Rhizobium leguminosarumHas a Second General Amino Acid Permease with Unusually Broad Substrate Specificity and High Similarity to Branched-Chain Amino Acid Transporters (Bra/LIV) of the ABC Family

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Abstract
Amino acid uptake by Rhizobium leguminosarum is dominated by two ABC transporters, the general amino acid permease (Aap) and the branched-chain amino acid permease (BraRl). Characterization of the solute specificity of BraRl shows it to be the second general amino acid permease of R. leguminosarum. Although BraRl has high sequence identity to members of the family of hydrophobic amino acid transporters (HAAT), it transports a broad range of solutes, including acidic and basic polar amino acids (l-glutamate, l-arginine, and l-histidine), in addition to neutral amino acids (l-alanine and l-leucine). While amino and carboxyl groups are required for transport, solutes do not have to be α-amino acids. Consistent with this, BraRl is the first ABC transporter to be shown to transport γ-aminobutyric acid (GABA). All previously identified bacterial GABA transporters are secondary carriers of the amino acid-polyamine-organocation (APC) superfamily. Also, transport by BraRl does not appear to be stereospecific as d amino acids cause significant inhibition of uptake of l-glutamate and l-leucine. Unlike all other solutes tested, l-alanine uptake is not dependent on solute binding protein BraCRl. Therefore, a second, unidentified solute binding protein may interact with the BraDEFGRl membrane complex during l-alanine uptake. Overall, the data indicate that BraRl is a general amino acid permease of the HAAT family. Furthermore, BraRl has the broadest solute specificity of any characterized bacterial amino acid transporter.