The C2 domains of Rabphilin3A specifically bind phosphatidylinositol 4,5-bisphosphate containing vesicles in a Ca²⁺-dependent manner: In vitro characteristics and possible significance

In the present study we investigated the lipid binding characteristics of the C2 domains of Rabphilin3a. We found that the tandem C2 domain of Rabphilin3a specifically bound lipid vesicles containing phosphatidyl- inositol 4,5-bisphosphate (PtdIns(4,5)P₂) in a Ca²⁺-dependent manner. There was little binding to vesicles containing PtdIns(3,4)P₂ in the presence or absence of Ca²⁺. Binding to phosphatidylinositol 3,4,5- triphosphate-containing vesicles was similar to binding to PtdIns(4,5)P₂- containing vesicles. The presence of physiological amounts of phosphatidylserine (PS) greatly potentiated the ability of PtdIns(4,5)P₂ to cause vesicle binding. As with the C2 domains together, the binding of individual C2 domain of Rabphilin3a was much greater to PtdIns(4,5)P₂- containing vesicles than PtdIns(3,4)P₂-containing vesicles. Both C2 domains also bound 29 mol % PS-containing vesicles in a Ca²⁺-dependent manner. Because of the importance of the C2B domain in the enhancement of secretion from chromaffin cells by Rabphilin3a, its biochemistry was further investigated. The mutation of aspartates 657 and 659 to asparagines in C2B decreased Ca²⁺-dependent and increased Ca²⁺-independent vesicle binding, indicating the Ca²⁺ dependence of the domain is provided by aspartic acid residues in the putative Ca²⁺-binding pocket. A peptide from the COOH- terminal region of the C2B domain specifically inhibited ATP-dependent secretion from permeabilized chromaffin cells and the binding of Rabphilin3a to phosphatidylcholine/PS/PtdIns(4,5)P₂-containing lipid vesicles, suggesting a role of this sequence in secretion through its ability to interact with acidic lipid vesicles.