Molecules that drive the exocytosis of human sperm dense core granule: second messengers, SNAREs and small G-proteins

BUSTOS MA; LUCCHESI O; QUEVEDO MF; RUETE MC; TOMES CN

Puerto Varas

Congreso; XII Congress of the Pan-American Association for Biochemistry and Molecular Biology (PABMB) junto a XLIX Annual Meeting of the Argentinean Society of Biochemistry and Molecular Biology (SAIB), the XXXVI Annual Meeting of the Society of Biochemistry and Mo; 2013

Pan-American Association for Biochemistry and Molecular Biology (PABMB)

Exocytosis is used by eukaryotic cells to release biological compounds and to insert specific lipids and proteins in the plasma membrane. Our lab is interested in unveiling the molecular mechanisms that drive exocytosis of the mammalian sperm´s acrosome,  a very large dense-core secretory granule  that is released near the egg at fertilization. We have established that exocytosis of the acrosome depends on members of a highly conserved proteinaceous fusion machinery that includes small GTPases (Rab3, Rab27, and Rap), SNAREs, proteins that interact with them, and the calcium sensor synaptotagmin. Sperm exocytosis also requires calcium, cAMP and its target Epac, a GEF for Rap. Sperm?s Rabs and SNAREs do not undergo constitutive activation/inactivation or assembly/disassembly cycles; rather, these reactions are unidirectional and coupled to the progress of exocytosis. Exposure to exocytotic inducers first increases and later decreases the population of cells exhibiting endogenous GTP-bound Rabs and Rap in the acrosomal region of the sperm head. The three small G proteins are activated in the sequence Rab27→Rab3→Rap during the exocytotic cascade. By means of chimaeric proteins, we have discovered that Rab3A-GTP is a molecule with positive and negative properties segregated in different domains of the polypeptide. These seemingly opposite attributes are expressed at different times during the secretory cascade.