A Beckman Optima XL-I analytical ultracentrifuge (Beckman Coulter Inc.) equipped with both absorbance and interference optical detection systems was used in the analytical ultracentrifugation experiment. Protein at an initial concentration of 1 mg/ml dissolved in 20 mM Tris-HCl (pH 8.0) and 50 mM NaCl was loaded into six-sector cells. Samples were centrifuged at 50,000 rpm to equilibrium at 20uC. Data were collected in continuous mode at a wavelength of 235 nm using a time interval of 300 s. The results were analyzed by the SEDFIT program (http://www.analyticalultracentrifugation.com) as described by Schuck [57] .