bThe mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase 5 (ERK5) plays a crucial role in cell proliferation, regulating gene transcription. ERK5 has a unique C-terminal tail which contains a transcriptional activation domain, and activates transcription by phosphorylating transcription factors and acting itself as a transcriptional coactivator. However, the molecular mechanisms that regulate its nucleocytoplasmatic traffic are unknown. We have used tandem affinity purification to identify proteins that interact with ERK5. We show that ERK5 interacts with the Hsp90-Cdc37 chaperone in resting cells, and that inhibition of Hsp90 or Cdc37 results in ERK5 ubiquitylation and proteasomal degradation. Interestingly, activation of cellular ERK5 induces Hsp90 dissociation from the ERK5-Cdc37 complex, leading to ERK5 nuclear translocation and activation of transcription, by a mechanism which requires the autophosphorylation at its C-terminal tail. Consequently, active ERK5 is no longer sensitive to Hsp90 or Cdc37 inhibitors. Cdc37 overexpression also induces Hsp90 dissociation and the nuclear translocation of a kinase-inactive form of ERK5 which retains transcriptional activity. This is the first example showing that ERK5 transcriptional activity does not require kinase activity. Since Cdc37 cooperates with ERK5 to promote cell proliferation, Cdc37 overexpression (as happens in some cancers) might represent a new, noncanonical mechanism by which ERK5 regulates tumor proliferation.
Mitogen-activated protein kinases (MAPKs) are a family of protein serine/threonine (Ser/Thr) kinases that transduce a wide range of extracellular stimuli into intracellular responses, and are activated in response to growth factors and different forms of stress. Phosphorylating a broad range of substrates, MAPKs regulate many cellular functions, including cell proliferation, differentiation, metabolism, and apoptosis (1, 2). In mammals, four subfamilies of conventional MAPKs have been characterized: extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinases (JNK), p38, and ERK5 (3).ERK5 is activated in response to a wide range of growth factors and oxidative and hyperosmotic stresses (4, 5). ERK5 phosphorylates several transcription factors, including the members of the myocyte enhancer factor family, MEF2A, -C, and -D (4-6), and is required for epidermal growth factor (EGF)-induced cell proliferation and progression through the cell cycle (7). ERK5 and its upstream activator, mitogen-activated protein kinase kinase 5 (MEK5), were independently cloned by different groups (8, 9). MEK5 activates ERK5 by dual phosphorylation of the Thr-Glu-Tyr (TEY) motif within the activation loop, and three findings indicate that MEK5 is the only kinase that activates ERK5: (i) MEK5 and ERK5 specifically interact with each other but not with other MAPKs (9); (ii) targeted deletion of the ERK5 and MEK5 genes renders identical phenotypes, with mice dying around embryonic day 10.5, due to angiogenic failure and cardiov...
