Chaperonin 60

Chaperonin 60 (GroEL) and β-galactosidase from Escherichia coli were purchased as lyophilized powders from Sigma-Aldrich. Both proteins were dissolved in 100 mM ammonium acetate at ~1 mg/mL. GroEL was precipitated by mixing 200 μL acetone in 100 μL protein solution and allowing the mixture to sit for 5 min. The mixture was centrifuged at 14,000 g for 5 minutes, the liquid was pipetted out, and the precipitate was resuspended in 100 μL of 100 mM NH4OAc. This solution was filtered three times using Amicon Ultra-0.5 centrifugal filters with a molecular weight cut-off of 100 kDa. Each rinse used 400 μL 100 mM NH4OAc. The protein was recovered by inverting the filter and running the centrifuge at 2,000 g for 2 min. β-galactosidase was filtered three times and collected using the same Amicon filters and the same centrifuge settings. Polypropylene glycol (PPG, molecular weight ~2700) was purchased from Sigma-Aldrich. TEM grids (carbon support film on 400 mesh copper) and 1% uranyl acetate staining solution were purchased from Electron Microscopy Sciences.