The molecular genetic basis of the P histo-blood group system has eluded characterization despite extensive studies of the biosynthesis of the P 1 , P, and P k glycolipids. The main controversy has been whether a single or two distinct UDP-Gal:Gal1-R 4-␣-galactosyltransferases catalyze the syntheses of the structurally related P 1 and P k antigens. The P 1 polymorphism is linked to 22q11.3-ter. Data base searches with the coding region of an ␣4GlcNAc-transferase identified a novel homologous gene at 22q13.2 designated ␣4Gal-T1. Expression of full coding constructs of ␣4Gal-T1 in insect cells revealed it encoded P k but not P 1 synthase activity. Northern analysis showed expression of the transcript correlating with P k synthase activity and antigen expression in human B cell lines. Transfection of P k -negative Namalwa cells with ␣4Gal-T1 resulted in strong P k expression. A single homozygous missense mutation, M183K, was found in six Swedish individuals of the rare p phenotype, confirming that ␣4Gal-T1 represented the P k gene. Sequence analysis of the coding region of ␣4Gal-T1 in P 1 ؉/؊ individuals did not reveal polymorphisms correlating with P 1 P 2 typing.The P histo-blood group system is the last of the known carbohydrate defined blood group systems for which the molecular genetic basis has not yet been clarified. The P blood group system involves two major blood group phenotypes, P 1 ϩ and P 1 Ϫ, with approximate frequencies of 80% and 20%, respectively (1, 2). P 1 Ϫ individuals normally express the P antigen (P 1 Ϫ is designated P 2 when P antigen expression is demonstrated), but the rare P k phenotype lacks the P antigen, while the rare p phenotype lack both P and P k antigens (for reviews, see Refs. 3-7). The P 1 ϩ phenotype is defined by expression of the neolacto-series glycosphingolipid P 1 (for structures, see Table I) (8). In contrast, the P, P k , and p antigens constitute intermediate steps in biosynthesis of globo-series glycolipids and give rise to P 1 k , P 2 k , and p phenotypes (9). Although the rare P k phenotype shows the same frequency of P 1 antigen expression as individuals expressing the P antigen, the p phenotype is always associated with lack of P 1 antigen expression. Extensive studies of the chemistry, biosynthesis, and genetics of the P blood group system identified the antigens as being exclusively found on glycolipids, with the blood group specificity being synthesized by at least two distinct glycosyltransferase activities; UDP-galactose:-D-galactosyl-1-R 4-␣-Dgalactosyltransferase (␣4Gal-T) 1 activity(ies) for P k and P 1 syntheses and UDP-GalNAc:Gb 3 3--N-acetylgalactosaminyltransferase activity (EC 2.4.1.79) for P synthesis (for reviews, see Refs. 6 and 7). At least two independent gene loci, P and P 1 P k , are involved in defining these antigens. The P blood group-associated LKE antigen, shown to be the extended sialylated Gal-globoside structure (10), may involve polymorphism in an ␣2,3-sialyltransferase activity.A long-standing controversy has been whether a single...
