c-kit associated with the transmembrane 4 superfamily proteins constitutes a functionally distinct subunit in human hematopoietic progenitors

The transmembrane 4 superfamily (TM4SF) has come into prominence for its association with a wide range of cell surface molecules, especially integrins. We report that TM4SF molecules CD9, CD63, and CD81 are physically associated with c-kit receptor tyrosine kinase in the human factor–dependent myeloid cell line, MO7e. We characterized this complex using coimmunoprecipitation and colocalization methods. The c-kit coimmunoprecipitated with anti-TM4SF antibodies showed several distinct phenotypes compared to the total c-kit immunoprecipitated with anti–c-kit antibody. These included: (1) higher basal level of tyrosine phosphorylation without elevated kinase activity in the absence of Steel factor (SLF), (2) deficient enhancement of tyrosine phosphorylation and kinase activity in response to SLF, (3) elevated binding rate of SLF shown in chemical cross-linking studies, and (4) little internalization and degradation after SLF treatment. Cocapping studies in living cells showed that c-kit colocalized with TM4SF molecules after SLF stimulation, suggesting confirmation of the biochemical data obtained by the coimmunoprecipitation studies. Colocalization of c-kit with CD81 by SLF was also observed in cord blood CD34+ cells, suggesting the existence of functional units of c-kit in TM4SF complexes in primary hematopoietic cells. This suggests that some TM4SF members may negatively modulate function of c-kit receptor tyrosine kinase and thus regulate receptor sensitivity to SLF in hematopoietic progenitors.