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 a-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 cysteineencoding 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).