The Fbw7 subunit of a Skp1–Cullin–F-box (SCF) type E3 ligase complex recognizes substrates that are subsequently ubiquitylated and degraded by the proteasome. Several substrates of this SCF complex are oncoproteins, and decreased abundance of Fbw7 is seen in various cancers. Min et al. found that the prolyl isomerase Pin1 acts on a phosphorylated residue in Fbw7, thus disrupting Fbw7 dimerization and causing Fbw7 to ubiquitylate itself. The abundance of Fbw7 was inversely correlated to that of Pin1 in colon cancer samples, and the abundance of Fbw7 was higher in Pin1–/– mouse embryonic fibroblasts (MEFs) than in wild-type MEFs. The abundance of ectopically expressed Fbw7 was increased by the proteasome inhibitor MG132, and this stabilization by MG132 was not seen with a form of Fbw7 lacking the F-box motif, which is necessary for ubiquitylation, suggesting that Fbw7 may auto-ubiquitylate. Fbw7 bound to the WW domain of Pin1, an interaction that required the phosphorylation of Thr205 in Fbw7. The half-life of Fbw7 was higher in Pin1–/– MEFs compared with wild-type MEFs, an effect that was attenuated by reconstitution with wild-type Pin1 but not with a double point mutant of Pin1 that cannot bind substrates and that is catalytically inactive (W34A/K63A). Thr205 is located near the D domain, which is required for dimerization of Fbw7, and dimerization of Fbw7 was increased in cells in which Pin1 was knocked down, an effect attenuated by expression of wild-type Pin1 but not W34A or K63A mutants. In contrast, the dimerization of an Fbw7 mutant (T205A) unable to bind to Pin1 was not altered by Pin1 depletion. The half-life of a mutant lacking the D domain was decreased compared with that of wild-type Fbw7, suggesting that Fbw7 is less stable when not dimerized. In vitro assays indicated that purified Pin1 increased self-ubiquitylation of Fbw7 and, in cells, overexpression of Pin1 decreased the ability of wild-type Fbw7, but not the T205A mutant, to bind to and ubiquitylate a substrate. Cells overexpressing Fbw7 or depleted of Pin1 formed fewer foci and colonies in soft agar, whereas cells expressing wild-type Pin1 (but not the W34/K63A mutant) formed more foci. Although these results indicate that Pin1 activity decreases the abundance of Fbw7 by disrupting the dimerization of Fbw7, the authors note that cancer-associated mutations of Fbw7 do not affect the F-box motif or D domain. However, forms of Fbw7 bearing one of three cancer-associated mutations into cells showed increased ubiquitylation when transfected into cells. Increased activity of Pin1, which is observed in several cancers, may have an additive effect in decreasing the abundance of Fbw7.
S.-H. Min, A. W. Lau, T. H. Lee, H. Inuzuka, S. Wei, P. Huang, S. Shaik, D. Y. Lee, G. Finn, M. Balastik, C.-H. Chen, M. Luo, A. E. Tron, J. A. DeCaprio, X. Z. Zhou, W. Wei, K. P. Lu, Negative regulation of the stability and tumor suppressor function of Fbw7 by the Pin1 prolyl isomerase. Mol. Cell 46, 771–783 (2012). [PubMed]