Only one isoform of Drosophila melanogaster CTP synthase forms the cytoophidium

Abstract

CTP synthase is an essential enzyme that plays a key role in energy metabolism. Several independent studies have demonstrated that CTP synthase can form an evolutionarily conserved subcellular structure termed cytoophidium. In budding yeast, there are two isoforms of CTP synthase and both isoforms localize in cytoophidium. However, little is known about the distribution of CTP synthase isoforms in Drosophila melanogaster. Here, we report that three transcripts generated at the CTP synthase gene locus exhibit different expression profiles, and three isoforms encoded by this gene locus show a distinct subcellular distribution. While isoform A localizes in the nucleus, isoform B distributes diffusely in the cytoplasm, and only isoform C forms the cytoophidium. In the two isoform C-specific mutants, cytoophidia disappear in the germline cells. Although isoform A does not localize to the cytoophidium, a mutation disrupting mostly isoform A expression results in the disassembly of cytoophidia. Overexpression of isoform C can induce the growth of the cytoophidium in a cell-autonomous manner. Ectopic expression of the cytoophidium-forming isoform does not cause any defect in the embryos. In addition, we identify that a small segment at the amino terminus of isoform C is necessary but not sufficient for cytoophidium formation. Finally, we demonstrate that an excess of the synthetase domain of CTP synthase disrupts cytoophidium formation. Thus, the study of multiple isoforms of CTP synthase in Drosophila provides a good opportunity to dissect the biogenesis and function of the cytoophidum in a genetically tractable organism.

Figure 1. The Drosophila melanogaster CG6854/CTPsyn gene locus encodes three isoforms.

(A) Genome Browser view of the CG6854/CTPsyn gene locus (www.flybase.org). In two protein trap lines (CA06746 and CA07332), GFP was trapped between the first and second exons of the CTPsyn isoform. (B) The protein map of three isoforms of CTPsyn. UTP binding sites are indicated by red triangles.

Figure 2. Expression profiles of Drosophila CTPsyn isoforms as revealed by qPCR.

(A) embryos and larvae. (B) S2 cells and pupae. (C) Adult flies and tissues. WPP, white pre-pupae. P1–P4, pupal stages 1–4.

Figure 3. Overexpression of different CTPsyn isoforms in germline cells using nos-GAL4::VP16 driver.

(A–C) Cytoophidia revealed by CTPsyn-GFP in an egg chamber from a protein trap line CA06746. (D–F) CTPsyn isoform A is localized in punctate structures in the nucleus and does not affect endogenous cytoophidia. (G–I) CTPsyn isoform B is dispersed in the cytoplasm of the egg chamber and does not affect endogenous cytoophidia. (J–L) C-terminal-tagged CTPsyn isoform C induced longer and more curved cytoophidia. (M–O) N-terminal-tagged CTPsyn isoform C affects cytoophidia morphology. Iso, Isoform. Scale bars, 20 µm.

Figure 4. CTPsyn mutants.

(A) Genetic map of CTPsyn and the insertion site of p-element in three mutants CTPsyn^d6099, CTPsyn^e01207 and CTPsyn^d07411. (B) Comparison of the expressions of CTPsyn isoforms in larvae from y w and three CTPsyn mutants (CTPsyn^d6099, CTPsyn^e01207 and CTPsyn^d07411). While dramatic decrease of isofrom C expression occurs in CTPsyn^d6099 and CTPsyn^e01207, isoform A expression is hugely diminished in CTPsyn^d07411.

Figure 5. CTPsyn mutants survive until larval stages.

CTPsyn^d6099 homozygous mutants (−/− in A–E) and CTPsyn^e01207 homozygous mutants (−/− in F–J) survive until 7 days after egg deposition (DAE) with slower growth compared to heterozygous mutants (+/−) or y w (+/+). (K–M) CTPsyn^d07411 homozygous mutants (−/−) show severely delayed development, in comparison with heterozygous mutants (+/−and y w (+/+). CTPsyn^d07411 (−/−) mutant larvae only survive until 5 days after egg deposition (DAE) with very little growth. Heterozygous mutants were balanced with TM6B Tb (+/− in all panels).

