Activated Raf induces the hyperphosphorylation of stathmin and the reorganization of the microtubule network

Josip Lovrić, Sascha Dammeier, Arnd Kieser, Harald Mischak, Walter Kolch

    Research output: Contribution to journalArticlepeer-review


    Raf kinases are regulators of cellular proliferation, transformation, differentiation, and apoptosis. To identify downstream targets of Raf-1 in vivo, we used NIH 3T3 fibroblasts expressing a Raf-1 kinase domain-estrogen receptor fusion protein (BXB-ER), whose activity can be acutely regulated by estrogen. Proteins differentially phosphorylated 20 min after BXB-ER activation in living cells were displayed by two-dimensional electrophoresis. The protein with the most prominent newly induced phosphorylation was identified as stathmin, a phosphorylation-sensitive regulator of microtubule dynamics. Stathmin is rapidly phosphorylated on two ERK phosphorylation sites (serines 25 and 38) upon BXB-ER activation. The mitogen-activated protein kinase/extracellular signal-regulated kinase-kinase (MEK) inhibitor PI)98059 abolished this phosphorylation, demonstrating that stathmin is targeted by BXB-ER via the MEK/ERK pathway. Prolonged BXB-ER activation resulted in the accumulation of a stathmin phosphoisomer with impaired microtubule- destabilizing activity. The appearance of this phosphoisomer after BXB-ER activation correlated with rearrangements in the microtubule network, resulting in the formation of long bundled microtubules extending toward the rim of the cells. Our results identify stathmin as a main target of the Raf/MEK/ERK kinase cascade in vivo and strongly suggest that ERK-mediated stathmin phosphorylation plays an important role for the microtubule reorganization induced by acute activation of Raf-1.
    Original languageEnglish
    Pages (from-to)22848-22855
    Number of pages7
    JournalJournal of Biological Chemistry
    Issue number35
    Publication statusPublished - 28 Aug 1998


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