KSR1 is required for cell cycle reinitiation following DNA damage.

KSR1 (kinase suppressor of Ras 1) is a molecular scaffold and positive regulator of the Raf/MEK/ERK phosphorylation cascade. KSR1 is required for maximal ERK activation induced by growth factors and by some cytotoxic agents. We show here that KSR1 is also required for maximal ERK activation induced by UV light, ionizing radiation, or the DNA interstrand cross-linking agent mitomycin C (MMC). We further demonstrate a role for KSR1 in the reinitiation of the cell cycle and proliferation following cell cycle arrest induced by MMC. Cells lacking KSR1 underwent but did not recover from MMC-induced G(2)/M arrest. Expression of KSR1 allowed KSR1(-/-) cells to re-enter the cell cycle following MMC treatment. However, cells expressing a mutated form of KSR1 unable to bind ERK did not recover from MMC-induced cell cycle arrest, demonstrating the requirement for the KSR1-ERK interaction. In addition, constitutive activation of ERK was not sufficient to promote cell cycle reinitiation in MMC-treated KSR1(-/-) cells. Only cells expressing KSR1 recovered from MMC-induced cell cycle arrest. Importantly, MMC-induced DNA damage was repaired in KSR1(-/-) cells, as determined by resolution of gamma-H2AX-containing foci. These data indicate that cell cycle reinitiation is not actively signaled in the absence of KSR1, even when DNA damage has been resolved. These data reveal a specific role for the molecular scaffold KSR1 and KSR1-mediated ERK signaling in the cellular response to DNA interstrand cross-links.