A single cystein-enriched phaseolin expressed in transplastomic tobacco plants accumulates as a biopolymer

Recently, transformation of chloroplast genome has been used for the production of heterologous proteins. We transformed tobacco chloroplasts with two different versions of the storage protein of Phaseolus vulgaris, phaseolin (with or without signal peptide), in which a cysteine residue has been added to its C-terminal region. This modification allows for the formation of inter-chain disulfide bonds, as previously demonstrated in our lab. Our purpose here is to demonstrate the different ability of chloroplast compartments (stroma and thylakoids) in the formation of phaseolin polypeptides held together by disulfide bonds. We observed that the presence of the signal peptide directs phaseolin into the thylakoid compartment, where the protein is able to form disulfide bridges and high molecular weight polymers, which represent about 0.05 % of the total soluble proteins. The formation of phaseolin polymers, not detected in P. vulgaris, could be very interesting for industrial purposes. The chloroplast could be utilized as a platform for the production of a biopolymer that derives from an edible protein. A possible application is the production of biodegradable films.