Extracellular signal regulated kinase 5 (ERK5) is a novel member of the mitogen-activated protein kinase (MAPK) family with a poorly defined physiological function. Since ERK5 and its upstream activator MEK5 are abundant in skeletal muscle we examined a function of the cascade during muscle differentiation. We show that ERK5 is activated upon induction of differentiation in mouse myoblasts and that selective activation of the pathway results in promoter activation of differentiation-specific genes. Moreover, myogenic differentiation is completely blocked when ERK5 expression is inhibited by antisense RNA. Thus, we conclude that the MEK5/ERK5 MAP kinase cascade is critical for early steps of muscle cell differentiation.

The inhibitor of apoptosis proteins (IAPs) have been shown to interact with a growing number of intracellular proteins and pathways to fulfil their anti-apoptotic role. In the search for novel IAP-interacting proteins we identified the neurotrophin receptor-interacting MAGE homologue (NRAGE) as being able to bind to the avian IAP homologue ITA. This interaction requires the RING domain of ITA. NRAGE additionally coimmunoprecipitates with XIAP. When overexpressed in 32D cells NRAGE augments interleukin-3 withdrawal induced apoptosis, possibly through binding endogenous XIAP. Moreover, NRAGE is able to overcome the anti-apoptotic effect of Bcl-2.

In the search for physiological substrates of MAPKactivated protein (MAPKAP) kinases, we identified the basic helix-loop-helix (bHLH) transcription factor E47as an interaction partner of chromosome 3p kinase (3pK) and MAPKAP-K2 (MK2). The E2A protein E47 is known to be involved in the regulation of tissue-specific gene expression and cell differentiation. E47 is a phosphoprotein, and we identified 3pK and MK2 as E47 kinases in vitro. Furthermore, the expression of either kinase results in a repression of the transcriptional activity of E47 on an E-box containing promoter. In summary, the MAPK-activated protein kinases 3pK and MK2 were identified to form an assembly with the bHLH protein E47 suggesting that these kinases are regulators of E47 activity and E47-dependent gene expression.Chromosome 3p kinase(3pK) 1 (21) and MK2 belong to a family of serine/threonine kinases that are activated by members of the mitogen-activated protein kinase (MAPK) family. 3pK, also known as MAPKAP kinase 3 (1) is unique in that it was shown to be activated by mitogenic inducers such as serum/TPA through the Raf/MEK/ERK cascade as well as by stress-inducing agents that are activators of stress-induced MAPK cascades, thereby leading to the phosphorylation of 3pK by either JNK/SAPK or p38 (2). Thus, the kinase is targeted by the corresponding cascade depending on the extracellular stimulus (2). Although the upstream activation pathways of 3pK are well documented, little is known about its downstream effectors.A close homologue of 3pK, MK2, is involved in stress response mediated through p38. Among the substrates of MK2 are the small heat shock proteins (Hsp27/25) (3, 4) and transcription factors such as CREB (5) and SRF (6).The basic helix-loop-helix (bHLH) transcription factors E12 and E47 are encoded by the E2A gene and are generated by differential splicing of E12-and E47-specific bHLH-encoding exons (7,8). These E-proteins are characterized by their ability to bind to the consensus DNA sequence CANNTG, referred to as E-box, either as homodimers in B-cells or as heterodimers with tissue-specific Class II HLH proteins in other cell types (9, 10). Initially identified in B-cells as immunoglobulin enhancerbinding proteins, the E2A gene products classified as Class I HLH proteins are also involved in cell differentiation, lineage commitment, and the expression of many tissue-specific genes (11). Through phosphorylation, DNA binding and transactivation of a variety of transcription factors are regulated (12). For example, the dimerization of myogenin with E2A products was shown to enhance phosphorylation of myogenin, thereby reducing the transcriptional activity of myogenin, suggesting that phosphorylation of this myocyte enhancer factor (MEF) negatively interferes with muscle-specific gene expression (13). The DNA-binding activity of MyoD homodimers but not MyoD-E12 heterodimers was inhibited by phosphorylation (14). Johnson et al. (16) demonstrated that overexpression of casein kinase II (CKII) increased the transcriptional activit...