| Research Abstract |
1. PotF protein is a periplasmic substrate-binding protein of the putrescine transport system in Eschenchia coli. We have determined the crystal structure of PotF protein in complex with the substrate at 2.3-_ resolution. The PotF molecule has dimensions of 54 x 42 x 30 _ and consists of two similar globular domains. Putrescine is tightly bound in the deep cleft between the two domains of PotF through 12 hydrogen bonds and 36 van der Waals interactions. The comparison of the PotF structure with that of PotD provides the insight into the differences in the specificity between the two proteins. The PotF structure, in combination with the mutational analysis, revealed the residues crucial for putrescine binding (Trp-37, Ser-85, Glu-185, Trp-244, Asp-247, and Asp-278) and the importance of water molecules for putrescine recognition.
2. Properties of a membrane protein encoded by YLL028w were examined using yeast cells transformed with the gene. The transformed cells became resistant to poly
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amine toxicity, and the resistance was overcome by bafilomycin A_1, an inhibitor of vacuolar H^+-ATPase. Although spermine uptake activity of the transformed cells was almost the same as that of wild type cells, the uptake activity of vacuolar membrane vesicles from the transformed cells was higher than that from wild type cells. The transformed cells became resistant to MGBG (methylglyoxal bis(guanylhydrazone)) and paraquat. but not Ni^<2+> and Co^<2+>, suggesting that the protein encoded by YLL028w is a transport protein specific for polyamines, The results indicate that a membrane protein encoded by YLL028w (TPO1) is a polyamine transport protein on the vacuolar membrane.
3. Inhibition of spermidine uptake in Escherichia coli, which occurs in the presence of accumulated polyamines, has been studied using the spermidine uptake operon consisting of the potA, -B, -C, and -D genes. Transcription of the potABCD operon was inhibited by PotD, a spermidine-binding protein usually found in the periplasm, and the inhibitory effect of PotD was increased by spermidine. In the presence of 8 mM spermidine, a 50% inhibition of transcription was observed with a molar ratio of approximately 1 : 500 of template DNA : PotD.It was found that PotD bound to regions -258 to -209 nucleotides upstream and +66 to +135 nucleotides downstream of the ATG initiation codon of the potA gene. Binding of PotD to the downstream site was stimulated by spermidine. Overexpression of PotD in Escherichia coli DH5alpha inhibited
the uptake of spermidine, the synthesis of PotABCD mRNA, and expression of a lacZ reporter gene fused downstream of a potA gene containing the PotD binding sites. In cells overexpressing PotD, a large amount of PotD existed as PotD precursor in spheroplasts. The amino acid residues in PotD that are involved in its interaction with the potABCD operon were determined using mutated PotD proteins. Thr-35 and Ser-85 of PotD were found to be important for this interaction. Less
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