Characterization of AtNST-KT1, a novel UDP-galactose transporter from Arabidopsis thaliana.

Nucleotide sugar transporters (NST) mediate the transfer of nucleotide sugars from the cytosol into the lumen of the endoplasmatic reticulum and the Golgi apparatus. Because the NSTs show similarities with the plastidic phosphate translocators (pPTs), these proteins were grouped into the TPT/NST superfamily. In this study, a member of the NST-KT family, AtNST-KT1, was functionally characterized by expression of the corresponding cDNA in yeast cells and subsequent transport experiments. The histidine-tagged protein was purified by affinity chromatography and reconstituted into proteoliposomes. The substrate specificity of AtNST-KT1 was determined by measuring the import of radiolabelled nucleotide mono phosphates into liposomes preloaded with various unlabelled nucleotide sugars. This approach has the advantage that only one substrate has to be used in a radioactively labelled form while all the nucleotide sugars can be provided unlabelled. It turned out that AtNST-KT1 represents a monospecific NST transporting UMP in counterexchange with UDP-Gal but did not transport other nucleotide sugars. The AtNST-KT1 gene is ubiquitously expressed in all tissues. AtNST-KT1 is localized to Golgi membranes. Thus, AtNST-KT1 is most probably involved in the synthesis of galactose-containing glyco-conjugates in plants.

The Arabidopsis mutant dct is deficient in the plastidic glutamate/malate translocator DiT2.

The Arabidopsis mutant dicarboxylate transport (dct) is one of the classic mutants in the photorespiratory pathway. It requires high CO2 levels for survival. Physiologic and biochemical characterization of dct indicated that dct is deficient in the transport of dicarboxylates across the chloroplast envelope membrane. Hence, re-assimilation of ammonia generated by the photorespiratory cycle is blocked. However, the defective gene in dct has not been identified at the molecular level. Here, we report on the molecular characterization of the defective gene in dct, on the complementation of the mutant phenotype with a wild-type cDNA, and on the functional characterization of the gene product, DiT2, in a recombinant reconstituted system. Furthermore, we provide the kinetic constants of recombinant DiT1 and DiT2, and we discuss these data with respect to their functions in ammonia assimilation. Moreover, an analysis of the transcript levels of DiT1 and DiT2 in different C3- and C4-type plant species is presented, and we demonstrate that the substrate specificity of DiT2 from the C4-plant Flaveria bidentis is similar to its counterpart from C3 plants.