Inactivation of the RB protein is one of the most fundamental events in cancer. Coming to a molecular understanding of its function in normal cells and how it impedes cancer development has been challenging. Historically, the ability of RB to regulate the cell cycle placed it in a central role in proliferative control, and research focused on RB regulation of the E2F family of transcription factors. Remarkably, several recent studies have found additional tumour-suppressor functions of RB, including alternative roles in the cell cycle, maintenance of genome stability and apoptosis. These advances and new structural studies are combining to define the multifunctionality of RB.

The detection of biological molecules and their interactions is a significant component of modern biomedical research. In current biosensor technologies, simultaneous detection is limited to a small number of analytes by the spectral overlap of their signals. We have developed an NMR-based xenon biosensor that capitalizes on the enhanced signal-to-noise, spectral simplicity, and chemical-shift sensitivity of laser-polarized xenon to detect specific biomolecules at the level of tens of nanomoles. We present results using xenon ''functionalized'' by a biotin-modified supramolecular cage to detect biotin-avidin binding. This biosensor methodology can be extended to a multiplexing assay for multiple analytes.R ecent biosensor technologies exploit surface plasmon resonance (1), fluorescence polarization (2), and fluorescence resonance energy transfer (3) as detection methods. Although the sensitivity of such techniques is excellent, extending these assays to multiplexing capabilities has proven challenging because of the difficulty in distinguishing signals from different binding events. Although NMR spectroscopy is able to finely resolve signals from different molecules and environments, the spectral complexity and low sensitivity of NMR spectroscopy normally preclude its use as a detector of molecular targets in complex mixtures. Notable successes (4, 5) in the application of NMR to such problems are still limited by long acquisition times or a limited number of detectable analytes. Laser-polarized xenon NMR benefits from good signal-to-noise and spectral simplicity with the added advantage of substantial chemical-shift sensitivity.Optical pumping (6) has enhanced the use of xenon as a sensitive probe of its molecular environment (7,8). Laserpolarized xenon is being developed as a diagnostic agent for medical magnetic resonance imaging (9) and spectroscopy (10) and as a probe for the investigation of surfaces and cavities in porous materials and biological systems. Xenon provides information both through direct observation of its NMR spectrum (11)(12)(13)(14)(15)(16)(17) and by the transfer of its enhanced polarization to surrounding spins (18,19). In a protein solution, weak xenonprotein interactions render the chemical shift of xenon dependent on the accessible protein surface and even allow the monitoring of the protein conformation (20). To use xenon as a specific sensor of molecules, it would be valuable to ''functionalize'' the xenon for the purpose of reporting specific interactions with a molecular target.In this report, we demonstrate an example of such a functionalized system that exhibits molecular target recognition. Fig. 1 shows the chemical principle used for our initial study, a biosensor molecule designed to bind both xenon and protein.The molecule consists of three parts: the cage, which contains the xenon; the ligand, which directs the functionalized xenon to a specific protein; and the tether, which links the ligand and the cage. The ligand and target can be any two molecules or constructs. In su...

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The retinoblastoma (Rb) protein negatively regulates the G1-S transition by binding to the E2F transcription factors, until cyclin-dependent kinases phosphorylate Rb, causing E2F release. The Rb pocket domain is necessary for E2F binding, but the Rb C-terminal domain (RbC) is also required for growth suppression. Here we demonstrate a high-affinity interaction between RbC and E2F-DP heterodimers shared by all Rb and E2F family members. The crystal structure of an RbC-E2F1-DP1 complex reveals an intertwined heterodimer in which the marked box domains of both E2F1 and DP1 contact RbC. We also demonstrate that phosphorylation of RbC at serines 788 and 795 destabilizes one set of RbC-E2F-DP interactions directly, while phosphorylation at threonines 821 and 826 induces an intramolecular interaction between RbC and the Rb pocket that destabilizes the remaining interactions indirectly. Our findings explain the requirement of RbC for high-affinity E2F binding and growth suppression and establish a mechanism for the regulation of Rb-E2F association by phosphorylation.

The p27 protein is a canonical negative regulator of cell proliferation and acts primarily by inhibiting cyclin-dependent kinases (CDKs). Under some circumstances, p27 is associated with active CDK4, but no mechanism for activation has been described. We found that p27, when phosphorylated by tyrosine kinases, allosterically activated CDK4 in complex with cyclin D1 (CDK4-CycD1). Structural and biochemical data revealed that binding of phosphorylated p27 (phosp27) to CDK4 altered the kinase adenosine triphosphate site to promote phosphorylation of the retinoblastoma tumor suppressor protein (Rb) and other substrates. Surprisingly, purified and endogenous phosp27-CDK4-CycD1 complexes were insensitive to the CDK4-targeting drug palbociclib. Palbociclib instead primarily targeted monomeric CDK4 and CDK6 (CDK4/6) in breast tumor cells. Our data characterize phosp27-CDK4-CycD1 as an active Rb kinase that is refractory to clinically relevant CDK4/6 inhibitors.

