Cell division is finely controlled by various molecules including small G proteins and kinases/phosphatases. Among these, Aurora B, RhoA, and the GAP MgcRacGAP have been implicated in cytokinesis, but their underlying mechanisms of action have remained unclear. Here, we show that MgcRacGAP colocalizes with Aurora B and RhoA, but not Rac1/Cdc42, at the midbody. We also report that Aurora B phosphorylates MgcRacGAP on serine residues and that this modification induces latent GAP activity toward RhoA in vitro. Expression of a kinase-defective mutant of Aurora B disrupts cytokinesis and inhibits phosphorylation of MgcRacGAP at Ser387, but not its localization to the midbody. Overexpression of a phosphorylation-deficient MgcRacGAP-S387A mutant, but not phosphorylation-mimic MgcRacGAP-S387D mutant, arrests cytokinesis at a late stage and induces polyploidy. Together, these findings indicate that during cytokinesis, MgcRacGAP, previously known as a GAP for Rac/Cdc42, is functionally converted to a RhoGAP through phosphorylation by Aurora B.

STAT transcription factors are tyrosine phosphorylated upon cytokine stimulation and enter the nucleus to activate target genes. We show that Rac1 and a GTPase-activating protein, MgcRacGAP, bind directly to p-STAT5A and are required to promote its nuclear translocation. Using permeabilized cells, we find that nuclear translocation of purified p-STAT5A is dependent on the addition of GTP-bound Rac1, MgcRacGAP, importin α, and importin β. p-STAT3 also enters the nucleus via this transport machinery, and mutant STATs lacking the MgcRacGAP binding site do not enter the nucleus even after phosphorylation. We conclude that GTP-bound Rac1 and MgcRacGAP function as a nuclear transport chaperone for activated STATs.

We recently demonstrated that STAT5 can induce a variety of biological functions in mouse IL-3-dependent Ba/F3 cells; STAT5-induced expression of pim-1, p21WAF/Cip1, and suppressor of cytokine signaling-1/STAT-induced STAT inhibitor-1/Janus kinase binding protein is responsible for induction of proliferation, differentiation, and apoptosis, respectively. In the present study, using a constitutively active STAT5A (STAT5A1*6), we show that STAT5 induces macrophage differentiation of mouse leukemic M1 cells through a distinct mechanism, autocrine production of IL-6. The supernatant of STAT5A1*6-transduced cells contained sufficient concentrations of IL-6 to induce macrophage differentiation of parental M1 cells, and STAT3 was phosphorylated on their tyrosine residues in these cells. Treatment of the cells with anti-IL-6 blocking Abs profoundly inhibited the differentiation. We also found that the STAT5A1*6 transactivated the IL-6 promoter, which was mediated by the enhanced binding of NF-κB p65 (RelA) to the promoter region of IL-6. These findings indicate that STAT5A cooperates with Rel/NF-κB to induce production of IL-6, thereby inducing macrophage differentiation of M1 cells in an autocrine manner. In summary, we have shown a novel mechanism by which STAT5 induces its pleiotropic functions. Cytokines

IntroductionThe signal transducer and activator of transcription (STAT) family members STAT1-4, STAT5A, STAT5B, and STAT6 are activated through phosphorylation by the Janus kinase (JAK) family upon cytokine stimulation. The phosphorylated STATs form homodimers or heterodimers and translocate into the nucleus, where they regulate expression of their target genes. [1][2][3][4] Among them, STAT3 is activated mainly by the interleukin-6 (IL-6) family cytokines including IL-6, oncostatin M, and leukemia inhibitory factor (LIF), and is implicated in a wide range of biologic processes, including nephrogenesis, gliogenesis, hepatogenesis, T-cell proliferation, inflammation, and oncogenesis. [5][6][7][8][9][10][11] The critical role of STAT3 in myeloid differentiation was demonstrated by the use of dominant negative mutants. [12][13][14] In contrast, in embryonic stem cells, STAT3 is required for self-renewal. [15][16][17][18][19] In addition, STAT3 is activated in a broad spectrum of human hematologic malignancies. 20 STAT3 can also be negatively regulated. Among the known inhibitors of STAT proteins are the suppressor of cytokine signaling (SOCS) proteins, 21 also known as Janus kinase binding (JAB) proteins 22 or STAT-induced STAT inhibitors (SSIs). 23 While SOCS proteins interact with JAKs and reduced their tyrosine kinase activity, 21-23 a STAT3 inhibitor protein inhibitor of activated STAT3 (PIAS3) directly binds to STAT3 and inhibits its activity. 24 A zinc finger protein Gfi-1 enhances STAT3 signaling by preventing this binding of PIAS3 to STAT3. 25 Several other molecules have been found to interact with activated STAT3. Among them, cellular Jun (c-Jun) forms a complex with STAT3 and activates the ␣ 2 -macroglobulin promoter that contains both STAT3-and c-Jun-binding sites. 26,27 Like many other transcription factors, STAT3 associates with a transcriptional cofactor, cAMP response element binding proteinbinding protein/p300 (CBP/p300), to form a transcriptional complex. 28 A protein called gene-associated with retinoid interferoninduced mortality (GRIM)-19 suppresses STAT3 activity through cytoplasmic retention of STAT3, whereas an endothelial cellderived zinc finger protein (EZI) enhances STAT3 activity through nuclear retention of STAT3. 29,30 There may be more such regulators in various steps of STAT3 action, such as translocation to the nucleus, induction of chromatin remodeling, and proteolysis.In a search for key molecules that prevent IL-6-induced terminal differentiation of murine myeloid leukemia M1 cells, we identified an antisense DNA for the full-length form of male germ cell Rac guanosine triphosphatase-activating protein (MgcRac-GAP) through functional cloning. 31 An N-terminus-truncated form of MgcRacGAP had been isolated and named male germ cell Rac GAP, as its expression was highest in testis. 32 Rac, Cdc42, and RhoA, the Rho family of small guanosine triphosphates (GTPases), play pleiotropic roles in a variety of cell functions such as transformation, migration, cytokinesis, and transcriptional...