Abstract
The aim of this article is to review the surface molecules that are involved in capacitation/acrosomal exocytosis and zona pellucida (ZP) binding in context of tyrosine phosphorylation leading to signal transduction in human sperm. During capacitation, at least 7 proteins (200, 112, 104, 48, 42, 31 and 25 kD) are phosphorylated as studied by the 32P metabolic labeling assay, and 14 proteins (122, 105, 95, 89, 73, 62, 48, 46, 40, 33, 30, 28, 25 and 22 kD) are autophosphorylated as demonstrated in the in vitro kinase assay. Of the 7-14 proteins, two proteins of 95 and 51 kD molecular identities were phosphorylated at tyrosine residues. Treatment with Talpha1 enhanced and anti-FA-1 monoclonal antibody completely blocked phosphorylation of all the relevant proteins. Sperm proteins belonging to four molecular regions, namely 95 kD (double band), 63 kD (one band), 51 kD (one band) and 14-18 kD (three bands) were involved in ZP binding. Three of these, namely 95 kD, 51 kD and 14-18 kD proteins demonstrated the presence of tyrosine phosphorylation, and the 51 kD protein (that is FA-1 antigen) also showed autophosphorylating activity. These findings, along with the other available data, indicate a vital role of protein tyrosine phosphorylation in sperm capacitation, acrosomal exocytosis and zona pellucida binding in humans. Since tyrosine phosphorylation is a primary/even exclusive indication of signal transduction, it appears that a signal transduction pathway is involved in fertilizability of human sperm.