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Characterization of a cDNA clone coding for human testis membrane cofactor protein (MCP, CD46)
F Cervoni1, P Fenichel, C Akhoundi
1Institut National de la Santé et de la Recherche Médicale, Unité 210, Faculté de Médecine, Nice, France.
Abstract:
Membrane cofactor protein (MCP) is a complement regulatory protein that acts as a cofactor for the cleavage of C3b and C4b by the serine protease factor I. We have previously reported the characterization of a functional MCP molecule on the acrosomal membrane. This protein migrated as a single band with a molecular weight of 40,000 Da, which is 10,000-20,000 Da smaller than the known MCP molecules, and is devoid of N- and O-linked sugars. We have proposed that the difference in molecular weight resulted from the lack of sugars. To investigate if this is due to the absence of glycosylation sites, we have characterized a cDNA clone from a human testis cDNA library. This cDNA corresponds to a peculiar MCP form previously described, which is characterized by the presence of the serine/threonine/proline-rich exon C (STPC) and the cytoplasmic tail known as CYT2, and we conclude that the absence of mature oligosaccharide of the sperm MCP cannot be totally attributed to a defect of N- and O-glycosylation sequences but rather reflects an alteration of the mechanisms of glycosylation in spermatozoa. The presence of functional MCP on the acrosomal membrane, as well as the other complement regulatory protein, decay-accelerating factor, strongly suggests that these proteins may act concomitantly to protect the acrosome-reacted spermatozoa from the attack of the complement present in the female genital tract.
Insights
Membrane cofactor protein (MCP) on sperm is smaller due to altered glycosylation, not just missing sites. This protein may protect sperm from complement attack in the female reproductive tract.
Area of Science:
- Immunology
- Reproductive Biology
- Molecular Biology
Background:
- Membrane cofactor protein (MCP) regulates complement pathways by aiding C3b and C4b cleavage.
- A functional MCP molecule on the acrosomal membrane was previously identified, exhibiting a lower molecular weight and lacking N- and O-linked sugars.
- The reduced molecular weight of sperm MCP was hypothesized to result from the absence of glycosylation.
Purpose of the Study:
- To investigate the molecular basis for the reduced molecular weight of sperm MCP.
- To determine if the lack of glycosylation is due to absent glycosylation sites.
- To understand the role of sperm MCP in protecting spermatozoa.
Main Methods:
- Characterization of a cDNA clone from a human testis cDNA library.
- Analysis of the identified cDNA for specific protein domains (STPC and CYT2).
- Comparison of the deduced protein sequence with known MCP forms.
Main Results:
- A cDNA clone corresponding to a unique MCP form containing the STPC exon and CYT2 cytoplasmic tail was identified.
- The absence of mature oligosaccharides on sperm MCP is not solely due to a defect in N- and O-glycosylation sequences.
- Glycosylation mechanisms in spermatozoa appear to be altered, leading to the observed lack of mature oligosaccharides.
Conclusions:
- The reduced molecular weight of sperm MCP is attributed to alterations in sperm glycosylation mechanisms, not just a lack of glycosylation sites.
- Functional MCP and decay-accelerating factor on the acrosomal membrane likely protect acrosome-reacted spermatozoa from complement-mediated damage.
- These findings highlight the importance of complement regulatory proteins in sperm protection within the female reproductive tract.