Pih1p-Tah1p Puts a Lid on Hexameric AAA+ ATPases Rvb1/2p

The Saccharomyces cerevisiae (Sc) R2TP complex affords an Hsp90-mediated and nucleotide-driven chaperone activity to proteins of small ribonucleoprotein particles (snoRNPs). The current lack of structural information on the ScR2TP complex, however, prevents a mechanistic understanding of this biological process. We characterized the structure of the ScR2TP complex made up of two AAA+ ATPases, Rvb1/2p, and two Hsp90 binding proteins, Tah1p and Pih1p, and its interaction with the snoRNP protein Nop58p by a combination of analytical ultracentrifugation, isothermal titration calorimetry, chemical crosslinking, hydrogen-deuterium exchange, and cryoelectron microscopy methods. We find that Pih1p-Tah1p interacts with Rvb1/2p cooperatively through the nucleotide-sensitive domain of Rvb1/2p. Nop58p further binds Pih1p-Tahp1 on top of the dome-shaped R2TP. Consequently, nucleotide binding releases Pih1p-Tah1p from Rvb1/2p, which offers a mechanism for nucleotide-driven binding and release of snoRNP intermediates. Tian et al. report that AAA+ ATPases, Rvb1/2p, though previously shown to form both dodecamer and hexamer, form a hexameric base for binding co-chaperone Pih1p-Tah1p. Tian et al. also showed that the binding interface is made of flexible and nucleotide-sensitive insertions within Rvb1/2p, which offers the basis for nucleotide-driven client release.