Structure of the C2A domain of rabphilin-3A.

Rabphilin-3A is a neuronal protein containing a C2-domain tandem. To date, only the structure of the C2B domain has been solved. The crystal structure of the Ca2+-free C2A domain has been solved by molecular replacement and refined to 1.92 A resolution. It adopts the classical C2-domain fold consisting of an eight-stranded antiparallel beta-sandwich with type I topology. In agreement with its Ca2+-dependent negatively charged membrane-binding properties, this C2 domain contains all the conserved acidic residues responsible for calcium binding. However, the replacement of a conserved aspartic acid residue by glutamic acid allows formation of an additional strong hydrogen bond, resulting in increased rigidity of calcium-binding loop 1. The electrostatic surface of the C2A domain consists of a large positively charged belt surrounded by two negatively charged patches located at both tips of the domain. In comparison, the structurally very similar C2A domain of synaptotagmin I has a highly acidic electrostatic surface, suggesting completely unrelated functions for these two C2A domains.