Ion occlusion/deocclusion partial reactions in individual palytoxin-modified Na/K pumps

Abstract

In P-type ion-motive ATPases, transported ions approach their binding sites from one membrane surface, become buried deep within "occluded" conformations in which the sites are inaccessible from either membrane side, and are then deoccluded and released to the opposite membrane surface. This describes an alternating-gate transport mechanism, in which the pump acts like an ion channel with two gates that open and close alternately. The occluded states ensure that one gate closes before the other can open, thus preventing the large electrodiffusive ion fluxes that would otherwise quickly undo the pump's electrochemical work. High-resolution crystal structures of two conformations of the SERCA (sarcoplasmic and endoplasmic reticulum Ca(2+)) P-type ATPase, together with mutagenesis results and analyses of structural models based on homology, have begun to provide a picture of the ion coordination sites in related P-type ATPases, including the Na/K pump. However, in no P-type ATPase are the structures and mechanisms of the gates known. The marine toxin, palytoxin (PTX), is known to bind to the Na/K pump and elicit a nonselective cation leak pathway, possibly by disrupting the strict coupling between the pump's inner and outer gates, allowing them to both be open. We recently found that ion flow through PTX-modified Na/K pump-channels appears to be modulated by two gates that can be regulated by the pump's physiological ligands in a manner suggesting that gating reflects underlying ion occlusion/deocclusion partial reactions. We review that work here and provide evidence that the pore of the PTX-induced pump-channel has a diameter > 6 A.