Metabolic engineering of Escherichia coli for enhanced production of native-sized recombinant spider dragline silk protein

Abstract

Spider silk, a protein-based fiber secreted by spider, has been extensively studied due to its superior mechanical properties and biocompatibility in comparison to other available synthetic materials. The cannibalism and aggressiveness of spiders made harvesting spider silk in large quantities through spider breeding impossible, and much effort has been made on recombinant production of recombinant spider dragline silk proteins in heterologous hosts and spinning them into artificial fibers. Nevertheless, such process has been severely hindered by the difficulty of producing recombinant spidroins in heterologous hosts, mainly due to the limitations of the recombinant hosts in production of the large, highly repetitive, glycine-rich recombinant silk proteins. This study aims to develop a metabolic engineering approach for enhanced production of a native-sized recombinant spider dragline silk protein as large as 284.9 kDa in Escherichia coli. Availability of tRNA$^{Gly}$ was improved through increasing tRNA$^{Gly}$ transcription. Translational elongation was enhanced by overexpressing elongation factors to cope with challenge on E. coli’s translation machinery for expression of large and repetitive spidroin. Recombinant spidroin expression level was further elevated by overexpressing chaperones that play a key role in folding of newly translated spidroins. Through fed-batch fermentation of the best strain, 3.68 g·$l^{-1}$ of this native-sized recombinant spider dragline silk protein was obtained, which is the highest titre ever reported for this size of recombinant spider dragline silk protein. The results in this study provide insight into metabolic engineering for enhanced production of other large, repetitive proteins in E. coli.