The mechanisms of transport and distribution of nucleotide sugars in the cell remain unclear. In an attempt to further characterize nucleotide sugar transporters (NSTs), we determined the subcellular localization of overexpressed epitope-tagged canine UDP-GlcNAc transporter, human UDP-Gal transporter splice variants (UGT1 and UGT2), and human SLC35B4 transporter splice variants (longer and shorter version) by indirect immunofluorescence using an experimental model of MDCK wild-type and MDCK-RCAr mutant cells. Our studies confirmed that the UDP-GlcNAc transporter was localized to the Golgi apparatus only and its localization was independent of the presence of endogenous UDP-Gal transporter. After overexpression of UGT1, the protein colocalized with the Golgi marker only. When UGT2 was overexpressed, the protein colocalized with the endoplasmic reticulum (ER) marker only. When UGT1 and UGT2 were overexpressed in parallel, UGT1 colocalized with the ER and Golgi markers and UGT2 with the ER marker only. This suggests that localization of the UDP-Gal transporter may depend on the presence of the partner splice variant. Our data suggest that proteins involved in nucleotide sugar transport may form heterodimeric complexes in the membrane, exhibiting different localization which depends on interacting protein partners. In contrast to previously published data, both splice variants of the SLC35B4 transporter were localized to the ER, independently of the presence of endogenous UDP-Gal transporter.
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