Decapentaplegic (dpp), a TGF beta-related ligand, plays a key role in Drosophila development. Although dpp receptors have been isolated, the downstream components of the signaling pathway remain to be identified. We have cloned the schnurri (shn) gene and show that it encodes a putative zinc finger transcription factor homologous to the human major histocompatibility complex-binding proteins 1 and 2. Mutations in shn affect multiple events that require dpp signaling as well as the transcription of dpp-responsive genes. Genetic interactions and the strikingly similar phenotypes of mutations in shn and the dpp receptors encoded by thick veins and punt suggest that shn plays a downstream role in dpp signaling.

The vitamin K-dependent ␥-carboxylation of glutamate to ␥-carboxyglutamate was originally well characterized in the mammalian blood clotting cascade. ␥-Carboxyglutamate has also been found in a number of other mammalian proteins and in neuropeptides from the venoms of marine snails belonging to the genus Conus, suggesting wider prevalence of ␥-carboxylation. We demonstrate that an open reading frame from a Drosophila melanogaster cDNA clone encodes a protein with vitamin K-dependent ␥-carboxylase activity. The open reading frame, 670 amino acids in length, is truncated at the C-terminal end compared with mammalian ␥-carboxylase, which is 758 amino acids. The mammalian gene has 14 introns; in Drosophila there are two much shorter introns but in positions precisely homologous to two of the mammalian introns. In addition, a deletion of 6 nucleotides is observed when cDNA and genomic sequences are compared. In situ hybridization to fixed embryos indicated ubiquitous presence of carboxylase mRNA throughout embryogenesis. Northern blot analysis revealed increased mRNA levels in 12-24-h embryos. The continued presence of carboxylase mRNA suggests that it plays an important role during embryogenesis. Although the model substrate FLEEL is carboxylated by the enzyme, a substrate containing the propeptide of a Conus carboxylase substrate, conantokin G, is poorly carboxylated. Its occurrence in vertebrates, molluscan systems (i.e. Conus), and Drosophila and the apparently strong homology between the three systems suggest that this is a highly conserved and widely distributed posttranslational modification in biological systems.The functions of proteins are coordinated physiologically by post-translational modification. For example, phosphorylationdephosphorylation cascades integrate the biochemistry of individual proteins into cellular physiology. In addition to posttranslational modifications that occur primarily within cells, post-translational modifications also occur on extracellular proteins. The most familiar of these are N-glycosylation of asparagine residues and O-glycosylation of serine and threonine residues.One of the most distinctive of the extracellular post-translational modifications is the vitamin K-dependent ␥-carboxylation of glutamate residues to give ␥-carboxyglutamate (1). When it was first characterized, ␥-carboxylation was thought to be a biochemical specialization of the mammalian blood-clotting cascade. However, several bone proteins (2, 3) as well as an extracellular ligand, gas6 (4), were subsequently identified as having the post-translational modification, although in the latter cases the precise mechanistic role of ␥-carboxylation for proper protein function has not been established definitively. In addition, two novel proline-rich ␥-carboxyglutamic acid-containing proteins, PRGP1 and PRGP2, of unknown function have been identified (5).Long after its characterization in blood-clotting factors, vitamin K-dependent ␥-carboxylation of glutamate residues was discovered in a phylogenetically distant...

We employed robotic methods and the whole-genome sequence of Drosophila melanogaster to facilitate a large-scale expression screen for spatially restricted transcripts in Drosophila embryos. In this screen, we identified a pair of genes, scylla (scyl) and charybde (chrb), that code for dorsal transcripts in early Drosophila embryos and are homologous to the human apoptotic gene RTP801. In Drosophila, both gene products are transcriptionally regulated targets of Dpp/Zen-mediated signal transduction and appear more generally to be downstream targets of homeobox regulation. Gene disruption studies revealed the functional redundancy of scyl and chrb, as well as their requirement for embryonic head involution. From the perspective of functional genomics, our studies demonstrate that global surveys of gene expression can complement traditional genetic screening methods for the identification of genes essential for development: beginning from their spatio-temporal expression profiles and extending to their downstream placement relative to dpp and zen, our studies reveal roles for the scyl and chrb gene products as links between patterning and cell death.

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