E-Syts, a family of membranous Ca2+-sensor proteins with multiple C2 domains.

Abstract

C(2) domains are autonomously folded protein modules that generally act as Ca(2+)- and phospholipid-binding domains and/or as protein-protein interaction domains. We now report the primary structures and biochemical properties of a family of evolutionarily conserved mammalian proteins, referred to as E-Syts, for extended synaptotagmin-like proteins. E-Syts contain an N-terminal transmembrane region, a central juxtamembranous domain that is conserved from yeast to human, and five (E-Syt1) or three (E-Syt2 and E-Syt3) C-terminal C(2) domains. Only the first E-Syt C(2) domain, the C(2)A domain, includes the complete sequence motif that is required for Ca(2+) binding in C(2) domains. Recombinant protein fragments of E-Syt2 that include the first C(2) domain are capable of Ca(2+)-dependent phospholipid binding at micromolar concentrations of free Ca(2+), suggesting that E-Syts bind Ca(2+) through their first C(2) domain in a phospholipid complex. E-Syts are ubiquitously expressed, but enriched in brain. Expression of myc-tagged E-Syt proteins in transfected cells demonstrated localization to intracellular membranes for E-Syt1 and to plasma membranes for E-Syt2 and E-Syt3. Structure/function studies showed that the plasma-membrane localization of E-Syt2 and E-Syt3 was directed by their C-terminal C(2)C domains. This result reveals an unexpected mechanism by which the C(2)C domains of E-Syt2 and E-Syt3 functions as a targeting motif that localizes these proteins into the plasma membrane independent of their transmembrane region. Viewed together, our findings suggest that E-Syts function as Ca(2+)-regulated intrinsic membrane proteins with multiple C(2) domains, expanding the repertoire of such proteins to a fourth class beyond synaptotagmins, ferlins, and MCTPs (multiple C(2) domain and transmembrane region proteins).