The SERCA residue Glu340 mediates inter-domain communication that guides Ca2+ transport

The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) transports Ca2+ ions from the cytosol into the SR/ER lumen. SERCA9s activity is dependent on a tight coupling between movements of the transmembrane helices that comprise the two Ca2+ binding sites and the cytosolic ATPase headpiece. We have addressed the molecular basis for this intramolecular communication by analysing the structure and functional properties of the SERCA mutant E340A. The conserved residue Glu340 is located at a strategic position in the ATPase, at the interface between the phosphorylation domain and the cytosolic ends of six out of SERCA9s ten transmembrane helices. The mutant displays a reduced rate of binding Ca2+ ions from the cytosol. The structure of E340A reveals a rotated headpiece, altered connectivity between the cytosolic domains and an altered interaction pattern around the mutated residue Glu340. Supported by molecular dynamics simulations, we conclude that the E340A mutation causes a stabilisation of SERCA in a more occluded state, causing slowed dynamics in the ion binding region. This finding underpins the crucial role of Glu340 in the inter-domain communication of SERCA.