The peripheral
protein quality control (PPQC) checkpoint removes improperly folded
proteins from the plasma membrane through a mechanism involving the
E3 ubiquitin ligase CHIP (carboxyl terminus of Hsc70 interacting
protein). PPQC limits the efficacy of some
cystic fibrosis (CF) drugs, such as
VX-809, that improve trafficking to the plasma membrane of misfolded mutants of the CF transmembrane conductance regulator (CFTR), including F508del-CFTR, which retains partial functionality. We investigated the PPQC checkpoint in lung epithelial cells with F508del-CFTR that were exposed to
VX-809. The conformation of the scaffold
protein NHERF1 (Na(+)/H(+) exchange regulatory factor 1) determined whether the PPQC recognized "rescued" F508del-CFTR (the portion that reached the cell surface in
VX-809-treated cells). Activation of the cytoskeletal regulator Rac1 promoted an interaction between the actin-binding adaptor
protein ezrin and NHERF1, triggering exposure of the second PDZ domain of NHERF1, which interacted with rescued F508del-CFTR. Because binding of F508del-CFTR to the second PDZ of NHERF1 precluded the recruitment of CHIP, the coexposure of airway cells to Rac1 activator nearly tripled the efficacy of
VX-809. Interference with the NHERF1-ezrin interaction prevented the increase of efficacy of
VX-809 by Rac1 activation, but the actin-binding domain of
ezrin was not required for the increase in efficacy. Thus, rather than mainly directing anchoring of F508del-CFTR to the actin cytoskeleton, induction of
ezrin activation by Rac1 signaling triggered a conformational change in NHERF1, which was then able to bind and stabilize misfolded CFTR at the plasma membrane. These insights into the cell surface stabilization of CFTR provide new targets to improve treatment of CF.