Abstract
The characteristics of a system of thiamine transport were studied in baker's yeast (Saccharomyces cerevisiae) . The transport was energy- and temperature-dependent process which has an optimal pH around 4.5. The apparent K_m for thiamine was 0.18μM and the transport was inhibited by several thiamine analogs. Two mutants resistant to the inhibitory action of pyrithiamine were isolated from S. cerevisiae and they were found to be, respectively, partially and almost totally defective in a common, carrier protein for thiamine and the analog. It was demonstrated that two thiamine-binding proteins are present in S. cerevisiae. A soluble thiamine-binding protein (s-TBP) in the periplasm was proved biochemically and genetically to be identical with thiamine repressible acid phosphatase which may be responsible for the hydrolysis of thiamine phosphates prior to the uptake of their thiamine moiety by yeast cells. Another thiamine-binding protein is membrane-bound (m-TBP) and located in the yeast plasma membrane. Several lines of evidence suggested that m-TBP is involved directly in the thiamine transport. Hydroxymethylpyrimidine was also taken up by the thiamine transport system in S. cerevisiae. whereas the uptake of hydroxyethylthiazole occurred by diffusion, followed by metabolic trapping due to hydroxyethylthiazole kinase-catalyzed phosphorylation. Possible mechanism of the regulation of the transport of thiamine and related compounds is dicussed.
