Identification and characterization of membrane cofactor protein of human spermatozoa.

Abstract

Membrane cofactor protein (MCP) regulates C activation by serving as a cofactor for the cleavage of C3b and C4b by the serine protease factor I. An MCP-like molecule on the inner acrosomal membrane of human spermatozoa has been characterized. Three mAb and a rabbit polyclonal antibody against MCP recognized the sperm protein. On SDS-PAGE, it migrated as a single band with a molecular mass of 38,000 and 44,000 Da under nonreducing or reducing conditions, respectively. The molecular mass was 10,000 to 20,000 Da less than the two forms of MCP expressed on others cells. The electrophoretic pattern, by one- and two-dimensional gel analysis, and the isoelectric point profile (4.5 to 5.0) of the sperm protein were similar among multiple individuals. In contrast to MCP of other cells, digestion with endoglycosidases did not alter either the m.w. or the pI of the protein, suggesting that it is a poorly or nonglycosylated form of MCP. The solubilized sperm protein bound C3 with broken thioester bond to Sepharose and possessed cofactor activity for factor I-mediated cleavage of C3 with the broken bond. A mAb that blocks the regulatory function of MCP inhibited the cofactor activity of the sperm lysate. Thus, the sperm protein is an antigenic and functional homologue of MCP but has the distinct structural features of a lower m.w. and an apparent lack of glycosylation. MCP may play an essential role in the survival of the acrosome-reacted spermatozoa by modulating C activation in the female genital tract.

Membrane Cofactor Protein: Importance ofN- andO-Glycosylation for Complement Regulatory Function

“…MCP also is involved in reproduction. For example, it is expressed on placental trophoblast and the inner acrosomal membrane of human spermatozoa (7)(8)(9), possibly having a role in both fertilization (10) and habitual abortion (11). Finally, MCP (12) and related human complement regulators are undergoing evaluation for therapeutic use as soluble inhibitors of complement activation and are being engineered into transgenic animals to prevent the hyperacute graft rejection that accompanies xenotransplantation (reviewed in Refs.…”

Alternative pathway activation in pregnancy, a measured amount “complements” a successful pregnancy, too much results in adverse events

“…This exocytosis event releases the acrosomal contents and exposes membrane cofactor protein (MCP, CD46), located on the inner acrosomal membrane of the sperm, to the reproductive tract environment 11 . While CD46 exists as multiple isoforms, with individual isoform expression determined by the tissue or cell type, it should be noted that the isoform of CD46 expressed on the acrosomal membrane is unique to spermatozoa, suggestive of a highly specific role 12,13 …”

“…11 While CD46 exists as multiple isoforms, with individual isoform expression determined by the tissue or cell type, it should be noted that the isoform of CD46 expressed on the acrosomal membrane is unique to spermatozoa, suggestive of a highly specific role. 12,13 It has been shown that acrosin, an acrosomal protease released during the AR, can cleave C3 to generate a C3b-like molecule. This nascent C3b has the transient ability to bind covalently to adjacent surfaces, including both the acrosomal and the ovum plasma membranes.…”

“… 11 While CD46 exists as multiple isoforms, with individual isoform expression determined by the tissue or cell type, it should be noted that the isoform of CD46 expressed on the acrosomal membrane is unique to spermatozoa, suggestive of a highly specific role. 12 , 13 …”

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“…The smMCP is reportedly localized to the inner acrosomal membrane and has been reported to be critical for the survival of the acrosome-reacted spermatozoa by regulation of complement activation and the interaction of spermatozoa with oocytes in the FRT (Cervoni et al, 1992;Riley et al, 2002). Notably, the CD-46 sperm-specific proteoform, smMCP has distinct structural features of a lower molecular weight and modifications (deglycosylation) in the N-linked sugars (Riley, 2002).…”