A conditional probability analysis of cystic fibrosis transmembrane conductance regulator gating indicates that ATP has multiple effects during the gating cycle

ATP-binding cassette (ABC) transporters bind and hydrolyze ATP. In the cyst ic fibrosis transmembrane conductance regulator Cl- channel, this interacti on with ATP generates a gating cycle between a closed (C) and two open (O1 and O2) conformations. To understand better how ATP controls channel activi ty, we examined gating transitions from the C to the O1 and O2 states and f rom these open states to the C conformation. We made three main observation s. First, we found that the channel can open into either the O1 or O2 state , that the frequency of transitions to both states was increased by ATP con centration, and that ATP increased the relative proportion of openings into O1 vs. O2. These results indicate that ATP can interact with the closed st ate to open the channel in at least two ways, which may involve binding to nucleotide-binding domains (NBDs) NBD1 and NBD2. Second, ATP prolonged the burst duration and altered the way in which the channel closed. These data suggest that ATP also interacts with the open channel. Third, the channel s howed runs of specific types of open-closed transitions. This finding sugge sts a mechanism with more than one cycle of gating transitions. These data suggest models to explain how ATP influences conformational transitions in cystic fibrosis transmembrane conductance regulator and perhaps other ABC t ransporters.