Characterization of a fusion protein composed of the extracellular domain of c-kit and the Fc region of human IgG expressed in a baculovirus system.

The extracellular domain of c-kit, which is the receptor for stem cell factor (SCF), was fused genetically to the Fc portion of human immunoglobulin G1. This chimeric protein, c-kitFc, was then expressed in the baculovirus system. The fusion product was secreted into the serum-free culture medium as a soluble dimeric form of approximately 210 Kda. The recombinant protein was easily purified by protein A affinity chromatography at approximately 25 micrograms protein per ml of medium. Binding assay and cross-linking assay showed that the fusion protein retained high affinity for binding SCF (Kd = 0.3 nM). Addition of the chimeric protein into the culture medium of SCF-dependent cells inhibited cell proliferation in a dose-dependent manner. These results suggest that the dimeric c-kitFc protein can be used as an antagonist of SCF for the study of hematopoietic progenitor cells.