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Ion selectivity of gram-negative bacterial porins
Insights
This study reconstituted twelve bacterial porins into membranes, revealing ion channels with conductances and diameters from 1.0-2.0 nm. Most porins were cation-selective, while three showed anion selectivity, indicating ion mobility similar to free solution.
Area of Science:
- Biophysics
- Molecular Biology
- Microbiology
Background:
- Gram-negative bacteria utilize porins for nutrient transport across their outer membranes.
- Understanding porin structure-function relationships is crucial for drug development and understanding cellular transport.
Purpose of the Study:
- To characterize the ion transport properties and structural parameters of twelve different porins.
- To investigate the ion selectivity and mobility within porin channels.
Main Methods:
- Reconstitution of twelve porins from four Gram-negative bacteria into lipid bilayer membranes.
- Single-channel conductance measurements in 1 M KCl to determine channel properties.
- Zero-current potential measurements with salt gradients to assess ion selectivity.
Main Results:
- Most porins formed ion-permeable channels (1.5-6 nS conductance) with effective diameters of 1.0-2.0 nm.
- Ion mobility within porin channels mirrored free solution mobility.
- Three porins (PhoE, NmpC, and P. aeruginosa protein P) exhibited anion selectivity, with protein P showing high anion selectivity.
Conclusions:
- Porin channel characteristics, including diameter and ion selectivity, vary significantly.
- Ion movement within porin channels is largely consistent with aqueous phase behavior.
- Specific porins like PhoE, NmpC, and protein P possess distinct anion-selective properties.
Abstract:
Twelve different porins from the gram-negative bacteria Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, and Yersinia pestis were reconstituted into lipid bilayer membranes. Most of the porins, except outer membrane protein P, formed large, water-filled, ion-permeable channels with a single-channel conductance between 1.5 and 6 nS in 1 M KCl. The ions used for probing the pore structure had the same relative mobilities while moving through the porin pore as they did while moving in free solution. Thus the single-channel conductances of the individual porins could be used to estimate the effective channel diameters of these porins, yielding values ranging from 1.0 to 2.0 nm. Zero-current potential measurements in the presence of salt gradients across lipid bilayer membranes containing individual porins gave results that were consistent with the conclusions drawn from the single-channel experiments. For all porins except protein P, the channels exhibited a greater cation selectivity for less mobile anions and a greater anion selectivity for less mobile cations, which again indicated that the ions were moving inside the pores in a fashion similar to their movement in the aqueous phase. Three porins, PhoE and NmpC of E. coli and protein P of P. aeruginosa, formed anion-selective pores. PhoE and NmpC were only weakly anion selective, and their selectivity was dependent on the mobility of the ions. In contrast, cations were unable to enter the selectivity filter of the protein P channel. This resulted in a high anion selectivity for all salts tested in this study. The other porins examined, including all of the known constitutive porins of the four gram-negative bacteria studied, were cation selective with a 3- to 40-fold preference for K+ ions over Cl- ions.