Characterization of a cDNA clone coding for human testis membrane cofactor protein (MCP, CD46)

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.