Gangliosides are acidic glycosphingolipids that contain sialic acid residues and are expressed in nearly all vertebrate cells. They are synthesized at the Golgi complex by a combination of glycosyltransferase activities followed by vesicular delivery to the plasma membrane, where they participate in a variety of physiological as well as pathological processes. Recently, a number of enzymes of ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane. In particular, it was observed that CMP-NeuAc:GM3 sialyltransferase (Sial-T2) is able to sialylate GM3 at the plasma membrane (ciscatalytic activity). In this work, we demonstrated that plasma membrane-integrated ecto-Sial-T2 also displays a trans-catalytic activity at the cell surface of epithelial and melanoma cells. By using a highly sensitive enzyme-linked immunosorbent assay combined with confocal fluorescence microscopy, we observed that ecto-Sial-T2 was able to sialylate hydrophobically or covalently immobilized GM3 onto a solid surface. More interestingly, we observed that ecto-Sial-T2 was able to sialylate GM3 exposed on the membrane of neighboring cells by using both the exogenous and endogenous donor substrate (CMP-N-acetylneuraminic acid) available at the extracellular milieu. In addition, the trans-activity of ecto-Sial-T2 was considerably reduced when the expression of the acceptor substrate was inhibited by using a specific inhibitor of biosynthesis of glycolipids, indicating the lipidic nature of the acceptor. Our findings provide the first direct evidence that an ecto-sialyltransferase is able to trans-sialylate substrates exposed in the plasma membrane from mammalian cells, which represents a novel insight into the molecular events that regulate the local glycosphingolipid composition.Glycosphingolipids are amphipathic molecules consisting of a ceramide lipid moiety linked to a glycan chain of variable length and structure. Among these are found gangliosides, which are sialosylated glycosphingolipids mainly located in the outer layer of the plasma membrane of vertebrate cells (1, 2) and have been implicated in many physiological processes, including growth, differentiation, migration and apoptosis through modulating both cell signaling processes and cell-tocell and cell-to-matrix interactions (3-6). Furthermore, gangliosides have been associated with a wide range of pathological processes, being receptors for both viruses and antibodies (7,8).The biosynthesis of gangliosides is mainly carried out in the lumen of the Golgi cisternae by a complex system of membrane-bound glycolipid acceptors, nucleotide sugar donors, glycosyltransferases, and nucleotide sugar transporters (1, 9). These neosynthesized gangliosides move through the Golgi complex to the plasma membrane via the lumenal surface of transport vesicles (10 -12). It is very well documented that glycosphingolipid expression, including gangliosides, is mainly regulated at the transcriptional and posttranscriptional levels of glycolipid glycosyltransferases a...