Tuning the membrane surface potential for efficient toxin import

Membrane surface electrostatic interactions impose structural constraints on imported proteins. An unprecedented sensitive dependence on these constraints was seen in the voltage-gated Import and channel formation by the C-terminal pore-forming domain of the bacteriocin, colicin El. At physiological ionic strengths, significant channel current was observed only in a narrow interval of anionic lipid content ([L-1]), with the maximum current (I-max) at 25-30 mol% (dioleoyl)-phosphatidylglycerol ([L-1](max)) corresponding to a surface potential of the lipid bilayer in the absence of protein, psi(0)(max) = -60 +/- 5 mV. Higher ionic strength shifted [L-1](max) to larger values, but psi(0)(max) remained approximately constant. It is proposed that the channel current (i) increases and (ii) decreases at \psi(0)\ values <55 mV and >65 mV, because of (i) electrostatic Interactions needed for effective insertion of the channel polypeptide and (ii) constraints due to electrostatic forces on the flexibility needed for cooperative insertion into the membrane. The loss of flexibility for \psi(0)\ much greater than 65 mV was demonstrated by the absence of thermally induced intraprotein distance changes of the bound polypeptide. The anionic lipid content, 25-30 mol%, corresponding to the channel current maxima, is similar to that of the target Escherichia coli cytoplasmic membrane and membranes of mesophilic microorganisms. This suggests that one reason the membrane surface potential is tuned in vivo is to facilitate protein import.