EX-10.5



 Exhibit 10.5 

PROTEOSTASIS THERAPEUTICS, INC. 

2008 EQUITY INCENTIVE PLAN 
  

	1.	Purpose 

 This Plan is intended to encourage ownership of Common Stock by employees,
consultants and directors of the Company and its Affiliates and to provide additional incentive for them to promote the success of the Company’s business. The Plan is intended to be an incentive stock option plan within the meaning of
Section 422 of the Code but not all Awards granted hereunder are required to be Incentive Options. 
  

	2.	Definitions 

 As used in the Plan the following terms shall have the respective meanings
set out below, unless the context clearly requires otherwise: 
 2.1. “Accelerate”, “Accelerated”,
and “Acceleration”, when used with respect to an Option, means that as of the time of reference such Option will become exercisable with respect to some or all of the shares of Common Stock for which it was not then otherwise
exercisable by its terms, and, when used with respect to Restricted Stock, means that the Risk of Forfeiture otherwise applicable to such Common Stock shall expire with respect to some or all of the shares of Restricted Stock then still otherwise
subject to the Risk of Forfeiture. 
 2.2. “Acquiring Person” means, with respect to any Transaction or any
acquisition described in clause (ii) of the definition of Change of Control, the surviving or acquiring person or entity in connection with such Transaction or acquisition, as the case may be, provided that if such surviving or
acquiring person or entity is controlled, directly or indirectly, by any other person or entity (an “Ultimate Parent Entity”) that is not itself controlled by any other entity or person that is not a natural person,
then the term “Acquiring Person” shall mean such Ultimate Parent Entity. 
 2.3. “Affiliate” means, with respect
to any person or entity, any other person or entity controlling, controlled by or under common control with the first person or entity. 

2.4. “Applicable Voting Control Percentage” means (i) at any time prior to the initial public
offering of the Company, a percentage greater than fifty percent (50%) and (ii) at any time from and after the initial public offering of the Company, a percentage equal to or greater than twenty percent (20%). 

2.5. “Award” means any grant or sale pursuant to the Plan of Options, Restricted Stock, or Stock Grants. 

2.6. “Award Agreement” means an agreement between the Company and the recipient of an Award, setting forth the terms
and conditions of the Award. 
 2.7. “Beneficial Ownership” has the meaning ascribed to such term in Rule 13d-3, or
any successor rule thereto, promulgated by the Securities and Exchange Commission pursuant to the Exchange Act. 
 2.8.
“Board” means the Company’s board of directors. 







 2.9. “Change of Control” means (i) the closing of any Sale of
the Company Transaction or (ii) the direct or indirect acquisition, in a single transaction or a series of related transactions, by any person or Group (other than the Company or a Controlled Affiliate of the Company) of Beneficial Ownership of
previously outstanding shares of capital stock of the Company if (A) immediately after such acquisition, such person or Group, together with their respective Affiliates, shall own or hold shares of capital stock of the Company possessing at
least the Applicable Voting Control Percentage of the total voting power of the outstanding capital stock of the Company and (B) immediately prior to such acquisition, such person or Group, together with their respective Affiliates, did not own
or hold shares of capital stock of the Company possessing at least the Applicable Voting Control Percentage of the total voting power of the outstanding capital stock of the Company. Notwithstanding anything expressed or implied in the foregoing
provisions of this definition to the contrary, any direct or indirect acquisition referred to in clause (ii) above in this definition shall not be treated as a Change of Control if, at any time prior to or after such direct or indirect
acquisition a majority of the members of the Board as constituted immediately prior to such direct or indirect acquisition consent in writing to exclude such direct or indirect acquisition from the scope of this definition. 

2.10. “Code” means the Internal Revenue Code of 1986, as amended from time to time, or any successor statute thereto, and any
regulations issued from time to time thereunder. 
 2.11. “Controlled Affiliate” means, with respect to any person
or entity, any other person or entity that is controlled by such person or entity. 
 2.12. “Committee” means any committee
of the Board delegated responsibility by the Board for the administration of the Plan, as provided in Section 5 of the Plan. For any period during which no such committee is in existence, the term “Committee” shall mean the Board and
all authority and responsibility assigned the Committee under the Plan shall be exercised, if at all, by the Board. 
 2.13.
“Common Stock” means common stock, par value $0.001 per share, of the Company. 
 2.14. “Company”
means Proteostasis Therapeutics, Inc., a corporation organized under the laws of the State of Delaware. 
 2.15. “Grant
Date” means the date as of which an Option is granted, as determined under Section 7.1(a). 
 2.16. “Exchange
Act” means the Securities and Exchange Act of 1934, as amended. 
 2.17. “Group” has the meaning ascribed to
such term in Section 13(d)(3) of the Exchange Act or any successor section thereto. 
 2.18. “Incentive Option”
means an Option which by its terms is to be treated as an “incentive stock option” within the meaning of Section 422 of the Code. 

2.19. “Market Value” means the value of a share of Common Stock on a particular date as determined by the Committee.

 2.20. “Nonstatutory Option” means any Option that is not an Incentive Option. 

  
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 2.21. “Option” means an option to purchase shares of Common Stock. 

2.22. “Optionee” means an employee, consultant or director of the Company to whom an Option shall have been granted under the
Plan. 
 2.23. “Participant” means any holder of an outstanding Award under the Plan or any holder of shares of Common
Stock issued upon exercise of any Option. 
 2.24. “Plan” means this 2008 Equity Incentive Plan of the Company, as amended
and in effect from time to time. 
 2.25. “Restricted Stock” means a grant or sale of shares of Common Stock to an
employee, consultant or director of the Company subject to a Risk of Forfeiture. 
 2.26. “Restriction Period” means
the period of time, established by the Committee in connection with an Award of Restricted Stock, during which the shares of Restricted Stock are subject to a Risk of Forfeiture described in the applicable Award Agreement. 

2.27. “Risk of Forfeiture” means a limitation on the right of a Participant to retain an Award of Restricted
Stock, including a right in the Company to reacquire such Restricted Stock at less than their then Market Value, arising because of the occurrence or non-occurrence of specified events or conditions. 

2.28. “Sale of the Company Transaction” means any Transaction in which the stockholders of the Company
immediately prior to such Transaction, together with any and all of such stockholders’ Affiliates, do not own or hold, immediately after consummation of such Transaction, shares of capital stock of the Acquiring Person in connection with such
Transaction possessing at least a majority of the total voting power of the outstanding capital stock of such Acquiring Person. 
 2.29.
“Securities Act” means the Securities Act of 1933, as amended. 
 2.30. “Stock Grant” means
the grant pursuant to the Plan of shares of Common Stock not subject to restrictions or other forfeiture conditions. 
 2.31.
“Stockholder Agreement” means one agreement, or, collectively, two or more agreements, by and among the Company and one or more stockholders of the Company (including, without limitation, any Participant or prospective
Participant that became a stockholder of the Company as a result of the receipt or exercise of any Award) setting forth, among other provisions, restrictions on transfer, rights of first refusal, lock-up arrangements, bring-along rights, voting
agreements and/or the granting of certain proxies or powers of attorney with respect to shares of capital stock. Any and all provisions of this Plan that apply to any Stockholder Agreement shall be deemed to be applicable to any such agreement or
agreements referred to in this definition even if such agreement or agreements are called or titled something other than Stockholder Agreement or Stockholders Agreement. 

2.32. “Ten Percent Owner” means a person who owns, or is deemed within the meaning of Section 422(b)(6) of
the Code to own, stock possessing more than 10% of the total combined voting power of all classes of stock of the Company (or any parent or subsidiary corporations of the Company, as defined in Section 424(e) and (f), respectively, of the
Code). Whether a person is a Ten Percent Owner shall be determined with respect to each Option based on the facts existing immediately prior to the Grant Date of such Option. 

