淋巴结（LN）转移是膀胱癌（BCa）患者死亡的主要原因，但其潜在机制仍然很大程度上未知。近年来，越来越多的研究证实，线粒体与细胞核的双向通讯对于维持线粒体的多种功能至关重要。然而，关于线粒体蛋白易位是否以及如何参与淋巴结转移的研究很少。在这项研究中，我们首先发现 SUMO E3 连接酶 MUL1 在 LN 转移性 BCa 组织中显着下调，并且与良好的预后相关。从机制上讲，MUL1 在 K612 残基处 SUMO 化 HSPA9，导致 HSPA9 从线粒体输出并与 SUZ12 和细胞核中相互作用。因此，MUL1 诱导泛素化介导的 SUZ12 和 EZH2 降解，并以 HSPA9 依赖性方式诱导下游 STAT3 通路抑制。重要的是，HSPA9 SUMO 缀合基序的突变限制了线粒体 HSPA9 的易位并阻断了 HSPA9-SUZ12 和 HSPA9-EZH2 相互作用。随着 HSPA9 K612 位点的突变，MUL1 过表达的抑制作用在 BCa 细胞中消失。进一步的体外和体内测定表明，MUL1 抑制 BCa 细胞的转移和增殖。总的来说，我们的研究揭示了 SUMO E3 连接酶在淋巴结转移中的新功能和分子机制。© 作者。

Lymph node (LN) metastasis is the dominant cause of death in bladder cancer (BCa) patients, but the underlying mechanism remains largely unknown. In recent years, accumulating studies have confirmed that bidirectional mitochondria-nucleus communication is essential for sustaining multiple function of mitochondria. However, little has been studied regarding whether and how the translocation of mitochondrial proteins is involved in LN metastasis. In this study, we first identified that the SUMO E3 ligase MUL1 was significantly downregulated in LN-metastatic BCa tissues and correlated with a good prognosis. Mechanistically, MUL1 SUMOylated HSPA9 at the K612 residue, leading to HSPA9 export from mitochondria and interaction with SUZ12 and in the nucleus. Consequently, MUL1 induced the ubiquitination-mediated degradation of SUZ12 and EZH2 and induced downstream STAT3 pathway inhibition in a HSPA9-dependent manner. Importantly, mutation of HSPA9 SUMO-conjugation motifs limited the translocation of mitochondrial HSPA9 and blocked the HSPA9-SUZ12 and HSPA9-EZH2 interactions. With mutation of the HSPA9 K612 site, the suppressive role of MUL1 overexpression was lost in BCa cells. Further in vitro and in vivo assays revealed that MUL1 inhibits the metastasis and proliferation of BCa cells. Overall, our study reveals a novel function and molecular mechanism of SUMO E3 ligases in LN metastasis.© The author(s).