Association of the cystic fibrosis transmembrane regulator with CAL: Structural features and molecular dynamics

The association of the cystic fibrosis transmembrane regulator (CFTR) with two PDZ-containing molecular scaffolds (CAL and EBP50) plays an important role in CFTR trafficking and membrane maintenance. The CFTR-molecular scaffold interaction is mediated by the association of the C-terminus of the transmembrane regulator with the PDZ domains. Here, we characterize the structure and dynamics of the PDZ of CAL and the complex formed with CFTR employing high-resolution NMR. On the basis of NMR relaxation data, the alpha 2 helix as well as the beta 2-beta 3 loop of CAL PDZ domain undergoes rapid dynamics. Molecular dynamics simulations suggest a concerted motion between the alpha 2 helix and the beta 1-beta 2 and beta 2-beta 3 loops, elements which define the binding pocket, suggesting that dynamics may play a role in PDZ-ligand specificity. The C-terminus of CFTR binds to CAL with the final four residues (-D-3-T-R-L-0) within the canonical PDZ-binding motif, between the beta 2 strand and the alpha 2 helix. The R-1 and D-3 side chains make a number of contacts with the PDZ domain; many of these interactions differ from those in the CFTR-EBP50 complex, suggesting sites that can be targeted in the development of PDZ-selective inhibitors that may help modulate CFTR function.