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Membrane and secretory forms of mouse membrane cofactor protein (CD46) generated from a single gene through
M Nomura1, A Tsujimura, K Shida
1Department of Immunology, Osaka Medical Center for Cancer and Cardiovascular Diseases, Japan.
Abstract:
A cDNA encoding a new secretory form of mouse membrane cofactor protein (MCP, CD46) was identified additionally to the membrane form cDNA. The secretory MCP, predicted from the cDNA sequence, consisted of the conserved four short consensus repeats (SCRs) plus a four amino acid-stretch. Unlike human MCP which comprises many isoforms, mouse MCP cDNA predicted a single isoform of membrane MCP with cytoplasmic tail 1 (CYT1) and serine/ threonine-rich domain C (ST(C)). To clarify the genomic origin and monomorphic alteration of these cDNAs, we cloned and analyzed a mouse genomic DNA harboring the full coding sequence of MCP from a 129/SV mouse genomic library. The mouse Mcp was a single gene approximately 50 kilobases long. Eleven of the 14 coding exons of the human MCP gene and intron-exon boundary sequences were found to be conserved in the mouse gene. The STC homologue but not the STA or STB homologue in the mouse exons was functional: the latter being due to deletions and lack of consensus sequences for splicing. The sequence equivalent to cytoplasmic tail 2 (CYT2) has not been identified in the Mcp genome. Thus, the three exons (ST(A), ST(B), and probably CYT2) responsible for the polymorphism of human MCP by differential splicing were missing in the mouse Mcp gene. Unlike the case in humans, no Mcp-related genes or pseudogenes were observed in the mouse genome. The single mouse Mcp gene was mapped to the R-positive H5 band of mouse Chromosome 1 by FISH. Strikingly, one alternative exon with 73 base pairs (encoding the four new amino acids and a TGA stop codon) was discovered between the SCRIV and the STC exons; alternative splicing causes the generation of the secretory form of mouse MCP. These results on mouse MCP, together with the information concerning other mouse SCR proteins, infer that the regulator of complement activation (RCA) gene cluster is genetically diverged between humans and mice.
Insights
Researchers discovered a new secretory form of mouse membrane cofactor protein (MCP, CD46), differing from the human protein. The mouse MCP gene structure explains its limited isoforms and evolutionary divergence from human MCP.
Area of Science:
- Immunology
- Genetics
- Molecular Biology
Background:
- Membrane cofactor protein (MCP, CD46) plays a crucial role in immune regulation.
- Human MCP exhibits significant isoform diversity due to alternative splicing.
- Understanding MCP gene structure in different species provides insights into immune system evolution.
Purpose of the Study:
- To identify and characterize the mouse membrane cofactor protein (MCP, CD46) gene.
- To compare the genomic structure and splicing mechanisms of mouse MCP with human MCP.
- To investigate the genetic basis for the observed differences in MCP isoforms between mice and humans.
Main Methods:
- cDNA sequencing to identify MCP forms.
- Genomic DNA cloning and analysis from a mouse library.
- Fluorescence in situ hybridization (FISH) for gene mapping.
- Comparative analysis of exon-intron structures and splice sites.
Main Results:
- A novel secretory form of mouse MCP (CD46) was identified alongside the membrane form.
- The mouse MCP gene is a single, large gene (~50 kb) with conserved exons compared to humans.
- Key exons (ST(A), ST(B), CYT2) responsible for human MCP polymorphism are absent in the mouse gene.
- Alternative splicing involving a unique exon generates the secretory mouse MCP form.
- The mouse MCP gene maps to Chromosome 1.
Conclusions:
- The mouse MCP gene structure inherently limits isoform diversity compared to humans.
- Differential gene structure and alternative splicing explain the divergence in MCP forms between mice and humans.
- The regulator of complement activation (RCA) gene cluster shows significant genetic divergence between humans and mice.