Identification of the Cadaverine Recognition Site on the Cadaverine-Lysine Antiporter CadB

Amino acid residues involved in cadaverine uptake and cadaverine-lysine antiporter activity were identified by site-directed mutagenesis of the CadB protein. It was found that Tyr(73), Tyr(89), Tyr(90), Glu(204), Tyr(235), Asp(303), and Tyr(423) were strongly involved in both uptake and excretion and that Tyr(55), Glu(76), Tyr(246), Tyr(310), Cys(370), and Glu(377) were moderately involved in both activities. Mutations of Trp(43), Tyr(57), Tyr(107), Tyr(366), and Tyr(368) mainly affected uptake activity, and Trp(41), Tyr(174), Asp(185), and Glu(408) had weak effects on uptake. The decrease in the activities of the mutants was reflected by an increase in the K(m) value. Mutation of Arg(299) mainly affected excretion, suggesting that Arg(299) is involved in the recognition of the carboxyl group of lysine. These results indicate that amino acid residues involved in both uptake and excretion, or solely in excretion, are located in the cytoplasmic loops and the cytoplasmic side of transmembrane segments, whereas residues involved in uptake were located in the periplasmic loops and the transmembrane segments. The SH group of Cys(370) seemed to be important for uptake and excretion, because both were inhibited by the existence of Cys(125), Cys(389), or Cys(394) together with Cys(370). The relative topology of 12 transmembrane segments was determined by inserting cysteine residues at various sites and measuring the degree of inhibition of transport through crosslinking with Cys(370). The results suggest that a hydrophilic cavity is formed by the transmembrane segments II, III, IV, VI, VII, X, XI, and XII.

Excretion and uptake of cadaverine by CadB and its physiological functions in Escherichia coli

The functions of the putative cadaverine transport protein CadB were studied in Escherichia coli. CadB had both cadaverine uptake activity, dependent on proton motive force, and cadaverine excretion activity, acting as a cadaverine-lysine antiporter. The Km values for uptake and excretion of cadaverine were 20.8 and 303 microM respectively. Both cadaverine uptake and cadaverine-lysine antiporter activities of CadB were functional in cells. Cell growth of a polyamine-requiring mutant was stimulated slightly at neutral pH by the cadaverine uptake activity and greatly at acidic pH by the cadaverine-lysine antiporter activity. At acidic pH, the operon containing cadB and cadA, encoding lysine decarboxylase, was induced in the presence of lysine. This caused neutralization of the extracellular medium and made possible the production of CO(2) and cadaverine and aminopropylcadaverine instead of putrescine and spermidine. The induction of the cadBA operon also generated a proton motive force. When the cadBA operon was not induced, the expression of the speF-potE operon, encoding inducible ornithine decarboxylase and a putrescine-ornithine antiporter, was increased. The results indicate that the cadBA operon plays important roles in cellular regulation at acidic pH.