Binding of 14-3-3β but not 14-3-3σ controls the cytoplasmic localization of CDC25B: Binding site preferences of 14-3-3 subtypes and the subcellular localization of CDC25B

The dual specificity phosphatase CDC25B positively controls the G2-M transition by activating CDK1/cyclin B. The binding of 14-3-3 to CDC25B has been shown to regulate the subcellular redistribution of CDC25B from the nucleus to the cytoplasm and may be correlated with the G2 checkpoint. We used a FLAG-tagged version of CDC25B to study the differences among the binding sites for the 14-3-3 subtypes, 14-3-3β, 14-3-3ε and 14-3-3σ, and the relationship between subtype binding and the subcellular localization of CDC25B. All three subtypes were found to bind to CDC25B. Site-directed mutagenesis studies revealed that 14-3-3β bound exclusively near serine-309 of CDC25B1, which is within a potential consensus motif for 14-3-3 binding. By contrast, 14-3-3σ bound preferentially to a site around serine-216, and the presence of serine-137 and -309 enhanced the binding. In addition to these binding-site differences, we found that the binding of 14-3-3β drove CDC25B to the cytoplasm and that mutation of serine-309 to alanine completely abolished the cytoplasmic localization of CDC25B. However, co-expression of 14-3-3σ and CDC25B did not affect the subeellular localization of CDC25B. Furthermore, serine-309 of CDC25B was sufficient to produce its cytoplasmic distribution with co-expression of 14-3-3β, even when other putative 14-3-3 binding sites were mutated. 14-3-3ε resembled 14-3-3β with regard to its binding to CDC25B and the control of CDC25B subcellujar localization. The results of the present study indicite that two 14-3-3 subtypes can control the subcellular localization of CDC25B by binding to a specific site and that 14-3-3σ has effects on CDC25B other than the control of its subcellular localization

Transcription of the Geminin gene is regulated by a negative-feedback loop

Geminin performs a central function in regulating cellular proliferation and differentiation in development and also in stem cells. Of interest, down-regulation of Geminin induces gene transcription regulated by E2F, indicating that Geminin is involved in regulation of E2F-mediated transcriptional activity. Because transcription of the Geminin gene is reportedly regulated via an E2F-responsive region (E2F-R) located in the first intron, we first used a reporter vector to examine the effect of Geminin on E2F-mediated transcriptional regulation. We found that Geminin transfection suppressed E2F1- and E2F2-mediated transcriptional activation and also mildly suppressed such activity in synergy with E2F5, 6, and 7, suggesting that Geminin constitutes a negative-feedback loop for the Geminin promoter. Of interest, Geminin also suppressed nuclease accessibility, acetylation of histone H3, and trimethylation of histone H3 at lysine 4, which were induced by E2F1 overexpression, and enhanced tri­methylation of histone H3 at lysine 27 and monoubiquitination of histone H2A at lysine 119 in E2F-R. However, Geminin5EQ, which does not interact with Brahma or Brg1, did not suppress accessibility to nuclease digestion or transcription but had an overall dominant-negative effect. These findings suggest that E2F-mediated activation of Geminin transcription is negatively regulated by Geminin through the inhibition of chromatin remodeling

Hoxa9 Transduction Induces Hematopoietic Stem and Progenitor Cell Activity through Direct Down-Regulation of Geminin Protein

