To elucidate the physiological significance of MEK5 in vivo, we have examined the effect of mek5 gene elimination in mice. Heterozygous mice appear to be healthy and were fertile. However, mek5 ؊/؊ embryos die at approximately embryonic day 10.5 (E10.5). The phenotype of the mek5 ؊/؊ embryos includes abnormal cardiac development as well as a marked decrease in proliferation and an increase in apoptosis in the heart, head, and dorsal regions of the mutant embryos. The absence of MEK5 does not affect cell cycle progression but sensitizes mouse embryonic fibroblasts (MEFs) to the ability of sorbitol to enhance caspase 3 activity. Further studies with mek5 ؊/؊ MEFs indicate that MEK5 is required for mediating extracellular signalregulated kinase 5 (ERK5) activation and for the regulation of the transcriptional activity of myocyte enhancer factor 2. Overall, this is the first study to rigorously establish the role of MEK5 in vivo as an activator of ERK5 and as an essential regulator of cell survival that is required for normal embryonic development.The mitogen-activated protein kinase (MAPK) cascades constitute a complex network of signaling pathways that are involved in the regulation of numerous cell functions (9). They consist of the sequential activation of protein kinases that include MAPKs, MAPK/extracellular signal-regulated kinase (ERK) kinases (MEKs or MKKs), and MEK kinases (MEKKs) (9). MAPKs are activated by dual phosphorylation on threonine (T) and tyrosine (Y) residues within a T-X-Y motif by MEKs. MEKs are activated by MEKKs. Two main mechanisms have been proposed to ensure specific transmission of the signals from upstream kinases to MAPKs (38, 40): (i) scaffold proteins that assemble the different components of a cascade; (ii) physical interactions between the components of a cascade. Both mechanisms may operate in parallel and allow different responses of the same MAPK signaling pathways to different stimuli.At least four MAPK subfamilies have been identified: ERK1/2, ERK5, c-Jun NH 2 -terminal protein kinases (JNKs), and p38 MAPKs. MAPK activators include MEK1 and MEK2 for ERK1/2, MEK5 for ERK5, MKK4 and MKK7 for JNKs, and MKK3 and MKK6 for p38 MAPKs (9). Targeted deletion of the mapk and mek/mkk genes has contributed substantially to our increased understanding of the physiological role of these pathways in development and pathogenesis. In particular, the recent elimination of the erk5 gene in mice has provided genetic evidence that ERK5 is required for normal cardiac development (31,34,42). ERK5, also known as big MAPK, is almost twice the size (815 amino acids) of the other MAPKs (45). Its unique COOH-terminal tail contains a myocyte enhancer factor 2 (MEF2)-interacting domain and a potent transcriptional activation domain (12). The ERK5 catalytic NH 2 -terminal domain is 50% identical to ERK2. The activity of a number of transcription factors has been shown to be regulated by ERK5, including MEF2, c-Fos and Fra-1, Sap1, c-Myc, and NF-B (6,11,13,15,28,37). In vitro, the ERK5 signaling pathway has...