Murine Binder of SPerm homolog 2 (BSPH2): the black sheep of the BSP superfamily.

Proteins of the Binder of SPerm superfamily are known to bind choline phospholipids on sperm membrane and promote sperm capacitation. The current study focuses on the biochemical and functional characterization of the murine Binder of SPerm homolog 2 (BSPH2). A recombinant protein (rec-BSPH2) was expressed in Escherichia coli Rosetta-gami B (DE3)pLysS cells using pET32a vector. It was purified by immobilized metal ion affinity chromatography and refolded on column using a decreasing urea gradient. Rec-BSPH2 was found to share some binding characteristics with other BSP proteins, such as binding to gelatin, heparin, and epididymal sperm. Rec-BSPH2 as well as murine recombinant BSPH1 were found to have different immunofluorescence patterns when bound to uncapacitated versus capacitated sperm, indicating a rearrangement of these proteins on sperm surface during or following capacitation. Surprisingly, rec-BSPH2 was unable to bind phosphorylcholine liposomes or promote sperm capacitation. It is the first time that such results are reported for proteins of the BSP family. The results indicate that murine BSPH1 and BSPH2 might not have redundant functions, as is the case with bovine BSPs. This study could lead to a better understanding of the role of BSP proteins in sperm functions and the existence of redundant BSP proteins in the reproductive tract.