Expression of plant inositol transporters in yeast

Inositol transporters (ITRs) are membrane proteins in the carbohydrate transporter super-family. We have characterized plant ITR-like coding regions from Mesembryanthemum and Arabidopsis by their expression in yeast strains deficient in myo-inositol (MI) biosynthesis and compromised in inositol uptake. Both AtITR1 and McITR1 were capable of complementing the uptake deficiency of the cells albeit at a lower efficiency when compared to complementation by yeast ITR. All ITRs used were also capable of taking up a methylated inositol derivative, D-ononitol. but McITR1 was more strongly inhibited in the uptake of D-ononitol in the presence of myo-inositol than yeast ITR, indicating that McITR1 could have evolved to transport MI under conditions where methylated compounds are produced in large quantities in this plant. The ITRs tested were affected in their transport capacity by the proton gradient at the plasma membrane (PM). At high pH, MI uptake was ineffective. but the addition of sodium at low concentration let to growth recovery. We discuss the possibility that the ITRs tested, and possibly the entire plant sub-family, act as sodium/inositol or proton/inositol symporters. (C) 2003 Elsevier Ireland Ltd. All rights reserved.