E2F5 and LEK1 translocation to the nucleus is an early event demarcating myoblast quiescence

Raf/MEK/ERK signaling in skeletal muscle cells affects several aspects of myogenesis that are correlated with the duration and intensity of the input signal. 23A2RafER^DD myoblasts directing elevated levels of Raf kinase for 24 h are mitotically inactive. Removal of the stimulus results in cell cycle re-entry and proliferation. Using a proteomic approach, E2F5 and LEK1 were detected in the nuclei of Raf-arrested myoblasts. Disruption of MEK1 activity prevents phosphorylation of ERK1/2 and nuclear translocation of E2F5 and LEK1. The pocket proteins, p107 and p130, remain in the cytoplasm of growth arrested myoblasts irrespective of Raf/ERK activation while pRb translocates to the nucleus. Importantly, both E2F5 and LEK1 are found in the nuclei of non-dividing satellite cells and myonuclei in vivo and in vitro. Our results indicate that Raf-arrested myoblasts may serve as a model system for satellite cell cycle studies and that E2F5 and LEK1 translocation to the nucleus is an important first step during entry into quiescence.