Cyclin-dependent kinase-mediated phosphorylation and the negative regulatory domain of transcription factor B-Myb modulate its DNA binding

A-MYB substitutes for B-MYB in activating cell cycle genes and in stimulating proliferation

A-MYB (MYBL1) is a transcription factor with a role in meiosis in spermatocytes. The related B-MYB protein is a key oncogene and a master regulator activating late cell cycle genes. To activate genes, B-MYB forms a complex with MuvB and is recruited indirectly to cell cycle genes homology region (CHR) promoter sites of target genes. Activation through the B-MYB-MuvB (MMB) complex is essential for successful mitosis. Here, we discover that A-MYB has a function in transcriptional regulation of the mitotic cell cycle and can substitute for B-MYB. Knockdown experiments in cells not related to spermatogenesis show that B-MYB loss alone merely delays cell cycle progression. Only dual knockdown of B-MYB and A-MYB causes G2/M cell cycle arrest, endoreduplication, and apoptosis. A-MYB can substitute for B-MYB in binding to MuvB. The resulting A-MYB-MuvB complex activates genes through CHR sites. We find that A-MYB activates the same target genes as B-MYB. Many of the corresponding proteins are central regulators of the cell division cycle. In summary, we demonstrate that A-MYB is an activator of the mitotic cell cycle by activating late cell cycle genes.

A dephosphorylation-dependent molecular switch for FT repression mediates flowering in Arabidopsis

The reproductive success of flowering plants relies greatly on precise timing of the floral transition, which is finely modulated by a complex network of floral regulators. As a main floral integrator, FLOWERING LOCUS T (FT) is also an essential constituent of the florigen that is transported from leaves to shoot apices to induce flowering. FT is specifically transcribed in leaf vascular tissues, where its production is suppressed by many flowering repressors, including the MYB transcription factor EARLY FLOWERING MYB PROTEIN (EFM). Here, we show that a plant CTD phosphatase, C-TERMINAL DOMAIN PHOSPHATASE-LIKE 2 (CPL2), suppresses FT expression in leaf vascular tissues by modulating the binding activity of EFM. CPL2 interacts with and dephosphorylates EFM to facilitate the binding of dephosphorylated EFM to FT chromatin, thereby inhibiting flowering. Our results suggest that CPL2-mediated dephosphorylation of the floral repressor EFM serves as a molecular switch, adding another layer of regulation to fine-tune FT transcription and ensure that flowering occurs at an appropriate time.

From modulation of cellular plasticity to potentiation of therapeutic resistance: new and emerging roles of MYB transcription factors in human malignancies

MYB transcription factors are encoded by a large family of highly conserved genes from plants to vertebrates. There are three members of the MYB gene family in human, namely, MYB, MYBL1, and MYBL2 that encode MYB/c-MYB, MYBL1/A-MYB, and MYBL2/B-MYB, respectively. MYB was the first member to be identified as a cellular homolog of the v-myb oncogene carried by the avian myeloblastosis virus (AMV) causing leukemia in chickens. Under the normal scenario, MYB is predominantly expressed in hematopoietic tissues, colonic crypts, and neural stem cells and plays a role in maintaining the undifferentiated state of the cells. Over the years, aberrant expression of MYB genes has been reported in several malignancies and recent years have witnessed tremendous progress in understanding of their roles in processes associated with cancer development. Here, we review various MYB alterations reported in cancer along with the roles of MYB family proteins in tumor cell plasticity, therapy resistance, and other hallmarks of cancer. We also discuss studies that provide mechanistic insights into the oncogenic functions of MYB transcription factors to identify potential therapeutic vulnerabilities.