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.