A Disorder-to-Order Transition Activates an ATP-Independent Membrane Protein Chaperone

•Substrate and activator drive cpSRP43 from an inactive, open state to an active, closed state.•The open state is characterized by disordered Ankyrin repeat motifs in the client binding domain.•Activator binding drives a disorder-to-order transition in the Ankyrin repeat domain.•The bridging helix mediates allosteric communication from ligand binding to the client binding domain.