Abstract
The alpha2,6-sialyltransferase is a terminal glycosyltransferase localized in the trans Golgi and trans Golgi network. Here we show that 30% of the total rat liver Golgi alpha2,6-sialyltransferase forms a disulfide-bonded 100-kDa species that can be converted to the 50-kDa monomer form of the enzyme upon reduction. Limited proteolysis of both enzyme forms demonstrates that the 100-kDa species is a disulfide-bonded homodimer of the alpha2,6-sialyltransferase. The alpha2,6-sialyltransferase disulfide-bonded dimer is found in bovine liver Golgi membranes and in Golgi membranes prepared and solubilized in the presence of 100 mM iodoacetamide, suggesting that it is not unique to rat liver or formed aberrantly upon membrane lysis. The dimer form of the enzyme possesses no significant catalytic activity and has a much lower affinity for CDP-hexanolamine-agarose compared with the monomer form. In contrast, both the alpha2,6-sialyltransferase monomer and the disulfide-bonded dimer bind strongly to galactose and galactose-terminated substrates. These results suggest that the alpha2,6-sialyltransferase disulfide-bonded dimer lacks catalytic activity due to a weak affinity for its sugar nucleotide donor, CMP-NeuAc, and that this catalytically inactive form of the enzyme may act as a galactose-specific lectin in the Golgi.
Highlights
Charide chains of the nascent protein [10]
Some studies have concluded that the primary Golgi localization signals are found in the transmembrane regions of these enzymes [17,18,19], other studies have suggested that sequences in the cytoplasmic tails, transmembrane regions, and luminal regions are required for efficient Golgi localization (20 –24)
Studies of the signals required for ␣2,6-ST Golgi localization have led to the identification of sequences in the cytosolic, transmembrane, and luminal regions that play a role in this process [17,18,19,20,21,22,23,24]
Summary
Charide chains of the nascent protein [10]. This arrangement ensures that each enzyme has access to the correct substrates and sugar nucleotide donors. Early observations favored this model [22], more recent experiments [21] and the observation that the transmembrane regions of Golgi and plasma membrane proteins overlap in length suggest that this cannot be the sole mechanism for Golgi retention. Because several Golgi proteins require sequences in their cytoplasmic tails, transmembrane regions, and luminal domains for their Golgi localization (20 –24), it seems likely that their localization signals are conformation-dependent. From this standpoint, one mechanism that seems attractive is oligomerization. Presented results that suggest that the formation of insoluble oligomers correlates with the Golgi retention of these proteins
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