X0306 4 2016-02-17 0 0001445283 PROTEOSTASIS THERAPEUTICS, INC. PTI 0001262326 Mirabelli Christopher C/O HEALTHCARE VENTURES LLC, 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 1 0 1 0 0001358990 HealthCare Ventures VIII, L.P. C/O HEALTHCARE VENTURES LLC, 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001013473 CAVANAUGH JAMES H C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001358992 HealthCare Partners VIII, LLC C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001358991 HealthCare Partners VIII, L.P. C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001262327 LAWLOR AUGUSTINE C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001013469 LITTLECHILD JOHN W C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 0001013603 WERNER HAROLD R C/O HEALTHCARE VENTURES LLC 47 THORNDIKE STREET, SUITE B1-1 CAMBRIDGE MA 02141 0 0 1 0 Common Stock 2016-02-17 4 C 0 1774295 A 1774295 I See Footnote Common Stock 2016-02-17 4 P 0 375000 8.00 A 2149295 I See Footnote Series A Convertible Redeemable Preferred Stock 2016-02-17 4 C 0 11600000 0.00 D Common Stock 1559707 0 I See Footnote Series B Convertible Redeemable Preferred Stock 2016-02-17 4 C 0 2319750 0.00 D Common Stock 214588 0 I See Footnote Series A Convertible Preferred Warrant (Right to Buy) 1.00 2016-02-17 4 J 0 160000 0.00 D 2008-07-08 2018-07-08 Series A Preferred Stock 160000 0 I See Footnote Common Stock Warrant (Right to Buy) 10.8102 2016-02-17 4 J 0 14800 0.00 A 2008-07-08 2018-07-08 Common Stock 14800 14800 I See Footnote This transaction represents the total number of shares of Common Stock of the Issuer (the "Issuer Common Stock") received upon the conversion of the Issuer's Series A Convertible Redeemable Preferred Stock (the "Series A Shares"), the payment of accrued dividends on the Series A Shares and the conversion of the Issuer's Series B Convertible Redeemable Preferred Stock ( collectively, the "Preferred Shares"). The securities are held of record by HealthCare Ventures VIII, L.P. ("HCVVIII"). HealthCare Partners VIII, L.P. ("HCPVIII") is the General Partner of HCVVIII and HealthCare Partners VIII, LLC (the "LLC") is the General Partner of HCPVIII. Each of James Cavanaugh, Harold Werner, John Littlechild, Christopher Mirabelli and Augustine Lawlor are the managing directors of the LLC and exercise shared voting and investment power with respect to the shares owned by HCVVIII. Each of the reporting persons disclaims beneficial ownership of such securities, except to the extent of his, her or its proportionate pecuniary interest therein. Dr. Mirabelli is also a director of the issuer. These securities were purchased in the initial public offering of the Issuer. The Preferred Shares automatically converted into Issuer Common Stock on a 10.8102-to-1 basis upon the closing of the initial public offering of the Issuer without payment or further consideration. The Preferred Shares were convertible at any time at the election of the Reporting Person and had no expiration date. These securities include shares of Issuer Common Stock received as payment for accrued dividends on the Series A Shares. This transaction represents a deemed disposition of warrants to purchase Series A Shares and a deemed acquisition of warrants to purchase shares of Issuer Common Stock on a 10.8102-to-1 basis, which occurred automatically upon the closing of the initial public offering of the Issuer without payment or further consideration. The transaction is listed solely for the purpose of reporting the change in the shares underlying the warrants. /s/ Jeffrey B. Steinberg, Attorney-in-Fact for Christopher Mirabelli 2016-02-17 /s/ Jeffrey B. Steinberg, Administrative Partner of HealthCare Ventures VII, L.P. 2016-02-17 /s/ Jeffrey B. Steinberg, Attorney-in-Fact for James H. Cavanaugh 2016-02-17 /s/ Jeffrey B. Steinberg, Administrative Officer of HealthCare Partners VIII LLC 2016-02-17 /s/ Jeffrey B. Steinberg, Administrative Partner of HealthCare Partners VIII, L.P. 2016-02-17 /s/ Jeffrey B. Steinberg, Attorney-in-Fact for Augustine Lawlor 2016-02-17 /s/ Jeffrey B. Steinberg, Attorney-in-Fact for John Littlechild 2016-02-17 /s/ Jeffrey B. Steinberg, Attorney-in-Fact for Harold Werner 2016-02-17