Generation of histo-blood group B transferase by replacing the N-acetyl-D-galactosamine recognition domain of human A transferase with the galactose-recognition domain of evolutionarily related murine alpha1,3-galactosyltransferase.
The alpha1,3-galactosyl epitope (alpha1-3Gal epitope), a major xenotransplant antigen, is synthesized by alpha1,3-galactosyltransferase (alpha1-3Gal transferase), which is evolutionarily related to the histo-blood group A/B transferases. We constructed structural chimeras between the human type A and murine alpha1-3Gal transferases and examined their activity and specificity. In many instances, a total loss of transferase activity was observed. Certain areas could be exchanged, with a potential diminishing of activity. With a few constructs, changes in acceptor substrate specificity were suspected. Unexpectedly, a functional conversion from A to B transferase activity was observed after replacing the short sequence of human A transferase with the corresponding sequence from murine alpha1-3Gal transferase. Because these two paralogous enzymes differ in 16 positions of the 38 amino acid residues in the replaced region, our finding may suggest that despite separate evolution and diversified acceptors, these glycosyltransferases still share the three-dimensional domain structure that is responsible for their sugar specificity, arguing against the functional requirement of a strong purifying selection playing a role in the evolution of the ABO family of genes.