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We tested the importance of the aspartate-any residue-aspartate (DXD) motif for the enzymatic activity and nucleotide binding capacity of the Golgi glycosyltransferase GM2 synthase. We prepared point mutations of the motif, which is found in the sequence 352-VLWVDDDFV, and analyzed cells that stably expressed the mutated proteins. Whereas the folding of the mutated proteins was not seriously disrupted as judged by assembly into homodimers, Golgi localization, and secretion of a soluble form of the enzyme, exchange of the highly conserved aspartic acid residues at position 356 or 358 with alanine or asparagine reduced enzyme activity to background levels. In contrast, the D356E and D357N mutations retained weak activity, while the activity of V352A and W354A mutants was 167% and 24% that of wild-type enzyme, respectively. Despite the major effect of the DXD motif on enzymatic activity, nucleotide binding was not altered in the triple mutant D356N/D357N/D358N as revealed by binding to UDP-beads and labeling with the photoaffinity reagent, P(3)-(4-azidoanilido)uridine 5'-triphosphate (AAUTP). In summary, rather than being critical for nucleotide binding, this motif may function during catalysis in GM2 synthase, as has been proposed elsewhere for the SpsA glycosyltransferase based on its crystal structure.

Ganglioside GM2 synthase and other enzymes required for complex ganglioside synthesis were localized recently to the trans Golgi network (TGN). However, there are conflicting reports as to the location of GM3 synthase; originally this enzyme was detected in the early Golgi of rat liver but a recent report localized it to the late Golgi. We have used chimeric forms of ganglioside GM2 synthase to determine if the location of lactosylceramide (LacCer) to GM3 conversion in Chinese hamster ovary (CHO) cells was the early or late Golgi. Our approach tested whether GM3 could be utilized as a substrate by GM2 synthase chimeras which were targeted to compartments earlier than the trans Golgi, i.e., GM3 produced in the cis Golgi should be utilized by GM2 synthase located anywhere in the Golgi whereas GM3 produced in the trans Golgi should only be used by GM2 synthase located in the trans Golgi or TGN. Comparison of cell lines stably expressing these chimeras revealed that the in vivo functional activity of GM2 synthase decreased progressively as the enzyme was targeted to earlier compartments; specifically, the percentage of GM3 converted to GM2 was 83-86% for wild type enzyme, 70% for the medial Golgi targeted enzyme, 13% for the ER and cis Golgi targeted enzyme, and only 1.7% for the ER targeted enzyme. Thus, these data are consistent with a late Golgi location for LacCer to GM3 conversion in these cells.

Brefeldin A reversibly disassembles the Golgi complex, causing mixing of the Golgi cisternae with the ER while the trans Golgi network persists as part of a separate endosomal membrane system. Because of this compartmental separation, Brefeldin A treatment has been used to map the sub-Golgi locations of several Golgi enzymes including GM2 synthase. We previously proposed that GM2 synthase might be located in a distal portion of the Golgi complex which in the presence of Brefeldin A would be separated from the substrate ganglioside GM3 present in the mixed ER-Golgi membrane system. In the present study we show using GM2 synthase chimeras that GM2 synthesis was blocked by Brefeldin A when GM2 synthase was distributed throughout all Golgi subcompartments or even when it was restricted to the medial Golgi. Because these findings opposed our speculation regarding a distal location of this enzyme, we sought an alternative explanation for the inhibition of ganglioside synthesis by Brefeldin A. However, Brefeldin A did not degrade GM2 synthase, prevent its homodimerization, or inhibit its in vitro activity. Brefeldin A did result in the conversion of a portion of membrane bound GM2 synthase into a soluble form which has minimal capability to produce GM2 in whole cells. However, this conversion was not sufficient to explain the nearly total loss of GM2 production in intact cells in the presence of Brefeldin A. Nevertheless, the results of this study indicate that Brefeldin A-induced inhibition of ganglioside synthesis cannot be used to deduce the location of GM2 synthase.

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 alpha1, 3-galactosyltransferase was comparable to its membrane bound counterpart in its ability to galactosylate newly synthesized glycoproteins (Cho,S.K. and 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, ss1,4 N-Acetylgalactosaminyltransferase (GM2/GD2/ GA2 synthase; GalNAcT) and beta galactoside alpha2,6 sialyltransferase (alpha2,6-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 alpha2,6-ST, cells expressing a soluble form of alpha2,6-ST contained 3-fold higher alpha2,6-ST mRNA levels and secreted 7-fold greater alpha2,6-ST activity as measured in vitro , but in striking contrast contained 2- to 4-fold less of the alpha2,6-linked sialic acid moiety in cellular glycoproteins in vivo . In summary these results suggest that unlike alpha1,3-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.