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

Chimpanzee erythrocytes express a 75,000 M sub(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 sub(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 sub(r) chimpanzee E-CR. A chimpanzee cell line was identified that expressed a 220,000 M sub(r) CR1, and a 75,000 M sub(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 sub(r) chimpanzee CR1. This nucleotide sequence was 98.8% homologous to that of the human 220,000 M sub(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.