Protocol for Spontaneous and Chaperonin-assisted in vitro Refolding of a Slow-folding Mutant of GFP, sGFP.

Understanding the folding pathway of any protein is of utmost importance for deciphering the folding problems under adverse conditions. We can obtain important information about the folding pathway by monitoring the folding of any protein from its unfolded state. It is usually very difficult to monitor the folding process in real time as the process is generally very fast, and we need a suitable read out. In this protocol, we have solved this issue by using a protein that is non-fluorescent in its unfolded state but fluoresces in its native state after folding. The kinetics of refolding can be monitored by following the increase in fluorescence in real time. Previously, this was generally achieved by either monitoring a protein's enzymatic activity or measuring the tryptophan fluorescence, where the signal output depends on well-described enzymatic activity or the frequency of tryptophan residues present in the proteins, respectively. Here, we describe a simple and real-time assay to monitor the refolding of sGFP, a recently described slow-folding mutant of yeGFP (yeast enhanced GFP). We unfold this protein using chemical denaturant and refold in a suitable buffer, monitoring the increase in fluorescence over time. GFP is fluorescent only when correctly folded; thus, using this technique, we can measure the true rate of protein refolding by following the increase in fluorescence over time. Therefore, sGFP can be used as an ideal model to study the in vitro protein folding process. Accordingly, the effects of different conditions and molecules on the protein folding pathway can be efficiently studied using sGFP as a model protein. Graphical abstract: Schematic of the steps involved in the sGFP refolding pathway. Native sGFP is unfolded by chemical denaturation using 6 M GuHCl at 25°C for 1 hour and then refolded in refolding buffer by 100-fold dilution.