Search

Signaling by TGF beta-related factors requires ligand-induced association between type I and type II transmembrane serine/threonine kinases. In Drosophila, the saxophone (sax) and thick veins (tkv) genes encode type I receptors that mediate signaling by decapentaplegic (dpp), a member of the bone morphogenetic protein (BMP) subgroup of TGF beta-type factors. In this report, we demonstrate that the Drosophila punt gene encodes atr-II, a previously described type II receptor that on its own is able to bind activin but not BMP2, a vertebrate ortholog of dpp. Mutations in punt produce phenotypes similar to those exhibited by tkv, sax, and dpp mutants. Furthermore, punt will bind BMP2 in concert with tkv or sax, forming complexes with these receptors. We suggest that punt functions as a type II receptors for dpp and propose that BMP signaling in vertebrates may also involve sharing of type II receptors by diverse ligands.

Drosophila sequences at chromosomal positions 25D (Brk25D) and 43E (Brk43E) are similar to the TGF beta type I receptor serine/threonine kinases and are expressed broadly during embryogenesis. Brk25D binds dpp protein and bone morphogenetic protein 2 with high affinity. Mutations affecting Brk25D map to the gene thick veins and block the expression of two decapentaplegic-responsive (dpp-responsive) genes, dpp and labial, in the embryonic midgut. Defects in Brk25D receptor function combined with reduced expression of dpp ligand produce mutant phenotypes in the embryo and adult. Brk43E is the product of the gene saxophone, which also interacts with dpp. We conclude that dpp signaling in vivo is mediated by at least two receptors, Brk25D and Brk43E.

Drosophila fasciclin I is a homophilic cell adhesion molecule expressed in the developing embryo on the surface of a subset of fasciculating CNS axons, all PNS axons, and some nonneuronal cells. We have identified protein-null mutations in the fasciclin I (fas I) gene, and show that these mutants are viable and do not display gross defects in nervous system morphogenesis. The Drosophila Abelson (abl) proto-oncogene homolog encodes a cytoplasmic tyrosine kinase that is expressed during embryogenesis primarily in developing CNS axons; abl mutants show no gross defects in CNS morphogenesis. However, embryos doubly mutant for fas I and abl display major defects in CNS axon pathways, particularly in the commissural tracts where expression of these two proteins normally overlaps. The double mutant shows a clear defect in growth cone guidance; for example, the RP1 growth cone (normally fas I positive) does not follow its normal path across the commissure.

The Abelson gene in Drosophila (abl) consists of ten exons extending over 26 kb of genomic DNA. The DNA sequence encodes a protein of 1520 amino acids with sequence homology to the human c-abl proto-oncogene product, beginning at the amino terminus and extending 656 amino acids through the region essential for tyrosine kinase activity. Mutant lesions in the abl gene were identified first by their failure to complement chromosomal deletions that overlap the abl DNA sequence and then by rescue of the mutant phenotypes with an abl minigene in transgenic flies. Elimination of abl zygotic function by mutations produces some recessive lethality at the pharate adult pupal stage, and mutant adults with reduced longevity, reduced fecundity, and an irregular pattern of retinal cells.

The decapentaplegic gene complex (2-4.0) in Drosophila melanogaster is defined by a series of allelic mutations affecting imaginal disk development. Decapentaplegic (dpp) mutant individuals exhibit a variety of pattern deficiencies and duplications in structures derived from one or more of the 15 major imaginal disks. Based on dpp mutant phenotypes, we suggest that the dpp gene complex is involved in the elaboration of positional information within developing epidermal tissue. The dpp mutations are recessive and fall into six phenotypic classes. Milder alleles (classes I and II) affect only one or a few disks while most alleles (classes III, IV, V and EL) affect all major imaginal disks. Class EL homozygotes are embryonic lethals; development is arrested before germ-band shortening late in gastrulation. Presently inseparable from EL, is a haplo-insufficient function (Hin-d) associated with the distal (left) end of the dpp gene complex. The dpp gene complex occupies most or all of 22F1--3, three densely staining polytene chromosome bands. A colinearity exists between map positions of the four identified functional units within the complex and the severities of mutant phenotypes caused by disruption of these functions. Most dpp mutations are gross chromosomal rearrangements; they exert polar effects on the decapentaplegic functions that are proximal to the rearrangement breakpoints in 22F. Many structural similarities exist between the decapentaplegic and bithorax gene complexes.

