Volume 272, Number 21, Issue of May 23, 1997 pp. 13622-13628
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

A Soluble Form of 1,3-Galactosyltransferase Functions within Cells to Galactosylate Glycoproteins

(Received for publication, January 6, 1997, and in revised form, March 4, 1997)

From the Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma Center for Molecular Medicine, Oklahoma City, Oklahoma 73190

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 1,3-galactosyltransferase expressed in human 293 cells is highly efficient and comparable to the full-length enzyme in -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 ⁷³KDWWFPSWFKNG. These results demonstrate that the soluble 1,3-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.

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