The L-arginine/agmatine transporter AdiC is part of the arginine-dependent extreme acid resistance system of the bacterium Escherichia coli and its pathogenic varieties such as strain E. coli O157:H7. At the present time, there is a lack of knowledge concerning the role of water molecules and networks for the structure and function of AdiC and solute transporters in general. The structure of the L-arginine/agmatine transporter AdiC was determined at 1.7 Å resolution by X-ray crystallography. This high resolution allowed for the identification of numerous water molecules buried in the structure. In combination with molecular dynamics (MD) simulations, we demonstrate that water molecules play an important role for stabilizing the protein and acting as placeholders for atoms of the AdiC substrates L-arginine and agmatine. Furthermore, a water-filled cavity was identified at the dimer interface of AdiC. The two monomers formed bridging interactions through water-mediated hydrogen bonds. The accessibility and presence of water molecules in this cavity was confirmed with MD simulations. Point mutations disrupting the interfacial water network validated the importance of water molecules for dimer stabilization. This work gives new insights into the role and importance of water molecules in the L-arginine/agmatine transporter AdiC for protein stabilization and substrate-binding site shaping, and as placeholders of substrate atoms. Finally, we identified a water-filled cavity at the dimeric interface that contributes to the stability of the amino acid transporter oligomer.
About the speaker:
Dimitrios José Fotiadis is professor of structural biology at the Institute of Biochemistry and Molecular Medicine, University of Bern, where he has established "The Fotiadis laboratory", focusing on the function, structure and supramolecular organization of membrane proteins. To this aim, biochemical and biophysical approaches as well as high-resolution microscopy and crystallography techniques are used.
He holds a PhD in biochemistry with the subject and the title of “Biochemical and structural analyses of membrane proteins in plants and animals” from the University of Basel in 2000.
During his postdoc in the group of Prof. Andreas Engel at the Biozentrum of the University of Basel, Dimitrios Fotiadis unsettled the dogma that the light receptor protein rhodopsin is monomeric in the retinal rod outer segments by direct visualization of rhodopsin dimers and higher oligomers in native disk membranes using electron and particularly atomic force microscopy. Furthermore, these results on the oligomeric state of rhodopsin were corroborated by biochemical and biophysical methods in Basel and during Dimitrios Fotiadis’ scientific stay in Kris Palczewski’s laboratory (University of Washington, Seattle, USA). Importantly, rhodopsin is just one example of a G protein-coupled receptor (GPCR) of which more than a thousand exist in the human body.
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