Figure 6.

Models for Ca²⁺-dependent and Ca²⁺-independent modes of membrane binding of syt1C2B. In the absence of Ca²⁺, the bilayer “protects” C2B from interactions with other components of the fusion machinery. The binding of Ca²⁺ to the domain alters the surface of C2B that interacts with the bilayer interface, and promotes the insertion of the Ca²⁺ binding loops into the bilayer. The Ca²⁺ bound orientation allows the polybasic face and other positively charged segments of C2B to interact with the SNAREs or with other bilayer surfaces.