The human C3b/C4b receptor or complement receptor type one (CR1) is an approximately 200-kD single chain membrane glycoprotein of human peripheral blood cells that mediates the binding, processing, and transport of C3b-bearing immune complexes and regulates the activity of the complement cascade. Analysis of partial cDNA clones has shown that the COOH terminus is composed predominantly of three tandemly repeated regions of 450 amino acids each (15). In this report, we present a cDNA sequence that encodes the NH2 terminus of CR1. It appears to have been derived from an alternatively processed transcript, caused by polyadenylation occurring at a site within an intron in the CR1 transcriptional unit. The resulting truncated messenger carries an open reading frame that would produce a short, secreted CR1 form. We present genomic sequences and Northern blots which support this hypothesis and we propose that the NH2-terminal end of CR1 is a likely location for active sites. In addition, we report evidence for a CR1-like sequence in the human genome and we present a model for the organization of CR1.

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