Homology Model of the Na+/Proline Transporter PutP of Escherichia coli and Its Functional Implications

Na+/solute symporters are essential membrane integrated proteins that couple the flow of Na+ ions driven by electrochemical Na+ gradients to the transport of solutes across biological membranes. Here, we used a combination of molecular modeling techniques and evolutionary conservation analysis to construct and validate a first model of the Na+/proline symporter PutP of Escherichia coli based on the crystal structure of the bacterial Na+/galactose symporter vSGLT. Ligand docking experiments were employed to gain information about residues involved in proline binding. The proposed model is consistent with the available experimental data and was further validated by amino acid substitutions and kinetic and protein chemical analyses. Combination of the results of molecular modeling and functional studies predicts the location and organization of the Na+ and proline binding sites. Remarkably, as proposed computationally and discovered here experimentally, residues Y140, W244, and Y248 of transmembrane segments 4 and 7 are found to be particularly important for PutP function and suggested to participate in proline binding and/or gating.