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RESEARCH PROJECTS AND FINDINGS-2


2. Polyamine Transport Systems in E. coli and S. cerevisiae


Fig. 2-1
2-1s
Fig. 2-2
2-2s
(Click the figure, then you can see the larger one.)

The polyamine content in cells is regulated by both polyamine biosynthesis and its transport. We have obtained and characterized three clones of polyamine transport genes (pPT104, pPT79 and pPT71) in Escherichia coli. The system encoded by pPT104 was the spermidine-preferential uptake system and that encoded by pPT79 the putrescine-specific uptake system. These two systems are ABC (ATP binding cassette) transporter consisting of four kinds of proteins: pPT104 clone encoded PotA, -B, -C, and -D and pPT79 clone encoded PotF, -G, -H, and -I. PotD and -F were periplasmic substrate binding proteins and PotA and -G membrane associated proteins having the nucleotide binding site. PotB and -C, and PotH and -I were transmembrane proteins probably forming channels for spermidine and putrescine, respectively. In contrast, the putrescine transport system encoded by pPT71 consisted of one membrane protein (PotE) having twelve transmembrane segments, and was active in both the uptake and excretion of putrescine. The uptake was dependent on membrane potential, and the excretion was due to the exchange reaction between putrescine and ornithine. PotE, a putrescine-ornithine antiporter, plays important roles for cell growth at acidic pH together with CadB, a cadaverine-lysine antiporter (Fig. 2-1). We are now studying the relationship between structure and function of PotABCD spermidine uptake system and PotE.
We thus far identified 9 polyamine transporters in S. cerevisiae. Polyamine uptake is mainly catalyzed by DUR3 and SAM3. Putrescine uptake into vacuole is catalyzed by UGA4. Excretion of polyamines is catalyzed by TPO1-5, located on either the plasma membrane or post-Golgi secretary vesicles. Proteins shown in red characters are more strongly involved in polyamine transport than those shown in black (Fig. 2-2). We are now studying the role of phosphorylation on the polyamine uptake proteins DUR3 and SAM3.

Last Update: 05/16/2007

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