A translucent matrix termed the zona pellucida (ZP) surrounds the mammalian oocyte. It plays a critical role in fertilization by acting as a "docking site" for binding of spermatozoa followed by induction of the acrosome reaction in the
zona bound sperm. Recent analyses of the genes of the human oocyte revealed that the ZP matrix is composed of four
glycoproteins, designated as ZP1, ZP2, ZP3 and ZP4, instead of 3 found in the mouse ZP. Comparison of the deduced
amino acid (aa) sequences of the human ZP
glycoproteins with those from various species, revealed that these are evolutionarily conserved. Phylogenetic analysis revealed that ZP1 and ZP4 may be related as these have the highest sequence identity at the aa level within a given species. Each
zona protein has a
signal sequence driving these
proteins to the endoplasmic reticulum, a aproximately 260 aa long 'ZP domain' comprising of 8-10 conserved
cysteine residues, a C-terminal, hydrophobic transmembrane-like region and a short cytoplasmic tail. In order to understand the structure-function relationship of human ZP
glycoproteins, our lab has cloned and expressed ZP2, ZP3 and ZP4
proteins both in E. coli as well as baculovirus expression systems. Simultaneously, our group has been able to amplify the
cDNA encoding human ZP1. Employing baculovirus-expressed recombinant ZP
glycoproteins; our group has provided evidence for the first time that in human, in addition to ZP3, ZP4 is also able to induce acrosomal exocytosis in the capacitated spermatozoa. ZP3 mediated induction of the acrosome reaction can be inhibited by
pertussis toxin suggesting the involvement of
G, protein in downstream signaling in contrast to ZP4, which follows a
G, protein independent pathway. Hence, elucidation of the role of individual ZP
glycoproteins in humans will provide a better insight into the gamete interaction culminating in fertilization.