2.33. “Transaction” means any merger or consolidation of the Company with or into another person or entity or the sale or
transfer of all or substantially all of the assets of the Company, in each case in a single transaction or in a series of related transactions. 

  
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	3.	Term of the Plan 

 Unless the Plan shall have been earlier terminated by the Board,
Awards may be granted under this Plan at any time in the period commencing upon the date of approval of the Plan by the Board and ending immediately prior to the tenth anniversary of the earlier of the adoption of the Plan by the Board and the
approval of the Plan by the Company’s stockholders. Awards granted pursuant to the Plan within such period shall not expire solely by reason of the termination of the Plan. Awards of Incentive Options granted prior to stockholder approval of
the Plan are hereby expressly conditioned upon such approval, but in the event of the failure of the stockholders to approve the Plan shall thereafter and for all purposes be deemed to constitute Nonstatutory Options. 

 

	4.	Stock Subject to the Plan 

 At no time shall the number of shares of Common Stock issued
pursuant to or subject to outstanding Awards granted under the Plan (including, without limitation, pursuant to Incentive Options) exceed One Million Six Hundred Thousand (1,600,000) shares of Common Stock; subject, however, to the
provisions of Section 8 of the Plan. For purposes of applying the foregoing limitation, (a) if any Option expires, terminates, or is cancelled for any reason without having been exercised in full, or if any Award of Restricted Stock is
forfeited by a Participant, the shares not purchased by the Optionee or forfeited by such Participant shall again be available for Awards thereafter to be granted under the Plan, and (b) if any Option is exercised by delivering previously owned
shares in payment of the exercise price therefor, only the net number of shares, that is, the number of shares issued minus the number received by the Company in payment of the exercise price, shall be considered to have been issued pursuant to an
Award granted under the Plan. Shares of Common Stock issued pursuant to the Plan may be either authorized but unissued shares or shares held by the Company in its treasury. 
  

	5.	Administration 

 The Plan shall be administered by the Committee; provided,
however, that at any time and on any one or more occasions the Board may itself exercise any of the powers and responsibilities assigned the Committee under the Plan and when so acting shall have the benefit of all of the provisions of the
Plan pertaining to the Committee’s exercise of its authorities hereunder; and provided further that the Committee may delegate to an executive officer or officers the authority to grant Awards hereunder to employees who are not officers,
and to consultants, in accordance with such guidelines as the Committee shall set forth at any time or from time to time. Subject to the provisions of the Plan, the Committee shall have complete authority, in its discretion, to make or to select the
manner of making all determinations with respect to each Award to be granted by the Company under the Plan in addition to any other determination allowed the Committee under the Plan including, without limitation: (a) the employee, consultant
or director to receive the Award; (b) the form of Award; (c) whether an Option (if granted to an employee) will be an Incentive Option or a Nonstatutory Option; (d) the 

  
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time of granting an Award; (e) the number of shares subject to an Award; (f) the exercise price of an Option or purchase price, if any, for shares of Restricted Stock or for a Stock
Grant and the method of payment of such exercise price or such purchase price; (g) the term of an Option; (h) the vesting period of shares of Restricted Stock and any acceleration thereof; (i) the exercise date or dates of an Option
and any acceleration thereof; and (j) the effect of termination of any employment, consulting or Board member relationship with the Company or any of its Affiliates on the subsequent exercisability of an Option or on the Risk of Forfeiture of
Restricted Stock. In making such determinations, the Committee may take into account the nature of the services rendered by the respective employees, consultants and directors, their present and potential contributions to the success of the Company
and its Affiliates, and such other factors as the Committee in its discretion shall deem relevant. Subject to the provisions of the Plan, the Committee shall also have complete authority to interpret the Plan, to prescribe, amend and rescind rules
and regulations relating to it, to determine the terms and provisions of the respective Award Agreements (which need not be identical), and to make all other determinations necessary or advisable for the administration of the Plan. The
Committee’s determinations made in good faith on matters referred to in this Plan shall be final, binding and conclusive on all persons having or claiming any interest under the Plan or an Award made pursuant hereto. 

 

	6.	Authorization and Eligibility 

 The Committee may grant from time to time and at any time
prior to the termination of the Plan one or more Awards, either alone or in combination with any other Awards, to any employee of or consultant to one or more of the Company and its Affiliates or to any non-employee member of the Board or of any
board of directors (or similar governing authority) of any Affiliate. However, only employees of the Company or of any parent or subsidiary corporations of the Company, as defined in Sections 424(e) and (f), respectively, of the Code, shall be
eligible for the grant of an Incentive Option. 
 Each grant of an Award shall be subject to all applicable terms and conditions of the Plan
(including but not limited to any specific terms and conditions applicable to that type of Award set out in Section 7 below), and such other terms and conditions, not inconsistent with the terms of the Plan, as the Committee may prescribe. No
prospective Participant shall have any rights with respect to an Award, unless and until such Participant has executed an agreement evidencing the Award, delivered a fully executed copy thereof to the Company, and otherwise complied with the
applicable terms and conditions of such Award. 
  

	7.	Specific Terms of Awards 

 7.1. Options. 

(a) Date of Grant. The granting of an Option shall take place at the time specified in the Award Agreement. Only if
expressly so provided in the applicable Award Agreement shall the Grant Date be the date on which the Award Agreement shall have been duly executed and delivered by the Company and the Optionee. 

(b) Exercise Price. The price at which shares of Common Stock may be acquired under each Incentive Option shall be not less than
100% of the Market Value of Common Stock on the Grant Date, or not less than 110% of the Market Value of Common Stock on the Grant Date if the Optionee is a Ten Percent Owner. The price at which shares may be acquired under each Nonstatutory Option
shall not be so limited solely by reason of this Section. 

  
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 (c) Option Period. No Incentive Option may be exercised on or after the tenth
anniversary of the Grant Date, or on or after the fifth anniversary of the Grant Date if the Optionee is a Ten Percent Owner. The Option period under each Nonstatutory Option shall not be so limited solely by reason of this Section. 

(d) Exercisability. An Option may be immediately exercisable or become exercisable in such installments, cumulative or non-cumulative,
as the Committee may determine. In the case of an Option not otherwise immediately exercisable in full, the Committee may Accelerate such Option in whole or in part at any time; provided, however, that in the case of an Incentive
Option, any such Acceleration of such Incentive Option would not cause such Incentive Option to fail to comply with the provisions of Section 422 of the Code or the Optionee consents to such Acceleration. 

