A role for Drosophila SMC4 in the resolution of sister chromatids in mitosis

BACKGROUND

Faithful segregation of the genome during mitosis requires interphase chromatin to be condensed into well-defined chromosomes. Chromosome condensation involves a multiprotein complex known as condensin that associates with chromatin early in prophase. Until now, genetic analysis of SMC subunits of the condensin complex in higher eukaryotic cells has not been performed, and consequently the detailed contribution of different subunits to the formation of mitotic chromosome morphology is poorly understood.

RESULTS

We show that the SMC4 subunit of condensin is encoded by the essential gluon locus in Drosophila. DmSMC4 contains all the conserved domains present in other members of the structural-maintenance-of-chromosomes protein family. DmSMC4 is both nuclear and cytoplasmic during interphase, concentrates on chromatin during prophase, and localizes to the axial chromosome core at metaphase and anaphase. During decondensation in telophase, most of the DmSMC4 leaves the chromosomes. An examination of gluon mutations indicates that SMC4 is required for chromosome condensation and segregation during different developmental stages. A detailed analysis of mitotic chromosome structure in mutant cells indicates that although the longitudinal axis can be shortened normally, sister chromatid resolution is strikingly disrupted. This phenotype then leads to severe chromosome segregation defects, chromosome breakage, and apoptosis.

CONCLUSIONS

Our results demonstrate that SMC4 is critically important for the resolution of sister chromatids during mitosis prior to anaphase onset.

Crystal structure of the VHS and FYVE tandem domains of Hrs, a protein involved in membrane trafficking and signal transduction

We have determined the 2 A X-ray structure of the 219-residue N-terminal VHS and FYVE tandem domain unit of Drosophila Hrs. The unit assumes a pyramidal structure in which the much larger VHS domain (residues 1-153) forms a rectangular base and the FYVE domain occupies the apical end. The VHS domain is comprised of an unusual "superhelix" of eight alpha helices, and the FYVE domain is mainly built of loops, two double-stranded antiparallel sheets, and a helix stabilized by two tetrahedrally coordinated zinc atoms. The two-domain structure forms an exact 2-fold-related homodimer through antiparallel association of mainly FYVE domains. Dimerization creates two identical pockets designed for binding ligands with multiple negative charges such as citrate or phosphatidylinositol 3-phosphate.

Identification of immune system and response genes, and novel mutations causing melanotic tumor formation in Drosophila melanogaster

We are using Drosophila as a model system for analysis of immunity and tumor formation and have conducted two types of screens using enhancer detector strains to find genes related to these processes; genes expressed in the immune system (type A; hemocytes, lymph glands and fat body) and genes increased in expression by bacterial infection (type B). For type A, tissue-specific reporter gene activity was determined. For type B, a variation of enhancer detection was devised in which beta-galactosidase is assayed spectrophotometrically with and without bacterial infection. Because of immune system involvement in melanotic tumor formation, a third type was hypothesized to be found among types A and B; genes that, when mutated, have a melanotic tumor phenotype. Enhancer detector strains (2800) were screened for type A, 900 for B, and 11 retained for further analysis. Complementation tests, cytological mapping, P-element mobilization, and determination of lethal phase and mutant phenotype have identified six novel genes, Dorothy, wizard, toto, viking, Thor and dappled, and one previously identified gene, Collagen IV. All are associated with reporter gene expression in at least one immune system tissue. Thor has increased expression upon infection. Mutations of wizard and dappled have a melanotic tumor phenotype.

The Drosophila 18 wheeler is required for morphogenesis and has striking similarities to Toll

We have isolated and characterized a novel gene, named 18 wheeler (18w) for its unique segmental expression pattern in Drosophila embryos and expression in cells that migrate extensively. 18 wheeler transcripts accumulate in embryos in a pattern reminiscent of segment polarity genes. Mutations in 18w cause death during larval development and early adulthood. Escaping mutant adults often display leg, antenna, and wing deformities, presumably resulting from improper eversion of imaginal discs. Sequence analysis indicates that 18w encodes a transmembrane protein with an extracellular moiety containing many leucine rich repeats and cysteine motifs, and an intracellular domain bearing homology to the cytoplasmic portion of the interleukin-1-receptor. Expression of 18W protein in non-adhesive Schneider 2 cells promotes rapid and robust aggregation of cells. Analysis of the expression of 18w in different mutant backgrounds shows that it is under control of segment polarity and homeotic genes. The data suggest that the 18W protein participates in the developmental program specified by segmentation and homeotic genes as a cell adhesion or receptor molecule that facilitates cell movements.

Neuromusculin, a Drosophila gene expressed in peripheral neuronal precursors and muscles, encodes a cell adhesion molecule

To unravel the molecular mechanisms of peripheral nervous system differentiation in Drosophila, we have screened for and identified genes that are expressed in sensory mother cells. Here, we describe a novel gene, neuromusculin (nrm), that is expressed in sensory mother cells and developing muscles. nrm encodes a member of the immunoglobulin superfamily. Immunoblots of Schneider 2 cells transfected with an nrm cDNA indicate that Nrm is present in a membrane-associated form and a secreted form. Cell aggregation assays suggest that Nrm is a homophilic cell adhesion molecule that is secreted or released after proteolysis, a mechanism that to our knowledge has not been described for immunoglobulin-like molecules. Genetic analyses indicate that nrm is an essential gene required for larval viability. We propose that Nrm may play a role as a cell adhesion molecule in clustering cells of the peripheral nervous system, neuronal fasciculation, and/or pathfinding.

Heterogeneity of Es-1 esterases in the rabbit (Oryctolagus cuniculus)

Analysis of the Es-1 system in the rabbit with polyacrylamide gel electrophoresis (PAGE) revealed a high degree of individual variation. In the liver the number of esterase bands found in the Es-1 region of the gels ranged from 2 to 16. The results indicate that one locus with three alleles is responsible for all of the esterase bands in the Es-1 region. The most plausible explanation for the observed heterogeneity is that each of the alleles codes for a protein (MW 65,000 +/- 2000) that is changed by posttranslational modifications, thus giving rise to two to five monomeric enzymes with esterase activity. Polymerization of these monomers then results in 1-11 dimers. Based on similarities with mouse Es-9, chromosomal homology between rabbit Es-1 and mouse Es-9 is proposed.