Characterization of the rat alpha(1,3)galactosyltransferase: evidence for two independent genes encoding glycosyltransferases that synthesize Galalpha(1,3)Gal by two separate glycosylation pathways

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Abstract
The important xenoepitope Galα(1,3)Gal was thought to be exclusively synthesized by a single α(1,3)galactosyltransferase. However, the cloning of the distant family member rat iGb3 synthase, which is also capable of synthesizing Galα(1,3)Gal as the glycolipid structure iGb3, challenges the notion that α(1,3)galactosyltransferase is the sole Galα(1,3)Gal-synthesizing enzyme. We describe the cloning of the rat homolog of α(1,3)galactosyltransferase, showing that indeed the rat expresses two distinct α(1,3)galactosyltransferases, α(1,3)GT and iGb3 synthase. Rat α(1,3)galactosyltransferase shows a high amino acid sequence identity with the α(1,3)galactosyltransferase of mouse (90%), pig (76%), and ox (75%), in contrast to the low amino acid sequence identity (42%) with iGb3 synthase. The rat α(1,3)galactosyltransferase is expressed in heart, brain, spleen, kidney, and liver and has a similar intron/exon structure to the mouse α(1,3)galactosyltransferase. Transfection studies show that in contrast to the iGb3 synthase, rat α(1,3)galactosyltransferase can synthesize Galα(1,3)Gal on glycoproteins but cannot synthesize the glycolipid iGb3, defining two separate glycosylation pathways for the synthesis of Galα(1,3)Gal. Furthermore iGb3 synthase was found to be distinct from α(1,3)GT with its ability to synthesize poly-α-Gal glycolipid structures.

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