The Mechanism of Sperm/egg interaction and involved factors.

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The proper folding of newly synthesized membrane proteins in the endoplasmic reticulum (ER) is required for formation of functional mature proteins. Calnexin is a ubiquitous ER chaperone that plays afrnajor role in quality control by retaining incompletely or misfolded proteins. In contrast to other known chaperones (Hsps, BiP and calreticulin), calnexin is an integral membrane protein. Calmegin is a testis-specific ER protein that is homologous to calnexin. We report here that calmegin binds to nascent polypeptides during spermatogenesis and the physiological function of calmegin has been analyzed by targeted disruption of its gene. Homozygous null males were nearly sterile even though spermatogenesis was morphologically normal and mating was normal. In vitro, sperm from homozygous null males did not adhere to the egg extracellular matrix (zona pellucida), and this defect may explain the observed infertility. These results suggest that calmegin functions as a chaperone for one or more sperm surface proteins that mediate sperm-egg interactions. The defective zona pellucida adhesion phenotype of sperm from calmegin deficient mice is reminiscent of certain cases of human male unexplained infertility.