Genetic and biochemical characterization of dTOR, the Drosophila homolog of the target of rapamycin

Abstract

The adaptation of growth in response to nutritional changes is essential for the proper development of all organisms. Here we describe the identification of the Drosophila homolog of the target of rapamycin (TOR), a candidate effector for nutritional sensing. Genetic and biochemical analyses indicate that dTOR impinges on the insulin signaling pathway by autonomously affecting growth through modulating the activity of dS6K. However, in contrast to other components in the insulin signaling pathway, partial loss of dTOR function preferentially reduces growth of the endoreplicating tissues. These results are consistent with dTOR residing on a parallel amino acid sensing pathway.

Figure 1

Identification of dTOR. Tissue-specific induction of dTOR^2L1 and dTOR^2L19 mutant clones using the ey-Flp technique (Newsome et al. 2000) produced flies with reduced head size relative to their body. (A) Dorsal view of y w ey-Flp; P(w+) l(2)2L-3.1 FRT40/CyO y⁺ (right) and y w ey-Flp; dTOR^2L1 FRT40/P(w+) l(2)2L-3.1 FRT40 (left). Flies of the following genotype were examined by SEM: (B) y w ey-Flp; dTOR^2L19 FRT40/P(w+) l(2)2L-3.1 FRT40; (C) y w ey-Flp; dTOR^2L1 FRT40/P(w+) l(2)2L-3.1 FRT40; (D) wild-type eye. Bar, 100 μm. (E) Genomic structure and mutants of dTOR. The grey region consists of the coding region, and the gaps indicate introns. The percentage amino acid identity of each domain to human TOR is shown in parentheses.

Figure 1

Identification of dTOR. Tissue-specific induction of dTOR^2L1 and dTOR^2L19 mutant clones using the ey-Flp technique (Newsome et al. 2000) produced flies with reduced head size relative to their body. (A) Dorsal view of y w ey-Flp; P(w+) l(2)2L-3.1 FRT40/CyO y⁺ (right) and y w ey-Flp; dTOR^2L1 FRT40/P(w+) l(2)2L-3.1 FRT40 (left). Flies of the following genotype were examined by SEM: (B) y w ey-Flp; dTOR^2L19 FRT40/P(w+) l(2)2L-3.1 FRT40; (C) y w ey-Flp; dTOR^2L1 FRT40/P(w+) l(2)2L-3.1 FRT40; (D) wild-type eye. Bar, 100 μm. (E) Genomic structure and mutants of dTOR. The grey region consists of the coding region, and the gaps indicate introns. The percentage amino acid identity of each domain to human TOR is shown in parentheses.

Figure 2

dTOR acts autonomously in the control of cell growth in the eye. (A,B) FACS analysis of dTOR^2L1/dTOR^l(2)k17004 mutant wing disks showing forward scatter (FSC) and cell cycle distribution. (C) Section through an eye containing a mitotic clone of dTOR^l(2)k17004. The area of the mutant clone is demarcated by the absence of red pigment. At the boundaries of the clone, ommatidia consist of mixed wild-type heterozygous and homozygous mutant cells. (D–F) Flies were examined by SEM. (D) y w control. (E). Loss of dPTEN function in the eye and head capsule results in a fly with a big head. (F) The large head caused by loss of dPTEN function in the eye can be suppressed by homozygosity for dTOR^EP(2)2353. Bar, 200 μm.

Figure 3

dTOR mutations and amino acid starvation affect the level and activity of dS6K. In vitro kinase assay to assess the levels of dS6K activity (top) and Western blot of the protein levels of dS6K and Drosophila eIF-4E (bottom).

Figure 4

dTOR larval and pupal mutant phenotypes. (A) Pupal sizes of the various mutant combinations. Pupae were collected just after puparium formation. (B) Pupal weights of the various mutant combinations. Gray bars are female, black bars are male. (C) Heteroallelic combinations of dTOR mutant larvae compared with mutants of other components in the Inr pathway. All larvae were taken from a staged collection at day 5. dTOR(w) is dTOR^2L1/dTOR^l(2)k17004, dTOR(s) is dTOR^2L1/dTOR^2L19, dPI3K is a null mutation (Weinkove et al. 1999), and dS6K is dS6K^l-1. (D) The size of mutant disks and salivary glands of hypomorphic dTOR mutants are distinct from dS6K and chico. (E) DAPI and phalloidin staining of salivary glands from y w, dS6K^l-1, chico, and dTOR^2L1/dTOR^l(2)k17004 mutant larvae. All disks and salivary glands were taken from larvae that had everted their spiracles just before pupariation.

Figure 5

Model showing the possible relationship of dTOR with the insulin pathway. The unexpected finding that hypomorphic mutations of dTOR, but not null mutations of chico, strongly inhibit dS6K activity may be explained by a two-threshold model. The threshold for dS6K inactivation by dTOR is low, whereas that for dS6K inactivation by the Inr pathway is high. Thus, a low level of PI3K signaling from Inr may be sufficient for dS6K activation in the absence of Chico. Dashed arrow indicates indirect or unclear interaction. Solid arrow indicates direct interaction.