Identification and Assays of Polyamine Transport Systems in Escherichia coli and Saccharomyces cerevisiae

Polyamine content in cells is regulated by biosynthesis, degradation, and transport. With regard to transport, uptake and excretion proteins exist in Escherichia coli and Saccharomyces cerevisiae . In E. coli , the uptake systems comprise a spermidine-preferential uptake system consisting of the PotA, B, C, and D proteins, and a putrescine-specific uptake system consisting of the PotF, G, H, and I proteins. Two other proteins, PotE and CadB, each containing 12 transmembrane segments, function as antiporters (putrescine-ornithine and cadaverine-lysine) and are important for cell growth at acidic pH. MdtJI was identified as a spermidine excretion system. When putrescine was used as energy source, PuuP functioned as a putrescine transporter. In S. cerevisiae , DUR3 and SAM3, containing 16 or 12 transmembrane segments, are the major polyamine uptake proteins, whereas TPO1 and TPO5, containing 12 transmembrane segments, are the major polyamine excretion proteins, and UGA4 is a putrescine transporter on the vacuolar membrane. The activities of DUR3 and TPO1 are regulated by phosphorylation of serine/threonine residues. The identification and assay procedures of these transporters are described in this chapter.