Seed-based expression systems for plant molecular farming.

The evolution of the seed system provides enormous adaptability to the gymnosperms and angiosperms, because of the properties of dormancy, nutrient storage and seedling vigour. Many of the unique properties of seeds can be exploited in molecular farming applications, particularly where it is desirable to produce large quantities of a recombinant protein. Seeds of transgenic plants have been widely used to generate a raw material for the extraction and isolation of proteins and polypeptides, which can be processed into valuable biopharmaceuticals. The factors that control high-level accumulation of recombinant proteins in seed are reviewed in the following paragraphs. These include promoters and enhancers, which regulate transcript abundance. However, it is shown that subcellular trafficking and targeting of the desired polypeptides or proteins play a crucial role in their accumulation at economically useful levels. Seeds have proven to be versatile hosts for recombinant proteins of all types, including peptides or short and long polypeptides as well as complex, noncontiguous proteins like antibodies and other immunoglobulins. The extraction and recovery of recombinant proteins from seeds is greatly assisted by their dormancy properties, because this allows for long-term stability of stored products including recombinant proteins and a decoupling of processing from the growth and harvest cycles. Furthermore, the low water content and relatively low bioload of seeds help greatly in designing cost-effective manufacturing processes for the desired active pharmaceutical ingredient. The development of cGMP processes based on seed-derived materials has only been attempted by a few groups to date, but we provide a review of the key issues and criteria based on interactions with Food and Drug Administration and European Medicines Agency. This article uses 'case studies' to highlight the utility of seeds as vehicles for pharmaceutical production including: insulin, human growth hormone, lysozyme and lactoferrin. These examples serve to illustrate the preclinical and, in one case, clinical information required to move these plant-derived molecules through the research phase and into the regulatory pathway en route to eventual approval.

Expression and purification of the chloroplast putative nitrogen sensor, PII, of Arabidopsis thaliana.

The bacterial PII protein was discovered over 30 years ago and is known to be a key player in orchestrating the coordination of nitrogen metabolism with changes in carbon flux. Bacterial PII is regulated by covalent modification and binding to effector molecules in response to the nitrogen/carbon status of the cell and appropriately coordinates the activity of glutamine synthetase and the transcription of a nitrogen sensitive regulon. Recently, a PII protein was identified in higher plants and the protein was found to be localized to the chloroplast. The Arabidopsis thaliana putative nitrogen sensor protein, PII, was cloned and overexpressed with a C-terminal 6-histidine tag. The full-length protein, which included the chloroplast transit peptide, was overexpressed in Escherichia coli, but was very susceptible to proteolytic degradation. Removal of the transit peptide yielded a highly pure, stable recombinant protein whose identity was established as PII by matrix assisted laser desorption ionization-time of flight mass spectrometry. Polyclonal antibodies generated against the recombinant protein effectively immunoprecipitated PII from an A. thaliana extract and the protein was confirmed to be 17 kDa in mass. The availability of milligram amounts of PII will allow a complete biophysical characterization of the protein and antibodies should aid in the identification of PII interacting proteins and the establishment of the higher plant PII signal transduction cascade.