Distinct classes of neurons are generated from progenitor cells distributed in characteristic dorsoventral patterns in the developing spinal neural tube. We define restricted neural progenitor populations by the discrete, nonoverlapping expression of Ngn1, Math1, and Mash1. Crossinhibition between these bHLH factors is demonstrated and provides a mechanism for the generation of discrete bHLH expression domains. This precise control of bHLH factor expression is essential for proper neural development since as demonstrated in both loss- and gain-of-function experiments, expression of Math1 or Ngn1 in dorsal progenitor cells determines whether LH2A/B- or dorsal Lim1/2-expressing interneurons will develop. Together, the data suggest that although Math1 and Ngn1 appear to be redundant with respect to neurogenesis, they have distinct functions in specifying neuronal subtype in the dorsal neural tube.

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An ERK2-binding site at the N terminus of MEK1 was reported to mediate their stable association. We examined the importance of this binding site in the feedback phosphorylation of MEK1 on Thr 292 and Thr 386 by ERK2, the phosphorylation and activation of ERK2 by MEK1, and the interaction of MEK1 with ERK2 and Raf-1. Deletion of the binding site from MEK1 reduced its phosphorylation by ERK2, but had no effect on its phosphorylation by p21-activated protein kinase-1 (PAK1). A MEK1 N-terminal peptide containing the binding site inhibited MEK1 phosphorylation by ERK2. However, it did not affect MEK1 phosphorylation by p21-activated protein kinase or myelin basic protein phosphorylation by ERK2. Deletion of the N-terminal ERK-binding domain of MEK1 also reduced its ability to phosphorylate ERK2 in vitro, to co-immunoprecipitate with ERK2, and to stimulate ERK2 activation in transfected cells, but it did not alter the association with endogenous Raf-1. Using ERK2-p38 chimeras and an ERK2 deletion mutant, a MEK1-binding site of ERK2 was localized to its N terminus.The MAP 1 kinases ERK1 and ERK2 are activated by a protein kinase cascade under the control of the small G protein Ras. Ras binds to Raf with high affinity and causes Raf translocation to the membrane, where it is activated by mechanisms including phosphorylation of serine and tyrosine residues and regulated interactions with other proteins (1). Raf activates MEK1 and MEK2, the dual-specificity protein kinases that directly phosphorylate and activate ERK1 and ERK2 (2, 3). Reconstitution experiments with purified proteins in vitro demonstrated that these MEK proteins are sufficient to activate ERK proteins (4). Mansour et al. (5) and others identified mutations that created constitutively active forms of MEK1 and MEK2. When introduced into mammalian cells, these activated MEK mutants increase ERK activity and induce effects associated with activation of the Ras/ERK pathway such as transformation of fibroblasts, development of tumors in nude mice, and outgrowth of neurites in PC12 cells (5-7).The multienzyme ERK cascade allows for amplification because MEK1 and MEK2 are present in considerable excess of Raf. However, the MEK and ERK proteins from this pathway are present at roughly equal concentrations and, in some cells, MEK proteins are in excess (8). This suggests that the MEK to ERK step exists not for amplification, but for kinetic regulation and to receive additional modulatory inputs (8 -10). The intermediate step introduces mechanisms for sensing coincident and distinct regulatory inputs (11).In the budding yeast pheromone response pathway, the MAP kinase cascade is organized on a scaffold Ste5p (12,13). No functional homolog of Ste5p has yet been identified for the mammalian ERK pathway, although the recently discovered protein MP-1 has been reported to enhance interactions between MEK1 and ERK1 and thereby increase signaling through the pathway (10). Specialized domains within the kinases themselves may also facilitate productive interactions among ...

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