Significant enhanced expression and solubility of human proteins in Escherichia coli by fusion with protein S from Myxococcus xanthus

Protein S is a major spore coat protein of Myxococcus xanthus, consisting of two homologous domains, the N-terminal domain (NTD) and the C-terminal domain, both of which contain a Ca²⁺-binding site. Protein S tightly binds to myxospores in a Ca²⁺-dependent manner. Here, we constructed a novel expression vector, pCold-PST, encoding two tandem repeat NTDs (PrS ₂). By using this vector, a number of human proteins that were expressed at low levels or in insoluble forms by a pET vector were expressed not only at high levels but also in soluble forms. We also demonstrated that an Escherichia coli protein tagged with PrS₂ fully retained its function, indicating that it is folded independently from the tag. This technology not only allows simple, one-step protein purification using myxospores, but can also be used for the identification of proteins interacting with a protein of interest and will prove immensely useful for structural studies of proteins which are difficult to produce or are insoluble.