Only One Isoform of Drosophila melanogaster CTP Synthase Forms the Cytoophidium

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.

CTP合酶（CTP synthase）是一种在能量代谢中发挥关键作用的必需酶。多项独立研究已证实，CTP合酶可形成一种进化保守的亚细胞结构，名为细胞蛇（cytoophidium）。在酿酒酵母中，CTP合酶存在两种同工型，且二者均定位于细胞蛇中。然而，目前对于黑腹果蝇（Drosophila melanogaster）中CTP合酶同工型的分布情况仍知之甚少。本研究发现，CTP合酶基因座产生的三种转录本具有不同的表达谱，该基因座编码的三种同工型则展现出独特的亚细胞定位模式：同工型A定位于细胞核，同工型B在细胞质中呈弥散分布，而仅同工型C可形成细胞蛇。在两种同工型C特异性突变体中，生殖细胞内的细胞蛇完全消失。尽管同工型A并不定位于细胞蛇，但一种主要干扰同工型A表达的突变会导致细胞蛇解体。同工型C的过表达可通过细胞自主方式诱导细胞蛇的形成。异位表达可形成细胞蛇的同工型不会对胚胎造成任何缺陷。此外，本研究鉴定出，同工型C氨基末端的一段小序列是细胞蛇形成所必需的，但并非充分条件。最后，我们证实过量表达CTP合酶的合成酶结构域会破坏细胞蛇的形成。因此，对果蝇中CTP合酶多种同工型的研究，为在一种易于进行遗传操作的模式生物中解析细胞蛇的生物发生与功能提供了良好的契机。