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DREAM complex

The dimerization partner, RB-like, E2F and multi-vulval class B (DREAM) complex is a protein complex responsible for the regulation of cell cycle-dependent gene expression. The complex is evolutionarily conserved, although some of its components vary from species to species. In humans, the key proteins in the complex are RBL1 (p107) and RBL2 (p130), both of which are homologs of RB (p105) and bind repressive E2F transcription factors E2F4 and E2F5DP1DP2 and DP3, dimerization partners of E2F; and MuvB, which is a complex of LIN9/37/52/54 and RBBP4.

Discovery

Genes encoding the MuvB complex were originally identified from loss-of-function mutation studies in C. elegans. When mutated, these genes produced worms with multiple vulva-like organs, hence the name ‘Muv’. Three classes of Muv genes were classified, with class B genes encoding homologues of mammalian RB, E2F, and DP1, and others such as LIN-54, LIN-37, LIN-7 and LIN-52, whose functions were not yet understood.

Studies in Drosophila melanogaster ovarian follicle cells identified a protein complex that bound to repeatedly amplifying chorion genes. The complex included genes that had close homology with the MuvB genes such as Mip130, Mip120 and Mip40. These Mip genes were identified as homologues of the MuvB genes LIN9, LIN54, and LIN37 respectively. Further studies in the fly embryo nuclear extracts confirmed the coexistence of these proteins with others such as the RB homologues Rbf1 and Rbf2, and others like E2f and Dp. The protein complex was thus termed as the Drosophila RBF, E2f2 and Mip (dREAM) complex. Disruption of the dREAM complex through RNAi knockdown of the components of dREAM complex led to higher expression of E2f regulated genes that are typically silenced, implicating dREAM’s role in gene down-regulation. Later in Drosophila melanogaster, there was also found a testis-specific paralog of the Myb-MuvB/DREAM complex known as tMAC (testis-specific meiotic arrest complex), which is involved in meiotic arrest.

A protein complex similar to dREAM was subsequently identified in C. elegans extract containing DP, RB, and MuvB, and was named as DRM. This complex included mammalian homologues of RB and DP, and other members of the MuvB complex.

The mammalian DREAM complex was identified following immunoprecipitation of p130 with mass-spectrometry analysis. The results showed that p130 was associated with E2F4, E2F5, the dimerization partner DP, and LIN9, LIN54, LIN37, LIN52, and RBBP4 that make up the MuvB complex. Immunoprecipitation of MuvB factors also revealed association of BMYB. Subsequent immunoprecipitation with BMYB yielded all the MuvB core proteins, but not other members of the DREAM complex – p130, p107, E2F4/5 and DP. This indicated that MuvB associated with BMYB to form the BMYB-MuvB complex or with p130/p107, E2F4/5 and DP to form the DREAM complex. The DREAM complex was found prevalent in quiescent or starved cells, and the BMYB-MuvB complex was found in actively dividing cells, hinting at separate functionalities of these two complexes.

MuvB-like complexes were also recently discovered in Arabidoposis that include E2F and MYB orthologs combined with LIN9 and LIN54 orthologs.

Function

The main function of the DREAM complex is to repress G1/S and G2/M gene expression during quiescence (G0). Entry into the cell cycle dissociates p130 from the complex and leads to subsequent recruitment of activating E2F proteins. This allows for the expression of E2F regulated late G1 and S phase genes. BMYB (MYBL2), which is repressed by the DREAM complex during G0 is also able to be expressed at this time, and binds to MuvB during S phase to promote the expression of key G2/M phase genes such as CDK1 and CCNB1FOXM1 is then recruited in G2 to further promote gene expression (e.g. AURKA). During late S phase BMYB is degraded via CUL1 (SCF complex), while FOXM1 is degraded during mitosis by the APC/C. Near the end of the cell cycle, the DREAM complex is re-assembled by DYRK1A to repress G1/S and G2/M genes.

