Molecular Dynamics of Selective Sugar Permeation through the Connexin26 Channel

Abstract

A signal property of connexin channels is the ability to mediate selective diffusive movement of molecules through plasma membrane, yet the movement of biological molecules through these channels has yet to be well-characterized in mechanistic or energetic terms. Different connexin channels have distinct molecular selectivities that cannot be explained on the basis of size or charge of the permeants; the forces that molecules experience within the pore determines which molecules are permeable and to what degree. The energetics of the movement of two derivatized sugars, one permeable and one impermeable, through an experimentally validated connexin26 (Cx26) structural model were explored using Hamiltonian Replica Exchange MD Umbrella Sampling (US/H-REMD) and Steered Molecular Dynamics (SMD), and associated analytic tools. Crucially, the Cx26 channel model, in explicit membrane/solvent, incorporates key post-translational charge changes shown by Brownian Dynamics to be required to reproduce the electrical conductance characteristics of the native channel [Kwon J.Gen.Physiol. 138:4751]. The results show energy profiles consistent with experimental results. The energetic barriers extend through most of the pore length, rather than being highly localized, as in ion-specific channels. There is little evidence for binding within the pore. Force decomposition reveals how, for each test molecule, interactions with water and with the Cx26 protein vary over the length of the pore, and reveal a significant contribution of interaction with K+ ions. The flexibility of pore width varies along its length, and the test molecules tend to widen the pore as they pass through. This work highlights factors involved in selective molecular permeation that may not be significant for atomic ions. The results suggest that this system can be used to explore the molecular basis by which connexin channels select among (potential) permeating molecules, and how mutations alter the permeation process.