CRRP: a guinea pig protein, identified by sequence homology to human CR1, which contains two short consensus repeat motifs and appears not to be transmembrane or secreted

Abstract cDNA from the C4b-binding site of the human C3b/C4b receptor (CR1) was used to find homologous sequences in the guinea pig. This cDNA identified an 18S mRNA species in guinea pig spleen, but not liver. Probing of a guinea pig spleen cDNA library identified clones with identical 1.5-kb inserts, which also hybridized to mRNA in spleen, but not liver. Sequence analysis of the insert revealed a single long open-reading frame coding for a 20,000 Mr protein consisting of two short consensus repeat motifs homologous to human CR1, and unique sequence at the amino- and carboxy-terminals of the short consensus repeats. This sequence did not encode peptides with features of transmembrane domains or signal peptides. Antibody to this complement receptor-related protein-β galactosidase fusion protein recognized a 20,000 Mr protein in SDS lysates of guinea pig spleen, lymph node, lymphocytes, neutrophils, and peritoneal macrophages. Immunoprecipitation of human serum by this antibody revealed an 180,000 Mr protein reacting both with the anti-guinea pig protein antibody and with anti-human CR1 antibody. Immunoprecipitation of guinea pig serum revealed no protein reacting with the anti-guinea pig protein antibody. Tissue staining of cultured peritoneal macrophages with this antibody showed intracellular staining, as opposed to membrane staining obtained with anti-guinea pig Ig antibody. The lack of membrane expression was confirmed by surface protein radiolabeling experiments and by fluorescent staining of surface proteins. Thus, we have identified a guinea pig protein with homology to human CR1, which may have an unusual property for this class of proteins in that it appears to be intracellular.