The activities of the endogenous galactosyltransferase 1 (GalT1) and sialyltransferase 1 (SialT1) of CHO-K1 cells were increased 1.4 and 2.3 fold, respectively, in cells (ST18 cells) stably expressing sialytransferase 2 (SialT2). The activation was not due to protein stabilization or transcriptional activation of GalT1 and SialT1 genes. The N-terminal domain of SialT2, which participates in the formation of a complex with GalT1 and SialT1, was not able to activate them, indicating that the C-terminal catalytic domain was necessary for the activation (SAIB 2007). In this presentation we analyze the products of SialT1 activity in wt and ST18 cells. While in wt cells GM3 was the main product, in ST18, which showed higher SialT1 activity, the products were GM3, GD3 and GT3. This indicates that the product of SialT1 activity is being efficiently used by SialT2 to produce GD3, and that the presence of SialT2 stimulates the conversion of LacCer into GM3 by SialT1. Also, they suggest that, in vitro, the two enzymes are still part of a complex existing in vivo. Results of co-inmunoprecipitation experiments in homogenates of cells co-expressing SialT1-Flag and SialT2-HA indicate an interaction between both enzymes. These results indicate a topological organization of glycosyltransferases that improves glycolipid synthesis by a functional docking of participating enzymes.
