Protein kinase A phosphorylation potentiates cystic fibrosis transmembrane conductance regulator gating by relieving autoinhibition on the stimulatory C terminus of the regulatory domain

Abstract

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel activated by protein kinase A (PKA) phosphorylation on the regulatory (R) domain. Phosphorylation at several R domain residues stimulates ATP-dependent channel openings and closings, termed channel gating. To explore the protein segment responsible for channel potentiation and PKA-dependent activation, deletion mutations were constructed by removing one to three protein segments of the R domain including residues 708–759 (ΔR _708–759), R _760–783, and R _784–835, each of which contains one or two PKA phosphorylation sites. Deletion of R _708–759 or R _760–783 had little effect on CFTR gating, whereas all mutations lacking R _784–835 reduced CFTR activity by decreasing the mean burst duration and increasing the interburst interval (IBI). The data suggest that R _784–835 plays a major role in stimulating CFTR gating. For ATP-associated regulation, ΔR _784–835 had minor impact on gating potentiation by 2′dATP, CaATP, and pyrophosphate. Interestingly, introducing a phosphorylated peptide matching R _809–835 shortened the IBI of ΔR _708–835-CFTR. Consistently, ΔR _815–835, but not ΔR _784–814, enhanced IBI, whereas both reduced mean burst duration. These data suggest that the entirety of R _784–835 is required for stabilizing the open state of CFTR; however, R _815–835, through interactions with the channel, is dominant for enhancing the opening rate. Of note, PKA markedly decreased the IBI of ΔR _708–783-CFTR. Conversely, the IBI of ΔR _708–814–CFTR was short and PKA-independent. These data reveal that for stimulating CFTR gating, PKA phosphorylation may relieve R _784–814–mediated autoinhibition that prevents IBI shortening by R _815–835. This mechanism may elucidate how the R domain potentiates channel gating and may unveil CFTR stimulation by other protein kinases.