Structural basis of the conformational and functional regulation of human SERCA2b, the ubiquitous endoplasmic...

Sarco/endoplasmic reticulum Ca²⁺ ATPase 2b (SERCA2b), a member of the SERCA family, is expressed ubiquitously and transports Ca²⁺ into the sarco/endoplasmic reticulum using the energy provided by ATP binding and hydrolysis. The crystal structure of SERCA2b in its Ca²⁺ - and ATP-bound (E1∙2Ca²⁺ -ATP) state and cryo-electron microscopy (cryo-EM) structures of the protein in its E1∙2Ca²⁺ -ATP and Ca²⁺ -unbound phosphorylated (E2P) states have provided essential insights into how the overall conformation and ATPase activity of SERCA2b is regulated by the transmembrane helix 11 and the subsequent luminal extension loop, both of which are specific to this isoform. More recently, our cryo-EM analysis has revealed that SERCA2b likely adopts open and closed conformations of the cytosolic domains in the Ca²⁺ -bound but ATP-free (E1∙2Ca²⁺ ) state, and that the closed conformation represents a state immediately prior to ATP binding. This review article summarizes the unique mechanisms underlying the conformational and functional regulation of SERCA2b.