Molecular origin of the cation selectivity in OmpF porin: single channel conductances vs. free energy calculation.
Ion current through single outer membrane protein F (OmpF) trimers was recorded and compared to molecular dynamics simulation. Unidirectional insertion was revealed from the asymmetry in channel conductance. Single trimer conductance showed particularly high values at low symmetrical salt solution. The conductance values of various alkali metal ion solutions were proportional to the monovalent cation mobility values in the bulk phase, LiCl<NaCl<KCl<RbCl approximately CsCl, but the conductance differences were quantitatively larger than conductivity differences in bulk solutions. Selectivity measurements at low concentration showed that OmpF channels favored permeation of alkali metal ions over chloride and suggested size preference for smaller cations. These results suggest that there are specific interactions between the permeating cation and charged residues lining the channel walls. This hypothesis was supported by computational study which predicted that monovalent cations bind to Asp113 at low concentration. Here, free energy calculations revealed that the affinity of the alkali metal ions to its binding site increased with their atomic radii, Li(+) approximately Na(+)<K(+) approximately Rb(+) approximately Cs(+). A detailed inspection of both experimental and computational results suggested that stronger binding at the central constriction of the channel increases the translocation rate of cations under applied voltage by increasing their local concentration relative to the bulk solution.
Danelon
Christophe
C
Institut de Pharmacologie et de Biologie Structurale, CNRS-UMR 5089, Universite P. Sabatier, 205 route de Narbonne, 31077 Toulouse, France.
Suenaga
Atsushi
A
Winterhalter
Mathias
M
Yamato
Ichiro
I
D001224:Aspartic Acid / Q000737:chemistry; D027682:Cation Transport Proteins / Q000737:chemistry / Q000502:physiology*; D002414:Cations, Monovalent / Q000737:chemistry / Q000378:metabolism*; D002586:Cesium / Q000737:chemistry / Q000378:metabolism; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism; D019295:Computational Biology; D004553:Electric Conductivity; D008051:Lipid Bilayers; D018021:Lithium Chloride / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D018408:Patch-Clamp Techniques; D018272:Porins / Q000737:chemistry / Q000502:physiology*; D011189:Potassium Chloride / Q000737:chemistry / Q000378:metabolism; D012413:Rubidium / Q000737:chemistry / Q000378:metabolism; D012965:Sodium Chloride / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
D027682:Cation Transport Proteins; D002414:Cations, Monovalent; D002712:Chlorides; D008051:Lipid Bilayers; C109237:OmpF protein; D018272:Porins; D002586:Cesium; D001224:Aspartic Acid; D012965:Sodium Chloride; D011189:Potassium Chloride; D018021:Lithium Chloride; C028019:cesium chloride; D012413:Rubidium; C032710:rubidium chloride