Blood group antigens Rb(a), Tr(a), and Wd(a) are located in the third ectoplasmic loop of erythroid band 3

Rb(a), Tr(a), and Wd(a) are three low-incidence blood group antigens that have not been assigned to a particular structure of the red cell membrane. Recent genetic and serologic data suggested erythroid band 3 as a possible carrier of these three antigens.

Ten band 3 gene exons that encode the membrane domain of band 3 were screened for single strand conformation polymorphism (SSCP). Exons displaying SSCP were cloned and sequenced, and the presence of the mutations was verified by restriction digestion.

Substitutions 548 Pro-->Leu, 551 Lys-->Asn, and 557 Val-->Met, all located in the third ectoplasmic loop of band 3, were detected in the subjects with Rb(a+), Tr(a+), and Wd(a+) red cells, respectively. The presence of the Rb(a) and Wd(a) mutations was confirmed in additional carriers of these blood group antigens. Chymotryptic cleavage at Tyr 553 and Tyr 555 abolished the agglutinability of Tr(a+) and Wd(a+) cells with the corresponding antisera, further demonstrating that the epitopes are located in the third ectoplasmic loop of band 3. Similar quantities of mRNA corresponding of the two band 3 alleles, a normal pattern of red cell membrane proteins, and normal DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, disodium salt)-inhibitable sulfate flux were detected, which suggests that the mutations do not affect band 3 mRNA stability or band 3 protein expression and transport function.

Wd(a) and Rb(a), and tentatively Tr(a), can be assigned to the Diego blood group system.