The Crystal Structure of the Ca2+-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 Ca2+-ATPase (SERCA). Here we present three crystal structures of Ca2+-ATPase 1 from Listeria monocytogenes (LMCA1). Structures with BeF3- 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 Ca2+ 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 Ca2+ 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 Ca2+ exchange pathways. The LMCA1 structures provide insight into the evolutionary divergence and conserved features of this important class of ion transporters.

Acidic Environment Induces Dimerization and Ligand Binding Site Collapse in the Vps10p Domain of Sortilin.

Sortilin is a neuronal receptor involved in transmembrane signaling, endocytosis, and intracellular sorting of proteins. It cycles through a number of cellular compartments where it encounters various acidic conditions. The crystal structure of the sortilin ectodomain has previously been determined at neutral pH. Here, we present the 3.5-Å resolution crystal structure of sortilin at pH 5.5, which represents an environment similar to that of late endosomes, where ligands are released. The structure reveals an overall distortion of the 10-bladed ß-propeller domain. This distortion and specific conformational changes, caused by protonation of a number of histidine residues, render the currently known binding sites unavailable for ligand binding. Access to the binding sites is furthermore blocked by a reversible and pH-dependent formation of tight sortilin dimers, also confirmed by electron microscopy, size-exclusion chromatography, and mutational studies. This study reveals how sortilin binding sites are disrupted and explains pH-dependent ligand affinity.