Cyclin F-dependent degradation of E2F7 is critical for DNA repair and G2-phase progression

E2F7 and E2F8 act as tumor suppressors via transcriptional repression of genes involved in S-phase entry and progression Previously, we demonstrated that these atypical E2Fs are degraded by APC/CCdh1 during G1 phase of the cell cycle. However, the mechanism driving the downregulation of atypical E2Fs during G2 phase is unknown. Here, we show that E2F7 is targeted for degradation by the E3 ubiquitin ligase SCFcyclin-F during G2. Cyclin F binds via its cyclin domain to a conserved C-terminal CY motif on E2F7. An E2F7 mutant unable to interact with SCFcyclin-F remains stable during G2. Furthermore, SCFcyclin-F can also interact and induce degradation of E2F8. However, this does not require the cyclin domain of SCFcyclin-F nor the CY motifs in the C terminus of E2F8, implying a different regulatory mechanism than for E2F7. Importantly, depletion of cyclin F causes an atypical-E2F-dependent delay of the G2/M transition, accompanied by reduced expression of E2F target genes involved in DNA repair. Live-cell imaging of DNA damage revealed that cyclin F-dependent regulation of atypical E2Fs is critical for efficient DNA repair and cell cycle progression.

E2F7与E2F8可通过转录抑制S期进入及进程相关基因，发挥肿瘤抑制因子的作用。此前本研究团队已证实，这类非典型E2F家族成员在细胞周期G1期会被后期促进复合物结合Cdh1（APC/CCdh1）降解。然而，目前尚不清楚G2期介导非典型E2F家族成员下调的具体机制。本研究发现，E2F7在G2期会被SCFcyclin-F型E3泛素连接酶（SCFcyclin-F）靶向降解。Cyclin F通过其cyclin结构域，与E2F7保守的C端CY基序相结合；无法与SCFcyclin-F结合的E2F7突变体在G2期仍保持稳定。此外，SCFcyclin-F同样可与E2F8结合并诱导其降解，但这一过程既不依赖SCFcyclin-F的cyclin结构域，也不依赖E2F8C端的CY基序，提示二者的调控机制存在差异。值得注意的是，敲低cyclin F会引发依赖非典型E2F的G2/M转换延迟，并伴随DNA修复相关E2F靶基因的表达水平下调。针对DNA损伤的活细胞成像实验结果显示，cyclin F对非典型E2F的调控对于高效完成DNA修复及细胞周期进程至关重要。