What ATP binding does to the Ca pump and how nonproductive phosphoryl transfer is prevented in the absence of Ca.

Under physiological conditions, most Ca-ATPase (SERCA) molecules bind ATP before binding the Ca transported. SERCA has a high affinity for ATP even in the absence of Ca, and ATP accelerates Ca binding at pH values lower than 7, where SERCA is in the E2 state with low-affinity Ca-binding sites. Here we describe the crystal structure of SERCA2a, the isoform predominant in cardiac muscle, in the E2·ATP state at 3.0-Å resolution. In the crystal structure, the arrangement of the cytoplasmic domains is distinctly different from that in canonical E2. The A-domain now takes an E1 position, and the N-domain occupies exactly the same position as that in the E1·ATP·2Ca state relative to the P-domain. As a result, ATP is properly delivered to the phosphorylation site. Yet phosphoryl transfer never takes place without the filling of the two transmembrane Ca-binding sites. The present crystal structure explains what ATP binding itself does to SERCA and how nonproductive phosphorylation is prevented in E2.