SUBSTITUTION OF 2 AMINO-ACIDS CONFERS C3B BINDING TO THE C4B BINDING-SITE OF CR-1 (CD35) - ANALYSIS BASED AN LIGAND-BINDING BY CHIMPANZEE ERYTHROCYTE COMPLEMENT RECEPTOR

The chimpanzee (Ch) E complement receptor type 1 (CR1(77)) appears to be an alternatively spliced product of the Ch CR1 gene transcript. Its cDNA-derived amino acid sequence contains complement protein-repeatin g modules (CP) 1-6, 28, 29, and 30 in tandem and is 98.8% homologous t o the corresponding regions of human (Hu) CR1. It differs from the C4b binding site of Hu CR1 only by two amino acids, Tyr for Ser37 in CP 1 and Asp for Gly79 in CP 2. However, in addition to binding C4b, Ch E binds C3b. As homologous substitution of one of these amino acids (Tyr far Ser37) was previously shown to not confer C3b binding, we reasone d that either single substitution of the other amino acid or a combina tion of the two amino acid changes would be required for C3b binding. To test this, using a truncated form of Hu CR1 that has a binding site only far C4b, we made these additional constructs. Single substitutio n of either amino acid did not affect the ligand binding or cofactor a ctivity. However, the double substitution induced C3b binding and incr eased cofactor activity for C3b without changing the C4b-binding prope rty. Of interest, these two amino acids are also found in the homologo us positions of CP 9 and 16, which form part of the C3b binding site o f Hu CR1. Thus, Ch E CR1(77), one-third of the size and with only a si ngle ligand binding site, by acquiring key amino acid substitutions, b inds C3b and C4b and functions analogous to Hu E CR1.