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<p><strong> STORAGE VACUOLES FOR PROTEIN ACCUMULATION </strong></p> <p>In the seeds of most plants, including the economically important grain legumes and oil seeds, storage proteins accumulate in vacuoles termed protein storage vacuoles (PSVs). The difference between PSVs and the "typical" vegetative vacuoles is not simply due to the presence of storage proteins in the former. PSVs originate de novo during seed maturation, and the mechanisms of protein sorting to the two types of vacuoles are, at least in part, distinct (<a href="http://www.biology-online.org/articles/figures-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm">Figure 1</a>) <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R53">53</a>)</em>. Young cells that are not yet differentiated often have both types of vacuoles, and during differentiation either of the two types becomes prevalent, depending on the role that the cell ultimately assumes. PSVs have a slightly higher pH and lower hydrolytic activity than vacuoles of vegetative tissues <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R58">58</a>)</em>. </p> <p>PSVs of seeds are therefore an attractive compartment of accumulation for recombinant proteins or peptides. In a pioneering experiment, the DNA sequence coding for the pentapeptide Leuenkephalin was inserted, flanked by tryptic cleavage sites, into the "2S storage protein" of <em>Arabidopsis thaliana (<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R59">59</a>)</em>. The peptide replaced five amino acids present in a region that is not conserved in storage proteins of the same class and is therefore probably not fundamental for correct folding. Transgenic oilseed rape plants were produced, and Leuenkephalin could be prepared from seeds by obtaining a protein fraction enriched in the recombinant 2S storage protein and subsequent protease digestion followed by chromatography <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R59">59</a>).</em> </p> <p>Recent experiments suggest that it may not always be necessary to perform sophisticated protein engineering to allow targeting and accumulation of recombinant proteins in seed storage compartments. Human lysozyme, which is a secreted protein, accumulates in ER-derived protein bodies and in PSVs when expressed in the endosperm of rice seeds (<a href="http://www.biology-online.org/articles/figures-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm">Figure 4</a>) <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R60">60</a>)</em>. Recombinant lysozyme produced in rice seeds is biologically active <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R61">61</a>)</em>. </p> Rice naturally contains both ER-stored and vacuolar storage proteins; it is therefore possible, although not proved, that the high synthesis of these proteins has a dominant effect on the localization of foreign proteins expressed in the secretory pathway, perhaps because the aggregation properties of storage proteins <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R53">53</a>)</em> can influence traffic in the endomembrane system. The dominant effect of storage proteins on subcellular localization has also been observed in transgenic wheat. Both human serum albumin engineered to contain the C-terminal KDEL ER-localization signal and <em>Asperigillus niger</em> phytase (another secreted protein) are delivered to protein storage vacuoles together with storage proteins when expressed in wheat endosperm <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R62">62</a>)</em>. In the leaf cells of the same transgenic plants <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R62">62</a>),</em> the human serum albumin–KDEL construct efficiently accumulates in the ER, and phytase expressed in tobacco leaves is secreted <em>(<a href="http://www.biology-online.org/articles/references-recombinant_pharmaceuticals_from_plants_the_plant_endomembrane_system_as_bioreactor.htm#R63">63</a>).</em> Therefore, the two proteins have the subcellular localization expected from their molecular properties when expressed in vegetative plant tissues, confirming that vacuolar delivery in wheat endosperm was tissue-specific.
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