Figure 3

Binding of radiolabeled proteins on cell surfaces. (a) Dose-dependent binding of iodinated core bone morphogenic protein (wild type) (coreBMP2wt) to C2C12 cells (total binding, black squares). Unspecific binding as determined by addition of a 1,000-fold excess of cold ligand (blue diamonds) was subtracted resulting in specific binding of the ligand (red stars). (b) Comparison of specific binding of coreBMP2wt to COS-7 cells transfected with either BMP receptor (BMPR)-IA or activin receptor (ActR)-IIB or cotransfected with both receptors chains. (c) Specific binding of the iodinated heterodimeric coreBMP2/L51P mutant to BMPR-IA transfected COS-7 cells using ligand concentration up to 4 nM. (d) Specific binding of a radiolabeled anti-BMPR-IA Fab fragment to either untransfected (black squares) or BMPR-IA transfected (red asterisks) COS-7 cells. At all cases specific binding was fitted to a one-site binding model resulting in the indicated values for K _D and B _max.