Euglena gracilis is a photosynthetic flagellate possessing chlorophyte-derived secondary plastids that are enclosed by only three enveloping membranes, unlike most secondary plastids, which are surrounded by four membranes. It has generally been assumed that the two innermost E. gracilis plastid envelopes originated from the primary plastid, while the outermost is of eukaryotic origin. It was suggested that nucleus-encoded plastid proteins pass through the middle and innermost plastid envelopes of E. gracilis by machinery homologous to the translocons of outer and inner chloroplast membranes, respectively. Although recent genomic, transcriptomic, and proteomic data proved the presence of a reduced form of the translocon of inner membrane, they failed to identify any outer-membrane translocon homologs, which raised the question of the origin of E. gracilis’s middle plastid envelope. Here, we compared the lipid composition of whole cells of the pigmented E. gracilis strain Z and two bleached mutants that lack detectable plastid structures, W10BSmL and WgmZOflL. We determined the lipid composition of E. gracilis strain Z mitochondria and plastids, and of plastid subfractions (thylakoids and envelopes), using HPLC high-resolution tandem mass spectrometry, thin-layer chromatography, and gas chromatography-flame ionization detection analytical techniques. Phosphoglycerolipids are the main structural lipids in mitochondria, while glycosyldiacylglycerols are the major structural lipids of plastids and also predominate in extracts of whole mixotrophic cells. Glycosyldiacylglycerols were detected in both bleached mutants, indicating that mutant cells retain some plastid remnants. Additionally, we discuss the origin of the E. gracilis middle plastid envelope based on the lipid composition of envelope fraction.

Euglena gracilis是一种光合鞭毛虫，具有叶绿素衍生的次级质体，仅被三个包膜包围，不像大多数次生质体被四个膜包围。一般认为，最里面的两个纤细纤毛虫质体包膜起源于初级质体，而最外面的则是真核起源的。有人提出核编码的质体蛋白穿过E. gracilis的中间和最里面的质体包膜通过分别与外叶绿体膜和内叶绿体膜的易位子同源的机制。尽管最近的基因组学、转录组学和蛋白质组学数据证明了内膜转位子的减少形式的存在，但他们未能识别出任何外膜转位子同源物，这引发了关于纤细纤毛虫中间质体包膜起源的问题. 在这里，我们比较了色素E. gracilis Z株和两个缺乏可检测质体结构的漂白突变体W10BSmL和WgmZOflL的全细胞的脂质组成。我们确定了E. gracilis Z菌株的脂质组成线粒体和质体，以及质体亚组分（类囊体和包膜），使用 HPLC 高分辨率串联质谱、薄层色谱和气相色谱-火焰电离检测分析技术。磷酸甘油脂是线粒体中的主要结构脂质，而糖基二酰基甘油是质体的主要结构脂质，并且在整个混合营养细胞的提取物中也占主导地位。在两个漂白的突变体中都检测到糖基二酰基甘油，表明突变体细胞保留了一些质体残余物。此外，我们根据包膜部分的脂质组成讨论了E. gracilis中间质体包膜的起源。