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(184) CALCIUM SENSITIVITY OF THE ACROSOME REACTION AND BINDING BETWEEN SPERM ISOFORMS OF SYNTAXIN AND SYNAPTOTAGMIN. Hutt, Darren1,2, Baltz, Jay1,2,3, Ngsee, Johnny1,2, 1 Dept of Cellular and Molecular Medicine, Ottawa, Canada2 Ottawa Health Research Institute, Ottawa, Canada3 Dept of OB/GYN (Div Reprod Med), Ottawa, Canada ABSTRACT- The acrosome is a large secretory granule that undergoes exocytosis when receptors on the sperm surface bind ligands in the egg extracellular matrix. Acrosomal exocytosis resembles stimulated secretion in neurons in that it is triggered by a rise in intracellular calcium. In neurons, a core complex composed of the plasma membrane SNAP receptors (SNARE) - syntaxin (Stx) and SNAP-25 - and the vesicular SNARE, VAMP or synaptobrevin is involved in synaptic vesicle fusion. Regulation of this protein complex is accomplished through the action of accessory proteins, including the synaptotagmin (Syt) family of proteins. Syts are calcium-binding proteins, which associate with the core complex via calcium-dependent binding to Stx. They are believed to serve as calcium sensors mediating fusion of membrane compartments. Recent evidence has revealed that isoforms of the SNARE proteins, Stx2, Syt VI and Syt VIII are expressed in sperm and localized to the acrosomal compartment where they might mediate exocytosis in response to calcium influx. We have utilized streptolysin O-permeabilized sperm to determine the calcium concentration dependence of the acrosome reaction and found that AR is half-maximal at 69.3 KEY WORDS: acrosome reaction, synaptotagmin, syntaxin, SNARE |
M Ca2+. In addition, we have determined that Stx2 binding to both Syt isoforms present in sperm - VI and VIII - is calcium dependent. This suggests that calcium-dependent interaction of these molecules may regulate fusion of the outer acrosomal membrane to the overlying plasma membrane in response to the influx of calcium during fertilization. Thus, Syt is a molecule that may transduce the calcium signal to the SNARE complex during the acrosome reaction.