It has been reported that GAP1 and AGP2 catalyze the uptake of polyamines together with amino acids in Saccharomyces cerevisiae. We have looked for polyamine-preferential uptake proteins in S. cerevisiae. DUR3 catalyzed the uptake of polyamines together with urea, and SAM3 was found to catalyze the uptake of polyamines together with S-adenosylmethionine, glutamic acid, and lysine. Polyamine uptake was greatly decreased in both DUR3-and SAM3-deficient cells. The K m values for putrescine and spermidine of DUR3 were 479 and 21.2 M, respectively, and those of SAM3 were 433 and 20.7 M, respectively. Polyamine stimulation of cell growth of a polyamine requiring mutant, which is deficient in ornithine decarboxylase, was not influenced by the disruption of GAP1 and AGP2, but it was diminished by the disruption of DUR3 and SAM3. Furthermore, the polyamine stimulation of cell growth of a polyamine-requiring mutant was completely inhibited by the disruption of both DUR3 and SAM3. The results indicate that DUR3 and SAM3 are major polyamine uptake proteins in yeast. We previously reported that polyamine transport protein kinase 2 regulates polyamine transport. It was found that DUR3 (but not SAM3) was activated by phosphorylation of Thr 250 , Ser 251 , and Thr 684 by polyamine transport protein kinase 2.Polyamines (putrescine, spermidine, and spermine) in cells, which are essential for cell growth, are regulated by biosynthesis, degradation, and transport (1-4). With regard to polyamine transport, the properties of four polyamine transport systems were characterized in Escherichia coli (5-8). They include spermidine-preferential and putrescine-specific uptake systems as well as PotE (involved in the excretion of putrescine by a putrescine-ornithine antiporter activity) and CadB (involved in the excretion of cadaverine by a cadaverine-lysine antiporter activity). The former two transport systems function at neutral pH (2), whereas the latter two transport systems at acidic pH (9). In Saccharomyces cerevisiae, we identified four genes that encode polyamine excretion proteins TPO1-TPO4, mainly located on the plasma membrane (10 -12). We also found that UGA4 (located on vacuoles) can catalyze the uptake of ␥-aminobutyric acid and putrescine (13), and TPO5 (located on Golgi or post-Golgi secretory vesicles) can catalyze the excretion of polyamines (14). Furthermore, we reported that GAP1, located on the plasma membrane, can catalyze the uptake of putrescine and spermidine together with the uptake of amino acids (15). Although it has been reported that AGP2 can selectively catalyze the uptake of spermidine (16), there is also a report that AGP2 functions as an amino acid permease (17). In this study, we looked for proteins that can preferentially catalyze the uptake of polyamines in S. cerevisiae. We found that DUR3 can catalyze the uptake of polyamines together with urea, and SAM3 (which belongs to the family of amino acid polyamine-organocation transporters (18)) can catalyze the uptake of putrescine and spermidine together wit...