Vitha Stanislav, Froehlich John E, Koksharova Olga, Pyke Kevin A, van Erp Harrie, Osteryoung Katherine W
Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA.
Plant Cell. 2003 Aug;15(8):1918-33. doi: 10.1105/tpc.013292.
Replication of chloroplasts is essential for achieving and maintaining optimal plastid numbers in plant cells. The plastid division machinery contains components of both endosymbiotic and host cell origin, but little is known about the regulation and molecular mechanisms that govern the division process. The Arabidopsis mutant arc6 is defective in plastid division, and its leaf mesophyll cells contain only one or two grossly enlarged chloroplasts. We show here that arc6 chloroplasts also exhibit abnormal localization of the key plastid division proteins FtsZ1 and FtsZ2. Whereas in wild-type plants, the FtsZ proteins assemble into a ring at the plastid division site, chloroplasts in the arc6 mutant contain numerous short, disorganized FtsZ filament fragments. We identified the mutation in arc6 and show that the ARC6 gene encodes a chloroplast-targeted DnaJ-like protein localized to the plastid envelope membrane. An ARC6-green fluorescent protein fusion protein was localized to a ring at the center of the chloroplasts and rescued the chloroplast division defect in the arc6 mutant. The ARC6 gene product is related closely to Ftn2, a prokaryotic cell division protein unique to cyanobacteria. Based on the FtsZ filament morphology observed in the arc6 mutant and in plants that overexpress ARC6, we hypothesize that ARC6 functions in the assembly and/or stabilization of the plastid-dividing FtsZ ring. We also analyzed FtsZ localization patterns in transgenic plants in which plastid division was blocked by altered expression of the division site-determining factor AtMinD. Our results indicate that MinD and ARC6 act in opposite directions: ARC6 promotes and MinD inhibits FtsZ filament formation in the chloroplast.
叶绿体的复制对于在植物细胞中实现并维持最佳质体数量至关重要。质体分裂机制包含内共生起源和宿主细胞起源的成分，但对于调控分裂过程的机制和分子机理却知之甚少。拟南芥突变体arc6在质体分裂方面存在缺陷，其叶片叶肉细胞仅含有一两个明显增大的叶绿体。我们在此表明，arc6叶绿体还表现出关键质体分裂蛋白FtsZ1和FtsZ2的异常定位。在野生型植物中，FtsZ蛋白在质体分裂位点组装成环，而arc6突变体中的叶绿体含有许多短的、杂乱无章的FtsZ丝状片段。我们鉴定出arc6中的突变，并表明ARC6基因编码一种定位于叶绿体包膜膜的类DnaJ蛋白。ARC6-绿色荧光蛋白融合蛋白定位于叶绿体中心的一个环，并挽救了arc6突变体中的叶绿体分裂缺陷。ARC6基因产物与Ftn2密切相关，Ftn2是蓝细菌特有的一种原核细胞分裂蛋白。基于在arc6突变体和过表达ARC6的植物中观察到的FtsZ丝状形态，我们推测ARC6在质体分裂FtsZ环的组装和/或稳定中发挥作用。我们还分析了转基因植物中FtsZ的定位模式，在这些转基因植物中，质体分裂因分裂位点决定因子AtMinD表达改变而受阻。我们的结果表明，MinD和ARC6的作用方向相反：ARC6促进而MinD抑制叶绿体中FtsZ丝的形成。
