c-Fos is regulated by phosphorylation and multiple turnover mechanisms. We found that c-Fos was ubiquitylated in the cytoplasm during IL-6/gp130 stimulation under MEK inhibition and sought the mechanisms involved in the regulation. We show that sustained ERK5 activity and the E3 ligase UBR1 regulate the stability and subcellular localization of c-Fos. UBR1, rapidly induced by STAT3, interacts with and ubiquitylates c-Fos in the cytoplasm for its accelerated degradation. ERK5 inhibits the nuclear export of c-Fos by phosphorylating Thr232 in the c-Fos NES(221-233) and disrupts the interaction of c-Fos with UBR1 by phosphorylating Ser32. Moreover, UBR1 depletion in HeLa cells, which constitutively express UBR1 at a high level, enhances both c-Fos expression and cell growth, whereas ERK5 depletion reduces both of them. Interestingly, an NES mutant of c-Fos, but not wild-type, substitutes ERK5 activity for HeLa cell proliferation. Thus, this spatiotemporal regulation of c-Fos by ERK5 and UBR1 is critical for the regulation of c-Fos/AP-1.
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