Kinetics, energetics and specificity of a general amino acid transporter from the ectomycorrhizal fungus Paxillus involutus

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Abstract
~~ ~~ ~ The kinetics, energetics and specificity of a general amino acid transporter were studied in the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. The uptake of amino acids showed features characteristic of active transport. After correction for a non-mediated transport component, the kinetics of glutamate, glutamine, alanine and aspartate uptake measured over a wide concentration range followed the simple Michaelis-Menten saturation curves. The apparent Km derived from the Eadie-Hofstee plots ranged from 7 pM for alanine to 27 pM for glutamate. Dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone and NaN, strongly inhibited amino acid uptake, whereas dicyclohexylcarbodiimide, vanadate and the ionophores monensin and nonactin had no effect on the uptake. Both pH dependence and inhibition by protonophores are consistent with a proton symport mechanism for amino acid uptake by P. involutus. Competition studies indicated a broad substrate recognition by the uptake system, which resembles the general amino acid permease of yeast. Dixon plots of the inhibition of glutamate uptake by alanine, lysine and methionine sulfoximine showed that inhibitions were competitive. The physiological importance of this transporter for the exchange of nitrogenous compounds between fungal and host plant cells in ectom ycorrh iza I associations is discussed.

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