Structure and Cl-Conductance Properties of the Open State of Human CFTR
The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel that plays a vital role in water and ion secretion on epithelial surfaces. Loss of function in CFTR causes the life-threatening disease cystic fibrosis (CF). The functionally open state of CFTR has so far eluded detailed structural characterization. Although multiple near-atomic resolution structures of CFTR have been solved under conditions that promote channel opening, they all lack a continuous ion conduction pathway. In recent molecular dynamics (MD) simulations, structural fluctuations of human CFTR in a hydrated lipid bilayer led to the observation of transient Cl- conducting states, but the stability and conduction properties of these putative open states were not established. Here, we conduct massively repeated simulations initiated from these Cl- permeable conformations. Reproducible structural relaxation of the pore leads to a stable open conformation featuring five symmetrically arranged pore-lining helices. Unlike previously reported structures, this novel penta-helical arrangement reproduces experimentally determined properties of the open pore, including a Cl- conductance close to that measured at physiological voltages. Together, our results support the validity of this newly identified pore conformation as a model of the fully open channel. Detailed analysis highlights the role of cationic pore-lining residues in the Cl- permeation mechanism and suggests that the kinks observed in several transmembrane helices play a role in channel gating.