Contrasting effects of molecular crowding on the membrane-perturbing and chaperone-like activities of major bovine seminal plasma protein, PDC-109.

Crowded environments inside cells and biological fluids greatly affect protein stability and activity. PDC-109, a polydisperse oligomeric protein of the bovine seminal plasma selectively binds choline phospholipids on the sperm cell surface and causes membrane destabilization and lipid efflux, leading to acrosome reaction. PDC-109 also exhibits chaperone-like activity (CLA) and protects client proteins against various kinds of stress, such as high temperature and low pH. In the present work, we have investigated the effect of molecular crowding on these two different activities of PDC-109 employing Dextran 70 (D70) - a widely used polymeric dextran - as the crowding agent. The results obtained show that presence of D70 markedly increases membrane destabilization by PDC-109. Isothermal titration calorimetric studies revealed that under crowded condition the binding affinity of PDC-109 for choline phospholipids increases approximately 3-fold, which could in turn facilitate membrane destabilization. In contrast, under identical conditions, its CLA was reduced significantly. The decreased CLA could be correlated to reduced surface hydrophobicity, which was due to stabilization of the protein oligomers. These results establish that molecular crowding exhibits contrasting effects on two different functional activities of PDC-109 and highlight the importance of microenvironment of proteins in modulating their functional activities.