(e) Effect of Termination of Employment, Consulting or Board Member
Relationship. Unless the Committee shall provide otherwise with respect to any Option, if the applicable Optionee’s association with the Company or any of its Affiliates as an employee, director or consultant ends for any reason or no
reason, regardless of whether the end of such association is effected by the Company, any such Affiliate or such Optionee (whether voluntarily or involuntarily, including because an entity with which such Optionee has any such association ceases to
be an Affiliate of the Company), and immediately following the end of any such association, such Optionee is not associated with the Company or any of its Affiliates as an employee, director or consultant, or if such Optionee dies, then any
outstanding Option initially granted to such Optionee, whether then held by such Optionee or any other Participant, shall cease to be exercisable in any respect not later than ninety (90) days following the end of such association or such death
and, for the period it remains exercisable following the end of such association or such death, shall be exercisable only to the extent exercisable on the date of the end of such association or such death. Military or sick leave or other bona fide
leave shall not be deemed a termination of employment, provided that it does not exceed the longer of ninety (90) days or the period during which the absent Optionee’s reemployment rights, if any, are guaranteed by statute or by
contract. 
 (f) Transferability. Except as otherwise provided in this subsection (f), Options shall not be transferable, and no
Option or interest therein may be sold, transferred, pledged, assigned, or otherwise alienated or hypothecated, other than by will or by the laws of descent and distribution (subject always to the provisions of subsection (e) above). Except as
otherwise provided in this subsection (f), all of a Participant’s rights in any Option may be exercised during the life of such Participant only by such Participant or such Participant’s legal representative. The Committee may (at or after
the grant of a Nonstatutory Option) provide that a Nonstatutory Option may be transferred by the applicable Participant to a family member; provided, however, that any such transfer is without payment of any consideration whatsoever and that
no transfer of a Nonstatutory Option shall be valid unless first approved by the Committee, acting in its sole discretion, unless such transfer is permitted under the applicable Award Agreement. For this purpose, “family member” means any
child, stepchild, grandchild, parent, stepparent, spouse, former spouse, sibling, niece, nephew, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law, or sister-in-law, including adoptive relationships, any person sharing the
applicable Participant’s household (other than a tenant or employee), a trust in which the foregoing persons and/or the applicable Participant have more than fifty percent (50%) of the beneficial interests, a foundation in which the
foregoing persons and/or the applicable Participant control the management of assets, and any other entity in which these persons and/or the applicable Participant own more than fifty percent (50%) of the voting


  
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interests. The Committee may at any time or from time to time delegate to one or more officers of the Company the authority to permit transfers of Nonstatutory Options to third parties pursuant
to this subsection (f), which authorization shall be exercised by such officer or officers in accordance with guidelines established by any Committee at any time and from time to time. 

(g) Method of Exercise. An Option may be exercised by a Participant giving written notice, in the manner provided in
Section 15, specifying the number of shares of Common Stock with respect to which the Option is then being exercised. The notice shall be accompanied by payment in the form of cash or check payable to the order of the Company in an amount equal
to the exercise price of the shares of Common Stock to be purchased or, subject in each instance to the Committee’s approval, acting in its sole discretion and subject to such conditions, if any, as the Committee may deem necessary to comply
with applicable laws, rules and regulations or to avoid adverse accounting effects to the Company, by delivery to the Company of (i) shares of Common Stock having a Market Value equal to the exercise price of the shares to be purchased, or
(ii) the Participant’s executed promissory note in the principal amount equal to the exercise price of the shares to. be purchased and otherwise in such form as the Committee shall have approved. Receipt by the Company of such notice and
payment in any authorized or combination of authorized means shall constitute the exercise of the Option. Within thirty (30) days thereafter but subject to the remaining provisions of the Plan, the Company shall deliver or cause to be delivered
to the Participant or his agent a certificate or certificates for the number of shares then being purchased. Such shares shall be fully paid and nonassessable. Notwithstanding any of the foregoing provisions in this subsection (g) to the
contrary, (A) no Option shall be considered to have been exercised unless and until all of the provisions governing such exercise specified in the Plan and in the relevant Award Agreement shall have been duly complied with; and (B) the
obligation of the Company to issue any shares upon exercise of an Option is subject to the provisions of Section 9.1 hereof and to compliance by the Optionee and the Participant with all of the provisions of the Plan and the relevant Award
Agreement. 
 (h) Limit on Incentive Option Characterization. An Incentive Option shall be considered to
be an Incentive Option only to the extent that the number of shares of Common Stock for which the Option first becomes exercisable in a calendar year do not have an aggregate Market Value (as of the date of the grant of the Option) in excess of the
“current limit”. The current limit for any Optionee for any calendar year shall be $100,000 minus the aggregate Market Value at the date of grant of the number of shares of Common Stock available for purchase for the first time in
the same year under each other Incentive Option previously granted to the Optionee under the Plan, and under each other incentive stock option previously granted to the Optionee under any other incentive stock option plan of the Company and its
Affiliates, after December 31, 1986. Any shares of Common Stock which would cause the foregoing limit to be violated shall be deemed to have been granted under a separate Nonstatutory Option, otherwise identical in its terms to those of the
Incentive Option. 
 (i) Notification of Disposition. Each person exercising any Incentive Option granted under the
Plan shall be deemed to have covenanted with the Company to report to the Company any disposition of such shares prior to the expiration of the holding periods specified by Section 422(a)(1) of the Code and, if and to the extent that the
realization of income in such a disposition imposes upon the Company federal, state, local or other withholding tax requirements, or any such withholding is required to secure for the Company an otherwise available tax deduction, to remit to the
Company an amount in cash sufficient to satisfy those requirements. 
 (j) Rights Pending Exercise. No person holding
an Option shall be deemed for any purpose to be a stockholder of the Company with respect to any of the shares of Common Stock issuable pursuant to such Option, except to the extent that such Option shall have been exercised with respect thereto
and, in addition, a certificate shall have been issued therefor and delivered to such person or his agent. 

  
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 7.2. Restricted Stock. 

(a) Purchase Price. Shares of Restricted Stock shall be issued under the Plan for such consideration, in cash, other property or
services, or any combination thereof, as is determined by the Committee. 
 (b) Issuance of Certificates. Subject to
subsection (c) below, each Participant receiving an Award of Restricted Stock shall be issued a stock certificate in respect of such shares of Restricted Stock. Such certificate shall be registered in the name of such Participant, and, if
applicable, shall bear an appropriate legend referring to the terms, conditions, and restrictions applicable to such Award substantially in the following form: 

(c) The transferability of this certificate and the shares represented by this certificate are subject to the terms and conditions of the
Proteostasis Therapeutics, Inc. 2008 Equity Incentive Plan and an Award Agreement entered into by the registered owner and of Proteostasis Therapeutics, Inc. Copies of such Plan and Agreement are on file in the offices of Proteostasis Therapeutics,
Inc. 
 (d) Escrow of Shares. The Committee may require that the stock certificates evidencing shares of Restricted
Stock be held in custody by a designated escrow agent (which may but need not be the Company) until the restrictions thereon shall have lapsed, and that the Participant deliver a stock power, endorsed in blank, relating to the Common Stock covered
by such Award. 
 (e) Restrictions and Restriction Period. During the Restriction Period applicable to shares of
Restricted Stock, such shares shall be subject to limitations on transferability and a Risk of Forfeiture arising on the basis of such conditions related to the performance of services, Company or Affiliate performance or otherwise as the Committee
may determine and provide for in the applicable Award Agreement. Any such Risk of Forfeiture may be waived or terminated, or the Restriction Period shortened, at any time by the Committee on such basis as it deems appropriate. 

(f) Rights Pending Lapse of Risk of Forfeiture or Forfeiture of
Award. Except as otherwise provided in the Plan or the applicable Award Agreement, at all times prior to lapse of any Risk of Forfeiture applicable to, or forfeiture of, an Award of Restricted Stock, the Participant shall have all of the
rights of a stockholder of the Company, including the right to vote the shares of Restricted Stock. 
 (g) Effect of
Termination of Employment, Consulting or Board Member Relationship. Unless otherwise determined by the Committee at or after grant and subject to the applicable provisions of the Award
Agreement, if the applicable original grantee’s association with the Company or any of its Affiliates as an employee, director or consultant ends for any reason or no reason during the Restriction Period, regardless of whether the end of such
association is effected by the Company, any such Affiliate or such original grantee (whether 

  
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voluntarily or involuntarily, including because an entity with which such original grantee has any such association ceases to be an Affiliate of the Company), and immediately following the end of
any such association, such original grantee is not associated with the Company or any of its Affiliates as an employee, director or consultant, or if such original grantee dies, then all outstanding shares of Restricted Stock initially granted to
such original grantee that are still subject to Risk of Forfeiture, whether then held by such original grantee or any other Participant, shall be forfeited or otherwise subject to return to or repurchase by the Company if and to the extent so
provided by, and subject to and in accordance with, the terms of the applicable Award Agreement; provided, however, that military or sick leave or other bona fide leave shall not be deemed a termination of employment, if it does not exceed
the longer of ninety (90) days or the period during which the absent original grantee’s reemployment rights, if any, are guaranteed by statute or by contract. 

