STIMULATION OF PROTEIN-TYROSINE PHOSPHORYLATION BY A PROGESTERONE-RECEPTOR ON THE CELL-SURFACE OF HUMAN SPERM

Mature human sperm initiate a rapid Ca2+ influx and the acrosomal exoc ytosis in response to progesterone. Recent evidence indicates that bot h events can be induced by antibody-mediated cross-linking of a sperm surface progesterone receptor. In many other systems in which signal i s generated by receptor cross-linking, protein phosphorylation on tyro sine residues is involved in the signal transduction across the plasma membrane. In this study we examined whether tyrosine phosphorylation is implicated in the function of the sperm surface progesterone recept or, too. The effect of progesterone on the phosphorylation of proteins from a sperm membrane lysate was evaluated by in vitro kinase assay a nd by phosphoamino acid analysis using [gamma-P-32]ATP as precursor. T hese experiments revealed a selective increase in the tyrosine phospho rylation of a 94-kilodalton phosphoprotein in the presence of progeste rone. To decide whether the progesterone-induced increase in protein t yrosine phosphorylation is actually due to the hormone action on the c ell surface, living sperm were treated with a cell-impermeant progeste rone receptor agonist, and the resulting changes in the cellular level of phosphotyrosine proteins were examined. These experiments showed a clear relationship between the agonist binding and an increase in the phosphotyrosine concentration in the respective cells. This relations hip was lost in the presence of genistein, which also efficiently inhi bited the phosphorylation of the 94-kilodalton protein and the progest erone-induced acrosomal exocytosis. These results lead to the hypothes is that protein tyrosine phosphorylation is involved in signal transdu ction through the sperm surface progesterone receptor and may be impli cated in nongenomic steroid effects in other cell types.