Abstract
Plastids are the defining organelles of all photosynthetic eukaryotes. They are the site of photosynthesis and of a large number of other essential metabolic pathways, such as fatty acid and amino acid biosyntheses, sulfur and nitrogen assimilation, and aromatic and terpenoid compound production, to mention only a few examples. The metabolism of plastids is heavily intertwined and connected with that of the surrounding cytosol, thus causing massive traffic of metabolic precursors, intermediates, and products. Two layers of biological membranes that are called the inner (IE) and the outer (OE) plastid envelope membranes bound the plastids of Archaeplastida. While the IE is generally accepted as the osmo-regulatory barrier between cytosol and stroma, the OE was considered to represent an unspecific molecular sieve, permeable for molecules of up to 10 kDa. However, after the discovery of small substrate specific pores in the OE, this view has come under scrutiny. In addition to controlling metabolic fluxes between plastid and cytosol, the OE is also crucial for protein import into the chloroplast. It contains the receptors and translocation channel of the TOC complex that is required for the canonical post-translational import of nuclear-encoded, plastid-targeted proteins. Further, the OE is a metabolically active compartment of the chloroplast, being involved in, e.g., fatty acid metabolism and membrane lipid production. Also, recent findings hint on the OE as a defense platform against several biotic and abiotic stress conditions, such as cold acclimation, freezing tolerance, and phosphate deprivation. Moreover, dynamic non-covalent interactions between the OE and the endomembrane system are thought to play important roles in lipid and non-canonical protein trafficking between plastid and endoplasmic reticulum. While proteomics and bioinformatics has provided us with comprehensive but still incomplete information on proteins localized in the plastid IE, the stroma, and the thylakoids, our knowledge of the protein composition of the plastid OE is far from complete. In this article, we report on the recent progress in discovering novel OE proteins to draw a conclusive picture of the OE. A “parts list” of the plastid OE will be presented, using data generated by proteomics of plastids isolated from various plant sources.
Highlights
Plastids are the eponymous cellular organelles of the Archaeplastida and they host the majority of anabolic pathways
These fatty acids are used for lipid biosynthesis in the plastid envelopes and in the endoplasmic reticulum (ER)
In this review we described the multitude of functions associated with the outer plastid envelope
Summary
Plastids are the eponymous cellular organelles of the Archaeplastida (i.e., photosynthetic eukaryotes that contain plastids of primary endosymbiotic origin, known as the Plantae) and they host the majority of anabolic pathways. 1, 6 billion years of co-evolution irreversibly integrated the photoautotrophic prokaryote into the host cell, creating the plant cell as we know it (Figure 1; Yoon et al, 2004; Reyes-Prieto et al, 2007; Tyra et al, 2007) Plastids derived from this initial event, primary plastids, are bound by two surrounding envelope membranes, the inner (IE) and the outer (OE) envelope membrane. The 2.5to 3-nm wide pore is created by at least two OEP24 proteins This homodimer facilitates the transport of triose phosphates (TP), hexose-phosphates, sugars, ATP, phosphates (Pi), dicarboxylates like 2-oxoglutarate, and charged amino acids (Table 1; Pohlmeyer et al, 1998). It is hypothesized that this defect is due to a lipid and energy deprivation during early pollen development
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