Glycobiology Advance Access originally published online on November 1, 2002
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Glycobiology, 2003, Vol. 13, No. 2 109-117
© 2003 Oxford University Press
The two rat 
2,6-sialyltransferase (ST6Gal I) isoforms: evaluation of catalytic activity and intra-Golgi localization
3 Department of Cell Biology and Anatomy, Finch University of Health Sciences/the Chicago Medical School, North Chicago, IL 60064, USA
4 Department of Biochemistry and Molecular Biology, University of Illinois at Chicago, College of Medicine, 1819 W. Polk Street, M/C 536, Chicago, IL 60612, USA
Received on June 14, 2002; revised on August 21, 2002; accepted on September 17, 2002
2,6-Sialyltransferase (ST6Gal I) functions in the Golgi to terminally sialylate the N-linked oligosaccharides of glycoproteins. Interestingly, rat ST6Gal I is expressed as two isoforms, STtyr and STcys, that differ by a single amino acid in their catalytic domains. In this article, our goal was to evaluate more carefully possible differences in the catalytic activity and intra-Golgi localization of the two isoforms that had been suggested by earlier work. Using soluble recombinant STtyr and STcys enzymes and three asialoglycoprotein substrates for in vitro analysis, we found that the STcys isoform was somewhat more active than the STtyr isoform. However, we found no differences in isoform substrate choice when these proteins were expressed in Chinese hamster ovary cells, and sialylated substrates were detected by lectin blotting. Immuno-fluorescence and immunoelectron microscopy revealed differences in the relative levels of the isoforms found in the endoplasmic reticulum (ER) and Golgi of transiently expressing cells but similar intra-Golgi localization. STtyr was restricted to the Golgi in most cells, and STcys was found in both the ER and Golgi. The ER localization of STcys was especially pronounced with a C-terminal V5 epitope tag. Ultrastructural and deconvolution studies of immunostained HeLa cells expressing STtyr or STcys showed that within the Golgi both isoforms are found in medial-trans regions. The similar catalytic activities and intra-Golgi localization of the two ST6Gal I isoforms suggest that the particular isoform expressed in specific cells and tissues is not likely to have significant functional consequences.
1 These authors contributed equally to this work.
2 To whom correspondence should be addressed; e-mail: karenc{at}uic.edu
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