(h) Lapse of Restrictions. If and when the Restriction Period expires without a prior forfeiture of the Restricted Stock,
the certificates for such shares shall be delivered to the Participant promptly if not theretofore so delivered. 
 (i)
Transferability. Except as otherwise provided in this subsection (h), shares of Restricted Stock shall not be transferable during the applicable Restriction Period, and no share of Restricted Stock or interest therein may be sold,
transferred, pledged, assigned, or otherwise alienated or hypothecated during the applicable Restriction Period, other than by will or by the laws of descent and distribution (subject always to the provisions of subsection (f) above). The
applicable Award Agreement or the Committee (at or after the grant of a share of Restricted Stock) may provide that such share of Restricted Stock may be transferred by the applicable Participant to a family member during the applicable Restriction
Period; provided, however, that any such transfer is without payment of any consideration whatsoever and that no transfer of a share of Restricted Stock during the applicable Restriction Period shall be valid unless first approved by the
Committee, acting in its sole discretion, unless such transfer is permitted under the applicable Award Agreement. For this purpose, “family member” means any child, stepchild, grandchild, parent, stepparent, spouse, former spouse, sibling,
niece, nephew, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law, or sister-in-law, including adoptive relationships, any person sharing the applicable Participant’s household (other than a tenant or employee), a trust
in which the foregoing persons and/or the applicable Participant have more than fifty percent (50%) of the beneficial interests, a foundation in which the foregoing persons and/or the applicable Participant control the management of assets, and
any other entity in which these persons and/or the applicable Participant own more than fifty percent (50%) of the voting interests. The Committee may at any time or from time to time delegate to one or more officers of the Company the
authority to permit transfers of shares of Restricted Stock during the applicable Restriction Period to third parties pursuant to this subsection (h), which authorization shall be exercised by such officer or officers in accordance with guidelines
established by the Committee at any time and from time to time. 
 7.3. Stock Grants. 

(a) In General. Stock Grants shall be issued for such consideration, in cash, other property or services, or any combination
thereof, as is determined by the Committee. Without limiting the generality of the foregoing, Stock Grants may be awarded in such circumstances as the Committee deems appropriate, including without limitation in recognition of significant
contributions to the success of the Company or its Affiliates or in lieu of compensation otherwise already due. Stock Grants shall be made without forfeiture conditions of any kind. 

(b) Issuance of Certificates. Each Participant receiving a Stock Grant shall be issued a stock certificate in respect of
such Stock Grant. Such certificate shall be registered in the name of such Participant, and, if applicable, shall bear an appropriate legend referring to the terms, conditions, and restrictions applicable to such Award substantially in the following
form: 
 The transferability of this certificate and the shares represented by this certificate are subject to the terms and conditions of
the Proteostasis Therapeutics, Inc. 2008 Equity Incentive Plan. A copy of such Plan is on file in the offices of Proteostasis Therapeutics, Inc. 

  
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 7.4. Awards to Participants Outside the United
States. The Committee may modify the terms of any Award under the Plan granted to a Participant who is, at the time of grant or during the term of the Award, resident or primarily employed outside of the United States in any manner deemed by
the Committee to be necessary or appropriate in order that such Award shall conform to laws, regulations, and customs of the country in which the Participant is then resident or primarily employed, or so that the value and other benefits of the
Award to the Participant, as affected by foreign tax laws and other restrictions applicable as a result of the Participant’s residence or employment abroad, shall be comparable to the value of such an Award to a Participant who is resident or
primarily employed in the United States. An Award may be modified under this Section 7.4 in a manner that is inconsistent with the express terms of the Plan, so long as such modifications will not contravene any applicable law or regulation.
The Committee may establish supplements to, or amendments, restatements, or alternative versions of the Plan for the purpose of granting and administrating any such modified Award. No such modification, supplement, amendment, restatement or
alternative version may increase the share limit of Section 4. 
  

	8.	Adjustment Provisions 

 8.1. Adjustment for Corporate
Actions. Subject to the provisions of Section 8.2, if subsequent to such date the outstanding shares of Common Stock (or any other securities covered by the Plan by reason of the prior application of this Section) are increased,
decreased, or exchanged for a different number or kind of shares or other securities, or if additional shares or new or different shares or other securities are distributed with respect to such shares of Common Stock or other securities, through
merger, consolidation, sale of all or substantially all the property of the Company, reorganization, recapitalization, reclassification, stock dividend, stock split, reverse stock split, or other distribution with respect to such shares of Common
Stock, or other securities, an appropriate and proportionate adjustment will be made in (i) the maximum numbers and kinds of shares provided in Section 4, (ii) the numbers and kinds of shares or other securities subject to the then
outstanding Awards, (iii) the exercise price for each share or other unit of any other securities subject to then outstanding Options (without change in the aggregate purchase price as to which such Options remain exercisable), and
(iv) the repurchase price of each share of Restricted Stock then subject to a Risk of Forfeiture in the form of a Company repurchase right. 

8.2. Change of Control. Subject to the applicable provisions of the Award Agreement, in the event of a Change of Control,
the Committee shall have the discretion, exercisable in advance of, at the time of, or (except to the extent otherwise provided below) at any time after, the Change of Control, to provide for any or all of the following (subject to and upon such
terms as the Committee may deem appropriate): (A) the Acceleration, in whole or in part, of any or all outstanding Options (including Options that are assumed or replaced pursuant to clause (C) 


  
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below) that are not exercisable in full at the time the Change of Control, such Acceleration to become effective at the time of the Change of Control, or at such time following the Change of
Control that the employment, consulting or Board member relationship of the applicable Optionee or Optionees with the Company and its Affiliates terminates, or at such other time or times as the Committee shall determine; (B) the lapse or
termination of the Risk of Forfeiture (including, without limitation, any or all of the Company’s repurchase rights) with respect to outstanding Awards of Restricted Stock, such lapse or termination to become effective at the time of the Change
of Control, or at such time following the Change of Control that the employment, consulting or Board member relationship with the Company and its Affiliates of the Participant or Participants that hold such Awards of Restricted Stock (or the person
to whom such Awards of Restricted Stock were initially granted) terminates, or at such other time or times as the Committee shall determine; (C) the assumption of outstanding Options, or the substitution of outstanding Options with equivalent
options, by the acquiring or succeeding corporation or entity (or an affiliate thereof); or (D) the termination of all Options (other than Options that are assumed or substituted pursuant to clause (C) above) that remain outstanding at the
time of the consummation of the Change of Control, provided that, the Committee shall have made the determination to effect such termination prior to the consummation of the Change of Control and the Committee shall have given, or caused to
be given, to all Participants written notice of such potential termination at least five business days prior to the consummation of the Change of Control, and provided, further, that, if the Committee shall have determined in its sole and
absolute discretion that the Company make payment or provide consideration to the holders of such terminated Options on account of such termination, which payment or consideration shall be on such terms and conditions as the Committee shall have
determined (and which could consist of, in the Committee’s sole and absolute discretion, payment to the applicable Optionee or Optionees of an amount of cash equal to the difference between the Market Value of the shares of Common Stock for
which the Option is then exercisable and the aggregate exercise price for such shares under the Option), then the Company shall be required to make such payment or provide such consideration in accordance with the terms and conditions so determined
by the Committee; otherwise the Company shall not be required to make any payment or provide any consideration in connection with, or as a result of, the termination of Options pursuant to the foregoing provisions of this clause (D). The provisions
of this Section 8.2 shall not be construed as to limit or restrict in any way the Committee’s general authority under Sections 7.1(d) or 7.2(d) hereof to Accelerate Options in whole or in part at any time or to waive or terminate at any
time any Risk of Forfeiture applicable to shares of Restricted Stock. Each outstanding Option that is assumed in connection with a Change of Control, or is otherwise to continue in effect subsequent to a Change of Control, will be appropriately
adjusted, immediately after the Change of Control, as to the number and class of securities and the price at which it may be exercised in accordance with Section 8.1. 

