Analyses of the ternary GM-CSF receptor complex by sedimentation equilibrium.

The individual proteins or protein complexes in 150 mM NaCl/50 mM sodium phosphate, pH 7.0, were centrifuged at 20°C at angular velocity, W rpm, for 16 hours. The equilibrium profiles are presented as (W/W1)2 Ln(c/c_o) versus the square of the radial distance, where c/c_o is the optical density at 280 nm divided by the initial optical density and W1 is 20 000 rpm. For a single species, this plot is linear with a slope proportional to the molecular weight of the sedimenting species. The initial concentrations were in the range 0.40 to 0.47 mg/mL, and samples were centrifuged at 20 000 rpm except for E21R, where the initial concentration was 0.2 mg/mL and the angular velocity 15 000 rpm. Panel A samples: GM-CSF (○), sGMRα (■), GM-CSF/sGMRα complex (●). Panel B samples: E21R (○), E21R/sGMRα complex (●). Panel C samples: purified sβc (■) was centrifuged at 15 000 rpm with an initial concentration of 0.45 mg/mL, whereas the GM-CSF/sGMRα/sβc ternary complex at an initial concentration of 0.47 mg/mL was centrifuged at either 8000 rpm (▴) or 15 000 rpm (●) for 16 hours at 20°C.