<div><p>Hoxb4, a 3′-located Hox gene, enhances hematopoietic stem cell (HSC) activity, while a subset of 5′-located Hox genes is involved in hematopoiesis and leukemogenesis, and some of them are common translocation partners for Nucleoporin 98 (Nup98) in patients with leukemia. Although these Hox gene derivatives are believed to act as transcription regulators, the molecular involvement of the Hox gene derivatives in hematopoiesis and leukemogenesis remains largely elusive. Since we previously showed that Hoxb4 forms a complex with a Roc1-Ddb1-Cul4a ubiquitin ligase core component and functions as an E3 ubiquitin ligase activator for Geminin, we here examined the E3 ubiquitin ligase activities of the 5′-located Hox genes, Hoxa9 and Hoxc13, and Nup98-Hoxa9. Hoxa9 formed a similar complex with the Roc1-Ddb1-Cul4a component to induce ubiquitination of Geminin, but the others did not. Retroviral transduction-mediated overexpression or siRNA-mediated knock-down of Hoxa9 respectively down-regulated or up-regulated Geminin in hematopoietic cells. And Hoxa9 transduction-induced repopulating and clonogenic activities were suppressed by Geminin supertransduction. These findings suggest that Hoxa9 and Hoxb4 differ from Hoxc13 and Nup98-Hoxa9 in their molecular role in hematopoiesis, and that Hoxa9 induces the activity of HSCs and hematopoietic progenitors at least in part through direct down-regulation of Geminin.</p> </div

Immunoprecipitation analysis of Hox derivatives and effect of Cul4a knock-down on Hoxa9-mediated down-regulation of Geminin protein in HEK-293 cells.

<p>(<b>A</b>) Either of Flag-Hoxa9, Flag-Hoxc13 or Flag-Nup98-Hoxa9 was transfected in HEK-293 cells, and the complex formation with endogenous Cul4a, Ddb1 and Roc1 was examined by means of immunoprecipitation analysis using an anti-Flag antibody. (<b>B</b>) Cul4a siRNA was transfected, and the effect on Hoxa9-mediated down-regulation of Geminin protein was examined. Down-regulation of Cul4a by siRNA was confirmed by immunoblot analysis, and the level was restored by transfection of myc-tagged Cul4a. Endogenous Cul4a was also detected in myc-tagged Cul4a-transfected cells even if cells were pre-treated with siRNA for Cul4a probably because exogenously overexpressed mRNA for Cul4a prevented siRNA from affecting endogenous Cul4a.</p

Effect on clonogenic activity.

<p>(<b>A</b>) Effect of transduction of Hox derivatives on clonogenic activity. Numbers and types of colonies are shown in the upper panel, and close-up photographs of representative colonies in the lower panel. (<b>B</b>) Effect of Geminin supertransduction on Hoxa9 transduction-mediated induction of clonogenic activity. Numbers and types of colonies are shown in the upper panel. Close-up photographs of representative colonies and images obtained with an inverted microscopy are shown in the lower panel. A cell cluster with more than 20 cells was counted as a colony under microscopy. (<b>C</b>) Effect of Geminin supertransduction on Hoxa9 transduction-mediated induction of replating activity. MEP and MPI, empty control vectors.</p

Manipulation of Cell Cycle and Chromatin Configuration by Means of Cell-Penetrating Geminin

<div><p>Geminin regulates chromatin remodeling and DNA replication licensing which play an important role in regulating cellular proliferation and differentiation. Transcription of the <i>Geminin</i> gene is regulated via an E2F-responsive region, while the protein is being closely regulated by the ubiquitin-proteasome system. Our objective was to directly transduce Geminin protein into cells. Recombinant cell-penetrating Geminin (CP-Geminin) was generated by fusing Geminin with a membrane translocating motif from FGF4 and was efficiently incorporated into NIH 3T3 cells and mouse embryonic fibroblasts. The withdrawal study indicated that incorporated CP-Geminin was quickly reduced after removal from medium. We confirmed CP-Geminin was imported into the nucleus after incorporation and also that the incorporated CP-Geminin directly interacted with Cdt1 or Brahma/Brg1 as the same manner as Geminin. We further demonstrated that incorporated CP-Geminin suppressed S-phase progression of the cell cycle and reduced nuclease accessibility in the chromatin, probably through suppression of chromatin remodeling, indicating that CP-Geminin constitutes a novel tool for controlling chromatin configuration and the cell cycle. Since Geminin has been shown to be involved in regulation of stem cells and cancer cells, CP-Geminin is expected to be useful for elucidating the role of Geminin in stem cells and cancer cells, and for manipulating their activity.</p></div