Toward CFTR Structural Dynamics During Ion Channel Gating

Abstract

While CFTR catalyses the irreversible hydrolysis of its physiological ligand (ATP), there is disagreement about whether CFTR channel gating is a thermally driven process (Aleksandrov & Riordan 1998 FEBS Lett. 431:97, Csanady et al., 2006 JGP128:509). Knowing the answer to this question is of fundamental importance to understand problems leading to CFTR functional failure in cystic fibrosis, an inherited disease of high morbidity and mortality. Although CFTR single channel recordings do not provide a direct measure of enzymatic activity, they do provide an opportunity to establish the relationship between ATP binding and hydrolysis at the nucleotide binding domains (NBDs) and channel gating. The use of Rate Equilibrium Free Energy Relationship (REFER) is a possible way to probe structural dynamics in an ion channel gating (Auerbach, 2005 PNAS 102:1408). We use this conceptual framework to reveal the role of ATP hydrolysis in CFTR ion channel function. The REFER graphs for the set of nucleotide ligands have slope Φ ≈1 for the openings and Φ ≈ 0 for the closings. This means that the nucleotide triphosphate interaction with the binding sites is required for channel opening. In contrast, the ion channel transition from the open to the closed state occurs independently of any events in the binding sites. This could not be interpreted by conventional mechanical models of molecular devices where the mobile species are levered from the initial to the final position by force generated in the binding sites. However this behavior fits very well to Brownian ratchet models where internal thermal diffusion is a key element of the process, and specific interactions induce no mechanical movements directly but instead are used to trap favorable fluctuations.