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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.

Hepatitis B virus (HBV) infection is one of the major causes of hepatocellular carcinoma (HCC) development. Hepatitis B virus X protein (HBx) is known to play a key role in the development of hepatocellular carcinoma (HCC). Several cellular proteins have been reported to be over-expressed in HBV-associated HCC tissues, but their role in the HBV-mediated oncogenesis remains largely unknown. Here, we explored the effect of the over-expressed cellular protein, a ribosomal protein S3a (RPS3a), on the HBx-induced NF-κB signaling as a critical step for HCC development. The enhancement of HBx-induced NF-κB signaling by RPS3a was investigated by its ability to translocate NF-κB (p65) into the nucleus and the knock-down analysis of RPS3a. Notably, further study revealed that the enhancement of NF-κB by RPS3a is mediated by its novel chaperoning activity toward physiological HBx. The over-expression of RPS3a significantly increased the solubility of highly aggregation-prone HBx. This chaperoning function of RPS3a for HBx is closely correlated with the enhanced NF-κB activity by RPS3a. In addition, the mutational study of RPS3a showed that its N-terminal domain (1–50 amino acids) is important for the chaperoning function and interaction with HBx. The results suggest that RPS3a, via extra-ribosomal chaperoning function for HBx, contributes to virally induced oncogenesis by enhancing HBx-induced NF-κB signaling pathway.

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