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* Epithelial Cell Biology Laboratory, The Electron Microscopy Unit, Imperial Cancer Research Fund, Lincoln's Inn Fields, London
WC2A 3PX, UK; § Department of Biochemistry, Hospital for Sick Children and University of Toronto, Toronto M5G 1X8,
Canada; and Sloan-Kettering Institute, New York 10021
2,3 sialyltransferase, 2,3 SAT (O),
catalyzes the transfer of sialic acid to Gal
1,3 N-acetyld-galactosamine (GalNAc) (core-1) in mucin type
O-glycosylation, and thus terminates chain extension. A
Core-2 branch can also be formed from core-1 by the core-2 1,6 N-acetyl-d-glucosamine transferase (1,6
GlcNAc T) that leads to chain extension. Increased levels of the 2,3 SAT (O) and decreased levels of the
core-2 1,6 GlcNAc T are seen in breast cancer cells
and correlate with differences in the structure of the
O-glycans synthesized (; ). Since in mucin type O-glycosylation sugars
are added individually and sequentially in the Golgi apparatus, the position of the transferases, as well as their
activity, can determine the final structure of the O-glycans synthesized. A cDNA coding for the human 2,3
SAT (O) tagged with an immunoreactive epitope from
the myc gene has been used to map the position of the
glycosyltransferase in nontumorigenic (MTSV1-7) and
malignant (T47D) breast epithelial cell lines. Transfectants were analyzed for expression of the enzyme at the
level of message and protein, as well as for enzymic activity. In T47D cells, which do not express core-2 1,6
GlcNAc T, the increased activity of the sialyltransferase correlated with increased sialylation of core-1
O-glycans on the epithelial mucin MUC1. Furthermore, in MTSV1-7 cells, which do express core-2 1,6
GlcNAc T, an increase in sialylated core-1 structures is accompanied by a reduction in the ratio of GlcNAc:
GalNAc in the O-glycans attached to MUC1, implying
a decrease in branching. Using quantitative immunoelectron microscopy, the sialyltransferase was mapped
to the medial- and trans-Golgi cisternae, with some being present in the TGN. The data represent the first
fine mapping of a sialyltransferase specifically active in
O-glycosylation and demonstrate that the structure of
O-glycans synthesized by a cell can be manipulated by
transfecting with recombinant glycosyltransferases.
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