Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Microtubule-associated proteins (MAP), such as tau, modulate the extent and rate of microtubule assembly and play an essential role in morphogenetic processes, such as axonal growth. We have examined the mechanism by which tau affects microtubule polymerization by examining the kinetics of microtubule assembly and disassembly through direct observation of microtubules using dark-field microscopy. Tau increases the rate of polymerization, decreases the rate of transit into the shrinking phase (catastrophe), and inhibits the rate of depolymerization. Tau strongly suppresses the catastrophe rate, and its ability to do so is independent of its ability to increase the elongation rate. Thus, tau generates a partially stable but still dynamic state in microtubules. This state is perturbed by phosphorylation by MAP2 kinase, which affects all three activities by lowering the affinity of tau for the microtubule lattice.

SUMMARY Small cell lung carcinoma (SCLC) is a high-grade pulmonary neuroendocrine tumor. The transcription factors ASCL1 and NEUROD1 play crucial roles in promoting malignant behavior and survival of human SCLC cell lines. We find ASCL1 and NEUROD1 identify heterogeneity in SCLC, bind distinct genomic loci, and regulate mostly distinct genes. ASCL1 but not NEUROD1 is present in mouse pulmonary neuroendocrine cells and only ASCL1 is required in vivo for tumor formation in mouse models of SCLC. ASCL1 targets oncogenic genes including MYCL1, RET, SOX2, and NFIB, while NEUROD1 targets MYC. ASCL1 and NEUROD1 regulate different genes that commonly contribute to neuronal function. ASCL1 also regulates multiple genes in the NOTCH pathway including DLL3. Together, ASCL1 and NEUROD1 distinguish heterogeneity in SCLC with distinct genomic landscapes and distinct gene expression programs.

The structure of the MAP kinase ERK2, a ubiquitous protein kinase target for regulation by Ras and Raf, has been solved in its unphosphorylated low-activity conformation to a resolution of 2.3 A. The two domains of unphosphorylated ERK2 are farther apart than in the active conformation of cAMP-dependent protein kinase and the peptide-binding site is blocked by tyrosine 185, one of the two residues that are phosphorylated in the active enzyme. Activation of ERK2 is thus likely to involve both global and local conformational changes.

Induction of the human c‐fos proto‐oncogene by mitogens depends on the formation of a ternary complex by p62TCF with the serum response factor (SRF) and the serum response element (SRE). We demonstrate that Elk‐1, a protein closely related to p62TCF in function, is a nuclear target of two members of the MAP kinase family, ERK1 and ERK2. Phosphorylation of Elk‐1 increases the yield of ternary complex in vitro. At least five residues in the C‐terminal domain of Elk‐1 are phosphorylated upon growth factor stimulation of NIH3T3 cells. These residues are also phosphorylated by purified ERK1 in vitro, as determined by a combination of phosphopeptide sequencing and 2‐D peptide mapping. Conversion of two of these phospho‐acceptor sites to alanine impairs the formation of ternary complexes by the resulting Elk‐1 proteins. Removal of these serine residues also drastically diminishes activation of the c‐fos promoter in epidermal growth factor‐treated cells. Analogous mutations at other sites impair activation to a lesser extent without affecting ternary complex formation in vitro. Our results indicate that phosphorylation regulates ternary complex formation by Elk‐1, which is a prerequisite for the manifestation of its transactivation potential at the c‐fos SRE.

The unique kinase-inhibitor interactions observed in these complexes originate from amino-acid replacements in the active site and replacements distant from the active site that affect the size of the domain interface. This structural information should facilitate the design of better MAP-kinase inhibitors for the treatment of inflammation and other diseases.

The adverse effects of lipopolysaccharide (LPS) are mediated primarily by tumor necrosis factor alpha (TNF-␣). TNF-␣ production by LPS-stimulated macrophages is regulated at the levels of both transcription and translation. It has previously been shown that several mitogen-activated protein kinases (MAPKs) are activated in response to LPS. We set out to determine which MAPK signaling pathways are activated in our system and which MAPK pathways are required for TNF-␣ gene transcription or TNF-␣ mRNA translation. We confirm activation of the MAPK family members extracellular-signal-regulated kinases 1 and 2 (ERK1 and ERK2), p38, and Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), as well as activation of the immediate upstream MAPK activators MAPK/ERK kinases 1 and 4 (MEK1 and MEK4). We demonstrate that LPS also activates MEK2, MEK3, and MEK6. Furthermore, we demonstrate that dexamethasone, which inhibits the production of cytokines, including TNF-␣, significantly inhibits LPS induction of JNK/SAPK activity but not that of p38, ERK1 and ERK2, or MEK3, MEK4, or MEK6. Dexamethasone also blocks the sorbitol but not anisomycin stimulation of JNK/SAPK activity. A kinase-defective mutant of SAPK␤, SAPK␤ K-A, blocked translation of TNF-␣, as determined by using a TNF-␣ translational reporting system. Finally, overexpression of wild-type SAPK␤ was able to overcome the dexamethasone-induced block of TNF-␣ translation. These data confirm that three MAPK family members and their upstream activators are stimulated by LPS and demonstrate that JNK/SAPK is required for LPS-induced translation of TNF-␣ mRNA. A novel mechanism by which dexamethasone inhibits translation of TNF-␣ is also revealed.

c-Jun transcriptional activity is stimulated by phosphorylation at two N-terminal sites: Ser-63 and -73. Phosphorylation of these sites is enhanced in response to a variety of extracellular stimuli, including growth factors, cytokines, and UV irradiation. New members of the mitogen-activated protein (MAP) kinase group of signal-transducing enzymes, termed JNKs, bind to the activation domain of c-Jun and specifically phosphorylate these sites. However, the N-terminal sites of c-Jun were also suggested to be phosphorylated by two other MAP kinases, ERK1 and ERK2. Despite these reports, we find that unlike the JNKs, ERK1 and ERK2 do not phosphorylate the N-terminal sites of c-Jun in vitro; instead they phosphorylate an inhibitory C-terminal site. Furthermore, the phosphorylation of c-Jun in vivo at the N-terminal sites correlates with activation of the JNKs but not the ERKs. The ERKs are probably involved in the induction of c-fos expression and thereby contribute to the stimulation of AP-1 activity. Our study suggests that two different branches of the MAP kinase group are involved in the stimulation of AP-1 activity through two different mechanisms.