Predicted localization of different D epitope clusters and hypothetical model of loop-loop interactions of the Rh D polypeptide. Based on the serological reaction of the different mutant K562 lines, a model for the loop-loop interactions that govern D epitope expression is presented. The model is a plan view of an Rh D protein monomer in which each membrane-spanning segment is represented by a circle (numbered 1 through 12) and each externalized loop as a cross-hatched line (numbered 1 through 6). Cytoplasmic-localized loops are represented as black lines. Each external loop is positioned such that adjacent membrane-spanning segments may interact, altering the confirmation and expression of D epitopes (see Discussion). The model considers that each epitope cluster is no more than 25 Å in diameter, based on the fact that an antibody paratope is of this dimension. It could be anticipated that the diameter of an Rh D protein monomer is greater than 50 Å if compared with the determined dimensions of 2 other erythrocyte membrane proteins aquaporin 130,31 and the anion transport protein.32 The locations of the predicted 6 Rh D epitope clusters are indicated by letters A through F, and the D epitopes expressed by each cluster (1-9 model) is indicated by each panel. (A) Loop 6-dependent epitopes require only anRHD loop 6 for antibody binding. (B) Loop 4+6-dependent epitopes require amino acids located on both these loops. (C) Loop 3, 4, and 6-dependent epitopes have contact points with all 3 of theseRHD-specific loops. (D) Loop 1, 2, 3, and 5-dependent epitopes have a relatively large contact area and are not expressed on partial D proteins with mutations in the Rh CcEe/D common loops 1, 2, and 5 (DMH, D^VII, and DHMi, respectively). Contact with loop 3RHD-specific residues gives the antibodies anti-D specificity, because partial D phenotypes with mutations in this loop lack epD8 (DFR and DOL). (E) Loop 6-dependent epitopes. These epitopes requireRHD-specific cytoplasmic/transmembrane residues to stabilize their configuration. (F) Loop 3+4- and loop 3 or 4-dependent epitopes. This group includes antibodies that require RHDresidues on both loops 3 and 4 and also those antibodies that can bind to both R_o^Har and DBT red blood cells.