SCF(β-TrCP) targets Ajuba for degradation in a GSK3β-dependent manner in colorectal cancer.

Ajuba (ajuba LIM protein, JUB) is a member of the Ajuba family, and its oncogenic biological functions in colorectal cancer (CRC) have been extensively reported including proliferation, metastasis and resistance to chemotherapy. Although considerable studies have reported the regulation of Ajuba at the transcriptional level, the potential mechanisms of regulating Ajuba protein stability have not been fully elucidated to date. Herein, we showed that the mRNA and protein expression of Ajuba is upregulated in CRC tissues, high protein level correlates with unfavorable prognosis. Importantly, we identified Ajuba as a novel substrate of GSK3β kinase and SCF(β-TrCP) E3 ubiquitin ligase. Mechanistically, GSK3β phosphorylates Ajuba at serine 163 in the highly conserved degron motif (TS(163)GIS), which determines the interaction between Ajuba and the C-terminal WD40 domain of β-TrCP, and subsequent ubiquitination and targeted proteasomal degradation of Ajuba by β-TrCP. Conversely, the S163A mutant significantly attenuates the ubiquitination level of Ajuba. Overall, our study reveals a novel regulatory mechanism of Ajuba at post-translational level and sheds light on the role of GSK3β-β-TrCP axis in the turnover of Ajuba in CRC.