HUMAN STEM-CELL FACTOR DIMER FORMS A COMPLEX WITH 2 MOLECULES OF THE EXTRACELLULAR DOMAIN OF ITS RECEPTOR, KIT

Stem cell factor (SCF) is a cytokine that is active toward hematopoiet ic progenitor cells and other cell types, including germ cells, melano cytes, and mast cells, which express its receptor, the tyrosine kinase , Kit. SCF exists as noncovalently associated dimer at concentrations where it has been possible to study its quaternary structure; it stimu lates dimerization and autophosphorylation of Kit at the cell surface, We have used recombinant versions of human SCF and human Kit extracel lular domain (sKit) to study SCF-Kit interactions, By size exclusion c hromatography, plus various physical chemical methods including light scattering, sedimentation equilibrium, and titration calorimetry, we d emonstrate the formation of complexes containing a dimer of SCF (ungly cosylated SCF1-165) plus two molecules of sKit, The concentrations of SCF and sKit in these studies were in the range of 0.35-16.2 mu M. The data are analyzed and discussed in the context of several possible mo dels for complex formation, In particular, the sedimentation data are not consistent with a model involving cooperative binding. The K-d est imate for SCF-sKit interaction, obtained by sedimentation equilibrium, is about 17 nM at 25 degrees C, With glycosylated SCF1-165, the K-d i s considerably higher.