alpha 1,3 galactosyltransferase: new sequences and characterization of conserved cysteine residues

Nucleotide sequences were determined for alpha1,3 galactosyltransferases (a lpha1,3 GalTs) from several species (bat, mink, dog, sheep, and dolphin) an d compared with those previously determined for this enzyme and members of the alpha1,3 galactosyl/ N-acetylgalactosyltransferase (alpha1,3 Gal(NAc)Ts ) family of enzymes. Sequence comparison of the newly characterized alpha1, 3 GalT nucleotide and predicted amino acid sequences with those previously characterized for other alpha1,3GalT enzymes demonstrated a remarkable leve l of sequence identity at the nucleotide and amino acid level. The identity of each sequence as an alpha1,3 GalT was confirmed by expressing the encod ed protein and characterizing the resulting enzyme. The alpha1,3 GalTs have a significant degree of sequence homology with A an d B transferases, the alpha1,3 GalNAcT that catalyzes the synthesis of Fors sman antigen, and the recently cloned iso-globotriaosylceramide synthase. A mong the conserved residues, there are two Cys residues. To determine if th ese conserved residues are free or involved in the formation of a disulfide bond, bovine alpha1,3 GalT was characterized by chemical modification and mass spectrometry. Each peptide containing a Cys residue was chemically lab eled with an alkylating reagent demonstrating that these enzymes do not con tain disulfide bonds. Similar results have recently been reported for A and B transferases (Yen et al., 2000, J. Mass. Spectrom., 35, 990-1002). Thus, the highly conserved Cys residues found in these members of the alpha1,3 G al(NAc)Ts family of enzymes are likely involved in other important aspects of enzyme structure/function within this enzyme family.