Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden.
PLoS One. 2013;8(4):e59898. doi: 10.1371/journal.pone.0059898. Epub 2013 Apr 1.
Proteins and lipids are known to be transported to targeted cytosolic compartments in vesicles. A similar system in chloroplasts is suggested to transfer lipids from the inner envelope to the thylakoids. However, little is known about both possible cargo proteins and the proteins required to build a functional vesicle transport system in chloroplasts. A few components have been suggested, but only one (CPSAR1) has a verified location in chloroplast vesicles. This protein is localized in the donor membrane (envelope) and vesicles, but not in the target membrane (thylakoids) suggesting it plays a similar role to a cytosolic homologue, Sar1, in the secretory pathway. Thus, we hypothesized that there may be more similarities, in addition to lipid transport, between the vesicle transport systems in the cytosol and chloroplast, i.e. similar vesicle transport components, possible cargo proteins and receptors. Therefore, using a bioinformatics approach we searched for putative chloroplast components in the model plant Arabidopsis thaliana, corresponding mainly to components of the cytosolic vesicle transport system that may act in coordination with previously proposed COPII chloroplast homologues. We found several additional possible components, supporting the notion of a fully functional vesicle transport system in chloroplasts. Moreover, we found motifs in thylakoid-located proteins similar to those of COPII vesicle cargo proteins, supporting the hypothesis that chloroplast vesicles may transport thylakoid proteins from the envelope to the thylakoid membrane. Several putative cargo proteins are involved in photosynthesis, thus we propose the existence of a novel thylakoid protein pathway that is important for construction and maintenance of the photosynthetic machinery.
蛋白质和脂质已知是通过囊泡被运输到靶向的细胞质隔室。类似的系统被认为存在于叶绿体中,将脂质从内囊体膜转移到类囊体。然而,对于可能的货物蛋白和构建叶绿体中功能性囊泡运输系统所需的蛋白,我们知之甚少。已经提出了一些成分,但只有一种(CPSAR1)在叶绿体囊泡中有经过验证的位置。这种蛋白定位于供体膜(包膜)和囊泡中,但不在靶膜(类囊体)中,表明它在分泌途径中发挥类似于细胞质同源物 Sar1 的作用。因此,我们假设除了脂质运输之外,细胞质和叶绿体中的囊泡运输系统之间可能还有更多的相似之处,即类似的囊泡运输成分、可能的货物蛋白和受体。因此,我们使用生物信息学方法在模式植物拟南芥中搜索叶绿体的可能成分,这些成分主要对应于细胞质囊泡运输系统的成分,可能与之前提出的 COPII 叶绿体同源物协同作用。我们发现了几个额外的可能成分,支持叶绿体中存在完整的囊泡运输系统的观点。此外,我们发现定位于类囊体的蛋白中的基序与 COPII 囊泡货物蛋白中的基序相似,支持叶绿体囊泡可能将类囊体蛋白从包膜运输到类囊体膜的假说。几个假定的货物蛋白参与光合作用,因此我们提出了一种新的类囊体蛋白途径的存在,该途径对构建和维持光合作用机器很重要。