Arrangement of PMCA4 in bovine sperm membrane fractions

The plasma membrane Ca(2+)-ATPase (PMCA) is the main restorer of Ca(2+) balance in sperm. Particularly, PMCA isoform 4 has an essential function in sperm fertility by its participation in gaining sperm hypermotility. PMCA activity is influenced by its lipid environment. Sperm membranes exhibit lipid raft microdomains or detergent-resistant membrane domains, enriched in sphingo-lipids and cholesterol, forming functional specialized areas. Lipid and protein composition of lipid rafts alters during the capacitation process, which is characterized by a cholesterol efflux. In this study, the localization of PMCA4 in lipid membrane fractions of the sperm plasma membrane was investigated. We identified PMCA4 in both the detergent-resistant membrane (DRM) and in the detergent-soluble (DS) fraction of caput and cauda sperm, respectively. Capacitation did not influence PMCA4 localization. In immunocytochemical studies PMCA4 was co-localized with the lipid raft/DRM marker caveolin in the mid piece of caput and cauda sperm. Functional studies with seminal vesicle major protein PDC-109 showed that the Ca(2+)-ATPase activity in DS fractions of cauda sperm and capacitated cauda sperm was stronger enhanced than in the DRMs. In both fractions the effect was statistically significant. In contrast, in lipid overlay experiments PDC-109 interacted stronger with the lipids extracted from DRMs than with lipids extracted from DS. Our results indicate a possible functional compartmentalization of PMCA in bull sperm membranes and point to a presumptive, yet unknown interaction partner of Ca(2+)-ATPase and PDC109, mediating the PDC-109 action from DRMs to the DS fraction of sperm plasma membrane.