The retinoblastoma (RB) family of proteins are found in organisms as distantly related as humans, plants, and insects. These proteins play a key role in regulating advancement of the cell division cycle from the G1 to S-phases. This is achieved through negative regulation of two important positive regulators of cell cycle entry, E2F transcription factors and cyclin dependent kinases. In growth arrested cells transcriptional activity by E2Fs is repressed by RB proteins. Stimulation of cell cycle entry by growth factor signaling leads to activation of cyclin dependent kinases. They in turn phosphorylate and inactivate the RB family proteins, leading to E2F activation and additional cyclin dependent kinase activity. This propels the cell cycle irreversibly forward leading to DNA synthesis. This review will focus on the basic biochemistry and cell biology governing the regulation and activity of mammalian RB family proteins in cell cycle control.

Terminally differentiated cell types are needed to live and function in a postmitotic state for a lifetime. Cellular senescence is another type of permanent arrest that blocks the proliferation of cells in response to genotoxic stress. Here we show that the retinoblastoma protein (pRB) uses a mechanism to block DNA replication in senescence that is distinct from its role in permanent cell cycle exit associated with terminal differentiation. Our work demonstrates that a subtle mutation in pRB that cripples its ability to interact with chromatin regulators impairs heterochromatinization and repression of E2F-responsive promoters during senescence. In contrast, terminally differentiated nerve and muscle cells bearing the same mutation fully exit the cell cycle and block E2F-responsive gene expression by a different mechanism. Remarkably, this reveals that pRB recruits chromatin regulators primarily to engage a stress-responsive G 1 arrest program.

i Mammalian DREAM is a conserved protein complex that functions in cellular quiescence. DREAM contains an E2F, a retinoblastoma (RB)-family protein, and the MuvB core (LIN9, LIN37, LIN52, LIN54, and RBBP4). In mammals, MuvB can alternatively bind to BMYB to form a complex that promotes mitotic gene expression. Because BMYB-MuvB is essential for proliferation, loss-of-function approaches to study MuvB have generated limited insight into DREAM function. Here, we report a gene-targeted mouse model that is uniquely deficient for DREAM complex assembly. We have targeted p107 (Rbl1) to prevent MuvB binding and combined it with deficiency for p130 (Rbl2). Our data demonstrate that cells from these mice preferentially assemble BMYB-MuvB complexes and fail to repress transcription. DREAM-deficient mice show defects in endochondral bone formation and die shortly after birth. Micro-computed tomography and histology demonstrate that in the absence of DREAM, chondrocytes fail to arrest proliferation. Since DREAM requires DYRK1A (dual-specificity tyrosine phosphorylation-regulated protein kinase 1A) phosphorylation of LIN52 for assembly, we utilized an embryonic bone culture system and pharmacologic inhibition of (DYRK) kinase to demonstrate a similar defect in endochondral bone growth. This reveals that assembly of mammalian DREAM is required to induce cell cycle exit in chondrocytes.C ellular differentiation is generally controlled by transcriptional activation or repression of specific genes. Consequently, a host of different molecular genetic events can shape the properties of cells during development. Recent evidence indicates that an evolutionarily conserved protein complex known as DREAM is capable of regulating diverse gene expression programs, thereby unifying many disparate events in development into a single molecular machine (1, 2).The DREAM complex was isolated, and its composition was determined from a number of different model organisms. Studies of aberrant growth factor signaling in Caenorhabditis elegans lead to the discovery of complementation groups that contribute to a multivulval (Muv) phenotype (3). Mutation of any two of the synthetic multivulval (synMuv) group A, B, or C genes resulted in worms with elevated numbers of vulvae (4). Group B contains a number of genes (the Lin-9, Lin-37, Lin-52, Lin-53/RBBP4, and Lin-54 genes) whose encoded proteins form the MuvB core complex (5-7). In addition, worm homologues of retinoblastoma protein (RB), E2F, and DP are also group B members (8, 9). The MuvB core was also found to interact with MYB in transcriptional control of cell cycle progression in fruit flies (6, 7). Isolation of MYB and RB revealed that they copurify with MuvB proteins, and this has formed the basis of the DREAM complex (Drosophila RB, E2F, and MuvB). In some organisms it also contains epigenetic readers and writers such as histone deacetylases (HDACs) and L3MBT (6,7,10). The model that has emerged is one in which the DREAM complex can confer both positive and negative regulation of transcripti...

BackgroundThe interaction between viral oncoproteins such as Simian virus 40 TAg, adenovirus E1A, and human papilloma virus E7, and the retinoblastoma protein (pRB) occurs through a well characterized peptide sequence, LXCXE, on the viral protein and a well conserved groove in the pocket domain of pRB. Cellular proteins, such as histone deacetylases, also use this mechanism to interact with the retinoblastoma protein to repress transcription at cell cycle regulated genes. For these reasons this region of the pRB pocket domain is thought to play a critical role in growth suppression.ResultsIn this study, we identify and characterize a tumor derived allele of the retinoblastoma gene (RB1) that possesses a discrete defect in its ability to interact with LXCXE motif containing proteins that compromises proliferative control. To assess the frequency of similar mutations in the RB1 gene in human cancer, we screened blood and tumor samples for similar alleles. We screened almost 700 samples and did not detect additional mutations, indicating that this class of mutation is rare.ConclusionsOur work provides proof of principal that alleles encoding distinct, partial loss of function mutations in the retinoblastoma gene that specifically lose LXCXE dependent interactions, are found in human cancer.