Regulation of cystic fibrosis transmembrane conductance regulator single-channel gating by bivalent PDZ-domain-mediated interaction

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-de pendent protein kinase- and ATP-regulated chloride channel, the activity of which determines the rate of electrolyte and fluid transport in a variety of epithelial tissues. Here we describe a mechanism that regulates CFTR cha nnel activity, which is mediated by PDZ domains, a family of conserved prot ein-interaction modules, The Na+/H+ exchanger regulatory factor (NHERF) bin ds to the cytoplasmic tail of CFTR through either of its two PDZ (PDZ1 and PDZ2) domains, A recombinant fragment of NHERF (PDZ1-2) containing the two PDZ domains increases the open probability (P-o) of single CFTR channels in excised membrane patches from a lung submucosal gland cell line, Both PDZ domains are required for this functional effect, because peptides containin g mutations in either domain are unable to increase channel P-o. The concen tration dependence of the regulation by the bivalent PDZ1-2 domain is bipha sic, i.e., activating at lower concentrations and inhibiting at higher conc entrations. Furthermore, either PDZ domain alone or together is without eff ect on P-o, but either domain can competitively inhibit the PDZ1-2-mediated stimulation of CFTR, Our results support a molecular model in which bivale nt NHERF PDZ domains regulate channel gating by crosslinking the C-terminal tails in a single dimeric CFTR channel, and the magnitude of this regulati on is coupled to the stoichiometry of these interactions.