SWITCHING AMINO-TERMINAL CYTOPLASMIC DOMAINS OF ALPHA(1,2)FUCOSYLTRANSFERASE AND ALPHA(1,3)GALACTOSYLTRANSFERASE ALTERS THE EXPRESSION OF HSUBSTANCE AND GAL-ALPHA(1,3)GAL

When alpha(1,2)fucosyltransferase cDNA is expressed in cells that norm ally express large amounts of the terminal carbohydrate Gal alpha(1,3) Gal, and therefore the alpha(1,3)galactosyltransferase (GT), the Gal a lpha(1,3)Gal almost disappears, indicating that the presence of the al pha(1,2)fucosyltransferase (HT) gene/enzyme alters the synthesis of Ga l alpha(1,3)Gal. A possible mechanism to account for these findings is enzyme location within the Golgi apparatus, We examined the effect of Gels localization by exchanging the cytoplasmic tails of HT and GT; i f Golgi targeting signals are contained within the cytoplasmic tail se quences of these enzymes then a ''tail switch'' would permit GT first access to the substrate and thereby reverse the observed dominance of HT, Two chimeric glycosyltransferase proteins were constructed and com pared with the normal glycosyltransferases after transfection into COS cells, The chimeric enzymes showed K-m values and cell surface carboh ydrate expression comparable with normal glycosyltransferases. Coexpre ssion of the two chimeric glycosyltransferases resulted in cell surfac e expression of Gal alpha(1,3)Gal, and virtually no HT product was exp ressed. Thus the cytoplasmic tail of HT determines the temporal order of action, and therefore dominance, of these two enzymes.