Plasma membrane sialyltransferase

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. 

Sialyltransferase activity has also been described in plasma membranes from different cells, including blood platelets.285 The function of the enzyme on cell surfaces is unknown, as resialylation of membrane components directly on the cell surface is improbable, owing to the presence of CMP-Neu5Ac hydrolase in the plasma membrane, and the lack of CMP-sialic acids in the extracellular space. Furthermore, evidence has never been obtained for a role of cell-surface gly-cosyltransferases in cellular interaction according to a hypothesis of Roseman s.281... 

The presence of glycosyltransferase activities at the surface of cells, i.e. located in the plasma membrane, remains controversial. Very low or no activity of sialyltransferases could be detected in rat kidney and liver plasma membrane... 

Total sialic acid content was found to be lower in several SV40-transformed clones of Swiss 3T3 mouse cells (Ohta et aL, 1968). The plasma membrane as well as other membrane fractions of similar cell lines had reduced amounts of sialic acid (Wu et aL, 1969). Isolated plasma membranes from Py-BHK cells had 60% less sialic acid than membranes from normal BHK cells (Makita and Seyama, 1971). These changes appear to be associated with reduced sialyltransferase activities in the transformed cells. Grimes (1970) determined the levels of sialic acid and sialyltransferase activity in normal and SV40-transformed Swiss and Balb 3T3 mouse cells. There was a close correlation between sialic acid content and sialyltransferase activity in each cell line even though enzyme levels were measured with exogenous acceptors such as desialylated fetuin or bovine submaxillary mucin. This correlation has been extended to a number of other cell lines (Grimes, 1973). 

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