DIFFUSION AND GATING IN MEMBRANE-PROTEINS

A fundamental mechanism for the transmission of information between living cells is provided by the ionic transport through channels across the cell membranes. These channels, which are proteins imbedded in the membranes, usually fluctuate between open (conducting) and closed (non-conducting) states. A defect-diffusion gating model inspired by the analogy between glasses and proteins has been formulated; its predictions are shown to be in good agreement with the observed distributions, f(t), of closed-state durations. In particular, the model predicts that f(t) behaves at long times either as a stretched exponential or as a power law, depending on the initial defect configuration. There is also considerable evidence supporting the view that the ionic pathway is lined by membrane-spanning protein segments folded to form alpha-helices. In this paper, a model is presented for the motion of an alpha-helix in the framework formed by the rest of the protein. The physically appealing possibility of interpreting the defects diffusing in the protein in terms of kink-like excitations of the helix is examined.