Fig. 4. Recovery of maturation and functional regulation in mutant CFTR. (a) Root mean square fluctuations (RMSF) of residues in WT NBD1 compared with S492P substitution in the context of ΔF508-I539T-NBD1 and ΔI507-I539T-NBD1 denote stabilization of NBD1 upon mutation of S492. (b) Network representation of the subgraph from ΔF508-PT shows that the effect of RI is no longer propagated via P492 or any other nodes, but rather distributed across multiple nodes, indicating dissipation of thermal fluctuations upon mutation and consequent stabilization of the domain. (c) Western blot analysis of lysates from HEK293 cells transiently expressing WT or mutant CFTR confirms expression of all forms but maturation of only ΔF508-PT and ΔI507-PT CFTR. (d) Single channel recordings of wild type CFTR and mutant constructs at 35 °C in symmetrical salt solution (300 mM Cl^–) under voltage-clamp at –75 mV. The upper arrow in each trace represents the closed state while the lower arrow is the open state of a single CFTR channel. We find partial recovery of WT function in only ΔF508-PT CFTR. Traces are plotted from data previously published¹⁷ (e) Iodide efflux measurements (Methods) in BHK cells stably expressing wild-type CFTR (solid circles), ΔF508-PT CFTR (open circles) or ΔF508-CFTR (solid triangles). Stimulation cocktail is added at time 0 to activate iodide efflux through CFTR channels. The values represent the mean ± standard deviation of the amount of iodide released from the cells during a one minute interval (n = 3). Efflux buffer containing 0.1% NP40 is added at the end of each assay (arrow) to release remaining iodide. We find that ΔF508-PT CFTR partially recovers WT activity as compared to ΔF508-CFTR.