Transporters of nucleotide sugars, nucleotide sulfate and ATP in the Golgi apparatus membrane: Where next?

Recently the UDP-GlcNAc, CMP-sialic acid and GDP-mannose transporters from the Golgi apparatus membrane were cloned (Abeijon ed al., 1996; Eckhardt et al., 1996; Ma et al., 1997). These transporters, as well as those for other nucleotide sugars, PAPS and ATP, are required for these nucleotide derivatives to reach to Golgi lumen from the cytosol and serve as substrates in glycosylation, sulfation, and phosphorylation of glycoproteins, proteoglycans, and glycolipids. Previously, biochemical and genetic evidence demonstrated that these transporters are highly specific for solute transport (Hirschberg, 1996; Hirschberg and Snider, 1987), are often organelle specific (Hirschberg, 1996), appear to play a regulatory role in the biosynthesis of Golgi lumenal macromolecules (Abeijon et al., 1993; Toma et al., 1996), and use antiporters with the corresponding nucleoside phosphate as the mechanism for concentration of solutes in the Golgi lumen (Hirschberg and Snider, 1987; Milla and Hirschberg, 1989; Waldman and Rudnick, 1990; Abeijon et al., 1993; Berninsone et al., 1994). The aim of this article is not to provide a comprehensive review of this topic, which has been done recently (Hirschberg, 1996), but to highlight some important unanswered questions and new avenues of experimentation of this topic.