石川 裕之

ELYS determines the subcellular localizations of Nucleoporins (Nups) during interphase and mitosis. We made loss-of-function mutations of Elys in Drosophila melanogaster and found that ELYS is dispensable for zygotic viability and male fertility but the maternal supply is necessary for embryonic development. Subsequent to fertilization, mitotic progression of the embryos produced by the mutant females is severely disrupted at the first cleavage division, accompanied by irregular behavior of mitotic centrosomes. The Nup160 introgression from D. simulans shows close resemblance to that of the Elys mutations, suggesting a common role for those proteins in the first cleavage division. Our genetic experiments indicated critical interactions between ELYS and three Nup107-160 subcomplex components; hemizygotes of either Nup37, Nup96 or Nup160 were lethal in the genetic background of the Elys mutation. Not only Nup96 and Nup160 but also Nup37 of D. simulans behave as recessive hybrid incompatibility genes with D. melanogaster An evolutionary analysis indicated positive natural selection in the ELYS-like domain of ELYS. Here we propose that genetic incompatibility between Elys and Nups may lead to reproductive isolation between D. melanogaster and D. simulans, although direct evidence is necessary.

Despite intensive research on kinases and protein phosphorylation, most studies focus on kinases localized to the cytosol and nucleus. Studies in Drosophila discovered a novel signaling pathway that regulates growth and planar cell polarity. In this pathway, the atypical cadherin Fat acts as a receptor, and the cadherin Dachsous (Ds) serves as its ligand. Genetic studies in Drosophila identified the four-jointed gene as a regulator of the Fat pathway. Four-jointed (Fj) resides in the Golgi and phosphorylates the cadherin domains of Fat and Ds. Fj-mediated phosphorylations promote the ability of Fat to bind to its ligand Ds and inhibit the ability of Ds to bind Fat, which is biased toward a stronger effect on Fat. Fj is expressed in a gradient in many developing tissues. The Fat-Ds-binding gradient can be explained by the graded activity of Fj that is sufficient to propagate the polarization of complexes across whole tissues. Recent studies revealed a new class of kinases that localize within the secretory pathway and the extracellular space, and phosphorylate proteins and sugar chains in the secretory pathway. Further, they appear to regulate extracellular processes. Mutations of the genes encoding these kinases cause human disease, thus underscoring the biological importance of phosphorylation events within the secretory pathway.

Notch (N) is a transmembrane receptor that mediates the cell-cell interactions necessary for many cell fate decisions. N has many epidermal growth factor-like repeats that are O-fucosylated by the protein O-fucosyltransferase 1 (O-Fut1), and the O-fut1 gene is essential for N signaling. However, the role of the monosaccharide O-fucose on N is unclear, because O-Fut1 also appears to have O-fucosyltransferase activity-independent functions, including as an N-specific chaperon. Such an enzymatic activity-independent function could account for the essential role of O-fut1 in N signaling. To evaluate the role of the monosaccharide O-fucose modification in N signaling, here we generated a knock-in mutant of O-fut1 (O-fut1(R245A knock-in)), which expresses a mutant protein that lacks O-fucosyltransferase activity but maintains the N-specific chaperon activity. Using O-fut1(R245A knock-in) and other gene mutations that abolish the O-fucosylation of N, we found that the monosaccharide O-fucose modification of N has a temperature-sensitive function that is essential for N signaling. The O-fucose monosaccharide and O-glucose glycan modification, catalyzed by Rumi, function redundantly in the activation of N signaling. We also showed that the redundant function of these two modifications is responsible for the presence of N at the cell surface. Our findings elucidate how different forms of glycosylation on a protein can influence the protein's functions.

In interspecific hybrids between Drosophila melanogaster and Drosophila simulans, the D. simulans nucleoporin-encoding Nup96(sim) and Nup160(sim) can cause recessive lethality if the hybrid does not also inherit the D. simulans X chromosome. In addition, Nup160(sim) leads to recessive female sterility in the D. melanogaster genetic background. Here, we conducted carefully controlled crosses to better understandthe relationship between Nup96(sim) and Nup160(sim). Nup96(sim) did not lead to female sterility in the D. melanogaster genetic background, and double introgression of Nup96(sim) and Nup160(sim) did not generally lead to lethality when one was heterozygous and the other homozygous (hemizygous). It appears that introgression of additional autosomal D. simulans genes is necessary to cause lethality and that the effect of the introgression is dominant to D. melanogaster alleles. Interestingly, the genetic background affected dominance of Nup96(sim), and double introgression carrying homozygous Nup96(sim) and hemizygous Nup160(sim) resulted in lethality. Thus, Nup96(sim) and Nup160(sim) seem to be two components of the same incompatibility.

BACKGROUND: Several lines of evidence associate misregulated genetic expression with risk factors for diabetes, Alzheimer's, and other diseases that sporadically develop in healthy adults with no background of hereditary disorders. Thus, we are interested in genes that may be expressed normally through parts of an individual's life, but can cause physiological defects and disease when misexpressed in adulthood. RESULTS: We attempted to identify these genes in a model organism by arbitrarily misexpressing specific genes in adult Drosophila melanogaster, using 14,133 Gene Search lines. We identified 39 "reduced-lifespan genes" that, when misexpressed in adulthood, shortened the flies' lifespan to less than 30% of that of control flies. About half of these genes have human orthologs that are known to be involved in human diseases. For about one-fourth of the reduced-lifespan genes, suppressing apoptosis restored the lifespan shortened by their misexpression. We determined the organs responsible for reduced lifespan when these genes were misexpressed specifically in adulthood, and found that while some genes induced reduced lifespan only when misexpressed in specific adult organs, others could induce reduced lifespan when misexpressed in various organs. This finding suggests that tissue-specific dysfunction may be involved in reduced lifespan related to gene misexpression. Gene ontology analysis showed that reduced-lifespan genes are biased toward genes related to development. CONCLUSIONS: We identified 39 genes that, when misexpressed in adulthood, shortened the lifespan of adult flies. Suppressing apoptosis rescued this shortened lifespan for only a subset of the reduced-lifespan genes. The adult tissues in which gene misexpression caused early death differed among the reduced-lifespan genes. These results suggest that the cause of reduced lifespan upon misexpression differed among the genes.