Evidence for a structurally homologous Rh-like polypeptide in Rh(null) erythrocytes

Human Rh(null) red blood cells fail to react with Rh antibodies, indicating that these cells are either devoid of Rh protein or, like other species, possess antigenically distinct variants. To determine whether Rh(null) cells possess an Rh-like polypeptide, 32-kDa proteins from D--, rr, and Rh(null) cells were labeled with the cysteine-specific probe, ¹²⁵I- labeled pyridyldithioethylamine. Size comparisons of labeled proteins in Triton X-100-solubilized membranes from Rh-bearing and Rh(null) cells showed similar sedimentation coefficients and Stoke's radii. Immunoprecipitated Rh(D) from D-- cells, Rh(c) from rr cells, and purified 32-kDa proteins from Rh(null) cells were digested with α-chymotrypsin and examined by high- performance liquid chromatography and by two-dimensional iodopeptide mapping. Analysis of ¹²⁵I-labeled chymotryptic fragments from immunoprecipitated Rh(D) and Rh(c) showed the labeled peptides from both phenotypes to be virtually identical. High-performance liquid chromatography profiles and iodopeptide maps of 32-kDa Rh(null) proteins yielded patterns identical to 32-kDa proteins isolated from D-- cells and rr cells with the exception of one missing ¹²⁵I-labeled peptide. Further analysis of the Rh-related fragments from Rh(null) cells showed significant homology with immunoprecipitated Rh(D) and Rh(c). DNA sequence analysis of cysteine- encoding regions from Rh-bearing and Rh(null) cells showed complete identity. These data suggest that Rh(null) red blood cells, although serologically distinct, possess an Rh-like protein that is structurally very similar to Rh(D) and Rh(c).