Primary sequence of an alternatively spliced form of CR1. Candidate for the 75,000 M(r) complement receptor expressed on chimpanzee erythrocytes.

Chimpanzee erythrocytes express a 75,000 M(r) complement receptor (E-CR) that binds C3b bearing immune complexes and is recognized by an anti-CR1 mAb (E11). Human erythrocytes express the type 1 CR (CR1), the most common form being 220,000 M(r) and consisting of 30 short consensus repeats (SCRs) for its entire extracellular region. The purpose of this investigation was to determine the structure of the 75,000 M(r) chimpanzee E-CR. A chimpanzee cell line was identified that expressed a 220,000 M(r) CR1, and a 75,000 M(r) molecule that was recognized by E11 and could bind human C3i. Utilizing this cell line, chimpanzee CR1 cDNA was amplified in overlapping segments by the PCR, using primer pairs specific for various regions of human CR1 cDNA. Direct sequencing of the PCR-amplified products revealed 6044 nucleotides encoding the entire 220,000 M(r) chimpanzee CR1. This nucleotide sequence was 98.8% homologous to that of the human 220,000 M(r) CR1. Amplification using a CR1 primer from the signal peptide and from the cytoplasmic region yielded a 1985-bp PCR product, termed CR1a. The CR1a sequence was identical with the sequence encoding SCRs 1 to 6, SCRs 28 to 30, and the transmembrane and cytoplasmic regions of chimpanzee CR1. This alternatively spliced product of chimpanzee CR1 would encode a protein of 71,000 peptide m.w. with six potential N-glycosylation sites. Amplification employing a CR1 primer from SCR 1 and from the 3' untranslated region yielded a second PCR product of 1731 bp. This sequence, termed CR1b, encoded eight SCRs, followed by a hydrophobic region that ended in a stop codon. The first six SCRs of CR1b were closer in homology to the first six SCRs of a human CR1-like genomic sequence (97.4%) than to those of the chimpanzee CR1 (94.8%). Taken together, these sequence data suggest that the 75,000 M(r) chimpanzee E-CR is encoded by CR1a, an alternative splice variant of chimpanzee CR1. The CR1b is presumably derived from an RNA species related to the CR1-like genomic sequence previously described only in humans.