Role of MgcRacGAP/Cyk4 as a regulator of the small GTPase Rho family in cytokinesis and cell differentiation

To identify the key molecules that regulate differentiation of hematopoietic cells, we carried out retrovirus-mediated functional screening for cDNAs whose expression suppresses IL-6-induced differentiation of mouse myeloid leukemic M1 cells. From this screening, we obtained a full length cDNA encoding a mouse homologue of human MgcRacGAP. Overexpression of the anti-sense MgcRacGAP profoundly inhibited IL-6-induced macrophage-differentiation of M1 cells. On the other hand, overexpression of the full-length form of MgcRacGAP alone enhanced macrophage differentiation of M1 cells in response to IL-6, and induced macrophage differentiation of HL-60 leukemic cells. To determine how this protein regulates differentiation and proliferation, an antibody against MgcRacGAP was prepared. Immunohistochemical studies revealed that MgcRacGAP mainly localizes in the nucleus in interphase, accumulates on the mitotic spindle in metaphase, and is condensed in the midbody during cytokinesis. Overexpression of an N-terminal domain deletion mutant, which lacks the ability to localize to the midbody through association with tubulins, or a GAP-inactive mutant resulted in the formation of multinucleated cells in HeLa cells as well as in hemopoietic cells. Interestingly, MgcRacGAP in the midbody was phosphorylated probably on serine and threonine residues. These results indicate that MgcRacGAP regulates cytokinesis and cellular differentiation as a regulator of Rho family of GTPase and suggest that this process is controlled by some serine/threonine kinases.