The ?-helix dipole in membranes: a new gating mechanism for ion channels

Abstract

Electric dipoles placed side by side attract each other if antiparallel and repel each other if parallel. The hydrophobic alpha-helical sections of proteins that span membranes are known to possess large electric dipole moments. The first part of the paper consists of a calculation of the interaction energies between such helices including screening effects. Interaction energies remain comparable with a typical thermal energy of KT up to separations of order 20 A. In addition it is shown that, due solely to its dipole moment, an alpha-helix which completely spans the membrane has an energy up to 5 KT lower than one which terminates within the membrane width. The second part of the paper describes the electrical interaction of the charge structure of a membrane channel and the protein helices that surround the pore. The gating charge transfer that is measured when a voltage sensitive ion channel switches, means that the dipole moment of the ion channel changes. This in turn results in a change in the radial forces that act between the pore and the alpha-helices that surround it. A change in these radial forces which tend to open or to close the pore constitutes an electrically silent gating mechanism that must necessarily act subsequent to the gating charge transfer. The gating mechanism could consist of the radial translation of the neighbouring proteins or in their axial rotation under the influence of the torque that would act on a pair of approximately equidistant but oppositely directed alpha-helices. An attempt to calculate the interaction energy of a typical pore and a single alpha-helix spanning the membrane results in an energy of many times KT.