RESUMEN
Myb-binding protein 1a (Mybbp1a) is a novel nuclear protein localized predominantly, but not exclusively, in nucleoli. Although initially isolated as a c-Myb interacting protein, Mybbp1a is expressed ubiquitously, associates with a number of different transcription factors, and may play a role in both RNA polymerase I- and II-mediated transcriptional regulation. However, its precise function remains unclear. In this study we show that Mybbp1a is a nucleocytoplasmic shuttling protein and investigate the mechanisms responsible for both nuclear import and export. The carboxyl terminus of Mybbp1a, which contains seven short basic amino acid repeat sequences, is responsible for both nuclear and nucleolar localization, and this activity can be transferred to a heterologous protein. Deletion mapping demonstrated that these repeat sequences appear to act incrementally, with successive deletions resulting in a corresponding increase in the proportion of protein localized in the cytoplasm. Glutathione S-transferase pulldown experiments showed that the nuclear receptor importin-alpha/beta mediates Mybbp1a nuclear import. Interspecies heterokaryons were used to demonstrate that Mybbp1a was capable of shuttling between the nucleus and the cytoplasm. Deletion analysis and in vivo export studies using a heterologous assay system identified several nuclear export sequences which facilitate Mybbp1a nuclear export of Mybbp1a by CRM1-dependent and -independent pathways.
Asunto(s)
Proteínas Portadoras/metabolismo , Núcleo Celular/metabolismo , Citoplasma/metabolismo , Fibroblastos/metabolismo , Carioferinas/metabolismo , Proteínas Nucleares/metabolismo , Receptores Citoplasmáticos y Nucleares , Células 3T3 , Transporte Activo de Núcleo Celular/fisiología , Secuencia de Aminoácidos/genética , Animales , Secuencia de Bases/genética , Nucléolo Celular/metabolismo , Nucléolo Celular/ultraestructura , Núcleo Celular/ultraestructura , Citoplasma/ultraestructura , Proteínas de Unión al ADN , Fibroblastos/citología , Humanos , Carioferinas/genética , Ratones , Datos de Secuencia Molecular , Proteínas de Transporte Nucleocitoplasmático , Estructura Terciaria de Proteína/fisiología , Proteínas de Unión al ARN , Factores de Transcripción , alfa Carioferinas/metabolismo , Proteína Exportina 1RESUMEN
p53 is known to repress transcription of a number of genes, but the mechanism of p53 recruitment to these target genes is unknown. The c-myb proto-oncogene product (c-Myb) positively regulates proliferation of immature hematopoietic cells, whereas p53 blocks cell cycle progression. Here, we demonstrate that p53 inhibits c-Myb-induced transcription and transformation by directly binding to c-Myb. The ability of c-Myb to maintain the undifferentiated state of M1 cells was also suppressed by p53. p53 did not affect the ability of c-Myb to bind to DNA but formed a ternary complex with the corepressor mSin3A and c-Myb. Thus, p53 antagonizes c-Myb by recruiting mSin3A to down-regulate specific Myb target genes.