The nucleolus, as the ‘nucleus of the nucleus’, is a prominent subcellular organelle in a eukaryocyte. The nucleolus serves as the centre for ribosome biogenesis, as well as an important site for cell-cycle regulation, cellular senescence, and stress response. The protein composition of the nucleolus changes dynamically through protein turnover to meet the needs of cellular activities or stress responses. Recent studies have identified a nucleolus-localized protein degradation pathway in zebrafish and humans, namely the Def-CAPN3 pathway, which is essential to ribosome production and cell-cycle progression, by controlling the turnover of multiple substrates (e.g., ribosomal small-subunit [SSU] processome component Mpp10, transcription factor p53, check-point proteins Chk1 and Wee1). This pathway relies on the Ca²⁺-dependent cysteine proteinase CAPN3 and is independent of the ubiquitin-mediated proteasome pathway. CAPN3 is recruited by nucleolar protein Def from cytoplasm to nucleolus, where it proteolyzes its substrates which harbor a CAPN3 recognition-motif. Def depletion leads to the exclusion of CAPN3 and accumulation of p53, Wee1, Chk1, and Mpp10 in the nucleolus that result in cell-cycle arrest and rRNA processing abnormality. Here, we summarize the discovery of the Def-CAPN3 pathway and propose its biological role in cell-cycle control and ribosome biogenesis.

核仁，作为“细胞核的细胞核”，是真核细胞中重要的亚细胞器。核仁是核糖体生物发生的中心，也是细胞周期调节、细胞衰老和应激反应的重要场所。核仁的蛋白质组成通过蛋白质周转动态变化，以满足细胞活动或应激反应的需要。最近的研究已经确定了斑马鱼和人类的核仁定位蛋白降解途径，即 Def-CAPN3 途径，它通过控制多种底物（例如核糖体小亚基SSU] 加工组成分 Mpp10、转录因子 p53、检查点蛋白 Chk1 和 Wee1）。该途径依赖于 Ca ²⁺依赖的半胱氨酸蛋白酶 CAPN3 并且不依赖于泛素介导的蛋白酶体途径。CAPN3 被核仁蛋白 Def 从细胞质募集到核仁，在那里它蛋白水解含有 CAPN3 识别基序的底物。Def 耗竭导致 CAPN3 的排除和 p53、Wee1、Chk1 和 Mpp10 在核仁中的积累，从而导致细胞周期停滞和 rRNA 加工异常。在这里，我们总结了 Def-CAPN3 通路的发现，并提出了它在细胞周期控制和核糖体生物发生中的生物学作用。