Calcium-dependent phospholipid binding to the C2A domain of a ubiquitous form of double C2 protein (Doc2 beta)

Rabphilin 3A and Doc2 alpha are synaptic vesicle-associated proteins, and are thought to function as Ca2+ sensors in neurotransmitter release. If either rabphilin 3A or Doc2 alpha plays a role in membrane trafficking, like the synaptotagmins, then non-neural forms should be present. Here we describe the isolation of a mouse cDNA which encodes a novel Doc2 homologue (Doc2 beta) that is present in all tissues. The encoded protein, which is highly homologous to human Doc2 alpha (70% identity), is composed of 412 amino acids with a calculated relative molecular mass (M(r)) of 45,837. The sequence identity is especially high in two C2 domains (74% in C2A and 84% in C2B). Northern and Western blot analyses have shown that Doc2 beta is expressed in all cell lines and tissues tested. Ca(2+)-dependent phospholipid binding assaying of recombinant fusion proteins revealed that the single C2A domain, but not the C2B domain, of Doc2 beta binds phosphatidycholine and phosphatidylserine (2.5:1, w/w) liposomes. The binding is Ca(2+)-dependent, with an EC50 value of approximately 1 microM and a Hill coefficient of approximately 3, which are comparable to those of synaptotagmins, rabphilin 3A and Doc2 alpha. Our results suggest that Doc2 beta is involved in constitutive membrane trafficking.