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Activation of the Raf kinase by GTP-bound Ras is a poorly understood step in receptor tyrosine kinase signaling pathways. One such pathway, the epidermal growth factor receptor (EGFR) pathway, is critical for cell differentiation, survival, and cell cycle regulation in many systems, including the Drosophila eye. We have identified a mutation in a novel gene, aveugle, based on its requirement for normal photoreceptor differentiation. The phenotypes of aveugle mutant cells in the eye and wing imaginal discs resemble those caused by reduction of EGFR pathway function. We show that aveugle is required between ras and raf for EGFR signaling in the eye and for mitogen-activated protein kinase phosphorylation in cell culture. aveugle encodes a small protein with a sterile alpha motif (SAM) domain that can physically interact with the scaffold protein connector enhancer of Ksr (Cnk). We propose that Aveugle acts together with Cnk to promote Raf activation, perhaps by recruiting an activating kinase.

The Wingless signaling pathway directs many developmental processes in Drosophila by regulating the expression of specific downstream target genes. We report here that the product of the trithorax group gene osa is required to repress such genes in the absence of the Wingless signal. The Wingless-regulated genes nubbin, Distal-less, and decapentaplegic and a minimal enhancer from the Ultrabithorax gene are misexpressed in osa mutants and repressed by ectopic Osa. Osa-mediated repression occurs downstream of the up-regulation of Armadillo but is sensitive both to the relative levels of activating Armadillo/Pangolin and repressing Groucho/Pangolin complexes present and to the responsiveness of the promoter to Wingless. Osa functions as a component of the Brahma chromatin-remodeling complex; other components of this complex are likewise required to repress Wingless target genes. These results suggest that altering the conformation of chromatin is an important mechanism by which Wingless signaling activates gene expression.

Dorsal closure in the Drosophila embryo occurs during the later stages of embryogenesis and involves changes in cell shape leading to the juxtaposition and subsequent adherence of the lateral epidermal primordia over the amnioserosa. Dorsal closure requires the activation of a conserved c-jun amino-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) module, as it is blocked by null mutations in JNK kinase [hemipterous (hep)] and JNK [basket (bsk)]. Drosophila JNK (DJNK) functions by phosphorylating and activating DJun, which in turn induces the transcription of decapentaplegic (dpp). We provide biochemical and genetic evidence that a Ste20-related kinase, misshapen (msn), functions upstream of hep and bsk to stimulate dorsal closure in the Drosophila embryo. Mammalian (NCK-interacting kinase [NIK]) and Caenorhabditis elegans (mig-15) homologs of msn have been identified; mig-15 is necessary for several developmental processes in C. elegans. These data suggest that msn, mig-15, and NIK are components of a signaling pathway that is conserved among flies, worms, and mammals to control developmentally regulated pathways.

In Drosophila, pattern formation at multiple stages of embryonic and imaginal development depends on the same intercellular signaling pathways. We have identified a novel gene, eyelid (eld), which is required for embryonic segmentation, development of the notum and wing margin, and photoreceptor differentiation. In these tissues, eld mutations have effects opposite to those caused by wingless (wg) mutations. eld encodes a widely expressed nuclear protein with a region homologous to a novel family of DNA-binding domains. Based on this homology and on the phenotypic analysis, we suggest that Eld could act as a transcription factor antagonistic to the Wg pathway.

The yeast MCM2, MCM3, and MCM5/CDC46 genes are required for DNA replication and have been proposed to act as factors that license the DNA for one and only one round of replication per cell cycle. We have identified a Drosophila gene, DmMCM2, that is highly homologous to MCM2. A P-element insertion into this gene, which prevents its transcription, inhibits proliferation of cells in the imaginal discs and central nervous system (CNS) and causes an apparent prolongation of S phase in the embryonic and larval CNS. DmMCM2 is expressed in the embryo in a pattern corresponding to that of S-phase cells. These results suggest that DmMCM2 plays a role in the regulation of DNA replication analogous to that of its yeast counterpart.