8.3. Dissolution or Liquidation. Upon dissolution or liquidation of the Company, each outstanding Option shall terminate,
but the Optionee (if at the time he or she has an employment, consulting or Board member relationship with the Company or any of its Affiliates) shall have the right, immediately prior to such dissolution or liquidation, to exercise the Option to
the extent exercisable on the date of such dissolution or liquidation. 
 8.4. Related Matters. Any adjustment in Awards made
pursuant to this Section 8 shall be determined and made, if at all, by the Committee and shall include any correlative modification of terms, including exercise prices, rates of vesting or exercisability, Risks of Forfeiture and applicable
repurchase prices, which the Committee may deem necessary or appropriate so as to ensure that the rights of the Participants in their respective Awards are not 

  
 - 11 - 








 
substantially diminished nor enlarged as a result of the adjustment and corporate action other than as expressly contemplated in this Section 8. No fraction of a share shall be purchasable
or deliverable upon exercise, but in the event any adjustment hereunder of the number of shares covered by an Award shall cause such number to include a fraction of a share, such number of shares shall be adjusted to the nearest smaller whole number
of shares. No adjustment of an Option exercise price per share pursuant to this Section 8 shall result in an exercise price which is less than the par value of the Common Stock. 

 

	9.	Settlement of Awards 

 9.1. Violation of Law. Notwithstanding any
other provision of the Plan or the relevant Award Agreement, if, at any time, in the reasonable opinion of the Company, the issuance of shares of Common Stock covered by an Award may constitute a violation of law, then the Company may delay such
issuance and the delivery of a certificate for such shares until (i) approval shall have been obtained from such governmental agencies, other than the Securities and Exchange Commission, as may be required under any applicable law, rule, or
regulation and (ii) in the case where such issuance would constitute a violation of a law administered by or a regulation of the Securities and Exchange Commission, one of the following conditions shall have been satisfied: 

(a) the shares are at the time of the issue of such shares effectively registered under the Securities Act; or 

(b) the Company shall have determined, on such basis as it deems appropriate (including an opinion of counsel in form and substance
satisfactory to the Company) that the sale, transfer, assignment, pledge, encumbrance or other disposition of such shares or such beneficial interest, as the case may be, does not require registration under the Securities Act or any applicable state
securities laws. 
 9.2. Corporate Restrictions on Rights in Stock. Any Common Stock to be issued
pursuant to Awards granted under the Plan shall be subject to all restrictions upon the transfer thereof which may be now or hereafter imposed by the Certificate of Incorporation and the By-laws of the Company, each as amended and in effect from
time to time. Whenever Common Stock is to be issued pursuant to an Award, if the Committee so directs at the time of the grant (or, if such Award is an Option, at any time prior to the exercise thereof), the Company shall be under no obligation,
notwithstanding any other provision of the Plan or the relevant Award Agreement to the contrary, to issue such Common Stock until such time, if ever, as the recipient of the Award (and any person who exercises any Option, in whole or in part), until
such time, if ever, as such Participant or prospective Participant shall have become a party to and bound by any agreement that the Committee shall require in its sole discretion, including, without limitation, any Stockholder Agreement. In
addition, any Common Stock to be issued pursuant to Awards granted under the Plan shall be subject to all stop-transfer orders and other restrictions as the Committee may deem advisable under the rules, regulations and other requirements of any
stock exchange upon which the Common Stock is then listed, and any applicable federal or state securities laws, and the Committee may cause a legend or legends to be put on any such certificates to make appropriate reference to such restrictions.

 9.3. Investment Representations. The Company shall be under no obligation to issue any shares covered by an Award unless
the shares to be issued pursuant to Awards granted under the Plan have been effectively registered under the Securities Act or the Participant shall have 

  
 - 12 - 








 
made such written representations to the Company (upon which the Company believes it may reasonably rely) as the Company may deem necessary or appropriate for purposes of confirming that the
issuance of such shares will be exempt from the registration requirements of that Act and any applicable state securities laws and otherwise in compliance with all applicable laws, rules and regulations, including but not limited to that the
Participant is acquiring shares for his or her own account for the purpose of investment and not with a view to, or for sale in connection with, the distribution of any such shares. 

9.4. Registration. If the Company shall deem it necessary or desirable to register under the Securities Act or other applicable
statutes any shares of Common Stock issued or to be issued pursuant to Awards granted under the Plan, or to qualify any such shares of Common Stock for exemption from the Securities Act or other applicable statutes, then the Company shall take such
action at its own expense. The Company may require from each recipient of an Award, or each holder of shares of Common Stock acquired pursuant to the Plan, such information in writing for use in any registration statement, prospectus, preliminary
prospectus or offering circular as is reasonably necessary for such purpose and may require reasonable indemnity to the Company and its officers and directors from such holder against all losses, claims, damage and liabilities arising from such use
of the information so furnished and caused by any untrue statement of any material fact therein or caused by the omission to state a material fact required to be stated therein or necessary to make the statements therein not misleading in the light
of the circumstances under which they were made. 
 9.5. Lock-Up. Without the prior written consent of the Company or the managing
underwriter in any public offering of shares of Common Stock, no Participant shall sell, make any short sale of, loan, grant any option for the purchase of, pledge or otherwise encumber, or otherwise dispose of, any shares of Common Stock or any
Awards during the one hundred-eighty (180) day period commencing on the effective date of the registration statement relating to any underwritten public offering of securities of the Company. The foregoing restrictions are intended and shall be
construed so as to preclude any Participant from engaging in any hedging or other transaction that is designed to or reasonably could be expected to lead to or result in, a sale or disposition of any shares of Common Stock during such period even if
such shares of Common Stock are or would be disposed of by someone other than such Participant. Such prohibited hedging or other transactions would include, without limitation, any short sale (whether or not against the box) or any purchase, sale or
grant of any right (including without limitation any put or call option) with respect to any shares of Common Stock or with respect to any security that includes, relates to, or derives any significant part of its value from any shares of Common
Stock. Without limiting the generality and applicability of the foregoing provisions of this Section 9.5, if, in connection with any underwritten public offering of securities of the Company, the managing underwriter of such offering requires
that the Company’s directors and officers enter into a lock-up agreement, then (a) each Participant (regardless of whether or not such Participant has complied or complies with the provisions of clause (b) below) shall be bound by,
and shall be deemed to have agreed to, the same lock-up terms as those to which the Company’s directors and officers are required to adhere; and (b) at the request of the Company or such managing underwriter, each Participant shall execute
and deliver a lock-up agreement in form and substance equivalent to that which is required to be executed by the Company’s directors and officers. 

