Two soluble glycosyltransferases glycosylate less efficiently in vivo than their membrane bound counterparts

Many Golgi glycosyltransferases are type II membrane proteins which are cleaved to produce soluble forms that are released from cells. Cho and Cummings recently reported that a soluble form of α1,3-galactosyltransferase was comparable to its membrane bound counterpart in its ability to galactosylate newly synthesized glycoproteins (Cho,S.K., Cummings,R.D. (1997) J. Biol. Chem., 272, 13622–13628). To test the generality of their findings, we compared the activities of the full length and soluble forms of two such glycosyltransferases, β1,4 N-Acetylgalactosaminyltransferase (GM2/GD2/GA2 synthase; GalNAcT) and beta galactoside α2,6 sialyl-transferase (α2,6-ST; ST6Gal I), for production of their glycoconjugate products in vivo. Unlike the full length form of GalNAcT which produced ganglioside GM2 in transfected cells, soluble GalNAcT did not produce detectable GM2 in vivo even though it possessed in vitro GalNAcT activity comparable to that of full length GalNAcT. When compared with cells expressing full length α2,6-ST, cells expressing a soluble form of α2,6-ST contained 3-fold higher α2,6-ST mRNA levels and secreted 7-fold greater α2,6-ST activity as measured in vitro, but in striking contrast contained 2-to 4-fold less of the α2,6-linked sialic acid moiety in cellular glycoproteins in vivo. In summary these results suggest that unlike α1,3-galactosyltransferase the soluble forms of these two glycosyltransferases are less efficient at glycosylation of membrane proteins and lipids in vivo than their membrane bound counterparts.