Fig. 6

A model for Drs2p-Cdc50p autoinhibition and activation by PI4P. a In the absence of an activation factor, the Drs2p-Cdc50p flippase exists in the membrane in an autoinhibited state. Autoinhibition is primarily achieved by the extended C-terminus wrapping around the cytosolic A, P, and N domains. Specifically, Phe-1275 and Phe-1277 occupy the ATP-binding pocket, preventing access of ATP to the catalytic site. b Activation of the flippase is initiated by the binding of PI4P in a positively charged pocket in the transmembrane region proximal to the cytosol. c The binding of PI4P leads to a 90° rotation of the helix switch away from the cytosolic domains, thereby pulling the C-terminal inhibition peptide out of the cytosolic domains, and activating the enzyme. Upon Drs2p activation, ATP and the substrate PS bind to Drs2p, but PI4P diffuses away from the flippase. The active state is likely sustained by additional factors such as Gea2p, which is known to bind to the Drs2p C-terminal inhibition loop. The three circles mark the ATP, PS, and PI4P binding sites in Drs2p