9.6. Placement of Legends; Stop Orders; etc. Each share of Common Stock to be issued pursuant to Awards granted under the
Plan may bear a reference to the investment representations made in accordance with Section 9.3 in addition to any other applicable restrictions under the Plan, the terms of the Award and, if applicable, under any Stockholder


  
 - 13 - 








 
Agreement or any other agreement between the Company and any Optionee and/or Participant, and to the fact that no registration statement has been filed with the Securities and Exchange Commission
in respect to such shares of Common Stock. All certificates for shares of Common Stock or other securities delivered under the Plan shall be subject to such stock transfer orders and other restrictions as the Committee may deem advisable under the
rules, regulations, and other requirements of any stock exchange upon which the Common Stock is then listed, and any applicable federal or state securities law, and the Committee may cause a legend or legends to be placed on any such certificates to
make appropriate reference to such restrictions. 
 9.7. Tax Withholding. Whenever shares of Common Stock are issued or to be
issued pursuant to Awards granted under the Plan, the Company shall have the right to require the recipient to remit to the Company an amount sufficient to satisfy federal, state, local or other withholding tax requirements if, when, and to the
extent required by law (whether so required to secure for the Company an otherwise available tax deduction or otherwise) prior to the delivery of any certificate or certificates for such shares. The obligations of the Company under the Plan shall be
conditional on satisfaction of all such withholding obligations and the Company shall, to the extent permitted by law, have the right to deduct any such taxes from any payment of any kind otherwise due to the recipient of an Award. However, in such
cases Participants may elect, subject to the approval of the Committee, acting in its sole discretion, to satisfy an applicable withholding requirement, in whole or in part, by having the Company withhold shares to satisfy their tax obligations.
Participants may only elect to have shares of Common Stock withheld having a Market Value on the date the tax is to be determined equal to the minimum statutory total tax which could be imposed on the transaction. All elections shall be irrevocable,
made in writing, signed by the Participant, and shall be subject to any restrictions or limitations that the Committee deems appropriate 
  

	10.	Reservation of Stock 

 The Company shall at all times during the term of the Plan and any
outstanding Options granted hereunder reserve or otherwise keep available such number of shares of Common Stock as will be sufficient to satisfy the requirements of the Plan (if then in effect) and such Options and shall pay all fees and expenses
necessarily incurred by the Company in connection therewith. 
  

	11.	No Special Service Rights 

 Nothing contained in the Plan or in any Award Agreement shall
confer upon any recipient of an Award any right with respect to the continuation of his or her employment, consulting or Board member relationship or other association with the Company (or any Affiliate), or interfere in any way with the right of
the Company (or any Affiliate), subject to the terms of any separate employment, consulting or Board member agreement or provision of law or corporate articles or by-laws to the contrary, at any time to terminate such employment, consulting or Board
member agreement or to increase or decrease, or otherwise adjust, the other terms and conditions of the recipient’s employment, consulting or Board member relationship or other association with the Company and its Affiliates. 

 

	12.	Nonexclusivity of the Plan 

 Neither the adoption of the Plan by the Board nor the
submission of the Plan to the stockholders of the Company shall be construed as creating any limitations on the power of the Board to adopt such other incentive arrangements as it may deem desirable, including without limitation, the granting of
stock options and restricted stock other than under the Plan, and such arrangements may be either applicable generally or only in specific cases. 

  
 - 14 - 









	13.	Termination and Amendment of the Plan 

 The Board may at any time terminate the Plan or
make such amendments or modifications of the Plan as it shall deem advisable. In the event of the termination of the Plan, the terms of the Plan shall survive any such termination with respect to any Award that is outstanding on the date of such
termination, unless the holder of such Award agrees in writing to terminate such Award or to terminate all or any of the provisions of the Plan that apply to such Award. Unless the Board otherwise expressly provides, any amendment or modification of
the Plan shall affect the terms of any Award outstanding on the date of such amendment or modification as well as the terms of any Award made from and after the date of such amendment or modification; provided, however, that, except to the extent
otherwise provided in the last sentence of this paragraph, (i) no amendment or modification of the Plan shall apply to any Award that is outstanding on the date of such amendment or modification if such amendment or modification would reduce
the number of shares subject to such Award, increase the purchase price applicable to shares subject to such Award or materially adversely affect the provisions applicable to such Award that relate to the vesting or exercisability of such Award or
of the shares subject to such Award, (ii) no amendment or modification of the Plan shall apply to any Incentive Option that is outstanding on the date of such amendment or modification if such amendment or modification would result in such
Incentive Option no longer being treated as an “incentive stock option” within the meaning of Section 422 of the Code and (iii) no amendment or modification of the Plan shall apply to any Award that is outstanding on the date of
such amendment or modification unless such amendment or modification of the Plan shall also apply to all other Awards outstanding on the date of such amendment or modification. In the event of any amendment or modification of the Plan that is
described in clause (i), (ii) or (iii) of the foregoing proviso, such amendment or modification of the Plan shall apply to any Award outstanding on the date of such amendment or modification only if the recipient of such Award consents in
writing thereto. 
 The Committee may amend or modify, prospectively or retroactively, the terms of any outstanding Award without amending
or modifying the terms of the Plan itself, provided that as amended or modified such Award is consistent with the terms of the Plan as in effect at the time of the amendment or modification of such Award, but no such amendment or modification of
such Award shall, without the written consent of the recipient of such Award, reduce the number of shares subject to such Award, increase the purchase price applicable to shares subject to such Award, adversely affect the provisions applicable to
such Award that relate to the vesting or exercisability of such Award or of the shares subject to such Award, or otherwise materially adversely affect the terms of such Award (except for amendments or modifications to the terms of such Award or of
the stock subject to such Award that are expressly permitted by the terms of the Plan or that result from any amendment or modification of the Plan in accordance with the provisions of the first paragraph of this Section 13), or, if such Award
is an Incentive Option, result in such Incentive Option no longer being treated as an “incentive stock option” within the meaning of Section 422 of the Code. Notwithstanding any of the foregoing provisions of this paragraph to the
contrary, the Committee is expressly authorized to amend any or all outstanding Options to effect a repricing thereof by lowering the purchase price applicable to the shares of Common Stock subject to such Option or Options without the approval of
the stockholders of the Company or the holder or holders of such Option or Options, and, in connection with such repricing, to amend or modify any of the other terms of the Option or Options so repriced,


  
 - 15 - 








 
including, without limitation, for purposes of reducing the number of shares subject to such Option or Options or for purposes of adversely affecting the provisions applicable to such Option or
Options that relate to the vesting or exercisability thereof, in each case without the approval of stockholders of the Company or the holder or holders of such Option or Options. 

In addition, notwithstanding anything express or implied in any of the foregoing provisions of this Section 13 to the contrary, the
Committee may amend or modify, prospectively or retroactively, the terms of any outstanding Award to the extent the Committee reasonably determines necessary or appropriate to conform such Award to the requirements of Section 409A of the Code
(concerning non-qualified deferred compensation), if applicable. 
  

	14.	Interpretation of the Plan 

 In the event of any conflict between the provisions of this
Plan and the provisions of any applicable Award Agreement, the provisions of this Plan shall control, except if and to the extent that the conflicting provision in such Award Agreement was authorized and approved by the Committee at the time of the
grant of the Award evidenced by such Award Agreement or is ratified by the Committee at any time subsequent to the grant of such Award, in which case the conflicting provision in such Award Agreement shall control. Without limiting the generality of
the foregoing provisions of this Section 14, insofar as possible the provisions of the Plan and such Award Agreement shall be construed so as to give full force and effect to all such provisions. In the event of any conflict between the
provisions of this Plan and the provisions of any applicable Stockholder Agreement or other agreement between the Company and the Optionee and/or Participant, the provisions of such Stockholder Agreement or other agreement shall control except as
required to fulfill the intention that this Plan constitute an incentive stock option plan within the meaning of Section 422 of the Code, but insofar as possible the provisions of the Plan and any such Stockholder Agreement or other agreement
shall be construed so as to give full force and effect to all such provisions. 
  

