Mechanism of the Calcium-dependent Multimerization of Synaptotagmin VII Mediated by Its First and Second C2 Domains

The Ca²⁺-dependent oligomerization activity of the second C2 (C2B) domain of synaptotagmin I (Syt I) has been hypothesized to regulate neurotransmitter release. We previously showed that the cytoplasmic domains of several other Syt isoforms also show Ca²⁺-dependent oligomerization activity (Fukuda, M., and Mikoshiba, K. (2000) J. Biol. Chem. 275, 28180-28185), but little is known about the involvement of their C2 domains in Ca ²⁺-dependent oligomerization. In this study, we analyzed the Ca ²⁺-dependent oligomerization properties of the first (C2A) and the second C2 (C2B) domains of Syt VII. Unlike Syt I, both C2 domains of Syt VII contribute to Ca²⁺-dependent homo- and hetero-oligomerization with other isoforms. For instance, the Syt VII C2A domain Ca ²⁺-dependently binds itself and the C2A domain of Syt VI but not its C2B domain, whereas the Syt VII C2B domain Ca²⁺-dependently binds itself and the C2B domain of Syt II but not its C2A domain. In addition, we showed by gel filtration that a single Syt VII C2 domain is sufficient to form a Ca²⁺-dependent multimer of very high molecular weight. Because of this "two handed" structure, the Syt VII cytoplasmic domain has been found to show the strongest Ca²⁺-dependent multimerization activity in the Syt family. We also identified Asn-328 in the C2B domain as a crucial residue for the efficient Ca²⁺-dependent switch for multimerization by site-directed mutagenesis. Our results suggest that Syt VII is a specific isoform that can cluster different Syt isoforms with two hands in response to Ca²⁺.