klein complaint, Case No. 2:17-cv-04353, was filed on June 12, 2017 in the United States District Court for the Central District of California. The Klein complaint alleges that the members of the CUB board of directors violated Sections 14(a) and 20(a) of the Exchange Act, based upon alleged omissions and misrepresentations in the initial registration statement, by approving the proposed merger for inadequate consideration and by entering into the merger agreement containing preclusive deal protection devices. The plaintiff in this action seeks injunctive relief prohibiting consummation of the merger, rescission and damages in the event the merger is consummated, an accounting of damages suffered by the plaintiff and the putative class, attorneys' fees and costs and other and further relief.

        At this stage, it is not possible to predict the outcome of the proceedings in the merger litigation or their impact on CUB, PacWest or the merger.

Comparison of Shareholders' Rights (page 104)

        The rights of CUB shareholders who continue as PacWest stockholders after the merger will be governed by the certificate of incorporation and bylaws of PacWest rather than by the articles of incorporation and bylaws of CUB. For more information, please see the section entitled "Comparison of Shareholders' Rights."

The Parties (page 42)

PacWest Bancorp
9701 Wilshire Boulevard, Suite 700
Beverly Hills, California 90212
Phone: (310) 887-8500

        PacWest Bancorp, a Delaware corporation, is a bank holding company registered under the Bank Holding Company Act of 1956, as amended, referred to as the BHC Act, with corporate headquarters in Beverly Hills, California. PacWest's principal business is to serve as the holding company for PacWest's wholly owned subsidiary, Pacific Western Bank. References to PacWest refer to PacWest together with Pacific Western Bank and its other subsidiaries on a consolidated basis.

        PacWest is focused on relationship-based business banking to small, middle-market and venture-backed businesses nationwide. It offers a broad range of loan and deposit products and services through 74 full-service branches located throughout the state of California, one branch located in Durham, North Carolina, and several loan production offices located in cities across the country. It also provides commercial banking services, including real estate, construction, and commercial loans, and comprehensive deposit and treasury management services to small and middle-market businesses. In addition, PacWest offers products and services through its CapitalSource and Square 1 Bank Divisions. The CapitalSource Division provides cash flow, asset-based, equipment, and real estate loans and treasury management services to established middle-market businesses on a national basis. The Square 1 Bank Division offers a comprehensive suite of financial services focused on entrepreneurial businesses and their venture capital and private equity investors, with offices located in key innovation hubs across the United States. PacWest also provides investment advisory and asset management services to select clients through Square 1 Asset Management, Inc., a wholly owned subsidiary of Pacific Western Bank and a SEC-registered investment adviser.

        PacWest Bancorp was established in October 1999 and has achieved strong market positions by developing and maintaining extensive local relationships in the communities it serves. By leveraging its business model, service-driven focus, and presence in attractive markets, as well as maintaining a highly efficient operating model and robust approach to risk management, PacWest has achieved significant and profitable growth, both organically and through disciplined acquisitions. PacWest has successfully completed 28 acquisitions since 2000 which have contributed to its growth and expanded its market presence throughout the United States.

        As of March 31, 2017, PacWest had total assets of $21.9 billion, loans and leases, net of deferred fees, of $15.6 billion, total deposits of $16.3 billion, and stockholders' equity of $4.5 billion.

CU Bancorp
818 West 7th Street, Suite 220
Los Angeles, California 90017
Phone: (213) 430-7000

        CU Bancorp, a California corporation, headquartered in downtown Los Angeles, California, is a bank holding company registered under the BHC Act and within the meaning of Section 1280 of the California Financial Code. CUB's principal business is to serve as the holding company for its wholly owned subsidiary, CU Bank. The common shares of CUB are listed on the NASDAQ Capital Market under the trading symbol "CUNB."

        CU Bancorp was incorporated as a California corporation on November 16, 2011, and became the holding company for CU Bank on July 31, 2012. CUB is a California state-chartered banking corporation and is not a member of the Federal Reserve System. CU Bank was incorporated on September 30, 2004, under the laws of the State of California, and commenced operations on May 23, 2005.

        CU Bank is a full-service commercial business bank offering a broad range of banking products and services including: deposit services, lending and cash management to small- and medium-sized businesses, to non-profit organizations, to business principals and entrepreneurs, to the professional community, including attorneys, certified public accountants, financial advisors, healthcare providers and investors. CU Bank is a Small Business Administration, or SBA, Preferred Lender. CU Bank's deposit products include demand, money market, and certificates of deposit. CU Bank's loan products include commercial, real estate construction, commercial real estate, SBA and personal loans. CU Bank also provides cash management services, online banking, commercial credit cards and other primarily business-oriented products. In addition to the Los Angeles headquarters office of CUB and CU Bank, CU Bank has eight additional full-service branches in the Ventura/Los Angeles/Orange County/San

Bernardino metropolitan area located in Encino, West Los Angeles, Valencia, Thousand Oaks, Gardena, Anaheim, Irvine/Newport Beach and Ontario.

        As of March 31, 2017, CUB had consolidated total assets of $3.1 billion, total loans of $2.0 billion, total deposits of $2.8 billion and total shareholders' equity of $346.4 million.

Risk Factors (page 28)

        Before voting at the special meeting, you should carefully consider all of the information contained in or incorporated by reference into this document, including the risk factors set forth in the section entitled "Risk Factors" or described in PacWest's and CUB's respective Annual Reports on Form 10-K for the year ended on December 31, 2016 and other reports filed with the SEC, which are incorporated by reference into this document. Please see "Where You Can Find More Information" and "Incorporation of Certain Documents by Reference."

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA FOR PACWEST

        The following table summarizes consolidated financial results of PacWest for the periods and at the dates indicated and should be read in conjunction with PacWest's consolidated financial statements and the notes to the consolidated financial statements contained in reports that PacWest has previously filed with the SEC. Historical financial information for PacWest can be found in its Annual Report on Form 10-K for the year ended December 31, 2016 and in its Quarterly Report on Form 10-Q for the three-month period ended March 31, 2017. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information that has been incorporated by reference. Financial amounts as of and for the three months ended March 31, 2017 and 2016 are unaudited and are not necessarily indicative of the results of operations for the full year or any other interim period, and management of PacWest believes that such amounts reflect all adjustments (consisting only of normal recurring adjustments) necessary for a fair presentation of its results of operations and financial position as of the dates and for the periods indicated. You should not assume the results of operations for past periods indicate results for any future period.

SELECTED HISTORICAL CONSOLIDATED
FINANCIAL DATA FOR CUB

        The following table summarizes consolidated financial results of CUB for the periods and at the dates indicated and should be read in conjunction with CUB's consolidated financial statements and the notes to the consolidated financial statements contained in reports that CUB has previously filed with the SEC. Historical financial information for CUB can be found in its Annual Report on Form 10-K for the year ended December 31, 2016 and in its Quarterly Report on Form 10-Q for the three-month period ended March 31, 2017. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information that has been incorporated by reference. Financial amounts as of and for the three months ended March 31, 2017 and 2016 are unaudited and are not necessarily indicative of the results of operations for the full year or any other interim period, and management of CUB believes that such amounts reflect all adjustments (consisting only of normal recurring adjustments) necessary for a fair presentation of its results of operations and financial position as of the dates and for the periods indicated.