Correlation between temperature-dependent cytoplasmic solubility and periplasmic export of a heterologous protein in Escherichia coli

The coding sequence of mature human tumor necrosis factor (hTNF) was fused to the signal-encoding sequence of beta-lactamase (Bla). Mature hTNF was exported into the periplasm of Escherichia coli. A mutant hTNF [Van Ostade et al., FEBS Lett. 238 (1988) 347-352], which displays a temperature-dependent intracellular solubility, was fused to the same Bla signal-encoding sequence. We found that the export competence of the mutated hTNF was correlated with the intracellular solubility of this protein. We postulate that the secretion proficiency of eukaryotic proteins, when fused to a prokaryotic export signal, depends on the ability of the mature protein to readily fold into a soluble conformation.