Glycosylation differentially modulates membranolytic and chaperone-like activities of PDC-109, the major protein of bovine seminal plasma

The major bovine seminal plasma protein, PDC-109, is a mixture of glycosylated (BSP-A1) and non-glycosylated (BSP-A2) isoforms of a 109-residue long polypeptide. It binds to spermatozoa by specifically recognizing choline phospholipids on the plasma membrane and destabilizes it by penetrating the hydrophobic interior, resulting in lipid efflux, which is necessary for sperm capacitation and successful fertilization. PDC-109 also acts as a molecular chaperone and protects target proteins from denaturation and aggregation induced by various types of stress. In order to investigate the role of glycosylation in these activities, we have separated BSP-A1 and BSP-A2 from PDC-109, and also cloned and expressed BSP-A2 in E. coli and purified the recombinant BSP-A2 (rBSP-A2) to homogeneity. Employing biophysical and biochemical approaches we have investigated the membrane-perturbing and chaperone-like activities (CLA) of PDC-109, BSP-A1, BSP-A2 and recombinant BSP-A2 (rBSP-A2). The results obtained demonstrate that glycan-lacking wild-type BSP-A2 and rBSP-A2 exhibit higher membrane-perturbing activity but decreased CLA as compared to PDC-109. In contrast, BSP-A1 exhibits significantly higher CLA than PDC-109, but its ability to destabilize membranes is considerably lower. This differential modulation of the membrane-perturbing and chaperone-like activities has been explained on the basis of higher membrane-penetrating ability and lower solubility of glycan-lacking BSP-A2 as compared to the glycosylated BSP-A1.