N-terminal domains of native multidomain proteins have the potential to assist de novo folding of their downstream domains in vivo by acting as solubility enhancers.

The fusion of soluble partner to the N terminus of aggregation-prone polypeptide has been popularly used to overcome the formation of inclusion bodies in the E. coli cytosol. The chaperone-like functions of the upstream fusion partner in the artificial multidomain proteins could occur in de novo folding of native multidomain proteins. Here, we show that the N-terminal domains of three E. coli multidomain proteins such as lysyl-tRNA synthetase, threonyl-tRNA synthetase, and aconitase are potent solubility enhancers for various C-terminal heterologous proteins. The results suggest that the N-terminal domains could act as solubility enhancers for the folding of their authentic C-terminal domains in vivo. Tandem repeat of N-terminal domain or insertion of aspartic residues at the C terminus of the N-terminal domain also increased the solubility of fusion proteins, suggesting that the solubilizing ability correlates with the size and charge of N-terminal domains. The solubilizing ability of N-terminal domains would contribute to the autonomous folding of multidomain proteins in vivo, and based on these results, we propose a model of how N-terminal domains solubilize their downstream domains.