Comparative study of solubilization tags to improve protein secretion in a baculovirus expression system

• Eleven tags were screened for their effects on the solubility of recombinant LepA. • Five tags, including BmThymosin, significantly enhanced protein secretion levels. • Purified rLepA proteins exhibited comparable activity to that of tag-free LepA. Although the silkworm-baculovirus expression system (BES) is known as an excellent recombinant secreted protein expression system, some recombinant proteins are difficult to mass produce. One reason for this is that the protein structure itself is unstable and protein aggregation occurs. In this study, we used the chub mackerel leptin-A (cmLepA), a protein that has been shown to induce gonadotropin secretion in fish, as a model protein and examined the effects of eleven fusion peptides that are expected to enhance the solubility of recombinant proteins. Our results showed that soluble secretion levels of the recombinant LepA (rLepA) were significantly increased in the serum of baculovirus-infected silkworms when BmThymosin, ScSUMO, BmSUMO, GB1, or T7SET solubility tags were fused. The purified rLepA proteins were directly assayed in fish cells for the biological activity without cleavage of the fusion tags, revealing that the BmThymosin-fused protein exhibited comparable activity to that of tag-free LepA. Collectively, our findings provide a useful set of fusion tags designed to improve the secretion and solubility of proteins of interest when using the silkworm-baculovirus expression system.