Abstract

SERCA2b is a Ca2+-ATPase that pumps Ca2+ from the cytosol into the ER and maintains the cellular calcium homeostasis. Herein, we present cryo-EM structures of human SERCA2b in E1{middle dot}2Ca2+-AMPPCP and E2-BeF3- states at 2.9 and 2.8 [A] resolutions, respectively. The structures revealed that the luminal extension tail (LE) characteristic of SERCA2b runs parallel to the lipid-water boundary near the luminal ends of transmembrane (TM) helices TM10 and TM7, and approaches the luminal loop flanked by TM7 and TM8. Upon deletion of the LE, the cytosolic- and TM-domain arrangement of SERCA2b resembled that of SERCA1a, resulting in multiple conformations. The LE regulates the conformational transition between the E1{middle dot}2Ca2+-ATP and E2P states, explaining the different kinetic properties of SERCA2b from other isoforms lacking the LE. One sentence summaryCryo-EM structures of SERCA2b at 2.8-2.9 [A] resolutions reveal how the luminal extension tail regulates its activity.