Fig. 8. Proposed model for the involvement of tyrosine-phosphorylated molecular chaperones in the acquisition of mammalian sperm fertilizing ability. We hypothesise that a zona pellucida (ZP) receptor complex present in the plasma membrane of spermatozoa might encompass a ZP-binding molecule and also the chaperone proteins heat shock protein 60 (hsp60) and endoplasmin (erp99). In freshly ejaculated spermatozoa, these molecules might be on the cytoplasmic side of the membrane. During capacitation, cholesterol efflux from the membrane promotes increased fluidity, which facilitates changes in protein distribution. Tyrosine phosphorylation of hsp60 and erp99 on the inner surface of the membrane might activate this receptor complex and allow conformational changes such that the chaperone proteins and ZP binding molecule are exposed to the cell surface. This would account for the appearance of phosphotyrosine residues observed on the sperm head following capacitation, and the association of this event with the attainment to recognise and bind the ZP.