Molecular cloning of a murine homologue of membrane cofactor protein (CD46): preferential expression in testicular germ cells

Abstract

Human membrane cofactor protein (MCP, CD46) has been suggested, although no convincing evidence has been proposed, to be a fertilization-associated protein, in addition to its primary functions as a complement regulator and a measles virus receptor. We have cloned a cDNA encoding the murine homologue of MCP. This cDNA showed 45% identity in deduced protein sequence and 62% identity in nucleotide sequence with human MCP. Its ectodomains were four short consensus repeats and a serine/threonine-rich domain, and it appeared to be a type 1 membrane protein with a 23-amino acid transmembrane domain and a short cytoplasmic tail. The protein expressed on Chinese hamster ovary cell transfectants was 47 kDa on SDS/PAGE immunoblotting, approximately 6 kDa larger than the murine testis MCP. It served as a cofactor for factor I-mediated inactivation of the complement protein C3b in a homologous system and, to a lesser extent, in a human system. Strikingly, the major message of murine MCP was 1.5 kb and was expressed predominantly in the testis. It was not detected in mice defective in spermatogenesis or with immature germ cells (until 23 days old). Thus, murine MCP may be a sperm-dominant protein the message of which is expressed selectively in spermatids during germ-cell differentiation.

Membrane cofactor protein (MCP; CD46) expression in transgenic mice

“…Another interesting variation in the MCP (CD46) mRNA splicing and protein expression pattern occurred between human and transgenic mouse testes. Since the murine MCP (CD46) homologue is only abundantly expressed in the testis [42], the transcription or translation of the endogenous mouse MCP (CD46) gene may interfere with the expression of the human gene. We are currently further characterizing MCP (CD46) expression in the testis.…”

Identification and characterization of a silencer regulatory element in the 3′-flanking region of the murine CD46 gene

“…Like human CD46, it consisted of four short consensus repeats, a serine\threonine-rich domain, a transmembrane domain and a short cytoplasmic tail. Strikingly, murine CD46 is expressed only in the testicular germ cells 29 days after birth [13]. Its distribution is limited to the inner acrosomal membrane of murine spermatozoa and its expression is induced by the acrosomal reaction (N. Inoue, M. Nomura, M. Okabe and T. Seya, unpublished work), as is the case with human CD46 expressed in testis [14,15].…”

“…Murine CD46 is selectively expressed in the testis, in contrast to human CD46 which is expressed ubiquitously [2,13]. Given the importance of promoters in the expression of a variety of testisspecific genes [30][31][32][33], we focused on the 5h-flanking region of murine CD46 to determine the regulatory mechanisms governing its expression.…”

“…The message size of the testis CD46 is 1.5 kb in both humans and mice, while that of the ubiquitously expressed form of CD46 is 4 kb in humans [2,13]. The murine equivalent of the 4-kb form is present only at very low levels in all organs tested, offering one possible explanation for the testis-specific expression of CD46.…”

“…Recently, a murine cDNA homologue of human CD46 was cloned in our laboratory [13]. The predicted mouse CD46 protein showed 45 % identity in the primary structure with human CD46.…”