Refolding Protocol of the Cysteine-Rich Domain of Frizzled8 from Inclusion Bodies

Properly folded proteins are essential for studying protein function and developing therapeutic applications. However, when recombinant proteins are overexpressed in bacterial systems such as Escherichia coli, they often accumulate as inclusion bodies, insoluble protein aggregates in non-native conformations. Here, we report a refolding protocol for mouse Frizzled8 cysteine-rich domain (mouse FZD8 CRD) recovered from inclusion bodies, yielding a soluble, folded CRD preparation suitable for NMR analysis. Because mouse FZD8 CRD contains multiple conserved cysteine residues that must form an appropriate intramolecular disulfide-bonding pattern, the refolding strategy combines denaturant-mediated solubilization, dilution into a redox-buffered refolding solution, concentration, dialysis, lyophilization, and HPLC purification. Lyophilization followed by HPLC purification enables the resolution of the desired CRD species from aggregated and misfolded products. In this workflow, HPLC served as a key purification step, providing the resolution needed to enrich a folded, multi-disulfide-bond-containing CRD species from closely related misfolded species. NMR spectroscopy supported the presence of a folded, conformationally homogeneous CRD preparation. This workflow was also applied to mouse secreted Frizzled-related protein 3 cysteine-rich domain (mouse sFRP3 CRD), suggesting that it may be useful for other Frizzled-type CRDs with conserved cysteine-rich architectures, although additional validation will be required before broader application to more distantly related cysteine-rich proteins.