The Ability to Enhance the Solubility of Its Fusion Partners Is an Intrinsic Property of Maltose-Binding Protein but Their Folding Is Either Spontaneous or Chaperone-Mediated

<div><p><em>Escherichia coli</em> maltose binding protein (MBP) is commonly used to promote the solubility of its fusion partners. To investigate the mechanism of solubility enhancement by MBP, we compared the properties of MBP fusion proteins refolded <em>in vitro</em> with those of the corresponding fusion proteins purified under native conditions. We fused five aggregation-prone passenger proteins to 3 different N-terminal tags: His₆-MBP, His₆-GST and His₆. After purifying the 15 fusion proteins under denaturing conditions and refolding them by rapid dilution, we recovered far more of the soluble MBP fusion proteins than their GST- or His-tagged counterparts. Hence, we can reproduce the solubilizing activity of MBP in a simple <em>in vitro</em> system, indicating that no additional factors are required to mediate this effect. We assayed both the soluble fusion proteins and their TEV protease digestion products (i.e., with the N-terminal tag removed) for biological activity. Little or no activity was detected for some fusion proteins whereas others were quite active. When the MBP fusions proteins were purified from <em>E. coli</em> under native conditions they were all substantially active. These results indicate that the ability of MBP to promote the solubility of its fusion partners <em>in vitro</em> sometimes, but not always, results in their proper folding. We show that the folding of some passenger proteins is mediated by endogenous chaperones <em>in vivo</em>. Hence, MBP serves as a passive participant in the folding process; passenger proteins either fold spontaneously or with the assistance of chaperones.</p>