CAPACITATION OF MOUSE SPERMATOZOA .1. CORRELATION BETWEEN THE CAPACITATION STATE AND PROTEIN-TYROSINE PHOSPHORYLATION

The molecular basis of mammalian sperm capacitation, defined functiona lly as those processes that confer on the sperm the acquisition of fer tilization-competence either in vivo in the female reproductive tract or in vitro, is poorly understood. We demonstrate here that capacitati on of caudal epididymal mouse sperm in vitro is accompanied by a time- dependent increase in the protein tyrosine phosphorylation of a subset of proteins of M(r) 40,000-120,000. Incubation of sperm in media devo id of bovine serum albumin, CaCl2 or NaHCO3, components which individu ally are required for capacitation, prevent the sperm from undergoing capacitation as assessed by the ability of the cells to acquire the pa ttern B chlortetracycline fluorescence, to undergo the zona pellucida- induced acrosome reaction and, in some cases, to fertilize metaphase I I-arrested eggs in vitro, In each of these cases the protein tyrosine phosphorylation of the subset of capacitation-associated proteins does not occur. Protein tyrosine phosphorylation of these particular prote ins, as well as sperm capacitation, can be recovered in media devoid o f each of these three constituents (bovine serum albumin, CaCl2 or NaH CO3) by adding back the appropriate component in a concentration-depen dent manner. The requirement of NaHCO3 for these phosphorylations is n ot due to an alkalinization of intracellular sperm pH or to an increas e in media pH. Caput epididymal sperm, which lack the ability to under go capacitation in vitro, do not display this capacitation-dependent s ubset of tyrosine phosphorylated proteins in complete media even after extended incubation periods, and do not fertilize metaphase II-arrest ed eggs in vitro. These data suggest that protein tyrosine phosphoryla tion in sperm may represent an important regulatory pathway that may u ltimately modulate events associated with capacitation.