Sperm ecto-protein kinase and its protein substrate: novel regulators of membrane fusion during acrosome reaction.

Previously we have purified and characterized a unique plasma membrane-specific cyclic AMP-independent ecto-protein kinase (ecto-CIK) as well as its ecto-phosphoprotein substrate (MPS) using caprine sperm model. This study reports for the first time the role of the sperm external surface protein phosphorylation system on sperm acrosome reaction, which is essential for fertilization. Calcium ionophore A23187 has been used to trigger the sperm acrosome reaction in vitro. Treatment of sperm cells with CIK antibody (dil: 1:500) causes a significant decrease (approx. 50%) in percentage of acrosome reacted sperm. Onset of the acrosome reaction causes a remarkable increase in the rate of acrosin release from the cells in the medium. However, CIK antibody inhibits significantly (approx. 50%) the acrosin release. The level of membrane-bound MPS as estimated by ELISA and the degree of its phosphorylation catalyzed by the endogenous ecto-CIK, increase significantly with the progress of the acrosome reaction. Both the parameters increase by approximately 100% during the 20 min of the reaction. MPS antibody (1:100 dilution) markedly decreases (approx. 75%) percentage of acrosome-reacted sperm. MPS antibody as well shows high efficacy to inhibit acrosin release from spermatozoa. The results demonstrate that the cell-surface protein kinase and its protein substrate are essential for membrane fusion component of acrosome reaction. The data are consistent with the view that MPS regulates acrosomal membrane fusion with the overlying plasma membrane by the mechanism of its phosphorylation and dephosphorylation.