Maltose-Binding Protein as a Solubility Enhancer

A major impediment to the production of recombinant proteins in Escherichia coli is their tendency to accumulate in the form of insoluble and biologically inactive aggregates known as inclusion bodies. Although it is sometimes possible to convert aggregated material into native, biologically-active protein, this is a time consuming, labor-intensive, costly, and uncertain undertaking (1 ). Consequently, many tricks have been employed in an effort to circumvent the formation of inclusion bodies (2 ). One approach that shows considerable promise is to exploit the innate ability of certain proteins to enhance the solubility of their fusion partners. Although it was originally thought that virtually any highly soluble protein could function as a general solubilizing agent, this has not turned out to be the case. In a direct comparison with glutathione S-transferase (GST) and thioredoxin, maltose-binding protein (MBP) was decidedly superior at solubilizing a diverse collection of aggregation-prone passenger proteins (3 ). Moreover, some of these proteins were able to fold into their biologically active conformations when fused to MBP. It is not entirely clear why MBP is such a spectacular solubilizing agent, but there is some evidence to suggest that it may be able to function as a general molecular chaperone in the context of a fusion protein by temporarily sequestering aggregation-prone folding intermediates of its fusion partners and preventing their self association (3 –6 ).