A New Type of Na+-Driven ATP Synthase Membrane Rotor with a Two-Carboxylate Ion-Coupling Motif

The anaerobic bacterium fusobacterium nucleatum uses glutamate decarboxylation to generate a transmembrane gradient of na+. here,we demonstrate that this ion-motive force is directly coupled to atp synthesis,via an f1fo-atp synthase with a novel na+ recognition motif,shared by other human pathogens. molecular modeling and free-energy simulations of the rotary element of the enzyme,the c-ring,indicate na+ specificity in physiological settings. consistently,activity measurements showed na+ stimulation of the enzyme,either membrane-embedded or isolated,and atp synthesis was sensitive to the na+ ionophore monensin. furthermore,na+ has a protective effect against inhibitors targeting the ion-binding sites,both in the complete atp synthase and the isolated c-ring. definitive evidence of na+ coupling is provided by two identical crystal structures of the c11 ring,solved by x-ray crystallography at 2.2 and 2.6 å resolution,at ph 5.3 and 8.7,respectively. na+ ions occupy all binding sites,each coordinated by four amino acids and a water molecule. intriguingly,two carboxylates instead of one mediate ion binding. simulations and experiments demonstrate that this motif implies that a proton is concurrently bound to all sites,although na+ alone drives the rotary mechanism. the structure thus reveals a new mode of ion coupling in atp synthases and provides a basis for drug-design efforts against this opportunistic pathogen. © 2013 schulz et al.