Insertion intermediates of pore-forming colicins in membrane two-dimensional space

The formation of integral membrane voltage-gated ion channels by the initially soluble C-terminal channel polypeptide (CP) of the pore-forming colicins is a fruitful area for studies on membrane protein import. The dependence of CP import on specific membrane parameters can be better understood using liposomes and planar membranes of defined lipid composition. The membrane surface and interfacial layer provide special conditions for the transition of a pore-forming colicin from the soluble to the integral membrane state. The colicin E1 CP is arranged in the membrane interfacial layer as a conformationally mobile helical array that is extended far more in the two dimensions parallel to the membrane surface than in the third dimension perpendicular to it. The α-helical content of CP(E1) increases by ∼30% upon binding to the membrane. The sequence of kinetically distinguishable events in the CP(E1)-membrane interaction is binding, unfolding to a subtended area of 4200 Å 2, helix extension, and insertion, the last three events overlapping in their time course (∼10 s –1). The extension into two dimensions and the interaction with the membrane surface may explain the reversible denaturation and refolding of secondary structure that occurs after boiling of the CP-membrane complex. Although DSC showed the presence of helix–helix interactions in the membrane-bound state, the change in secondary structure and the extended surface area argue against a molten-globule intermediate in the CP-membrane interaction. However, the surface-bound state is mobile, as surface conformational mobility is a necessary prerequisite for insertion of CP trans-membrane helices into the bilayer. The requirement for this surface protein mobility, described by “thermal melting” FRET experiments, may provide the explanation for the precipitous decrease in the voltage-gated CP channel formation at high values of surface potential of planar bilayer membranes. Thus, the membrane interfacial layer, with the CP backbone situated near the acyl chain carbonyls, provides a favorable environment for the structure changes necessary for the transition from the soluble to the membrane-inserted state.