Glycophorin SAT of the human erythrocyte membrane is specified by a hybrid gene reciprocal to glycophorin Dantu gene

Previous studies of two unrelated Japanese families showed that two isoforms of glycophorin were associated with the expression of SAT antigen on the erythrocyte membrane. Here we report the molecular basis for one form of this MNSs-related surface marker that displayed an altered immunoblotting pattern. Evidence is presented that glycophorin SAT (GPSAT) is encoded by a hybrid gene resulting from unequal homologous recombination between GPA and GPB genes. Analysis of SAT genomic DNAs by Southern blots showed gross alterations in the glycophorin gene cluster. Those restriction fragments characteristic of the GPA 3′ and GPB 5′ ends were absent from the SAT homozygote and showed reduced intensity in SAT heterozygotes. Reticulocyte RNA polymerase chain reaction showed the presence in the SAT homozygote of GPSAT and GPE transcripts but no GPA and GPB transcripts. Direct sequencing of the amplified SAT cDNA showed that its sequence from exon I to exon IV was identical with the N allele of GPA, whereas its 3′ portion, including exon V and exon VI, was derived from the GPB gene. The GPSAT protein in its mature form should contain 104 amino acid residues and bear a novel sequence, Ser-Glu-Pro-Ala-Pro-Val, encoded by the junction of GPA exon IV and GPB exon V. This sequence interfaces the extracellular and transmembrane domains and could be the epitope site of the SAT antigen. The formation of such a hybrid junction not only explains why SAT homozygous erythrocytes lack S, s, and U antigens but also shows a reciprocal arrangement with respect to the B-A hybrid GPDantu gene.