Arabidopsis INOSITOL TRANSPORTER2 Mediates H+ Symport of Different Inositol Epimers and Derivatives across the Plasma Membrane1[C][OA]

Of the four genes of the Arabidopsis ( Arabidopsis thaliana ) INOSITOL TRANSPORTER family ( AtINT family) so far only AtINT4 has been described. Here we present the characterization of AtINT2 and AtINT3 . cDNA sequencing revealed that the AtINT3 gene is incorrectly spliced and encodes a truncated protein of only 182 amino acids with four transmembrane helices. In contrast, AtINT2 codes for a functional transporter. AtINT2 localization in the plasma membrane was demonstrated by transient expression of an AtINT2 - GREEN FLUORESCENT PROTEIN fusion in Arabidopsis and tobacco ( Nicotiana tabacum ) epidermis cells and in Arabidopsis protoplasts. Its functional and kinetic properties were determined by expression in yeast ( Saccharomyces cerevisiae ) cells and Xenopus laevis oocytes. Expression of AtINT2 in a Δitr1 (inositol uptake)/Δino1 (inositol biosynthesis) double mutant of bakers' yeast complemented the deficiency of this mutant to grow on low concentrations of myoinositol. In oocytes, AtINT2 mediated the symport of H + and several inositol epimers, such as myoinositol, scylloinositol, d -chiroinositol, and mucoinositol. The preference for individual epimers differed from that found for AtINT4. Moreover, AtINT2 has a lower affinity for myoinositol ( K m = 0.7–1.0 m m ) than AtINT4 ( K m = 0.24 m m ), and the K m is slightly voltage dependent, which was not observed for AtINT4. Organ and tissue specificity of AtINT2 expression was analyzed in AtINT2 promoter/reporter gene plants and showed weak expression in the anther tapetum, the vasculature, and the leaf mesophyll. A T-DNA insertion line (Atint2.1) and an Atint2.1/Atint4.2 double mutant were analyzed under different growth conditions. The physiological roles of AtINT2 are discussed.