2 SOLUBLE GLYCOSYLTRANSFERASES GLYCOSYLATE LESS EFFICIENTLY IN-VIVO THAN THEIR MEMBRANE-BOUND COUNTERPARTS

Many Golgi glycosyltransferases are type II membrane proteins which ar e cleaved to produce soluble forms that are released from cells. Cho a nd Cummings recently reported that a soluble form of alpha 1,3-galacto syltransferase was comparable to its membrane bound counterpart in its ability to galactosylate newly synthesized glycoproteins (Cho,S.K. an d Cummings,R.D. (1997) J, Biol, Chem., 272, 13622-13628), To test the generality of their findings, we compared the activities of the full l ength and soluble forms of two such glycosyltransferases, beta 1,4 N-A cetylgalactosaminyltransferase (GM2/GD2/GA2 synthase; GalNAcT) and bet a galactoside alpha 2,6 sialyltransferase (alpha 2,6-ST; ST6Gal I), fo r 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 tho ugh it possessed in vitro GalNAcT activity comparable to that of full length GalNAcT. When compared with cells expressing full length alpha 2,6-ST, cells expressing a soluble form of a2,6-ST contained 3-fold hi gher a2,6-ST mRNA levels and secreted 7-fold greater a2,6-ST activity as measured in vitro, but in striking contrast contained 2- to l-fold less of the alpha 2,6-linked sialic acid moiety in cellular glycoprote ins in vivo. In summary these results suggest that unlike al,3-galacto syltransferase the soluble forms of these two glycosyltransferases are less efficient at glycosylation of membrane proteins and lipids in vi vo than their membrane bound counterparts.