Acrosomal exocytosis is regulated at different stages by complexin and synaptotagmin.

ROGGERO CM, DE BLAS GA, DAI H, TOMES CN, RIZO J, MAYORGA LS.

Mar del Plata, Argentina

Congreso; XLIII Reunión Anual de la Soc. Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2007

Sociedad Argentina de Investigaciones en Bioquímica y Biología Molecular

Regulated secretion is a fundamental process for many cell types. In particular, acrosomal exocytosis in mammalian sperm is essential for egg fertilization. Regulated secretion requires SNARE proteins and, in neurons, also synaptotagmin I and complexin. Complexin imposes a fusion block that is released by Ca2+ and synaptotagmin I. However, no direct evidence for this model in secreting cells has been provided and whether this complexin/synaptotagmin interplay functions in other cells is unknown. We show that the C2B domain of synaptotagmin VI and anti-complexin antibody blocked the formation of trans SNARE complexes in permeabilized human sperm, and this effect was reversed by complexin. In contrast, an excess of complexin stopped exocytosis at a later step, when SNAREs were in loose trans complexes. This blockage was released by addition of the synaptotagmin VI C2B and Ca2+. Previously we demonstrated that this domain is regulated by PKC-mediated phosphorylation. Here, we show that a phosphomimetic mutation in the polybasic region of the C2B strongly affects its Ca2+ and phospholipids binding properties. This mutation abrogates its ability to rescue the complexin block. Our results show that the functional interplay between complexin and synaptotagmin has a central role in a physiological secretion event, and this interplay can be modulated by phosphorylation of the C2B domain.