Protagonist Therapeutics Inc — Additional Proxy Materials (DEFA14A)

Your Vote Counts! Smartphone users Point your camera here and vote without entering a control number For complete information and to vote, visit www.ProxyVote.com Control # V95678-P50686 *Please check the meeting materials for any special requirements for meeting attendance. PROTAGONIST THERAPEUTICS, INC. 7707 GATEWAY BLVD. SUITE 140 NEWARK, CA 94560 PROTAGONIST THERAPEUTICS, INC. 2026 Annual Meeting Vote by June 16, 2026 11:59 PM ET Vote Virtually at the Meeting* June 17, 2026 10:00 AM PT Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 3, 2026. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. Vote Before the Meeting by Internet: To vote now by internet, go to www.proxyvote.com. Have the control number that is printed in the box available and follow the instructions. Vote Before the Meeting by Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. Vote Virtually at the Meeting: To vote during the Meeting go to www.virtualshareholdermeeting.com/PTGX2026. Have the control number that is printed in the box available and follow the instructions. You invested in PROTAGONIST THERAPEUTICS, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 17, 2026.

Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. Voting Items Board Recommends V95679-P50686 THIS IS NOT A VOTABLE BALLOT This is an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials at www.proxyvote.com or easily request a paper copy. We encourage you to access and review all of the important information contained in the proxy materials before voting. Please follow the instructions on the reverse side to vote on these important matters. 1. To elect the Class I director nominees named below to hold office until the 2029 Annual Meeting of Stockholders and until their successors are duly elected and qualified. Nominees: For 01) Dinesh V. Patel, Ph.D. 02) Lewis T. “Rusty” Williams, M.D., Ph.D. 2. To approve, on a non-binding, advisory basis, the compensation of the Company’s named executive officers. For 3. To ratify the selection of Ernst & Young LLP as the Company’s independent registered public accounting firm for its fiscal year ending December 31, 2026. For 4. To approve the adoption of the Company’s 2026 Equity Incentive Plan. For NOTE: Such other business as may properly come before the meeting or any adjournment or postponement thereof. The Board of Directors knows of no other matters that are expected to be presented for consideration at the 2026 Annual Meeting of Stockholders. If any other matters are properly brought before the 2026 Annual Meeting of Stockholders or any postponement or adjournment thereof, it is the intention of the proxies named in the proxy card to vote on such matters in accordance with their best judgment.