Sperad is a novel sperm-specific plasma membrane protein homologous to a family of cell adhesion proteins.
A hallmark of fertilization is a high degree of species specificity, implying gamete-specific recognition signals. To identify sperm-specific plasma membrane proteins, an antiserum to sperm plasma membranes was produced in female guinea pigs. The screening of a testis cDNA expression library with this antiserum resulted in the isolation of two clones encoding a predicted protein containing two extracellular immunoglobulin-like domains, a transmembrane segment, and an intracellular proline-rich domain. The predicted protein (named sperad) is closely related to a large family (biliary glycoproteins) of putative cell adhesion molecules. Sperad is first expressed by the haploid spermatid and is localized to the plasma membrane overlying the acrosome, supportive of a role in cell adhesion/signaling. However, sperad expression in Sf9 cells does not result in Sf9 cell aggregation or in sperm adhesion to the infected insect cells, suggesting that sperad is involved in heterotypic interactions. The open reading frame of the two cDNA clones predicts proteins of either 32.2 or 33.3 kDa. Antibody produced to sperad recognizes three sperm plasma membrane proteins on immunoblots (Mr 55,000, 36,000, and 28,000), but the lower molecular weight proteins are degradation products; deglycosylation confirmed that the Mr 55, 000 sperm plasma membrane represents the full-length protein encoded by the clone. Induction of the acrosome reaction does not appear to alter the molecular weight of sperad but does result in its loss from the sperm cells. Thus, sperad is likely involved in heterotypic interactions prior to interaction of spermatozoa with the egg plasma membrane.
Quill
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Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75235, USA.
Garbers
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D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015815:Cell Adhesion Molecules / Q000737:chemistry*; D002462:Cell Membrane / Q000737:chemistry; D005260:Female; D006168:Guinea Pigs; D008297:Male; D008969:Molecular Sequence Data; D008970:Molecular Weight; D016415:Sequence Alignment; D013737:Testis / Q000737:chemistry
D015815:Cell Adhesion Molecules