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

Abstract

Summary A cDNA coding for a vitamin H (biotin) transport protein from Arabidopsis was 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‐dependent biotin‐H+ symport mechanism. The transport activity could contribute to the so‐far uncharacterized plant sucrose‐H+ symporter AtSUC5 which mediates the energy‐dependent transport of biotin and sucrose, and restores growth of the biotin transport‐deficient yeast mutant on medium with low biotin concentrations. Functional comparison of the AtSUC5 transporter with previously characterized plant sucrose or monosaccharide transporters revealed that biotin transport may be a general and specific property of all plant sucrose transporters (sucrose/biotin‐H+ symporters). This first report on a transporter with dual substrate specificity for two structurally unrelated molecules has a major impact on general thinking concerning the specificity of membrane transporters. The physiological relevance of this finding is discussed.

Mechanisms of Biotin Transport

“…Proton dependency of this permease has been confirmed using trans-membrane proton gradient uncouplers [74] and activators [76]. VHT1 consists of 593 amino acids (586 in S. pombe) that assemble into 12 trans-membrane helices [74,75].…”

“…Proton dependency of this permease has been confirmed using trans-membrane proton gradient uncouplers [74] and activators [76]. VHT1 consists of 593 amino acids (586 in S. pombe) that assemble into 12 trans-membrane helices [74,75]. This transporter contains a long (120 amino acid residues) N-terminal hydrophilic domain, with both N-and C-termini in the cytoplasm [74,75].…”

“…The VHT1 is a membrane transporter protein employed by biotinauxotrophic yeast Schizosaccharomyces pombe and Saccharomyces cerevisiae to import biotin via a proton (H+)-biotin symport mechanism [73][74][75]. Proton dependency of this permease has been confirmed using trans-membrane proton gradient uncouplers [74] and activators [76].…”

“…In S. pombe the trans-membrane helices are segregated into 2 sets of 6 helices by a cytoplasmic hydrophilic loop consisting of 71 residues between helix 6 and 7 [75]. Two extracellular asparagine residues, present in the conserved sequence for N-glycosylation in the S. cerevisiae VHT1, N146 (between helix 1 and 2) and N406 (between helix 7 and 8) are proposed to be glycosylated during secretion [74]. No mutational studies have been performed on VHT1 to establish if the glycosylation is essential for activity like it is for SMVT.…”

“…VHT1 consists of 593 amino acids (586 in S. pombe) that assemble into 12 trans-membrane helices [74,75]. This transporter contains a long (120 amino acid residues) N-terminal hydrophilic domain, with both N-and C-termini in the cytoplasm [74,75]. In S. pombe the trans-membrane helices are segregated into 2 sets of 6 helices by a cytoplasmic hydrophilic loop consisting of 71 residues between helix 6 and 7 [75].…”