Plant sucrose-H+ symporters mediate the transport of vitamin H

A cDNA coding for a vitamin H (biotin) transport protein from Arabidopsis w as identified by genetic complementation of a biotin uptake-deficient yeast mutant. Vitamin H transport by this protein was sensitive to the SH-group inhibitor p-chloromercuribenzene sulfonic acid (PCMBS) and to the uncoupler carbonyl cyanide-m-chlorophenylhydrazone (CCCP), suggesting an energy-depe ndent biotin-H+ symport mechanism. The transport activity could contribute to the so-far uncharacterized plant sucrose-H+ symporter AtSUC5 which media tes the energy-dependent transport of biotin and sucrose, and restores grow th of the biotin transport-deficient yeast mutant on medium with low biotin concentrations. Functional comparison of the AtSUC5 transporter with previ ously characterized plant sucrose or monosaccharide transporters revealed t hat biotin transport may be a general and specific property of all plant su crose transporters (sucrose/biotin-H+ symporters). This first report on a t ransporter with dual substrate specificity for two structurally unrelated m olecules has a major impact on general thinking concerning the specificity of membrane transporters. The physiological relevance of this finding is di scussed.