Factors Influencing the Chaperone-Like Activity of Major Proteins of Mammalian Seminal Plasma, Equine HSP-1/2 and Bovine PDC-109: Effect of Membrane Binding, pH and Ionic Strength.

HSP-1/2 and PDC-109 belong to a family of fibronectin type II proteins, present in high concentrations in bovine and equine seminal plasma, respectively. These proteins act as extracellular small heat shock proteins and protect target/client proteins against various kinds of stress. They also exhibit characteristic binding to choline phospholipids present on the sperm plasma membrane and cause efflux of choline phospholipids and cholesterol, resulting in sperm capacitation. The current study demonstrates that hypersaline conditions decrease the chaperone-like activity (CLA) of HSP-1/2. On the other hand, lipoprotein aggregates formed by the binding of choline phospholipids to this protein exhibit higher CLA than HSP-1/2 alone in vitro; the increased CLA can be correlated to the increased surface hydrophobicity of the lipoprotein aggregates. Presence of cholesterol in the membrane was found to decrease such enhancement in the CLA. We have also observed that salinity of the medium affects the chaperone activity by altering the polydisperse nature of the HSP-1/2. Together these results indicate that hydrophobicity and polydispersity are important for the chaperone-like activity of HSP-1/2 and factors that can alter these properties of HSP-1/2 can modulate its CLA. Further, studies on PDC-109 show that the chaperone-like and membrane-destabilizing activities of this protein are differentially affected by change in pH.