BLOCK BY MOPS REVEALS A CONFORMATION CHANGE IN THE CFTR PORE PRODUCEDBY ATP HYDROLYSIS

ATP hydrolysis by the cystic fibrosis transmembrane conductance regula tor (CFTR) Cl- channel predicts that energy from hydrolysis might caus e asymmetric transitions in the gating cycle. We found that 3-(N-morph olino)propanesulfonic acid (MOPS) blocked the open channel by binding to a site 50% of the way through the electrical field. Block by MOPS r evealed two distinct states, O1 and O2, which showed a strong asymmetr y during bursts of activity; the first opening in a burst was in the O 1 state and the last was in the O2 state. Addition of a nonhydrolyzabl e nucleoside triphosphate prevented the transition to the O2 state and prolonged the O1 state. These data indicate that ATP hydrolysis by th e nucleotide-binding domains drives a series of asymmetric transitions in the gating cycle. They also indicate that ATP hydrolysis changes t he conformation of the pore, thereby altering MOPS binding.