SYNAPTOTAGMIN-I IS A HIGH-AFFINITY RECEPTOR FOR CLATHRIN-AP-2 - IMPLICATIONS FOR MEMBRANE RECYCLING

In nerve terminals, Ca2+-stimulated synaptic vesicle exocytosis is rapidly followed by endocytosis. Synaptic vesicle endocytosis requires clathrin-coated pits similar to receptor-mediated endocytosis in fibroblasts. Binding of clathrin AP-2 (adaptor complex) to an unidentified high affinity membrane receptor appears to be necessary for coated pit assembly in fibroblasts. We now show that synaptic vesicles have a high affinity AP-2 site (K-D, similar to 1 x 10(-10) M) similar to the one observed in fibroblasts. Using a combination of competition and direct binding assays, we demonstrate that synaptotagmin I, an intrinsic membrane protein of synaptic vesicles, has all of the properties of the AP-2 receptor and that AP-2 binds to the second C-2 domain in the molecule. Thus, synaptotagmin I may be a multifunctional protein with a function in endocytosis in addition to the previously proposed role in exocytosis.