New Putative Chloroplast Vesicle Transport Components and Cargo Proteins Revealed Using a Bioinformatics Approach: An Arabidopsis Model

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.

IM30 triggers membrane fusion in cyanobacteria and chloroplasts

“…However, it appears likely that additional proteins with tethering activity are involved in vivo to induce and control selective membrane fusion. These proteins might involve dynamin-related GTPases, such as FZL, or SNARE-like proteins, which have been identified in chloroplasts and cyanobacteria 15,16,49,50 . The notion that the IM30 fusion activity depends on Mg 2 þ agrees well with the observation that Mg 2 þ is generally involved in enzyme regulation within chloroplasts and cyanobacteria 51 .…”

“…Whereas in cold-adapted chloroplasts vesicles appeared, which resemble COPII vesicles found in the cytosol 1,12,13 , the existence of vesicles in cyanobacteria remains the subject of debate 14 . However, in recent bioinformatics analyses, chloroplast-localized components of a vesicular transfer system have been identified, and some of the identified proteins are also conserved in cyanobacteria 15,16 . Thus, several recent observations indicate that vesicular transfer processes are potentially involved in TM biogenesis and maintenance.…”

ArabidopsisATG8-INTERACTING PROTEIN1 Is Involved in Autophagy-Dependent Vesicular Trafficking of Plastid Proteins to the Vacuole

“…Also, it suggests that in addition to stroma-derived proteins (Figures 5 and 6), transmembrane proteins from the envelope and thylakoid membranes might also be incorporated into the ATI-PS bodies following their interaction with ATI1. Interestingly, NPQ4 was recently suggested to be a cargo component within plastid-localized vesicles (Khan et al, 2013). To test this in planta, we chose two candidate chloroplast-localized ATI1-interacting proteins, NPQ4 and APE1, for bimolecular fluorescence complementation (BiFC) assays.…”

“…Indeed, a recent report employing a computational approach (Khan et al, 2013) identified in Arabidopsis almost all of the plastid proteins that share sequence similarity with proteins associated with COPII vesicles that transport cargo from the ER to the Golgi apparatus. Interestingly, one of the 21 chloroplast proteins identified using this computational approach as a potential cargo protein carried by intraplastid vesicles was NPQ4, which was also recognized as an ATI1 interacting protein in our split ubiquitin Y2H assay (Table 1).…”

Proteins affecting thylakoid morphology – the key to understanding vesicle transport in chloroplasts?

“…14 Three RTNLBs have been predicted to be chloroplast localized: AtRTNLB17, AtRTNLB18, and AtRTNLB21, 15 but notably none of these corresponds to the two identified by Interactome 3D.…”