The crystal structure of the Ca ²⁺ -ATPase 1 from Listeria monocytogenes reveals a pump primed for dephosphorylation

Many bacteria export intracellular calcium using active transporters homologous to the sarco/endoplasmic reticulum Ca ²⁺ -ATPase (SERCA). Here we present three crystal structures of Ca ²⁺ -ATPase 1 from Listeria monocytogenes (LMCA1). Structures with BeF ₃ ^- mimicking a phosphoenzyme state reveal a closed state, which is intermediate between the outward-open E2P and the proton-occluded E2-P* conformations known for SERCA. It suggests that LMCA1 in the E2P state is pre-organized for dephosphorylation upon Ca ²⁺ release, consistent with the rapid dephosphorylation observed in single-molecule studies. An arginine side-chain occupies the position equivalent to calcium binding site I in SERCA, leaving a single Ca ²⁺ -binding site in LMCA1, corresponding to SERCA site II. Observing no putative transport pathways dedicated to protons, we infer a direct proton counter transport through the Ca ²⁺ exchange pathways. The LMCA1 structures provide insight into the evolutionary divergence and conserved features of this important class of ion transporters.