Multiple Signals Are Required for α2,6-Sialyltransferase (ST6Gal I) Oligomerization and Golgi Localization

Fiona H Fenteany,Karen J Colley

doi:10.1074/jbc.m412396200

Fiona H Fenteany, Karen J Colley

Open Access

https://doi.org/10.1074/jbc.m412396200

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Abstract

A single amino acid difference in the catalytic domain of two isoforms of the alpha2,6-sialyltransferase (ST6Gal I) leads to differences in their trafficking, processing, and oligomerization. The STtyr isoform is transiently localized in the Golgi and is ultimately cleaved and secreted, whereas the STcys isoform is stably localized in the Golgi and is not cleaved and secreted. The stable localization of STcys is correlated with its enhanced ability to oligomerize. To test the hypothesis that multiple signals can mediate Golgi localization and further evaluate the role of oligomerization in the localization process, we evaluated the effects of individually and simultaneously altering the cytosolic tail and transmembrane region of the STcys isoform. We found that the localization, processing, and oligomerization of STcys were not substantially changed when either the core amino acids of the cytosolic tail were deleted or the sequence and length of the transmembrane region were altered. In contrast, when these changes were made simultaneously, the STcys isoform was converted into a form that was processed, secreted, and weakly oligomerized like STtyr. We propose that STcys oligomerization is a secondary event resulting from its concentration in the Golgi via mechanisms independently mediated by its cytosolic tail and transmembrane region.

Highlights

The glycan structures that modify proteins and lipids play a variety of roles during development, in the normal adult animal, and in various disease states
These results suggest that the single amino acid difference in the catalytic domain of the ST6Gal I isoforms leads to subtle differences in the conformation of this region that allow the STcys isoform to form more stable oligomers than the STtyr isoform
We considered the possibility that the cytosolic tail of the ST6Gal I was acting as a redundant signal for Golgi localization

Summary

Introduction

The glycan structures that modify proteins and lipids play a variety of roles during development, in the normal adult animal, and in various disease states (for review see Ref. 1). The majority of the STcys isoform is recovered as insoluble oligomers at pH 6.3, the pH of the late Golgi, less than 20% of STtyr was found as insoluble oligomers under these conditions [9] These results suggest that the single amino acid difference in the catalytic domain of the ST6Gal I isoforms leads to subtle differences in the conformation of this region that allow the STcys isoform to form more stable oligomers than the STtyr isoform. This in turn correlates with the differences in the efficiency of Golgi localization of the two proteins. The localization of Golgi enzymes is maintained by continuous retrograde transport, in contrast to the retention of enzymes predicted by the vesicular transport model

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