Abstract
The genome of S. cerevisae encodes at least twenty hexose transporter-like proteins. Despite extensive research, the functions of Hxt8-Hxt17 have remained poorly defined. Here, we show that Hxt13, Hxt15, Hxt16 and Hxt17 transport two major hexitols in nature, mannitol and sorbitol, with moderate affinities, by a facilitative mechanism. Moreover, Hxt11 and Hxt15 are capable of transporting xylitol, a five-carbon polyol derived from xylose, the most abundant pentose in lignocellulosic biomass. Hxt11, Hxt13, Hxt15, Hxt16 and Hxt17 are phylogenetically and functionally distinct from known polyol transporters. Based on docking of polyols to homology models of transporters, we propose the architecture of their active site. In addition, we determined the kinetic parameters of mannitol and sorbitol dehydrogenases encoded in the yeast genome, showing that they discriminate between mannitol and sorbitol to a much higher degree than the transporters.
Highlights
Sufficient to relieve the repression of genes required for mannitol assimilation in S. cerevisiae[12]
VW400010 was transformed with plasmids encoding Hxt[1], Hxt[4], Hxt[7], Hxt[8], Hxt[9], Hxt[10], Hxt[11], Hxt[13], Hxt[14], Hxt[15], Hxt[16] or Hxt[17] and appropriate polyol dehydrogenases (YNR073C for mannitol or Sor[1] for sorbitol)
The transport of polyols is likely to be their true function, since HXT15 and HXT16 genes are co-localized with sorbitol dehydrogenase genes SOR2 and SOR1, while HXT13 and HXT17 are co-localized with DSF1 and YNR073C, which encode mannitol dehydrogenases
Summary
Sufficient to relieve the repression of genes required for mannitol assimilation in S. cerevisiae[12]. Among the most prominently upregulated genes in tup1-cyc[8] mutants, the authors found HXT10, another hexose transporter-like protein of unknown function and speculated that it might act as a mannitol permease. Due to the overlapping transcriptional pattern, the individual role of Hxt8-Hxt[17] as potential polyol transporters was not clear. The assessment of their true function is complicated by the fact that Hxt[9], Hxt[11] and Hxt[13] were earlier implicated in drug resistance processes[13,14]. We systematically investigated the ability of Hxt8-Hxt[17] proteins to transport mannitol, sorbitol and xylitol in strains devoid of all known hexose transporter family members
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