	15.	Notices and Other Communications 

 Any notice, demand, request or other communication
hereunder to any party shall be deemed to be sufficient if contained in a written instrument delivered in person or duly sent by first class registered, certified or overnight mail, postage prepaid, or telecopied with a confirmation copy by regular,
certified or overnight mail, addressed or telecopied, as the case may be, (i) if to the recipient of an Award, at his or her residence address last filed with the Company and (ii) if to the Company, at its principal place of business,
addressed to the attention of its Chief Executive Officer, or to such other address or telecopier number, as the case may be, as the addressee may have designated by notice to the addressor. All such notices, requests, demands and other
communications shall be deemed to have been received: (i) in the case of personal delivery, on the date of such delivery; (ii) in the case of mailing, when received by the addressee; and (iii) in the case of facsimile transmission,
when confirmed by facsimile machine report. 
  

	16.	Governing Law 

 The Plan and all Award Agreements and actions taken thereunder shall be
governed, interpreted and enforced in accordance with the laws of the Commonwealth of Massachusetts, without regard to the conflict of laws principles thereof. 

  
 - 16 - 









	17.	Adoption of Plan 

 This 2008 Equity Incentive Plan was approved and adopted on August 15,
2008 by each of the Board of Directors and Stockholders. 

  
 - 17 - 








 ATTACHMENT A 

TO 
 2008
EQUITY INCENTIVE PLAN 
 Provisions Applicable to Award Recipients 

Resident in California 

Until such time as the Company’s Stock has been effectively registered under the Securities Act and if required by any applicable law,
the following additional terms shall apply to Awards, and Stock issued pursuant to such Awards, granted under the Plan to persons resident in California as of the date of grant of the Award (each such person, a “California
Recipient”). Capitalized terms not defined in this Attachment shall have the respective meanings set forth in the Plan. 
 1. No
Option or other right to acquire Stock pursuant to an Award issued to any California Recipient, that is otherwise transferable pursuant to the terms of the Plan, shall be transferable other than by gift or domestic relations order to an immediate
family member as that term is defined under applicable California and Federal securities law or to a revocable trust (or by will or the laws of descent and distribution). 

2. The following limitations shall apply to the early expiration of Options granted California Recipients on account of termination of
employment (unless employment is terminated for cause as defined by applicable law): 
 (a) Subject to Section 2(b) below, in the event
the employment or other association with the Company and its Affiliates of an Optionee who is a California Resident is terminated, whether voluntary or otherwise and including on account of an entity ceasing to be an Affiliate of the Company, such
California Recipient shall have at least 30 days after the date of such termination (but in no event later than the expiration of the term of such Option as set forth in the Award Agreement) to exercise such Option to the extent exercisable as of
the date of such termination. 
 (b) In the event that the employment or association with the Company and its Affiliates of an Optionee who
is a California Resident is terminated as a result of death or disability, such California Recipient shall have at least 6 months after the date of such termination (but in no event later than the expiration of the term of such Option as set forth
in the Award Agreement) to exercise such Option to the extent exercisable as of the date of such termination. 
 3. The Plan must be
approved by a majority of the outstanding securities entitled to vote within 12 months before or after the later of (i) the date the Plan is adopted by the Company and (ii) the date on which any Option or other Award is granted to a
California Recipient. 

  
 - 18 - 








 PROTEOSTASIS THERAPEUTICS, INC. 

FIRST AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 1,600,000 shares of Common Stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (the “Plan”); and 

WHEREAS, the Board of Directors of the Corporation, by written consent effective as of April 13, 2009, and the stockholders of the
Corporation, by written consent effective as of April 13, 2009, approved and authorized this First Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be increased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended and restated as follows: 

In Section 4 of the Plan, the number “One Million Six Hundred Thousand (1,600,000)” is hereby deleted and replaced with the
number “Four Million Five Hundred Thousand (4,500,000)”. 
 Except as expressly set forth above, all of the terms and provisions
of the Plan shall remain in full force and effect and all references to the Plan shall hereinafter be deemed to be references to the Plan as amended by this First Amendment. 







 PROTEOSTASIS THERAPEUTICS, INC. 

SECOND AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 4,500,000 shares of Common Stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (the “Plan”); and 

WHEREAS, the Board of Directors of the Corporation, by written consent effective as of December 1, 2010, and the stockholders of the
Corporation, by written consent effective as of December 1, 2010, approved and authorized this Second Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be increased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended and restated as follows: 

In Section 4 of the Plan, the number “Four Million Five Hundred Thousand (4,500,000)” is hereby deleted and replaced with the
number “Seven Million Eight Hundred Fifty Thousand (7,850,000)”. 
 Except as expressly set forth above, all of the terms and
provisions of the Plan shall remain in full force and effect and all references to the Plan shall hereinafter be deemed to be references to the Plan as amended by this Second Amendment. 

[End of Document] 







 PROTEOSTASIS THERAPEUTICS, INC. 

THIRD AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 7,850,000 shares of Common Stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (the “Plan”); and 

WHEREAS, the Board of Directors of the Corporation, by written consent effective as of July 5, 2011, and the stockholders of the
Corporation, by written consent effective as of September 8, 2011, approved and authorized this Third Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be increased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended and restated as follows: 

In Section 4 of the Plan, the number “Seven Million Eight Hundred Fifty Thousand (7,850,000)” is hereby deleted and replaced
with the number “Fourteen Million Nine Hundred Fifty Thousand (14,950,000)”. 
 Except as expressly set forth above, all of the
terms and provisions of the Plan shall remain in full force and effect and all references to the Plan shall hereinafter be deemed to be references to the Plan as amended by this Third Amendment. 

[End of Document] 







 PROTEOSTASIS THERAPEUTICS, INC. 

FOURTH AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 14,950,000 shares of Common Stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (as amended, the “Plan); and 

WHEREAS, the Board of Directors of the Corporation, at a regular meeting held on March 15, 2012, and the stockholders of the Corporation, by
written consent effective as of May 23, 2012, approved and authorized this Fourth Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be increased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended and restated as follows: 

In Section 4 of the Plan, the number “Fourteen Million Nine Hundred Fifty Thousand (14,950,000)” is hereby deleted and replaced
with the number “Seventeen Million Eight Hundred Forty Seven Thousand Three Hundred Seventy Five (17,847,375)”. 
 Except as
expressly set forth above, all of the terms and provisions of the Plan shall remain in full force and effect and all references to the Plan shall hereinafter be deemed to be references to the Plan as amended by this Fourth Amendment. 

[End of Document] 







 PROTEOSTASIS THERAPEUTICS, INC. 

FIFTH AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 17,847,375 shares of common stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (as amended, the “Plan); and 

WHEREAS, the Board of Directors of the Corporation, by written consent effective as of July 31, 2014, and the stockholders of the
Corporation, by written consent effective as of July 31, 2014, approved and authorized this Fifth Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be increased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended and restated as follows: 

In Section 4 of the Plan, the number “Seventeen Million Eight Hundred Forty Seven Thousand Three Hundred Seventy Five
(17,847,375)” is hereby deleted and replaced with the number “Forty Million Three Hundred Forty Seven Thousand Three Hundred Seventy Five (40,347,375)”. 

Except as expressly set forth above, all of the terms and provisions of the Plan shall remain in full force and effect and all references to
the Plan shall hereinafter be deemed to be references to the Plan as amended by this Fifth Amendment. 
 [End of Document] 







 PROTEOSTASIS THERAPEUTICS, INC. 

