A biotechnological approach for the production of new protein bioplastics

The future of biomaterial production will leverage biotechnology based on thedomestication of cells as biological factories. Plants, algae, and bacteria can producelow-environmental impact biopolymers. Here, two strategies were developedto produce a biopolymer derived from a bioengineered vacuolar storage protein ofthe common bean (phaseolin; PHSL). The cys-added PHSL* forms linear-structuredbiopolymers when expressed in the thylakoids of transplastomic tobacco leaves byexploiting the formation of inter-chain disulfide bridges. The same protein withoutsignal peptide (?PHSL*) accumulates in Escherichia coli inclusion bodies as high-molarmassspecies polymers that can subsequently be oxidized to form disulfide crosslinkingbridges in order to increase the stiffness of the biomaterial, a valid alternative to theuse of chemical crosslinkers. The E. coli cells produced 300 times more engineeredPHSL, measured as percentage of total soluble proteins, than transplastomic tobaccoplants. Moreover, the thiol groups of cysteine allow the site-specific PEGylation of?PHSL*, which is a desirable functionality in the design of a protein-based drug carrier.In conclusion, ?PHSL* expressed in E. coli has the potential to become an innovativebiopolymer.