During Drosophila embryogenesis, the Abelson tyrosine kinase (abl) is localized in the axons of the central nervous system (CNS). Mutations in abl have no detectable effect on the morphology of the embryonic CNS, and the mutant animals survive to the pupal and adult stages. In the absence of abl function, however, heterozygous mutations or deletions of disabled (dab) exert dominant effects, disrupting axonal organization and shifting the lethal phase of the animals to embryonic and early larval stages. Embryos that are homozygous mutant for both abl and dab fail to develop any axon bundles in the CNS, although the peripheral nervous system and the larval cuticle appear normal. The genetic interaction between these two genes begins to define a process in which both the abl tyrosine kinase and the dab gene product participate in establishing axonal connections in the embryonic CNS of Drosophila.

We have isolated phage clones containing the D. melanogaster sequence homologous to the v-abl oncogene, and two types of phage clones containing sequences homologous to the v-src probe. The D. melanogaster abl clone (lambda Dabl1) and one of the src clones (lambda Dsrc1) hybridize with both v-abl and v-src probes, and both map in situ to the same chromosomal position, 73B, on chromosome arm 3L. The second D. melanogaster src clone (lambda Dsrc2) does not react with the v-abl probe and hybridizes in situ to chromosomal position 64B. The hybridization pattern suggests that the src and abl cellular oncogenes have evolved from a common prototype sequence. The homologous sequences in D. melanogaster exhibit hybridization to regions in the vertebrate v-abl and v-src that are important for kinase activity and transforming potential of the viral gene products.

Two cloned Drosophila sequences, Dash and Dsrc, were previously isolated by hybridization to the viral oncogenes v-abl and v-src. We report that the Drosophila DNA sequences are more than 50% homologous to 700 base pairs of the vertebrate oncogenes that are essential for kinase activity of the v-abl and the v-src gene products. Alignment of the predicted amino acid sequences in the homologous regions shows amino acid sequence identities of 74% between Dash and the 5' portion of v-abl and of 54% between Dsrc and 3' portion of v-src. Comparison of the Drosophila and vertebrate sequences identifies amino acids that may be essential for the distinct functions of the c-src and the c-abl gene products and places the gene duplication event that has generated the two genes prior to the Chordate-Arthropod divergence.

Three Drosophila genes homologous to the Ha-ras probe were isolated and mapped to positions 85D, 64B, and 62B on chromosome 3. Two of these genes (termed Dras 1 and Dras 2) were sequenced. In the case of Dras 1, which contains multiple introns, a cDNA clone was isolated and sequenced. In the case of Dras2, the nucleotide sequence fo the genomic clone was determined. Each gene codes for a protein with a predicted molecular weight of 21.6 kd. Alignment of the amino acid sequence of Dras 1 with the vertebrate Ha-ras protein shows that at the amino terminus and central portion (residues 1-121 and 137-164) the two proteins are remarkably similar, and have an overall homology of 75%. The Dras 2 gene lacks significant homology to the vertebrate counterpart at the extreme amino terminus and is homologous only between positions 28-120 and 139-161 (overall homology of 50%). This result suggests that the N terminus of p21 forms a distinct regulatory or functional domain. At the carboxy terminus, the major region of variability among the vertebrate ras proteins, the two Drosophila sequences also display considerable variability. However, both appear to be more similar to exon 4B of the Ki-ras gene.