Murine complement receptor gene family. II. Identification and characterization of the murine homolog (Cr2) to human CR2 and its molecular linkage to Crry.

CR2, a 145,000 to 150,000 Mr protein which binds specific breakdown products of C3, has been identified on the surface of both human and murine B cells. In order to understand the evolutionary relatedness of the human and murine proteins, we have used the coding sequences from the human CR2 gene to investigate those homologous sequences of murine Cr2. Human CR2 cDNA sequences were used as probes on a cDNA library derived from BALB/c spleen mRNA to identify cross-reacting cDNA sequences. A number of putative cDNA clones encoding murine Cr2 have been isolated and examined. DNA sequence analysis of these Cr2 cDNA clones indicates that they represent the murine homolog to human CR2. mRNA analysis with these Cr2 cDNA clones has revealed a transcription pattern similar to, but distinct from that seen for CR2. Whereas human CR2 coding sequences identify a single mRNA species of approximately 5 kb from human tonsillar mRNA, the murine counterpart identifies four transcripts from murine spleen of approximately 3, 5, 9 and 11 kb in size. The Cr2 cDNA clones which detect the four forms of spleen mRNA overlap in coding sequences and contain exons mapping to three colinear fragments as defined by EcoRI digestion. This suggests that the 3- 5-, 9-, and 11-kb mRNA forms arise by alternative splicing from a single gene. Use of these murine Cr2-specific cDNA clones to isolate their respective genomic sequences has allowed for the linkage of the 3' end of the Cr2 gene to the 5' end of the Crry gene, the evolutionary homolog to human CR1.