Sialyltransferases membrane glycoconjugates

Further studies using NeuSAc have been related to the metabolism of extracellular CMP-NeuSAc and the detection of ectosialyltransferase activity. On the basis of a lag period observed for NeuSAc uptake and incorporation into glycoconjugates, and the absence of such a lag for CMP-NeuSAc incorporation, two routes have been proposed. The uptake of NeuSAc and metabolism as described above has been studied in hamster and mouse fibroblasts, and cell surface labelling of glycoprotein and glycolipid demonstrated (Datta 1974). The breakdown of CMP-NeuSAc was shown, and incorporation due to NeuSAc uptake rather than direct CMP-NeuSAc transfer proposed (Hirschberg et al. 1976). The uptake of CMP-NeuSAc into the cells (NIL, BHK and 3T3 fibroblasts) could be ruled out, and the K , for NeuSAc uptake was estimated to be 10 mM. Other experiments with CMP-NeuSAc and intact cell cultures (Painter and White 1976, Cerven 1977, see section III.9) pointed to surface sialyltransferase. Further studies by Fan and Datta (1980) provided evidence that both transfer and transport occur, by localization of acceptors within the cell and on the cell surface (plasma membrane), and direct demonstration of the presence of a plasma membrane sialyltransferase. The sialylation due to NeuSAc uptake occurs (at least initially) with different acceptors in comparison with CMP-NeuSAc plasma membrane sialylation. 

Figure 3. Two models for the O-acetylation of Sia in Golgi-membranes of bovine submandibular glands. A, as CMP-glycoside, before the transfer onto nascent glycoconjugates (mucin) and B, after this transfer. OAT, sialate-7(9)-0-acetyltransferase ST, sialyltransferase M, O-acetylisomerase. For further details see the text.

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