The G1/S transition is a stage in the cell cycle at the boundary between the G1 phase, in which the cell grows, and the S phase, during which DNA is replicated. It is governed by cell cycle checkpoints to ensure cell cycle integrity and the subsequent S phase can pause in response to improperly or partially replicated DNA. During this transition the cell makes decisions to become quiescent (enter G0), differentiate, make DNA repairs, or proliferate based on environmental cues and molecular signaling inputs. The G1/S transition occurs late in G1 and the absence or improper application of this highly regulated checkpoint can lead to cellular transformation and disease states such as cancer. During this transition, G1 cyclin D-Cdk4/6 dimer phosphorylates retinoblastoma releasing transcription factor E2F, which then drives the transition from G1 to S phase. The G1/S transition is highly regulated by transcription factor p53 in order to halt the cell cycle when DNA is damaged. It is a “point of no return” beyond which the cell is committed to dividing; in yeast this is called the Start point, and in multicellular eukaryotes it is termed the restriction point (R-Point). If a cell passes through the G1/S transition the cell will continue through the cell cycle regardless of incoming mitogenic factors due to the positive feed-back loop of G1-S transcription. Positive feed-back loops include G1 cyclins and accumulation of E2F.

The G2-M DNA damage checkpoint is an important cell cycle checkpoint in eukaryotic organisms that ensures that cells don’t initiate mitosis until damaged or incompletely replicated DNA is sufficiently repaired. Cells with a defective G2-M checkpoint will undergo apoptosis or death after cell division if they enter the M phase before repairing their DNA. The defining biochemical feature of this checkpoint is the activation of M-phasecyclin-CDK complexes, which phosphorylate proteins that promote spindle assembly and bring the cell to metaphase. The cell cycle is driven by proteins called cyclin dependent kinases that associate with cyclin regulatory proteins at different checkpoints of the cell cycle. Different phases of the cell cycle experience activation and/or deactivation of specific cyclin-CDK complexes. CyclinB-CDK1 activity is specific to the G2/M checkpoint. Accumulation of cyclin B increases the activity of the cyclin dependent kinase Cdk1 human homolog Cdc2 as cells prepare to enter mitosis. Cdc2 activity is further regulated by phosphorylation/dephosphorylation of its corresponding activators and inhibitors. Through a positive feedback loop, CyclinB-Cdc2 activates the phosphatase Cdc25 which in turn deactivates the CyclinB-Cdc2 inhibitors, Wee1 and Myt1. Cdc25 activates the complex through the removal of phosphates from the active site while Wee1 inactivates the complex through the phosphorylation of tyrosine residues, specifically tyrosine-15. This loop is further amplified indirectly through the coordinated interaction of the Aurora A kinase and the Bora cofactor. During the G2 phase, Bora accumulates and forms an activation complex with Aurora A. This complex then regulates the activation of Polo-like kinase 1 (Plk1). Plk1 phosphorylates Wee1, targeting it for degradation through the SCF ubiquitin ligase complex (SCF complex), and activates Cdc25 through phosphorylation with combined action activating Cdc2. The combined activity and complex of Cdc2, Cdc25, and Plk1 with the accumulation of cyclin B activates the CyclinB-Cdc2 complex, promoting entry into mitosis. Many proteins involved in this positive feedback loop drive the activation of the CyclinB-Cdc2 complex because entry into mitosis requires an all-or-none response. The Novak-Tyson model is a mathematical model used to explain such regulatory loop that predicted the irreversible transition into mitosis driven by hysteresis. Through experiments in Xenopus laevis cell-free egg extracts, such model was confirmed as the basis for entry into mitosis. Once cyclin concentration reaches a certain minimum activation threshold, Cdc2 is rapidly activated. It remains in this state until activity falls below a separate inactivation threshold at which it is abruptly inactivated through tyrosine phosphorylation by Wee1 and Myt1. In the case of unreplicated DNA, the cyclin concentration threshold for Cdc2 activation is further increased. Through this mechanism, there exists two separate steady-state conditions separated by an unstable steady state. The bistable and hysteretic nature of CyclinB-Cdc2 ensures a highly regulated nature of the G2/M checkpoint.

