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J Biol Chem. 2002 Aug 9;277(32):29315-20. Epub 2002 May 28.

The calcium-binding loops of the tandem C2 domains of synaptotagmin VII cooperatively mediate calcium-dependent oligomerization.

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  • 1Fukuda Initiative Research Unit, RIKEN (the Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. mnfukuda@brain.riken.go.jp

Abstract

Synaptotagmin VII (Syt VII), a proposed regulator for Ca2+-dependent exocytosis, showed a robust Ca2+-dependent oligomerization property via its two C2 domains (Fukuda, M., and Mikoshiba, K. (2001) J. Biol. Chem. 276, 27670-27676), but little is known about its structure or the critical residues directly involved in the oligomerization interface. In this study, site-directed mutagenesis and chimeric analysis between Syt I and Syt VII showed that three Asp residues in Ca2+-binding loop 1 or 3 (Asp-172, Asp-303, and Asp-357) are crucial to robust Ca(2+)-dependent oligomerization. Unlike Syt I, however, the polybasic sequence in the beta4 strands of the C2 structures (so-called "C2 effector domain") is not involved in the Ca2+-dependent oligomerization of Syt VII. The results also showed that the Ca2+-binding loops of the two C2 domains cooperatively mediate Syt VII oligomerization (i.e. the presence of redundant Ca2+-binding site(s)) as well as the importance of Ca2+-dependent oligomerization of Syt VII in Ca2+-regulated secretion. Expression of wild-type tandem C2 domains of Syt VII in PC12 cells inhibited Ca2+-dependent neuropeptide Y release, whereas mutant fragments lacking Ca2+-dependent oligomerization activity had no effect. Finally, rotary-shadowing electron microscopy showed that the Ca2+-dependent oligomer of Syt VII is "a large linear structure," not an irregular aggregate. By contrast, in the absence of Ca2+ Syt VII molecules were observed to form a globular structure. Based on these results, we suggest that the linear Ca2+-dependent oligomer may be aligned at the fusion site between vesicles and plasma membrane and modulate Ca2+-regulated exocytosis by opening or dilating fusion pores.

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