SIXTH AMENDMENT 
 TO THE 

2008 EQUITY INCENTIVE PLAN 

WHEREAS, 40,347,375 shares of common stock, par value $0.001 per share, of Proteostasis Therapeutics, Inc., a Delaware corporation (the
“Corporation”), are currently reserved under the Corporation’s 2008 Equity Incentive Plan (as amended, the “Plan”); and 

WHEREAS, the Board of Directors of the Corporation, by adoption of resolutions on March 27, 2015, and the stockholders of the
Corporation, by action by written consent effective as of May 8, 2015, approved and authorized this Sixth Amendment to the Plan, pursuant to which the number of shares reserved under the Plan shall be decreased as set forth herein. 

NOW THEREFORE, the Plan is hereby amended as follows: 

In Section 4 of the Plan, the number “Forty Million Three Hundred Forty Seven Thousand Three Hundred Seventy Five (40,347,375)”
is hereby deleted and replaced with the number “Seventeen Million Eight Hundred Forty Seven Thousand Three Hundred Seventy Five (17,847,375)”. 

Except as expressly set forth above, all of the terms and provisions of the Plan shall remain in full force and effect and all references to
the Plan shall hereinafter be deemed to be references to the Plan as amended by this Sixth Amendment. 
 [End of document] 







 PROTEOSTASIS THERAPEUTICS, INC. 

NONSTATUTORY STOCK OPTION AGREEMENT 

NONSTATUTORY STOCK OPTION AGREEMENT, dated as of [DATE] (this “Agreement”), by and between Proteostasis Therapeutics, Inc., a
Delaware corporation (the “Company”), and [NAME] (the “Optionee”). 
  

	 	1.	Grant of Option. 

 Pursuant to the Company’s 2008 Equity Incentive Plan (as amended
from time to time, the “Plan”), a copy of which is attached hereto as Exhibit A, the Company grants to the Optionee an option (the “Option”) to purchase from the Company all or any part of a total
of [NUMBER] shares (the “Option Shares”) of the Company’s common stock, $0.001 par value per share, at a price of $ [PRICE] per share. The Option is granted as of [BOARD MEETING DATE] (the “Grant Date”).

  

	 	2.	Character of Option. 

 The Option is not intended to be treated as an “incentive
stock option” within the meaning of Section 422 of the Internal Revenue Code of 1986, as amended (the “Code”), but as a nonstatutory stock option within the meaning of the Code. 

 

	 	3.	Duration of Option. 

 Unless subject to earlier expiration or termination pursuant to the
terms of the Plan, the Option shall expire on [DATE TEN (10) YEARS FROM GRANT DATE]. 
  

	 	4.	Exercise of Option. 

 Until its expiration or termination pursuant to Section 3
hereof, the Option may be exercised in the manner specified in Section 7.1(d) of the Plan and the Option Shares shall become exercisable, on a cumulative basis, over a 4-year period, as follows: 

[(a) 25% of the Option Shares vesting on [DATE] 

(b) 6.25% of the Option Shares vesting each quarter thereafter.] 

Notwithstanding anything express or implied to the contrary in the foregoing provisions of this Section 4, any portion or installment of
the Option Shares for which the Option is scheduled to become exercisable on any given date pursuant to this Section 4, shall never become exercisable (either on such date or on any other date) if on such date the Optionee is no longer a
consultant or advisor to, or an employee of, the Company for any reason or no reason (including, without limitation, termination by Optionee or the Company of the relationship for any reason or no reason). 

 

	 	5.	Transfer of Options. 

 The Option may not be transferred except by prior written consent
of the Company. 

  
 2008-{option grant number} 









	 	6.	Incorporation of Plan Terms. 

 The Option is granted subject to all of the applicable
terms and provisions of the Plan, including but not limited to the limitations on the Company’s obligation to deliver the Option Shares upon exercise thereof as set forth in Section 9 of the Plan. 

 

	 	7.	Miscellaneous. 

 This Agreement shall be construed and enforced in accordance with the
laws of the Commonwealth of Massachusetts and shall be binding upon and inure to the benefit of any successor or assign of the Company and any assign, executor, administrator, trustee, guardian, or other legal representative of the Optionee. 

IN WITNESS WHEREOF, the parties have executed this Non-Statutory Stock Option Agreement as a sealed instrument as of the date first above
written. 
  

							
	PROTEOSTASIS THERAPEUTICS, INC.				OPTIONEE
				
	By:		  
				  

	Name:						Name: [NAME]
	Title:						
							Address:
				
							  

				
							  

				
							  



  
 2008-{option grant number} 








 PROTEOSTASIS THERAPEUTICS, INC. 

INCENTIVE STOCK OPTION AGREEMENT 

INCENTIVE STOCK OPTION AGREEMENT, dated as of [DATE] (this “Agreement”), by and between Proteostasis Therapeutics, Inc., a
Delaware corporation (the “Company”), and [NAME] (the “Optionee”). 
  

	 	1.	Grant of Option. 

 Pursuant to the Company’s 2008 Equity Incentive Plan (as amended
from time to time, the “Plan”), a copy of which is attached hereto as Exhibit A, the Company grants to the Optionee an option (the “Option”) to purchase from the Company all or any part of a total
of [NUMBER] shares (the “Option Shares”) of the Company’s common stock, $0.001 par value per share, at a price of $ [PRICE] per share. The Option is granted as of [BOARD MEETING DATE] (the “Grant Date”).

  

	 	2.	Character of Option. 

 The Option is intended to be treated as an “incentive stock
option” within the meaning of Section 422 of the Internal Revenue Code of 1986, as amended (the “Code”). 
  

	 	3.	Duration of Option. 

 Unless subject to earlier expiration or termination pursuant to the
terms of the Plan, the Option shall expire on the earlier of (i) the tenth anniversary of the Grant Date and (ii) the 90th day following the termination of the Optionee’s employment
with the Company for any reason. 
  

	 	4.	Exercise of Option. 

 Until its expiration or termination pursuant to Section 3
hereof, the Option may be exercised in the manner specified in Section 7.1(d) of the Plan and the Option Shares shall become exercisable, on a cumulative basis, over a 4-year period, as follows: 

[(a) 25% of the Option Shares vesting on [DATE] (the one-year anniversary of the Optionee’s employment start date) and 

(b) 6.25% of the Option Shares vesting each quarter thereafter.] 

Any portion or installment of the Option Shares for which the Option is scheduled to become exercisable on any given date pursuant to this
Section 4, shall never become exercisable (either on such date or on any other date) if on such date the Optionee is no longer an employee of the Company for any reason or no reason (including, without limitation, termination of employment by
Optionee or the Company for any reason or no reason). 
  

	 	5.	Transfer of Options. 

 The Option may not be transferred except by will or the laws of
descent and distribution, and, during the lifetime of the Optionee, may be exercised only by the Optionee. 

  
 2008-[insert grant number] 









	 	6.	Incorporation of Plan Terms. 

 The Option is granted subject to all of the applicable
terms and provisions of the Plan, including but not limited to the limitations on the Company’s obligation to deliver the Option Shares upon exercise thereof as set forth in Section 9 of the Plan. 

 

	 	7.	Miscellaneous. 

 This Agreement shall be construed and enforced in accordance with the
laws of the Commonwealth of Massachusetts and shall be binding upon and inure to the benefit of any successor or assign of the Company and any executor, administrator, trustee, guardian, or other legal representative of the Optionee. 

IN WITNESS WHEREOF, the parties have executed this Incentive Stock Option Agreement as a sealed instrument as of the date first above written.

  

							
	PROTEOSTASIS THERAPEUTICS, INC.				OPTIONEE
				
	By:		  
				  

	Name:						Name: [NAME]
	Title:						
							Address:
				
							  

				
							  

				
							  



  
 2008-[insert grant number]