Characterization of a novel cell-surface protein expressed on human sperm.

ABSTRACT

BACKGROUND: Precise sperm-oocyte interaction is a critical event for successful fertilization. However, the identity of molecules involved in this process in humans remains largely unknown. This report describes the identification and characterization of a novel cell-surface protein and its potential role in human sperm-oocyte interaction. METHODS AND RESULTS: We previously identified an orphan guanylyl cyclase receptor (mouse GC-G) highly enriched in mouse testis and involved in sperm activation. By using a comparative genomic approach, we found the homologue gene in human (hGC-G) composed of 21 exons, spanning a minimum of 48 kb on chromosome 10q25. Real-time RT-PCR analysis revealed hGC-G mRNA selectively expressed in testis but with low or no expression in all other tissues examined. Compared with mGC-G, the hGC-G transcript contains three 1-bp deletions and two in-frame termination codons, which results in a short putative receptor-like polypeptide. Western blot analysis with an anti-hGC-G-specific antibody confirmed the protein expression of hGC-G in human sperm lysate. Flow cytometry and confocal immunofluorescence analysis demonstrated the localization of hGC-G protein on the acrosome cap and equatorial segment of mature human sperm. In addition, an integrin-binding Arg-Gly-Asp (RGD) motif was found in the extracellular domain of hGC-G. Pre-incubation of the hGC-G RGD peptide with zona pellucida-free oocytes greatly decreased the binding of human sperm to hamster oocytes, which suggests a role for hGC-G role in sperm-oocyte interaction. CONCLUSIONS: hGC-G is a novel surface protein on human sperm and potentially mediates sperm-oocyte interaction through its RGD-containing motif.