The C2A-C2B Linker Defines the High Affinity Ca2+ Binding Mode of Rabphilin-3A

The Ca(2+) binding properties of C2 domains are essential for the function of their host proteins. We present here the first crystal structures showing an unexpected Ca(2+) binding mode of the C2B domain of rabphilin-3A in atomic detail. Acidic residues from the linker region between the C2A and C2B domains of rabphilin-3A interact with the Ca(2+)-binding region of the C2B domain. Because of these interactions, the coordination sphere of the two bound Ca(2+) ions is almost complete. Mutation of these acidic residues to alanine resulted in a 10-fold decrease in the intrinsic Ca(2+) binding affinity of the C2B domain. Using NMR spectroscopy, we show that this interaction occurred only in the Ca(2+)-bound state of the C2B domain. In addition, this Ca(2+) binding mode was maintained in the C2 domain tandem fragment. In NMR-based liposome binding assays, the linker was not released upon phospholipid binding. Therefore, this unprecedented Ca(2+) binding mode not only shows how a C2 domain increases its intrinsic Ca(2+) affinity, but also provides the structural base for an atypical protein-Ca(2+)-phospholipid binding mode of rabphilin-3A.