Chaperones of F₁-ATPase

Mitochondrial F₁-ATPase contains a hexamer of alternating α and β subunits. The assembly of this structure requires two specialized chaperones, Atp11p and Atp12p, that bind transiently to α and β. In the absence of Atp11p and Atp12p, the hexamer is not formed, and α and β precipitate as large insoluble aggregates. An early model for the mechanism of chaperonemediated F₁ assembly (Wang, Z. G., Sheluho, D., Gatti, D. L., and Ackerman, S. H. (2000) EMBO J. 19, 1486-1493) hypothesized that the chaperones themselves look very much like the α and β subunits, and proposed an exchange of Atp11p for α and of Atp12p for β; the driving force for the exchange was expected to be a higher affinity of α and β for each other than for the respective chaperone partners. One important feature of this model was the prediction that as long as Atp11p is bound to β and Atp12p is bound to α, the two F₁ subunits cannot interact at either the catalytic site or the noncatalytic site interface. Here we present the structures of Atp11p from Candida glabrata and Atp12p from Paracoccus denitrificans, and we show that some features of the Wang model are correct, namely that binding of the chaperones to α and β prevents further interactions between these F₁ subunits. However, Atp11p and Atp12p do not resemble α or β, and it is instead the F₁ γ subunit that initiates the release of the chaperones from α and β and their further assembly into the mature complex.