Degradation of FBXO31 by APC/C is regulated by AKT- and ATM-mediated phosphorylation.

The F-box protein FBXO31 is a tumor suppressor that is encoded in 16q24.3, for which there is loss of heterozygosity in various solid tumors. FBXO31 serves as the substrate-recognition component of the SKP/Cullin/F-box protein class of E3 ubiquitin ligases and has been shown to direct degradation of pivotal cell-cycle regulatory proteins including cyclin D1 and the p53 antagonist MDM2. FBXO31 levels are normally low but increase substantially following genotoxic stress through a mechanism that remains to be determined. Here we show that the low levels of FBXO31 are maintained through proteasomal degradation by anaphase-promoting complex/cyclosome (APC/C). We find that the APC/C coactivators CDH1 and CDC20 bind to a destruction-box (D-box) motif present in FBXO31 to promote its polyubiquitination and degradation in a cell-cycle-regulated manner, which requires phosphorylation of FBXO31 on serine-33 by the prosurvival kinase AKT. Following genotoxic stress, phosphorylation of FBXO31 on serine-278 by another kinase, the DNA damage kinase ATM, results in disruption of its interaction with CDH1 and CDC20, thereby preventing FBXO31 degradation. Collectively, our results reveal how alterations in FBXO31 phosphorylation, mediated by AKT and ATM, underlie physiological regulation of FBXO31 levels in unstressed and genotoxically stressed cells.

FBXO32 activates NF-κB through IκBα degradation in inflammatory and genotoxic stress.

In response to diverse stresses, the canonical NF-κB pathway gets activated primarily to protect the cells and maintain their genomic integrity. It activates the cell cycle checkpoints allowing the cells with limited damage to restore a normal life cycle. One of the key events in activation of the canonical NF-κB pathway is the selective proteasomal degradation of IκBα. It has been previously shown that F-box protein ßTRCP1 has limited role in directing the proteasomal degradation of IκBα during stress conditions. Here, we report another member of F-box family proteins, FBXO32, as a potential activator of NF-κB signaling during genotoxic stress and inflammatory response. Following genotoxic or inflammatory stress, FBXO32 is stabilized, which leads to polyubiquitination and proteasome mediated degradation of IκBα. We also found that FBXO32 is required for physiological regulation of IκBα levels in unstressed cells. Thus, we decipher the new role of FBXO32 in regulation of NF-κB signaling pathway.