Steiner Jürgen M, Löffelhardt Wolfgang
Max F. Perutz Laboratories, University Departments at the Vienna Biocenter, Department of Biochemistry and Molecular Cell Biology, Dr. Bohrgasse 9, 1030 Vienna, Austria.
Mol Membr Biol. 2005 Jan-Apr;22(1-2):123-32. doi: 10.1080/09687860500041411.
The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria in morphology, pigmentation and, especially, in the presence of a peptidoglycan wall situated between the inner and outer envelope membranes. However, it is now clear that cyanelles in fact are primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high plastid gene content of the Porphyra purpurea rhodoplast and the peptidoglycan wall of glaucocystophyte cyanelles. This means that the import apparatus of all primary plastids should be homologous. Indeed, heterologous in vitro import can now be shown in both directions, provided a phenylalanine residue essential for cyanelle import is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved explaining the efficient heterologous import of native precursors from C. paradoxa. With respect to conservative sorting in cyanelles, both the Sec and Tat pathways could be demonstrated. Another cyanobacterial feature, the dual location of the Sec translocase in thylakoid and inner envelope membranes, is also unique to cyanelles. For the first time, protease protection of internalized lumenal proteins could be shown for cyanobacteria-like, phycobilisome-bearing thylakoid membranes after import into isolated cyanelles.
蓝隐藻门藻类蓝氏拟甲色球藻（Cyanophora paradoxa）的蓝小体在形态、色素沉着方面，尤其是在内包膜和外膜之间存在肽聚糖壁这一点上，类似于内共生蓝细菌。然而，现在已经清楚，蓝小体实际上是原始质体。对质体、核和线粒体基因的系统发育分析支持单一的一次内共生事件。在这种情况下，蓝小体和所有其他质体类型都源自一个祖先光合细胞器，它结合了紫菜红质体的高质体基因含量和蓝隐藻门蓝小体的肽聚糖壁。这意味着所有原始质体的输入装置应该是同源的。实际上，现在可以证明异源体外输入在两个方向上都可行，前提是将蓝小体输入所必需的苯丙氨酸残基工程化到叶绿体转运肽的N端部分。可以设想蓝小体以及可能还有红质体的输入装置是具有单一受体的原型，该受体对N端苯丙氨酸有这种要求。在叶绿体中，已经进化出了具有重叠且不太严格特异性的多个受体，这解释了来自蓝氏拟甲色球藻的天然前体的高效异源输入。关于蓝小体中的保守分选，可以证明Sec和Tat途径都存在。Sec转位酶在类囊体膜和内膜中的双重定位这一蓝细菌特征，也是蓝小体所特有的。首次在将内化的类囊体腔蛋白导入分离的蓝小体后，对具有蓝细菌样、带有藻胆体的类囊体膜进行了蛋白酶保护实验。
