Flipping the fusion switch

Calcium and calmodulin transfer a critical region of the vesicle protein VAMP from donor to target membrane, suggest Luc de Haro, Michael Seagar (INSERM, Marseille, France), and colleagues. The transfer may help to fuse the two membranes together.

VAMP is a v-SNARE: a helical protein sticking out of donor vesicles that zippers up with t-SNAREs on target membranes. Seagar has previously found that a membrane-proximal section of VAMP shows mutually exclusive binding to calmodulin or acidic phospholipids. Interaction with lipids in the donor vesicle membrane may inhibit fusion.

Now, Seagar finds that Ca²⁺/calmodulin alleviates this inhibition, at least as far as the binding step. Ca²⁺/calmodulin binds to VAMP-loaded vesicles, and is needed for target membranes to bind to the VAMP on VAMP-loaded vesicles....