TRiC folds the giant ciliary protein IFT172 via a non-canonical open-state mechanism

作者：Qiaoyu Zhao, Jiayi Li, Yujie Tong, Yurui Li, Wenyu Han, Zuyang Li, Yanxing Wang, Yue Yin, Jun Fang, Wanying Jiang, Qianqian Song, Shengyou Huang, Yidong Shen, Yao Cong · 发表于：bioRxiv (Cold Spring Harbor Laboratory) · 年份：2026 · DOI：10.64898/2026.03.28.714460 · 被引用次数：1 · 研究领域：Genetic and Kidney Cyst Diseases、Protist diversity and phylogeny、Microtubule and mitosis dynamics

Abstract The eukaryotic chaperonin TRiC/CCT is essential for folding complex proteins, yet how it folds substrates that exceed its closed chamber capacity remains a longstanding paradox. Here, we define the folding pathway of IFT172, the largest subunit (∼200 kDa) of the intraflagellar transport (IFT) machinery, and uncover a “divide-and-conquer” mechanism. TRiC and HSP70 engage IFT172 concurrently but on distinct domains: TRiC captures the N-terminal WD40 β-propellers within its chamber, whereas HSP70 independently stabilizes the C-terminal TPR domain in the cytosol. To accommodate this oversized client, specific TRiC subunits (CCT4, CCT2, and CCT7) undergo pronounced Z-shaped outward bending, thereby expanding the chamber. Unexpectedly, the first WD40 domain reaches a near-folded state within the open, ATP-bound chamber, and subsequent TRiC ring closure triggers substrate ejection rather than encapsulation. This non-canonical “fold-and-eject” mechanism challenges the classical view that the closed chamber is an obligate folding cage. We further demonstrate that this pathway is essential for ciliary functions in vivo, and reveal a conserved mode of chaperonin recognition among IFT components bearing tandem WD40-TPR architectures. Together, our findings establish a new paradigm for the folding of oversized, multi-domain proteins and identify TRiC as a central proteostasis hub in ciliary biogenesis, with direct implications for ciliopathy pathogenesis.