Abstract
Phosphorylated cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels require nucleoside triphosphates, such as ATP, to open. As the concentration of intracellular ATP increases, the probability of the channel being open (Po) increases. To better understand how ATP regulates the channel, we studied excised inside-out membrane patches that contained single, phosphorylated CFTR Cl- channels and examined the kinetics of gating at different concentrations of ATP. As the ATP concentration increased from 0.1 to 3 mM the mean closed time decreased, but mean open time did not change. Analysis of the data using histograms of open- and closed-state durations, the maximum likelihood method, and the log-likelihood ratio test suggested that channel behavior could be described by a model containing one open and two closed states (C1<==>C2<==>O). ATP regulated phosphorylated channels at the transition between the closed states C1 and C2: as the concentration of ATP increased, the rate of transition from C1 to C2 (C1-->C2) increased. In contrast, transitions from C2 to C1 and between C2 and the open state (O) were not significantly altered by ATP. Addition of ADP in the presence of ATP decreased the transition rate from C1 to C2 without affecting other transition rates. These data suggest that ATP regulates CFTR Cl- channels through an interaction that increases the rate of transition from the closed state to a bursting state in which the channel flickers back and forth between an open and a closed state (C2). This transition may reflect ATP binding or perhaps a step subsequent to binding.