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This review focuses on the peculiar features of plant cells that allow …
Biology Articles » Biotechnology » Red Biotechnology » Recombinant Pharmaceuticals from Plants: The Plant Endomembrane System as Bioreactor » Figures
Figure 1. The secretory pathways of plant cells versus animal cells: Opportunities to exploit plants as bioreactors. Secretory proteins enter the ER (delimited here by the broken line, indicating membrane-bound ribosomes) in both cell types and travel to the Golgi complex via vesicular trafficking. In the absence of ER-retention signals, soluble proteins are then secreted (blue arrows). Vesicular sorting to the inner hydrolytic compartments (lytic vacuoles in plants, lysosomes in animals; green arrows) requires signals that differ in plants and animals. In addition to lytic vacuoles, plant cells may also have protein storage vacuoles (PSV), which develop according to specific signals and protein sorting mechanisms (orange arrow) and may be useful in the biotechnological production of recombinant pharmaceutical proteins. Vast amounts of proteins of cereals (e.g., the prolamin storage proteins) form protein bodies (PB) within the ER in aggregation processes that may also be exploited for biotechnology; the aggregation mechanisms are an area of active research and may involve prolonged interactions with chaperones, the formation of inter-chain disulfide bonds, and selective insolubility (28). Oil bodies and lipid droplets originate from the ER by the deposition of lipids in the space between the two monolayers of the ER membrane (gray arrows).
Figure 2. A domain of a maize storage protein can be used recombinantly to form ER-located protein bodies. Electron micrograph of a thin section of a young leaf of transgenic tobacco that expresses "zeolin," a fusion between the N-terminal domain of the maize prolamin -zein and the vacuolar protein phaseolin. The recombinant protein forms electron-dense protein bodies (PB) not normally found in leaf cells. Zeolin was visualized with antibodies against phaseolin and secondary antibody-gold complex (black dots inside the PBs). The large empty area that occupies most of the cell is the lytic vacuole. [Ch, chloroplast; Mt, mitochondrion; CW, cell wall. Bar = 500 nm. Reproduced from (35).]
Figure 3. Strategies to accumulate high amounts of recombinant pharmaceuticals in the ER and ER-derived oil bodies. Through the use of plant protein domains (e.g., zein domains, oleosins, or tail-anchors), multiple protein fusions have been examined. Zein domains allow protein body formation within the ER lumen. Oleosin fusion products are first inserted into the external monolayer of the ER bilayer and then integrated into a nascent oil body (yellow). The tail anchor "anchors" recombinant proteins to the ER surface; upon overexpression, the ER can be commandeered in this way to develop distinct topologies that increase the stability of recombinant proteins. The hydrophobic domains of oleosin and of tail-anchored proteins are indicated in violet and red, respectively. Note that the N-terminal and C-terminal domains of oleosin are in the cytosol, and the very short C-terminal sequence of the tail-anchor is instead luminal.
Figure 4. Human lysozyme is sorted into protein bodies and storage vacuoles in transgenic rice seeds. Electron micrographs of developing seed tissue (i.e., endosperm) of non-transgenic A. and transgenic B. rice seeds. In non-transgenic tissue A. the storage proteins accumulate in ER-derived protein bodies (PB-I) and in protein storage vacuoles (PB-II). In transgenic rice expressing human lysozyme B. the recombinant protein (visualized by immunogold labeling) accumulates in both PB-I-like (arrowheads) and PB-II-like (arrows) structures. Notice the ribosomes decorating the membrane of PB-I, indicating the ER origin. [Reproduced from (60).]
Source: Molecular Interventions 5:216-225, (2005).
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