A TOPOLOGICAL MODEL FOR THE GENERAL AROMATIC AMINO-ACID PERMEASE, AROP, OF ESCHERICHIA-COLI

The general aromatic amino acid permease, AroP, of Escherichia coli is responsible for the active transport of phenylalanine, tyrosine, and tryptophan, A proposed topological model for the AroP permease, consis ting of 12 hydrophobic transmembrane spans connected by hydrophilic lo ops, is very similar to that of the closely related phenylalanine-spec ific permease, The validity of this model and its similarity to that o f the PheP permease were investigated by studying fusion proteins of A roP permease and alkaline phosphatase. Based on the results obtained f rom the AroP-alkaline phosphatase sandwich fusions, we have significan tly revised the proposed topological model for AroP in two regions, In this modified AroP topological model, the three charged residues E151 , E153, and K160 are repositioned within the membrane in span 5. These three residues are conserved in a large family of amino acid transpor t proteins, and site-directed mutagenesis identifies them as being ess ential for transport activity, It is postulated that these residues to gether with E110 in transmembrane span 3 may be involved in a proton r elay system.