Ras regulates SCF-beta-TrCP activity and specificity via its effector NORE1A [Cell Biology]

September 12th, 2014 by Schmidt, M. L., Donninger, H., Clark, G. J.

Ras is the most frequently activated oncogene found in human cancer but its mechanisms of action remain only partially understood. Ras activates multiple signaling pathways in order to promote transformation, however, Ras can also exhibit a potent ability to induce growth arrest and death. NORE1A (RASSF5) is a direct Ras effector that acts as a tumor suppressor by promoting apoptosis and cell cycle arrest. Expression of NORE1A is frequently lost in human tumors and its mechanism of action remains unclear. Here we show that NORE1A forms a direct, Ras regulated complex with β-TrCP, the substrate recognition component of the SCF-β-TrCP ubiquitin ligase complex. This interaction allows Ras to stimulate the ubiquitin ligase activity of SCF-β-TrCP towards its target β-catenin, resulting in degradation of β-catenin by the 26S proteasome. However, the action of Ras/NORE1A/β-TrCP is substrate specific, as IkB, another substrate of SCF-β-TrCP, is not sensitive to NORE1A promoted degradation. Thus we identify a completely new signaling mechanism for Ras that allows for the specific regulation of SCF-β-TrCP targets. We show that the NORE1A levels in a cell may dictate the effects of Ras on the Wnt/β-catenin pathway. Moreover, as NORE1A expression is frequently impaired in tumors, we provide an explanation for the observation that β-TrCP can act as a tumor suppressor or an oncogene in different cell systems.