The COP9 signalosome inhibits the activity of Cullin-RING E3 ubiquitin ligases by removing Nedd8 modifications from their Cullin subunits. Neddylation renders these complexes catalytically active, but deneddylation is also necessary for them to exchange adaptor subunits and avoid auto-ubiquitination. Although deneddylation is thought to be the primary function of the COP9 signalosome, additional activities have been ascribed to some of its subunits. We recently showed that COP9 subunits protect the transcriptional repressor and tumor suppressor Capicua from two distinct modes of degradation. Deneddylation by the COP9 signalosome inactivates a Cullin 1 complex that ubiquitinates Capicua following its phosphorylation by MAP kinase in response to Epidermal Growth Factor Receptor signaling. The CSN1b subunit also stabilizes unphosphorylated Capicua to control its basal level, independently of the deneddylase function of the complex. Here we further examine the importance of deneddylation for COP9 functions in vivo. We use an uncleavable form of Nedd8 to show that preventing deneddylation does not reproduce the effects of loss of COP9. In contrast, in the presence of COP9, conjugation to uncleavable Nedd8 renders Cullins unable to promote the degradation of their substrates. Our results suggest that irreversible neddylation prolongs COP9 binding to and inhibition of Cullin-based ubiquitin ligases.

COP9信号小体通过从其Cullin亚基中去除Nedd8修饰来抑制Cullin-RING E3泛素连接酶的活性。交联作用使这些复合物具有催化活性，但交联作用对于它们交换衔接子亚基并避免自体泛素化也是必需的。尽管树突化被认为是COP9信号小体的主要功能，但其某些亚基也具有额外的活性。我们最近表明，COP9亚基可以保护转录阻遏物和抑癌基因Capicua免受两种不同的降解方式的影响。COP9信号小体的树突化使Cullin 1复合物失活，该复合物在Capipua被MAP激酶磷酸化后响应表皮生长因子受体信号转导而泛素化。CSN1b亚基还可以稳定未磷酸化的Capicua，以控制其基础水平，而与复合物的树枝化酶功能无关。在这里，我们进一步研究了COP9功能的树突化的重要性体内。我们使用不可切割的Nedd8形式来表明，预防树突状复制不会重现COP9丢失的影响。相反，在存在COP9的情况下，与不可切割的Nedd8的结合使Cullins无法促进其底物的降解。我们的结果表明，不可逆的糊化作用延长了COP9与基于Cullin的泛素连接酶的结合并对其抑制。