A SOLUBLE FORM OF ALPHA-1,3-GALACTOSYLTRANSFERASE FUNCTIONS WITHIN CELLS TO GALACTOSYLATE GLYCOPROTEINS

It has been assumed that membrane bound glycosyltransferases function within the Golgi apparatus to glycosylate glycoproteins. We now report , however, that a truncated, soluble recombinant form of the murine al pha 1,3-galactosyltransferase expressed in human 293 cells is highly e fficient and comparable to the full-length enzyme in alpha-galactosyla ting both newly synthesized membrane-associated and secreted glycoprot eins. Although the soluble enzyme was secreted by cells as expected, w e also found that the full-length, membrane-associated form was secret ed. Unexpectedly, both secreted forms are cleaved identically at two p rimary sites within the stem region by endogenous protease(s) at the i ndicated positions in the sequence (KDWW)-K-73 down arrow FPS down arr ow WFKNG. These results demonstrate that the soluble alpha 1,3-galacto syltransferase is functional within the cell and that specific proteol ysis occurs in the stem region. The widespread occurrence of different soluble glycosyltransferases secreted by cells suggests that normal g lycoconjugate biosynthesis may involve cooperation between membrane-bo und and soluble enzymes.