Co-translational determination of quaternary structures in chaperone factories

The HSP90/R2TP quaternary chaperone assembles key cellular machines, including the three nuclear RNA polymerases and many non-coding RNPs. Here, we show that the R2TP binds many partners co-translationally. Remarkably, its co-translational interactome reveals many novel potential clients, and side-by-side comparison of RIP-seq and quantitative proteomics identifies clients bound only co-translationally, only post-translationally or both. For pairs of subunits assembling together and bound co-translationally by R2TP, only a marginal proportion of their mRNAs is co-localized and co-translated. Instead, the HSP90 and R2TP chaperones induce the formation of dedicated factories, which are cytoplasmic R2TP foci accumulating client mRNAs, thus favoring co-translational interactions. The R2TP alternates between co- and post-translational steps in a cycle regulated by ATP: following co-translational loading, it accompanies completed subunits towards their partners and becomes released following ATP hydrolysis. Assembly of protein complexes is thus initiated early by chaperones and this mechanism, dubbed co-translational chaperone channeling (cha-cha), substitutes for the rarity of co-localized/co-translated mRNAs.