Arabidopsis INOSITOL TRANSPORTER2 Mediates H⁺ Symport of Different Inositol Epimers and Derivatives across the Plasma Membrane

Abstract

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 Ditr1 (inositol uptake)/Dino1 (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 1 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 5 0.7-1.0 mM) than AtINT4 (K m 5 0.24 mM), 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.

Cyclitols in maturing grains of wheat, triticale and barley

“…Until now, there have been no data available on the transporters facilitating uptake of D-chiro-inositol and D-pinitol by developing embryos. In vegetative tissues of Arabidopsis different forms of plasma membrane localized transporters of cyclitols have been found: AtINT4, transporting D-pinitol beside myo-inositol and AtINT2, transporting also D-chiro--inositol (Schneider et al 2007). Our experiments indicated that maturing grains of barley accumulated more cyclitols than grains of triticale and wheat (Fig.…”

Myoinositol Oxygenase Controls the Level of Myoinositol in Arabidopsis, But Does Not Increase Ascorbic Acid

“…Indeed, the myoinositol concentration in these lines is lower than in wild-type plants, suggesting that higher Miox activity metabolizes most of the available myoinositol. Myoinositol is readily taken up by plant cells using several transporters in the plasma membrane (Schneider et al, 2007). Feeding myoinositol to leaf discs therefore strongly increases the internal concentration.…”

A diverse set of microRNAs and microRNA-like small RNAs in developing rice grains

“…The Os06g21900 siRNAs are predominantly expressed in 6-10 DAF grain, so could be involved in regulating a transitional step in the developmental program of the grain. The confirmed mRNA target of an Os06g21900 siRNA, Os04g43210, encodes a putative plasma membrane myoinositol transporter (closely related to Arabidopsis INOSITOL TRANSPORTER 2) (Schneider et al 2007). This protein could be involved in transport of myoinositol or its derivatives, which could act in either metabolic or signaling roles.…”