Evidence supporting the requirement for two proline residues for expression of c

BACKGROUND: In humans, c antigen expression is associated with a proline re sidue at amino acid position 103 in the second extracellular loop of the CE protein. Comparison of nonhuman primate Rh proteins suggested that c react ivity might actually involve two proline residues. It has been shown that t he RBCs of New World capuchin monkeys (Cebus apella) react with anti-c. To further define the amino acid residues involved in c expression, Rh cDNA fr om the capuchin was analyzed. STUDY DESIGN AND METHODS: Rh transcripts were amplified by reverse transcri ption PCR from RNA isolated from the reticulocytes of a capuchin monkey and were cloned and sequenced. RESULTS: Rh transcripts from the capuchin monkey, whose RBCs react with ant i-c, were found to encode adjacent proline residues at 102 and 103. CONCLUSION: Sequencing of Rh transcripts from the capuchin monkey supports the hypothesis that the expression of c requires two adjacent proline resid ues. Proline causes bends or loops in proteins, which, in this case, might form a unique, stable structure resistant to perturbations induced by chang es in upstream or downstream residues. This would explain the scarcity in h umans of c variants as compared to the other major Rh antigen variants, and the preservation of c reactivity despite 24-percent divergence between the human and capuchin Rh proteins.