Figure 6. Clonal analysis of three CTPsyn mutations in adult ovaries.

(A–C) CTPsyn^d6099. (D–F) CTPsyn^e01207. (G–I) CTPsyn^d07411 In all three mutations, no cytoophidia are detected in mutant mitotic clone (dotted lines) as indicated by lack of GFP, while cytoophidia (labelled by an antibody against CTPsyn in red) are clearly seen in adjacent wild-type germline cells (GFP positive). Scale bar, 20 µm.

Figure 7. Overexpression of CTPsyn isoform C induces the assembly of cytoophidia in Drosophila follicle cells.

(A, B) Cytoophidia revealed by CTPsyn-GFP in protein trap line CA06746. (C, D) Long cytoophidia are induced in stage-10 follicle cells overexpressing CTPsyn isoform C. (E, F) Each follicle cell contains only one long cytoophidium in a stage-7 egg chamber. The cell boundary is outlined by membrane protein Hu-li tai shao (Hts). (G) Within the same stage 10 egg chamber, cytoophidia in follicle cells overexpressing CTPsyn isoform C (GFP+ cells) are much longer and thicker than those in neighbouring wild-type follicle cells (GFP− cells). While the average length of cytoophidia in GFP− cells is only 3.97±0.61 µm (n = 41), cytoophidia in GFP+ cells are 20.76±5.60 µm long on average (n = 32). Note that cytoophidia in GFP+ cells in E are much longer than those in C and D, which could be due to different expression of GAL4 drivers in follicle cells (actin-GAL4 in C, D and tubulin-GAL4 in E). Scale bars, 20 µm.

Figure 8. Overexpression of CTPsyn isoform C induces macro-cytoophidia in Drosophila embryos.

(A, C, E) No macro-cytoophidium formation can be detected in y w embryos at stage 1 (A), 12 (C) and 15 (E). (B, D, F) Overexpressing CTPsyn isoform C induces macro-cytoophidia in embryos at stage 1 (B), 12 (D), and 15 (F). Segments in embryos are labeled by a transcription factor Engrailed (red). IsoC, isoform C. Scale bars, 20 µm.

Figure 9. Ectopic expression of transgenes in egg chambers using MTD-GAL4 driver.

(A–C) UAS-Venus-N-term which consists of the 56 amino acids shows dispersed cytoplasmic expression. (D–I) UAS-Trun isoC-Venus forms punctate structures in the cytoplasm. The punctate structures are more visible in egg chambers at later stages (G–I) than in those at early stages (D–F). (J–L) UAS-Venus-Trun isoC shows dispersed distribution in the cytoplasm. (M–O) Ectopic expression of UAS-SD-Venus disrupts endogenous cytoophidium formation in the germline cells. Note that there is no expression of UAS-SD-Venus in follicle cells in which cytoophida are not affected. (P–R) UAS-GAT-Venus shows disperse distribution in the cytoplasm. See Figure S6 for the structures of the constructs. N-term, a N-terminal segment (56-aa) of CTPsyn isoform C; Trun IsoC, truncated isoform C. SD, synthetase domain; GAT, type 1 glutamine amidotransferase domain. Scale bars, 20 µm.

Figure 10. Clonal expression of CTPsyn domains in follicle cells using inducible driver.

(A–F) Ectopic expression of UAS-SD-Venus disrupts endogenous cytoophidium formation in follicle cells (GFP+ cells), while cytoophidia are detectable in wild-type follicle cells (GFP− cells, within dotlined-area). (A–C) Surface view. (D–F) Lateral view. (G–I) Ectopic expression of UAS-Venus-GAT has no obvious effect on cytoophidium formation in follicle cells. Note that cytoophidia are detectable in both GFP+ (where the transgene is expressed) and GFP− cells (dotlined, wild-type). SD, synthetase domain; GAT, type 1 glutamine amidotransferase domain. Venus (C), cytoplasmic signal from Venus. GFP (N), nuclear signal from GFP. Scale bars, 20 µm.