Arg149 is involved in switching the low affinity, open state of the binding protein AfProX into its high affinity, closed state.

Abstract The substrate binding protein Af ProX from the Archaeoglobus fulgidus ProU ATP binding cassette transporter is highly selective for the compatible solutes glycine betaine (GB) and proline betaine, which confer thermoprotection to this hyperthermophilic archaeon. A detailed mutational analysis of the substrate binding site revealed the contribution of individual amino acids for ligand binding. Replacement of Arg149 by an Ala residue displayed the largest impact on substrate binding. The structure of a mutant Af ProX protein (substitution of Tyr111 with Ala) in complex with GB was solved in the open liganded conformation to gain further insight into ligand binding. In this crystal structure, GB is bound differently compared to the GB closed liganded structure of the wild-type Af ProX protein. We found that a network of amino acid side chains communicates the presence of GB toward Arg149, which increases ligand affinity and induces domain closure of Af ProX. These results were corroborated by molecular dynamics studies and support the view that Arg149 finalizes the high-affinity state of the Af ProX substrate binding protein.