Chaperonin TRiC/CCT supports mitotic exit and entry into endocycle in <i>Drosophila</i>

Endocycle is a commonly observed cell cycle variant through which cells undergo repeated rounds of genome DNA replication without mitosis. Endocycling cells arise from mitotic cells through a switch of the cell cycle mode, called the mitotic-to-endocycle switch (MES), to initiate cell growth and terminal differentiation. However, the underlying regulatory mechanisms of MES remain unclear. Here we used the Drosophila steroidogenic organ, called the prothoracic gland (PG), to study regulatory mechanisms of MES, which is critical for the PG to upregulate biosynthesis of the steroid hormone ecdysone. We demonstrate that PG cells undergo MES through downregulation of mitotic cyclins, which is mediated by Fizzy-related (Fzr). Moreover, we performed a RNAi screen to further elucidate the regulatory mechanisms of MES, and identified the evolutionarily conserved chaperonin TCP-1 ring complex (TRiC) as a novel regulator of MES. Knockdown of TRiC subunits in the PG caused a prolonged mitotic period, probably due to impaired nuclear translocation of Fzr, which also caused loss of ecdysteroidogenic activity. These results indicate that TRiC supports proper MES and endocycle progression by regulating Fzr folding. We propose that TRiC-mediated protein quality control is a conserved mechanism supporting MES and endocycling, as well as subsequent terminal differentiation.核内周期（Endocycle）是一种被广泛观测到的细胞周期变异形式，在此过程中细胞会进行多轮基因组DNA复制却不经历有丝分裂。进入核内周期的细胞由有丝分裂细胞通过细胞周期模式转换而来，该转换被称为有丝分裂-核内周期转换（mitotic-to-endocycle switch, MES），其启动细胞生长与终末分化。然而，MES背后的调控机制仍不明晰。本研究以果蝇的类固醇生成器官——前胸腺（prothoracic gland, PG）为模型，探究MES的调控机制——该机制对于前胸腺上调类固醇激素蜕皮激素（ecdysone）的生物合成至关重要。我们证实，前胸腺细胞通过由Fizzy相关蛋白（Fizzy-related, Fzr）介导的有丝分裂周期蛋白下调，完成MES过程。此外，我们通过RNA干扰（RNAi）筛选进一步阐明了MES的调控机制，并将进化保守的分子伴侣TCP-1环复合体（TCP-1 ring complex, TRiC）鉴定为MES的新型调控因子。在前胸腺中敲低TRiC亚基会导致有丝分裂时期延长，这可能是由于Fzr的核转位功能受损所致，同时该过程还会导致蜕皮激素合成活性丧失。上述结果表明，TRiC通过调控Fzr的折叠，助力正常的MES过程与核内周期推进。我们提出，TRiC介导的蛋白质质量控制是一种进化保守的机制，可支持MES过程、核内周期运行以及后续的终末分化。