The protein solubility was measured using the method described by
Li, Xiong, and Chen (2012) with some modifications. One milliliter protein
solution (5 mg/mL, dissolved in 20 mM PBS pH 6.0 containing 0.6 M
NaCl) mixed with 4 mL distilled water to get 1 mg/mL protein solution.
After 30 min of stirring at room temperature, the samples were centrifuged
(10,000 ×g, 4 °C, 15 min). The protein concentrations in the
supernatants were measured by a bicinchoninic acid (BCA) protein
quantification assay (Thermo Fisher Scientific, Germany).