Comparison of the three rat GDP‐L‐fucose:β‐D‐galactoside 2‐α‐L‐fucosyltransferases FTA, FTB and FTC

The complete coding sequences of three rat α1,2fucosyltransferase genes were obtained. Sequence analysis revealed that these genes, called FTA, FTB and FTC, were homologous to human FUT1, FUT2 and Sec1, respectively. A distance analysis between all α1,2fucosyltransferase sequences available showed that the two domains of the catalytic region evolved differently with little divergence between the FUT2 and Sec1 N‐terminal domains, quite distant from that of FUT1. At variance, FUT1 and FUT2 C‐terminal domains were less distant while a high evolutionary rate was noted for Sec1 C‐terminal domain. Whereas FTA and FTB encode typical glycosyltransferases, FTC lacks the homologous start codon and encodes a protein devoid of intracellular and transmembrane domains. It is located on rat chromosome 1q34. Transfection experiments revealed that unlike FTA and FTB, FTC does not generate enzyme activity. Analysis by flow cytometry showed that H type 2 epitopes were synthesized in Chinese hamster ovary cells transfected by both FTA and FTB cDNA, but only FTB transfectants possessed H type 3 determinants. In REG rat carcinoma cells, both FTA and FTB allowed synthesis of H type 2 and H type 3 at the cell surface. Western blots showed that, in both cell types, FTA was able to synthesize H type 2 epitopes on a larger set of glycoproteins than FTB. Analysis of the kinetic parameters obtained using small oligosaccharides revealed only a slight preference of FTA for type 2 over other types of acceptor substrates, whereas FTB was barely able to fucosylate this substrate.