Structural insights into the Ca2+ and PI(4,5)P2 binding modes of the C2 domains of rabphilin 3A and synaptotagmin 1

Proteins containing C2 domains are the sensors for Ca2+ and PI (4,5)P2 in a myriad of secretory pathways. Here, the use of a freemounting system has enabled us to capture an intermediate state of Ca 2+ binding to the C2A domain of rabphilin 3A that suggests a different mechanism of ion interaction. We have also determined the structure of this domain in complex with PI(4,5)P2 and IP3 at resolutions of 1.75 and 1.9 A, respectively, unveiling that the polybasic cluster formed by strands β3-β4 is involved in the interaction with the phosphoinositides. A comparative study demonstrates that the C2A domain is highly specific for PI(4,5)P2/PI (3,4,5)P3, whereas the C2B domain cannot discriminate among any of the diphosphorylated forms. Structural comparisons between C2A domains of rabphilin 3A and synaptotagmin 1 indicated the presence of a key glutamic residue in the polybasic cluster of synaptotagmin 1 that abolishes the interaction with PI (4,5)P2. Together, these results provide a structural explanation for the ability of different C2 domains to pull plasma and vesicle membranes close together in a Ca2+-dependent manner and reveal how this family of proteins can use subtle structural changes to modulate their sensitivity and specificity to various cellular signals.
Work in Barcelona was supported by Grant BIO2011-24333 [Ministerio de Economía y Competitividad (MINECO), Spain-Fondo Europeo de Desarrollo Regional (FEDER)] and work in Murcia by Grants BFU2011-22828 (MINECO, Spain-FEDER) and 08700/PI/08 (Fundación Seneca, Region de Murcia).C.F.-O. is recipient of a Junta para la Amplificación de Estudios postdoctoral contract from Consejo Superior de Investigaciones Cientificas-Fondo Social Europeo. J.G. is recipient of a Juan de la Cierva postdoctoral contract from MINECO