A soluble form of α1,3-galactosyltransferase functions within cells to galactosylate glycoproteins

Abstract

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 α1,3- galactosyltransferase expressed inhuman 293 cells is highly efficient and comparable to the full-length enzyme in α-galactosylating both newly synthesized membrane-associated and secreted glycoproteins. Although the soluble enzyme was secreted by cells as expected, we also found that the full-length, membrane-associated form was secreted. Unexpectedly, both secreted forms are cleaved identically at two primary sites within the stem region by endogenous protease(s) at the indicated positions in the sequence 73KDWW ↓ FPS ↓ WFKNG. These results demonstrate that the soluble α1,3- galactosyltransferase is functional within the cell and that specific proteolysis occurs in the stem region. The widespread occurrence of different soluble glycosyltransferases secreted by cells suggests that normal glycoconjugate biosynthesis may involve cooperation between membrane-bound and soluble enzymes.