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

Many Golgi glycosyltransferases are type II membrane proteins which arecleaved to produce soluble forms that are released from cells. Cho andCummings recently reported that a soluble form of alpha1, 3-galactosyltransferase was comparable to its membrane bound counterpart inits ability to galactosylate newly synthesized glycoproteins (Cho,S.K. andCummings,R.D. (1997) J. Biol. Chem., 272, 13622-13628). To test thegenerality of their findings, we compared the activities of the full lengthand soluble forms of two such glycosyltransferases, ss1,4N-Acetylgalactosaminyltransferase (GM2/GD2/ GA2 synthase; GalNAcT) and betagalactoside alpha2,6 sialyltransferase (alpha2,6-ST; ST6Gal I), forproduction of their glycoconjugate products in vivo . Unlike the fulllength form of GalNAcT which produced ganglioside GM2 in transfected cells,soluble GalNAcT did not produce detectable GM2 in vivo even though itpossessed in vitro GalNAcT activity comparable to that of full lengthGalNAcT. When compared with cells expressing full length alpha2,6-ST, cellsexpressing a soluble form of alpha2,6-ST contained 3-fold higheralpha2,6-ST mRNA levels and secreted 7-fold greater alpha2,6-ST activity asmeasured in vitro , but in striking contrast contained 2- to 4-fold less ofthe alpha2,6-linked sialic acid moiety in cellular glycoproteins in vivo .In summary these results suggest that unlike alpha1,3-galactosyltransferasethe soluble forms of these two glycosyltransferases are less efficient atglycosylation of membrane proteins and lipids in vivo than their membranebound counterparts.