Cryo-EM analysis provides new mechanistic insight into ATP binding to Ca -ATPase SERCA2b.

Sarco/endoplasmic reticulum Ca -ATPase (SERCA) 2b is a ubiquitous SERCA family member that conducts Ca uptake from the cytosol to the ER. Herein, we present a 3.3 Å resolution cryo-electron microscopy (cryo-EM) structure of human SERCA2b in the E1·2Ca state, revealing a new conformation for Ca -bound SERCA2b with a much closer arrangement of cytosolic domains than in the previously reported crystal structure of Ca -bound SERCA1a. Multiple conformations generated by 3D classification of cryo-EM maps reflect the intrinsically dynamic nature of the cytosolic domains in this state. Notably, ATP binding residues of SERCA2b in the E1·2Ca state are located at similar positions to those in the E1·2Ca -ATP state; hence, the cryo-EM structure likely represents a preformed state immediately prior to ATP binding. Consistently, a SERCA2b mutant with an interdomain disulfide bridge that locks the closed cytosolic domain arrangement displayed significant autophosphorylation activity in the presence of Ca . We propose a novel mechanism of ATP binding to SERCA2b.