Structural visualization of the tubulin folding pathway directed by human chaperonin TRiC/CCT

The ATP-dependent ring-shaped chaperonin TRiC/CCT is essential for cellular proteostasis. To uncover why some eukaryotic proteins can only fold with TRiC assistance, we reconstituted the folding of b-tubulin using human prefoldin and TRiC. We find unstructured b-tubulin is delivered by prefoldin to the open TRiC chamber followed by ATP-dependent chamber closure. Cryo-EM resolves four near-atomic-resolution structures con-taining progressively folded b-tubulin intermediates within the closed TRiC chamber, culminating in native tubulin. This substrate folding pathway appears closely guided by site-specific interactions with conserved regions in the TRiC chamber. Initial electrostatic interactions between the TRiC interior wall and both the folded tubulin N domain and its C-terminal E-hook tail establish the native substrate topology, thus enabling C-domain folding. Intrinsically disordered CCT C termini within the chamber promote subsequent folding of tubulin's core and middle domains and GTP-binding. Thus, TRiC's chamber provides chemical and topolog-ical directives that shape the folding landscape of its obligate substrates.

Automated summary

The ATP-dependent ring-shaped chaperonin TRiC/CCT plays a crucial role in cellular proteostasis. It has been found that TRiC guides the folding pathway of its substrates through site-specific interactions with conserved regions in its chamber. The chamber of TRiC provides chemical and topological directives that shape the folding landscape of its obligate substrates.