Structure of the Janus-faced C2B domain of rabphilin

C-2 domains are widespread protein modules that often occur as tandem repea ts in many membrane-trafficking proteins such as synaptotagmin and rabphili n. The first and second C-2 domains (C(2)A and C2B, respectively) have a hi gh degree of homology but also specific differences. The structure of the C (2)A domain of synaptotagmin I has been extensively studied but little is k nown about the C2B domains. We have used NMR spectroscopy to determine the solution structure of the C2B domain of rabphilin, The overall structure of the C2B domain is very similar to that of other C-2 domains, with a rigid beta-sandwich core and loops at the top (where Ca2+ binds) and the bottom. Surprisingly, a relatively long alpha-helix is inserted at the bottom of th e domain and is conserved in all C2B domains. Our results, together with th e Ca2+-independent interactions observed for C2B domains, indicate that the se domains have a Janus-faced nature, with a Ca2+-binding top surface and a Ca2+-independent bottom surface.