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.

Receptor-bound growth factors elicit intracellular signals that lead to the phosphorylation and activation of numerous intracellular kinases and transcription factors with consequent changes in patterns of gene expression. Several oncogene products are able to mimic these signals, resulting in cell transformation and proliferation. For example, the introduction of oncogenic forms of Raf-1 kinase into fibroblasts induces transformation and leads to the constitutive expression of, among others, the c-fos proto-oncogene. Here it is shown that the elevation of c-fos promoter activity brought about by v-raf is mediated by TCF/Elk-1, which forms a ternary complex with SRF at the serum response element and is a substrate for mitogen-activating protein kinases in vitro. In NIH 3T3 fibroblasts, v-raf activates Erk2, and overexpression of an interfering mutant of Erk2 both blocks the ability of v-raf to activate the c-fos promoter and suppresses transformation. Mutation of individual mitogen-activating protein kinase phosphoacceptor sites in TCF/Elk-1 also compromises v-raf-activated expression of a Gal-Elk/Gal-chloramphenicol acetyltransferase reporter system. However, in at least one instance the introduction of glutamate, but not aspartate, at a phosphoacceptor site is compatible with activation. These results provide compelling evidence that phosphorylation of TCF/Elk-1 by Erk2 is a major link in the Raf-1 kinase-dependent signal transduction pathway that activates c-fos expression.

Because the catalytic domain of dual leucine zipperbearing kinase (DLK) bears sequence similarity to members of the mitogen-activated protein (MAP) kinase kinase kinase subfamily, this protein kinase was investigated for its ability to activate MAP kinase pathways. While work in mammalian systems established the importance of the ERK pathway in signal transduction from RTKs, it has become clear from studies in yeast that multiple mammalian MAPK pathways exist in parallel (50). Using both genetic and biochemical approaches in mammalian cells, the components of several additional MAPK pathways have now been identified. Best characterized is the stress-activated protein kinase (SAPK) pathway. This pathway is thought to lead from the activated Rho subfamily small GTPases Rac1 and Cdc42Hs, to activation of the MAP kinase kinase kinase kinase, p65 PAK , to the MAP kinase kinase kinase, MEKK1, to the dual specificity MAP kinase kinase, MKK4/SEK1, and, finally, to activation of the MAP kinases p46/p54 SAPK . SAPKs were discovered as the principal c-Jun NH 2 -terminal phosphorylating kinases and therefore have also been termed JNKs (6). Distinct from the ERK cascade, the SAPK pathway is predominantly activated by stress-inducing signals such as heat shock, ultraviolet irradiation, anisomycin, proinflammatory cytokines (tumor necrosis factor ␣ and interleukin 1␤), and hyperosmolarity (7). Although G-protein-coupled receptors can signal through pathways leading to the activation of SAPK (8), the upstream signaling events by which the SAPK pathway becomes activated are largely unmapped.The mixed lineage kinase or MLK subfamily of protein kinases is a recently described subfamily of protein kinases that share two common structural features (9, 10). First, each has a distinctive kinase catalytic domain whose primary structure is hybrid between those found in serine/threonine and tyrosine protein kinases. Second, closely juxtaposed COOH-terminal to the catalytic domain, each MLK protein has a domain that is predicted to form two leucine/isoleucine zippers separated by a short spacer region. Additionally, each has both NH 2 -and COOH-terminal motifs suggestive of protein-protein interaction domains. Despite the hybrid structure of the catalytic domains, two members of the family (DLK and MLK3/SPRK) have been shown to exhibit serine/threonine-specific kinase autocatalytic activity in vitro (10 -12).

The rapid and transient induction of the human proto-oncogene c-fos in response to a variety of stimuli depends on the serum response element (SRE). In vivo footprinting experiments show that this promoter element is bound by a multicomponent complex including the serum response factor (SRF) and a ternary complex factor such as Elk-1. SRF is thought to recruit a ternary complex factor monomer into an asymmetric complex. In this report, we describe a quaternary complex over the SRE which, in addition to an SRF dimer, contains two Elk-1 molecules. Its formation at the SRE is strictly dependent on phosphorylation of S-383 in the Elk-1 regulatory domain and appears to involve a weak intermolecular association between the two Elk-1 molecules. The influence of mutations in Elk-1 on quaternary complex formation in vitro correlates with their effect on the induction of c-fos reporter expression in response to mitogenic stimuli in vivo.Both viral and cellular Fos proteins have been implicated in the regulation of genes involved in tumor invasiveness (15,21). These findings have been corroborated by the observation that skin tumors do not progress efficiently in mice lacking Fos (40). The functional nature of Fos as a transcription factor suggests that its constitutive presence in cells leads to the deregulated expression of its target genes. In resting cells, the cellular c-fos gene is transcriptionally inactive but can be induced rapidly and transiently by a large number of extracellular stimuli. Critical for this response is the serum response element (SRE). Several proteins have been shown to interact with the SRE in vitro (reviewed in reference 48). However, the DNA footprint observed on the central and left part of the SRE in vivo is most closely reproduced in vitro by the binding of the serum response factor (SRF) and a ternary complex factor (TCF) (16,44). Several TCFs, i.e., Elk-1, SAP1, and ERP/NET/SAP2, which belong to the ets family of transcription factors, have been cloned (4,11,20,27,38). While SRF binds as a dimer to the central CArG box of the SRE, TCFs have been detected only as monomers in this nucleoprotein complex. Moreover, TCFs are unable to bind to the SRE directly, presumably because their recruitment involves limited DNA contacts and extensive interactions with SRF (35,43).Numerous agents that stimulate c-fos transcription act through a conserved signalling cascade that involves sequential activation of the small G-protein Ras, Raf-1 kinase, the mitogen-activated protein (MAP) kinase activators MEK/MKK, and the MAP kinases ERK1 and ERK2 (25, 36). The latter phosphorylate and activate the TCF Elk-1 (8,9,22,26,31). Transcription of the c-fos gene requires the interaction of TCFs with SRF at the SRE. Mutations in the SRE that prevent TCF binding but leave the SRF binding site intact have been reported to prevent c-fos induction in response to several mitogens (12, 44). Recently, we and others have also demonstrated that Elk-1 phosphorylation by members of the stressactivated protein kinase subfamily...

The stromal compartment of the bone marrow is composed of various cell types that provide trophic and instructive signals for hematopoiesis. The mesenchymal stem cell is believed to give rise to all major cellular components of the bone marrow microenvironment. Nemo‐like kinase, Nlk, is a serine‐threonine kinase that connects MAP kinase and Wnt signaling pathways; its in vivo function in mouse is unknown. We have generated mice with a targeted disruption of Nlk and find that the complex phenotype significantly varies with the genetic background. Whereas C57BL/6 mice lacking Nlk die during the third trimester of pregnancy, the 129/Sv background supports survival into adolescence; such mice are growth retarded and suffer from various neurological abnormalities. We show here that the Nlk deficiency syndrome includes aberrant differentiation of bone marrow stromal cells. Varying degrees of morphological abnormality, such as increased numbers of adipocytes, large blood sinuses and absence of bone‐lining cells are observed in the bone marrow of mutant mice. Nlk deficient mice thus provide a novel model to study the genetic requirements for bone marrow stromal differentiation.