Tyrosine phosphorylated proteins in mammalian spermatozoa: molecular and functional aspects

During mammalian fertilization, spermatozoa must undergo capacitation and the acrosome reaction. These processes of sperm function are critically associated with various molecular events and one such process is protein tyrosine phosphorylation (PYP). This event is downstream of increases in intracellular Ca2+ and activities of HCO3- activated adenylate cyclase, cAMP-dependent-protein kinase-A and reactive oxygen species. Though, PYP is known to be mediated by tyrosine kinases and phosphatases, only a few of them have been identified and characterized in spermatozoa. Since most identified tyrosine kinases are soluble proteins from somatic cells, it is believed that distinct mechanisms could exist in spermatozoa for PYP. Such sperm-specific protein tyrosine kinases/ phosphatases still remain to be thoroughly characterized in most species, including hamsters. Nevertheless, a few tyrosine phosphorylated sperm proteins have been identified in hamsters and in other mammals as well. There is very limited information available on our understanding of the molecular and ultrastructural localization, as well as the characteristics of tyrosine phosphorylated proteins. Functionally, how sperm motility is regulated by PYP is also poorly understood. Knowledge of tyrosine phoshorylated proteins and how they regulate sperm function is of immense significance in our understanding of male (in)fertility and clinical management of fertility; especially, in the light of studies that implicate the hypo-tyrosine phosphorylated state of sperm proteins with asthenozoospermic condition in humans. This article provides a comprehensive review on PYP and its regulation by kinases and phosphatases.