Structures of the peptidase-containing ABC transporter PCAT1 under equilibrium and nonequilibrium conditions

Significance Recent advances in cryo-electron microscopy (cryo-EM) enabled us to determine multiple structures from the same sample. At equilibrium, the Boltzmann distribution law can be applied to identify the lowest energy state of the protein. We found that the conformational distributions of PCAT1, the peptidase-containing ATP-binding cassette (ABC) transporter 1, are completely different in the presence and absence of the Mg 2+ ion. This difference reflects energy inflow from ATP hydrolysis, shifting the system out of equilibrium. The conformational distribution under ATP turnover condition is determined by the transition rates along the transport pathway rather than the energy of each state. This study demonstrates how cryo-EM data can be used to understand thermodynamic and kinetic properties of an active transporter.