Multisite phosphorylation modulates the function of regulatory proteins with complex signaling properties and outputs. The retinoblastoma tumor suppressor protein (Rb) is inactivated by Cyclin-dependent kinase (Cdk) phosphorylation in normal and cancer cell cycles, so understanding the molecular mechanisms and effects of Rb phosphorylation is imperative. Rb functions in diverse processes regulating proliferation, and it has been speculated that multisite phosphorylation might act as a code in which discrete phosphorylations control specific activities. The idea of an Rb phosphorylation code is evaluated here in light of recent studies of Rb structure and function. Rb inactivation is discussed with an emphasis on how multisite phosphorylation changes Rb structure and associations with protein partners.

The retinoblastoma tumor suppressor (RB) is a central cell cycle regulator and tumor suppressor. RB cellular functions are known to be regulated by a diversity of post-translational modifications such as phosphorylation and acetylation, raising the possibility that RB may also be methylated in cells. Here we demonstrate that RB can be methylated by SMYD2 at lysine 860, a highly conserved and novel site of modification. This methylation event occurs in vitro and in cells, and it is regulated during cell cycle progression, cellular differentiation, and in response to DNA damage. Furthermore, we show that RB monomethylation at lysine 860 provides a direct binding site for the methylbinding domain of the transcriptional repressor L3MBTL1. These results support the idea that a code of post-translational modifications exists for RB and helps guide its functions in mammalian cells.The retinoblastoma tumor suppressor gene RB 2 is mutated in a large spectrum of human cancers (1, 2). In tumor cells where RB is not directly mutated, and in normal cells during cell cycle progression, the RB protein is functionally inactivated by phosphorylation by cyclin/CDK complexes (3). RB phosphorylation results in the release of E2F transcription factors, allowing cells to progress into the S phase of the cell cycle (4). In addition to phosphorylation by cyclin/CDK complexes, RB activity is controlled by other post-translational events. For instance, Chk2 phosphorylates RB in response to DNA damage (5). In addition, RB is acetylated (6 -9), sumoylated (10), and ubiquitinated (11-13) in response to various cellular signals. The consequences of these modifications involve changes in RB protein levels and in the binding affinity for proteins that interact with RB, such as E2F, chromatin-remodeling enzymes, and other regulators of cell cycle progression and cellular differentiation. For example, RB acetylation is thought to inhibit its phosphorylation and to promote its binding to MDM2, which results in the subsequent degradation of EID-1, an inhibitor of differentiation (8,9).Recent evidence that non-histone proteins can be methylated supports the idea that methylation may affect gene expression and cellular functions not only by modifying histone tails (14 -17) but also by changing the activity of transcription factors, including the p53 tumor suppressor (18 -27). These observations suggest that, similar to the "histone code" (28), combinations of post-translational modifications may define codes that affect the function of key regulators of transcription. Based on these observations and evidence suggesting that RB directly interacts with chromatin-modifying agents, including methyltransferases (29 -33), it is not surprising that RB was recently shown to be a target for lysine methylation by SET9 (34). Nevertheless, the extent of RB methylation in cells and its consequences for RB function are still largely unknown. In this study, we report that SMYD2 methylates RB at lysine 860. We show that this modification permits direct binding of R...

The DREAM complex represses cell cycle genes during quiescence through scaffolding MuvB proteins with E2F4/5 and the Rb tumor suppressor paralog p107 or p130. Upon cell cycle entry, MuvB dissociates from p107/p130 and recruits B-Myb and FoxM1 for up-regulating mitotic gene expression. To understand the biochemical mechanisms underpinning DREAM function and regulation, we investigated the structural basis for DREAM assembly. We identified a sequence in the MuvB component LIN52 that binds directly to the pocket domains of p107 and p130 when phosphorylated on the DYRK1A kinase site S28. A crystal structure of the LIN52-p107 complex reveals that LIN52 uses a suboptimal LxSxExL sequence together with the phosphate at nearby S28 to bind the LxCxE cleft of the pocket domain with high affinity. The structure explains the specificity for p107/p130 over Rb in the DREAM complex and how the complex is disrupted by viral oncoproteins. Based on insights from the structure, we addressed how DREAM is disassembled upon cell cycle entry. We found that p130 and B-Myb can both bind the core MuvB complex simultaneously but that cyclin-dependent kinase phosphorylation of p130 weakens its association. Together, our data inform a novel target interface for studying MuvB and p130 function and the design of inhibitors that prevent tumor escape in quiescence.