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

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.