The SERCA residue Glu340 mediates interdomain communication that guides Ca2+ transport

Abstract

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is a P-type ATPase that transports Ca²⁺ from the cytosol into the sarco(endo)plasmic reticulum (SR/ER) lumen, driven by ATP. This primary transport activity depends on tight coupling between movements of the transmembrane helices forming the two Ca²⁺-binding sites and the cytosolic headpiece mediating ATP hydrolysis. We have addressed the molecular basis for this intramolecular communication by analyzing the structure and functional properties of the SERCA mutant E340A. The mutated Glu340 residue is strictly conserved among the P-type ATPase family of membrane transporters and is located at a seemingly strategic position at the interface between the phosphorylation domain and the cytosolic ends of 5 of SERCA's 10 transmembrane helices. The mutant displays a marked slowing of the Ca²⁺-binding kinetics, and its crystal structure in the presence of Ca²⁺ and ATP analog reveals a rotated headpiece, altered connectivity between the cytosolic domains, and an altered hydrogen bonding pattern around residue 340. Supported by molecular dynamics simulations, we conclude that the E340A mutation causes a stabilization of the Ca²⁺ sites in a more occluded state, hence displaying slowed dynamics. This finding underpins a crucial role of Glu340 in interdomain communication between the headpiece and the Ca²⁺-binding transmembrane region.

Keywords: Ca2+ binding; P-type ATPase; SERCA; molecular dynamics simulations; tryptophan fluorescence.