The effect of dS6K on growth and the identification of interactors of dS6K in Drosophila melanogaster

Cell size is a process that is tightly regulated by several signaling pathways. The fact that several known regulators of growth are amplified or misexpressed in human tumors has led scientists to view these proteins as potential therapeutic targets. The S6 kinase (dS6K) protein in Drosophila melanogaster belongs to the insulin receptor (InR) and target of rapamycin (TOR) pathways. These two pathways are involved in the regulation of organismal size, which is mediated by their effects on cell size and cell proliferation. The InR and TOR pathways regulate protein synthesis at the level of ribosome biogenesis and translation initiation. One goal of this dissertation was to identify potential new interactors of dS6K. To do so, several transgenic fly lines expressing different dS6K alleles were produced. The effects of several dS6K alleles on growth were studied at the organismal level and at the cellular level. All the dS6K alleles affected cell size without modifying cell proliferation rate or cell cycle phasing. Two of the dS6K alleles were used in a deficiency screen that led to the identification of several potential positive and negative regulators of dS6K activity. The screen yielded the identification of three potential negative interactors of dS6K. One of these negative interactors is the catalytic subunit of the protein phosphatase 2A, which could be involved in the pathway regulating dS6K. The second gene identified encodes the thiolase protein, which is involved in fatty acid beta oxidation. The third gene, very small puparia, is involved in the regulation of pupal size. Of the six potential positive regulators of dS6K, three were genes of unknown function. One of the genes identified was the farnesyl pyrophosphatase gene, which is involved in isoprenoid biosynthesis. The second gene encodes the nucleosome assembly protein 1, which is involved in nucleosome formation and the regulation of mitosis. Lastly, an L7-like ribosomal protein was identified. The L7 protein is located in the large subunit of the ribosome and might have extra-ribosomal function in mammalian cells. The screen led to the identification of nine genes which might be involved in the regulation of organ and/or cell size.