Abstract
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.
Highlights
Chloroplasts in plants contain three distinct membrane systems and three compartments with soluble contents
Initiation and Budding Cytosolic vesicle budding is initiated by the recruitment of Sar1 to the donor membrane following its activation to a GTP-bound state from the inactive GDP-bound state by a guanine nucleotide exchange factor (GEF)
As we found several transmembrane cargo proteins with an diacidic motif that are involved in the photosystem II (PSII) complex, and THF1 has suggested responsibility for PSII complex biogenesis [143], fusion of vesicles transporting PSII complex proteins might be facilitated by THF, which is important for thylakoid formation being localized both in stroma and thylakoids [23].VIPP1 is just like FZL associated with both the inner envelope and thylakoids, consistent with a trafficking function
Summary
Chloroplasts in plants contain three distinct membrane systems and three compartments with soluble contents. The compartments with soluble contents are the intermembrane space between the inner and outer envelope membranes, the stroma between the inner envelope membrane and the thylakoid membrane, and the lumen enclosed by the thylakoids. Chloroplast-localized proteins are derived from both the chloroplast and nuclear genomes, but the vast majority (,95%) are nucleusencoded, targeted to the chloroplast and imported across the envelope membranes [1]. This import is facilitated by TOC/TIC translocons at the outer/inner envelope membranes of chloroplasts [2]. Lumen proteins are transported across the thylakoid membrane via the Sec or Tat pathway, whereas integral thylakoid membrane proteins are transported via the SRP or spontaneous [3,4,5]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
