Cyclic GMP-AMP synthase (cGAS) senses aberrant DNA during infection, cancer and inflammatory disease, and initiates potent innate immune responses through the synthesis of 2′3′-cyclic GMP-AMP (cGAMP)^{1,2,3,4,5,6,7}. The indiscriminate activity of cGAS towards DNA demands tight regulatory mechanisms that are necessary to maintain cell and tissue homeostasis under normal conditions. Inside the cell nucleus, anchoring to nucleosomes and competition with chromatin architectural proteins jointly prohibit cGAS activation by genomic DNA^{8,9,10,11,12,13,14,15}. However, the fate of nuclear cGAS and its role in cell physiology remains unclear. Here we show that the ubiquitin proteasomal system (UPS) degrades nuclear cGAS in cycling cells. We identify SPSB3 as the cGAS-targeting substrate receptor that associates with the cullin–RING ubiquitin ligase 5 (CRL5) complex to ligate ubiquitin onto nuclear cGAS. A cryo-electron microscopy structure of nucleosome-bound cGAS in a complex with SPSB3 reveals a highly conserved Asn-Asn (NN) minimal degron motif at the C terminus of cGAS that directs SPSB3 recruitment, ubiquitylation and cGAS protein stability. Interference with SPSB3-regulated nuclear cGAS degradation primes cells for type I interferon signalling, conferring heightened protection against infection by DNA viruses. Our research defines protein degradation as a determinant of cGAS regulation in the nucleus and provides structural insights into an element of cGAS that is amenable to therapeutic exploitation.

环 GMP-AMP 合酶 (cGAS) 在感染、癌症和炎症性疾病期间感知异常 DNA，并通过合成 2'3'-环 GMP-AMP (cGAMP) ^1,2,3,4,5启动有效的先天免疫反应^,6,7 。 cGAS 对 DNA 的无差别活性需要严格的调节机制，这是维持正常条件下细胞和组织稳态所必需的。在细胞核内，锚定到核小体以及与染色质结构蛋白的竞争共同抑制基因组 DNA 的 cGAS 激活^{8,9,10,11,12,13,14,15} 。然而，核 cGAS 的命运及其在细胞生理学中的作用仍不清楚。在这里，我们证明泛素蛋白酶体系统 (UPS) 会降解循环细胞中的核 cGAS。我们将 SPSB3 确定为 cGAS 靶向底物受体，它与 cullin-RING 泛素连接酶 5 (CRL5) 复合物结合，将泛素连接到核 cGAS 上。与 SPSB3 复合物中核小体结合的 cGAS 的冷冻电镜结构揭示了 cGAS C 末端高度保守的 Asn-Asn (NN) 最小降解决定子基序，指导 SPSB3 募集、泛素化和 cGAS 蛋白稳定性。干扰 SPSB3 调节的核 cGAS 降解可以启动细胞 I 型干扰素信号传导，从而增强对 DNA 病毒感染的保护。我们的研究将蛋白质降解定义为细胞核中 cGAS 调节的决定因素，并为适合治疗开发的 cGAS 元素提供了结构见解。