Production of complex heterologous proteins and protein assemblies using E. coli-based cell-free protein synthesis [electronic resource]
Keywords: Chemical Engineering

The Swartz lab has put much effort into understanding the underlying principles of E. coli-based cell-free protein synthesis (CFPS), and the technology has developed into a scalable, affordable platform for producing a wide range of protein targets. Key breakthroughs have included activating central metabolism, stabilization of critical amino acids, controlling the redox environment to produce proteins containing disulfide bonds, and using scale-up technologies to produce proteins at milligram quantities. My work has sought to expand this CFPS technology for producing valuable and complex eukaryotic protein targets by manipulating and optimizing the folding of these proteins in the heterologous CFPS environment. Furthermore, I have sought to apply these advances to specific applications of interest. By modifying a key molecular chaperone native to the eukaryotic endoplasmic reticulum (ER), the Hsp70-family chaperone, BiP, soluble production was increased in CFPS reactions for specific proteins normally secreted through this organelle, namely those from the immunoglobulin superfamily which includes antibodies, T-cell receptors, and many membrane receptors. First, the functional properties of BiP were compared to that of the E. coli Hsp70, DnaK. A fusion protein was then constructed between BiP and the ribosome-binding portion of the E. coli protein, trigger factor, to localize BiP to translating ribosomes. This replicated the native function of BiP, which provides co-translational folding assistance to nascent polypeptides. After verifying its bioactivity, this fusion protein was utilized in CFPS reactions to indicate that the chaperone functions of BiP are specific to proteins normally secreted through the eukaryotic ER, whereas DnaK demonstrates a more general chaperone mechanism. Since the discovery that somatic cells could be reprogrammed back to a pluripotent state through the viral expression of a specific set of transcription factors, there has been great interest in reprogramming using a safer and more clinically relevant protein-based approach. Production of these transcription factor proteins was greatly increased by fusing them to the C-terminus of the solubility partner, IF2 domain 1 (IF2D1). While the fusions provided marginal benefit in molar yields using a CFPS approach, in vivo E. coli expression produced the transcription factors in soluble form. The fusion proteins could be purified in milligram quantities from liter shake-flask cultures, whereas essentially no soluble protein accumulated without the fusion partner. The transcription factor fusions bound specifically to their consensus DNA sequences and partially activated some of their downstream gene targets. Another application utilizing CFPS technology is an enhanced luciferase mutant from the marine copepod, Gaussia princeps (GLuc). GLuc is both the smallest and brightest known luciferase, and previous work from our lab demonstrated that this protein could be produced at higher volumetric yields and specific activities in CFPS compared to conventional protein expression systems. By mutating key residues in the Gaussia luciferase sequence, the luminescence half-life was shown to increase over ten-fold while maintaining the initial specific activity of the wild-type. This improved mutant provides a valuable imaging agent to use in fusions and bioconjugates with other proteins such as those that recognize cell surface markers on cancer cells. In a final application, influenza vaccines were produced using CFPS by isolating specific fragments of the protein hemagglutinin (HA), a viral surface protein. Specific mutations as well as a C-terminal trimerization domain were critical for producing this protein fragment in both its monomeric and native trimeric forms. By using the CFPS platform to incorporate non-natural amino acids (nnAAs) with alkyne functional groups, the HA proteins were covalently 'clicked' to virus-like particles (VLPs) that had surface exposed nnAAs with azide functionality. The VLPs provide an immunogenic delivery platform that efficiently traffics to lymph nodes and allows for co-attachment of other adjuvants in addition to the primary HA antigen. This vaccine platform was characterized and tested in mouse models and compared to both a standard influenza vaccine as well as free HA protein fragments. In summary, CFPS is a valuable and robust method of protein production for a variety of targets. My thesis has sought to use this platform as a means to better understand folding pathways of complex, eukaryotic proteins and improve production of these proteins. To this end, CFPS has been shown to be a valuable method for elucidating and manipulating chaperone function, producing difficult proteins with enhanced function, and as a platform to produce novel vaccines.