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Glycobiology Advance Access published online on November 1, 2002
Glycobiology, doi:10.1093/glycob/cwg015
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Submitted on June 14, 2002
Revised on August 21, 2002
Accepted on September 17, 2002

© 2002 Oxford University Press

ORIGINAL ARTICLES

The two rat {alpha}2, 6-sialyltransferase (ST6Gal I) isoforms: evaluation of catalytic activity and intra-golgi localization

Tung-Ling L. Chen 1*, Chun Chen 2, Nyahne Q. Bergeron 2, Brett E. Close 2, Tracy J. Bohrer 2, Barbara M. Vertel 1, Karen J. Colley 2

1 The Department of Cell Biology and Anatomy, The Finch University of Health Sciences/The Chicago Medical School, North Chicago, IL 60064
2 The Department of Biochemistry and Molecular Biology, University of Illinois at Chicago, College of Medicine, Chicago, IL 60612

* To whom correspondence should be addressed. E-mail: karenc{at}uic.edu.

Abstract

The {alpha}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 study, our goal was to more carefully evaluate possible differences in the catalytic activity and intra-Golgi localization of the two isoforms that had been suggested by earlier studies. 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 CHO cells and sialylated substrates were detected by lectin blotting. Immunofluorescence and immunoelectron microscopy revealed differences in the relative levels of the isoforms found in the ER and Golgi of transiently expressing cells, but similar intra-Golgi localization. The STtyr was restricted to the Golgi in most cells, while the STcys was found in both the endoplasmic reticulum (ER) and Golgi. The ER localization of the 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.


Keywords: sialyltransferase/Golgi/cellular localization/catalytic activity/isoform
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The CMP-sialic Acid Transporter Is Localized in the Medial-Trans Golgi and Possesses Two Specific Endoplasmic Reticulum Export Motifs in Its Carboxyl-terminal Cytoplasmic Tail
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Multiple Signals Are Required for {alpha}2,6-Sialyltransferase (ST6Gal I) Oligomerization and Golgi Localization
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