Outer membrane protein K of Escherichia coli: purification and pore-forming properties in lipid bilayer membranes

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Abstract
Protein K, an outer membrane protein correlated with encapsulation in E. coli, was purified to apparent homogeneity. Purification was based on the noncovalent association of protein K with peptidoglycan; the purified protein formed sodium dodecyl sulfate-resistant oligomers on polyacrylamide gels. Incorporation of small amounts (10-10-10-11 M) of purified protein K into artificial lipid bilayers resulted in a large increase in membrane conductance. The increased conductance resulted from the formation of large, water-filled, ion-permeable channels exhibiting single-channel conductance in 1.0 M KCl of 1.83 nS [nanoSiemens]. The membrane conductance showed a linear relationship between current and applied voltage and was not voltage induced or regulated. The channel was permeable to large organic ions (e.g, Tris+ Cl-) and, based on a pore length of 7.5 nm, a minimum channel diameter of 1.2 nm was estimated; these properties resemble values for other enteric porins. The possible biological role of the pores produced by protein K is discussed.