The Chloroplast Protein Import Apparatus, Its Components, and Their Roles

The Significance of Calcium in Photosynthesis

As a secondary messenger, calcium participates in various physiological and biochemical reactions in plants. Photosynthesis is the most extensive biosynthesis process on Earth. To date, researchers have found that some chloroplast proteins have Ca²⁺-binding sites, and the structure and function of some of these proteins have been discussed in detail. Although the roles of Ca²⁺ signal transduction related to photosynthesis have been discussed, the relationship between calcium and photosynthesis is seldom systematically summarized. In this review, we provide an overview of current knowledge of calcium’s role in photosynthesis.

New putative chloroplast vesicle transport components and cargo proteins revealed using a bioinformatics approach: an Arabidopsis model

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.

Dimerization of TOC receptor GTPases and its implementation for the control of protein import into chloroplasts

Pre-protein import into chloroplasts is facilitated by multiprotein translocon complexes in the envelope membranes. Major components of the TOC (translocon at the outer envelope membrane of chloroplasts) complex are the receptor proteins Toc33 and Toc159. These two receptors are related GTPases, and they are predicted to engage in homodimerization and/or heterodimerization. Although such dimerization has been studied extensively, its exact function in vivo remains elusive. In this issue of the Biochemical Journal, Oreb et al. present evidence that homodimerization of Toc33 prevents nucleotide exchange, thereby locking the receptor in the GDP-loaded state and preventing further activity. Pre-protein arrival is proposed to release this lock, through disruption of the dimer and subsequent nucleotide exchange. The Toc33-bound pre-protein is then able to progress to downstream steps in the translocation mechanism, with GTP hydrolysis defining another important control point as well as preparing the receptor for the next pre-protein client. These new results are discussed in the context of previous findings pertaining to TOC receptor dimerization and function.

Rapid isolation of Arabidopsis chloroplasts and their use for in vitro protein import assays

In vitro chloroplast protein import assays have been performed since the late 1970s, initially with plant species (e.g., pea and spinach) that readily provide an abundant source of starting material and also, subsequently, a good yield of chloroplasts for import assays. However, the sequencing of the Arabidopsis genome paved the way for an additional model system that is more amenable to genetic analysis, as a complement to the more biochemically orientated models such as pea and spinach. A prerequisite for this change was an efficient and reliable protocol for the isolation of chloroplasts for use in protein import assays, enabling biochemical approaches to be combined with the genetic potential of the plant. The method described here was developed as a rapid and low-cost procedure that can be accessed by everyone due to its simplicity. Despite its rapidity and simplicity, the method yields highly pure chloroplasts, and in addition works well with mutant plants that exhibit pale or chlorotic phenotypes. The protocol is also optimized for work with material from young plants (10-14 days old), when protein import is believed to be at its peak, and so plant growth can be conducted in vitro on Murashige and Skoog medium. The isolation method has been used not only for protein import assays, but also for proteomic analysis and further subfractionation studies.

Genes co-expressed with CPSAR1 identified using ATTED-II

Identification of interacting proteins will help to investigate further the relationship between CPSAR1 and the vesicle transport system or the ribosomes. Thus, we adopted a bioinformatic approach, using the publicly available Arabidopsis thaliana trans-factor and cis-element prediction database, ATTED-II (http://atted.jp/), to identify putative protein interactors. The proteins directly linked to CPSAR1 were almost exclusively nucleus encoded and several were involved in protein synthesis of which three were thylakoid localized. The list of putative interacting proteins does not exclude any of the previous proposed actions of CPSAR1 but encourage more detailed examination of the role of CPSAR1.

The TOC complex: preprotein gateway to the chloroplast

Photosynthetic eukaryotes strongly depend on chloroplast metabolic pathways. Most if not all involve nuclear encoded proteins. These are synthesized as cytosolic preproteins with N-terminal, cleavable targeting sequences (transit peptide). Preproteins are imported by a major pathway composed of two proteins complexes: TOC and TIC (Translocon of the Outer and Inner membranes of the Chloroplasts, respectively). These selectively recognize the preproteins and facilitate their transport across the chloroplast envelope. The TOC core complex consists of three types of components, each belonging to a small family: Toc34, Toc75 and Toc159. Toc34 and Toc159 isoforms represent a subfamily of the GTPase superfamily. The members of the Toc34 and Toc159 subfamily act as GTP-dependent receptors at the chloroplast surface and distinct members of each occur in defined, substrate-specific TOC complexes. Toc75, a member of the Omp85 family, is conserved from prokaryotes and functions as the unique protein-conducting channel at the outer membrane. In this review we will describe the current state of knowledge regarding the composition and function of the TOC complex.