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Evidence supporting the requirement for two proline residues for expression of c
C M Westhoff1, L E Silberstein, D E Wylie
1Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA. Westhoff@mail.med.upenn.edu
Background:
In humans, c antigen expression is associated with a proline residue at amino acid position 103 in the second extracellular loop of the CE protein. Comparison of nonhuman primate Rh proteins suggested that c reactivity might actually involve two proline residues. It has been shown that the 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 from the capuchin was analyzed.
Study Design And Methods:
Rh transcripts were amplified by reverse transcription 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 anti-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 residues. Proline causes bends or loops in proteins, which, in this case, might form a unique, stable structure resistant to perturbations induced by changes in upstream or downstream residues. This would explain the scarcity in humans 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.