Cells of the human adenocarcinoma line A431 secrete a truncated form of the EGF receptor (sEGF-R). This soluble receptor with a molecular mass of ca. 110 kDaltons has been isolated from different culturing time points ranging from 40 to 710 days. Carbohydrate analysis of sEGF-R indicates the presence of one to two O-linked oligosaccharides and at least 6 N-linked oligosaccharides per mole of protein. IEF gel analysis of the sEGF-R from various culture time-points suggests significant changes in pI occurred at about 200 days as indicated by an acidic shift, i.e. , from a pI range of pH 6.3 to 6.8 to a pI range of 4.9 to 6.5. The determined ligand-receptor equilibrium dissociation constants using the equilibrium dialysis method for the receptor with acidic and near neutral pIs are 315 plus/minus 40nM respectively. Although the sEGF-R is observed to be refractory toward protease digestion by trypsin, SV8, and Chymotrypsin at 1:20 enzyme substrate ratio at 37 degrees C up to 24 hrs, the receptor becomes protease sensitive in the presence of 2M GuHC1, condition under which the sEGF-R is known to undergo a denaturation transition as determined by CD.
