Molecular Dynamics Flexible Fitting Simulations Identify New Models of the Closed State of the Cystic Fibrosis Transmembrane Conductance Regulator Protein.

Cystic fibrosis (CF) is a lethal, genetic disease found in particular in humans of European origin which is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The search for CF therapies acting by modulating the impaired function of mutant CFTR will be greatly advanced by high resolution structures of CFTR in different states. To date, two medium resolution electron microscopy (EM) structures of CFTR are available (one of a distant zebrafish (Danio rerio) CFTR ortholog and one of human CFTR). The two models are nearly identical to one another, and both correspond to the inward-facing, nucleotide binding domains (NBDs) separated, closed state of the channel. In addition, lower resolution structural data are available for human CFTR in an alternative conformation which likely features associated NBDs and thus geometrically resembles the conducting state of the channel. Multiple homology models of human CFTR in multiple states have been developed over the years, yet their correspondence to the existing structural information is unexplored. In this work we use molecular dynamics flexible fitting (MDFF) simulations to refine two previously described CFTR models based on the available cryo-EM map of the human protein. This map was recorded in the absence of ATP and consequently represents closed-state CFTR yet its features likely correspond to an NBD associated conformation of the protein. Accordingly, the resulting models feature dimerized NBDs yet with no membrane traversing pore. Moreover, the open probability of the new models as deduced from the MDFF trajectories is significantly lower than that deduced from control MD trajectories initiated from the starting models. We propose that the new models correspond to a CFTR conformation which to date was largely unexplored yet is one that is relevant to the gating cycle of the protein. In particular this conformation may participate in rapid channel opening and closing through small allosteric movements controlled by nucleotide binding and dissociation events. Analyzing the resulting trajectories (and not only the final models as is usually the case), we demonstrate that the refined models have good stereochemical properties and are also in favorable agreement with multiple experimental data. Moreover, despite different starting points, the final models share many common features. Finally, we propose that the combination of high resolution cryo-EM maps, which are currently emerging from multiple laboratories, and MDFF simulations will be of value for the development of yet more reliable CFTR models as well as for the identification of binding sites for CFTR modulators.