The SERCA protein is a membrane protein that actively transports calcium from the cytosol to the endoplasmic reticulum. It belongs to the family of P-type ATPases, which transport ions (types P1 to P3), lipids (P4) or polyamines (P5).

Calcium transport by SERCA is achieved by conformational changes in the protein, allowing access to the calcium binding sites alternately from the cytosol and from the lumen of the endoplasmic reticulum. There are numerous crystal structures in SERCA which have led to a better understanding of the conformational changes occurring during the reaction cycle. They have highlighted the strong coupling between the movements of the transmembrane helixes which form the calcium binding sites and the cytosolic "headpiece", the site of ATP hydrolysis and composed of three distinct domains: nucleotide binding, phosphorylation and dephosphorylation domains.

In a new study published in PNAS, researchers from the Universities of Oxford (England), Aarhus (Denmark) and the Joliot Institute (LPSM, department I2BC, University Paris-Saclay/CEA/CNRS) have studied the structural properties, by X-ray crystallography and molecular dynamics simulations, and functional properties of SERCA, whose glutamate residue 340 has been mutated into alanine. E340 is strictly preserved in the family of P-type ATPases and is located in a position that seems strategic for relaying information between the catalytic and transmembrane domains: at the interface between the phosphorylation domain of the cytosolic cap and the cytosolic end of five of the ten transmembrane helices.

The researchers show that the E340A mutant has a strongly slowed calcium binding kinetics. The crystallographic structure of the mutant in the presence of calcium and an ATP analogue reveals that the headpiece is not in its normal conformation but has rotated. This change alters the connectivity between the cytosolic domains of the cuff and the network of hydrogen bonds around the E340 residue. Simulation studies using molecular dynamics lead the authors to conclude that the E340A mutation stabilises the calcium binding sites in an occluded conformation (they are not accessible from either side of the membrane), leading to a slowing down of their dynamics.

This discovery underlies a crucial role for glutamate 340 in the inter-domain communication between the cuff and the transmembrane calcium binding region. E340 helps maintain the structural flexibility necessary for rapid calcium exchange at the binding sites and to relay this flexibility to the phosphorylation site. Highly conserved in the family of P-type ATPases, it helps to ensure the delicate balance between the dynamics of binding, occlusion and release of the transported ion.