Mammalian DREAM complex in G0 and early G1

G0

During quiescence, the DREAM complex represses G1/S and G2/M gene expression. In mammalian systems, chromatin-immunoprecipitation (ChIP) studies have revealed that DREAM components are found together at promoters of genes that peak in G1/S or G2/M phase. Abrogation of the DREAM complex on the other hand, led to increased expression of E2F regulated genes normally repressed in the G0 phase. Contrary to mammalian cells, the fly dREAM complex was found at almost one-third of all promoters, which may reflect a broader role for dREAM in gene regulation, such as programmed cell death of neural precursor cells.

Docking of the DREAM complex to promoters is achieved by binding of LIN-54 to regions known as cell cycle genes homology region (CHR). These are specific sequence of nucleotides that are commonly found in the promoters of genes expressed during late S phase or G2/M phase. Docking can also be achieved via E2F proteins binding to sequences known as cell cycle-dependent element sites (CDEs). Some cell cycle dependent genes have been found where both CHRs and CDEs are in proximity to one another. Because p130-E2F4 can form stable associations with the MuvB complex, the proximity of CHRs to CDEs suggests that affinity of binding of the DREAM complex to target genes is cooperatively improved by association with both the binding sites.

When DREAM is docked onto the promoter, p130 is bound to LIN52, and this association inhibits LIN52 binding to chromatin modifier proteins. Therefore, unlike RB-E2F, the DREAM complex is unlikely to directly recruit chromatin modifiers to repress gene expression, although some associations have been suggested. DREAM complex may instead down-regulate gene expression by affecting nucleosome positioning. Compacted DNA at transcription start sites inhibit gene expression by blocking the docking of RNA polymerase. In worms for example, loss of a MuvB complex protein, LIN35, leads to loss of repressive histone associations and high expression of cell cycle dependent genes. However, direct evidence for the link between repressive histones and the DREAM complex remains to be elucidated.

Mammalian DREAM complex disassembly in G1/S

G1/S

Like its counterpart, RB-E2F, the DREAM complex is also affected by similar growth stimuli and subsequent cyclin-CDK activity. Increasing cyclin D-CDK4 and cyclin E-CDK2 activity dissociates the DREAM complex from the promoter by phosphorylation of p130. Hyper-phosphorylated p130 is subsequently degraded  and E2F4 exported from the nucleus. Once the repressive E2Fs are vacated, activating E2Fs bind to the promoter to up-regulate G1/S genes that promote DNA synthesis and transition of the cell cycle. BMYB is also up-regulated during this time, which then binds to genes that peak at G2/M phase. Binding of BMYB to late cell cycle genes is dependent on its association with the MuvB core to form the BMYB-MuvB complex, which is then able to up-regulate genes in the G2/M phase.

Late mitosis

Near the end of mitosis, p130 and p107 are dephosphorylated from their hyperphosphorylated state by the phosphatase PP2a. Inhibition of PP2a activity reduced promoter binding of some of the proteins of the DREAM complex in the subsequent G1 phase and de-repression of gene expression.

Other components have been shown to be phosphorylated for DREAM complex assembly to occur. Of these, LIN52 phosphorylation on its S28 residue is the most well-understood. Substitution of this serine to alanine led to reduced binding of the MuvB core to p130 and impaired the ability of cells to enter quiescence. This indicates that LIN52 S28 phosphorylation is required for proper association and function of the DREAM complex via binding with p130. One known regulator of phosphorylation of the S28 residue is the DYRK1A. The loss of this kinase leads to decreased phosphorylation of the S28 residue and association of p130 with MuvB. DYRK1A was also found to degrade cyclin D1, which would increase p21 levels – both of which contribute to cell cycle exit.

The DREAM complex was also shown to regulate cytokinesis through GAS2L3.

Cancer therapy

Due to its regulatory role in the cell cycle, targeting the DREAM complex might enhance anticancer treatments such as imatinib.

See also

References

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Further reading

  • Rashid, NN; Yusof, R; Watson, RJ (November 2014). “A B-myb–DREAM complex is not critical to regulate the G2/M genes in HPV-transformed cell lines”. Anticancer Research34 (11): 6557–63. PMID 25368258.

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