PRIMARY SEQUENCE OF AN ALTERNATIVELY SPLICED FORM OF CR-1 - CANDIDATEFOR THE 75,000-M(R) COMPLEMENT RECEPTOR EXPRESSED ON CHIMPANZEE ERYTHROCYTES

Chimpanzee erythrocytes express a 75,000 M(r) complement receptor (E-C R) that binds C3b bearing immune complexes and is recognized by an ant i-CR1 mAb (E11). Human erythrocytes express the type 1 CR (CR1), the m ost common form being 220,000 M(r) and consisting of 30 short consensu s repeats (SCRs) for its entire extracellular region. The purpose of t his investigation was to determine the structure of the 75,000 M(r) ch impanzee 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 E1 1 and could bind human C3i. Utilizing this cell line, chimpanzee CR1 c DNA was amplified in overlapping segments by the PCR, using primer pai rs specific for various regions of human CR1 cDNA. Direct sequencing o f the PCR-amplified products revealed 6044 nucleotides encoding the en tire 220,000 M(r) chimpanzee CR1. This nucleotide sequence was 98.8% h omologous to that of the human 220,000 M(r) CR1. Amplification using a CR1 primer from the signal peptide and from the cytoplasmic region yi elded a 1985-bp PCR product, termed CR1a. The CR1a sequence was identi cal with the sequence encoding SCRs 1 to 6, SCRs 28 to 30, and the tra nsmembrane and cytoplasmic regions of chimpanzee CR1. This alternative ly spliced product of chimpanzee CR1 would encode a protein of 71,000 peptide m.w. with six potential N-glycosy[ation sites. Amplification e mploying a CR1 primer from SCR 1 and from the 3' untranslated region y ielded a second PCR product of 1731 bp. This sequence, termed CR1b, en coded eight SCRs, followed by a hydrophobic region that ended in a sto p codon. The first six SCRs of CR1b were closer in homology to the fir st 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 su ggest that the 75,000 M(r) chimpanzee E-CR is encoded by CR1a, an alte rnative splice variant of chimpanzee CR1. The CR1b is presumably deriv ed from an RNA species related to the CR1-like genomic sequence previo usly described only in humans.