The Molecular Basis for the B(A) Allele: An Amino Acid Alteration in the Human Histoblood Group B α-(1,3)-Galactosyltransferase Increases Its Intrinsic α-(1,3)-N-Acetylgalactosaminyltransferase Activity

The formation of the subgroup B(A) phenotype is thought to be due to an overlapping specificity of the human blood group A and B transferases. A new molecular basis for the B(A) allele, resulting from the C-700 to G substitution which predicts the alteration of Pro(234) to Ala, just ahead of the second of the four amino acid residues which differentiates the specificities of the A and B transferases, is reported here. Compared to normal group B sera, a relatively lower B-transferase activity was demonstrated in the B(A) serum, which correlated well with the observation of a smaller amount of B antigen on the B(A) red cells. Also a much higher A-transferase activity was demonstrated in the B(A) serum in contrast to the minute amount of A-transferase activity found in normal group B sera. The formation of the B(A) phenotype in this report is most likely due to the shifting of the specificity of the B transferase rather than an enhanced B-transferase activity which was previously presumed to be responsible for the formation of this phenotype. The Pro(234) to Ala alteration is suggested to be responsible for the shifting of the specificity with a subsequent increase in A- but a decrease in B-transferase activity. This new B(A) allele shows that not only the four critical residues but also the neighboring areas may influence the specificity of the A and B transferases. (C) 1999 Academic Press.