HDAC activity is dispensable for repression of cell-cycle genes by DREAM and E2F:RB complexes

Histone deacetylases (HDACs) play a crucial role in transcriptional regulation and are implicated in various diseases, including cancer. They are involved in histone tail deacetylation and canonically linked to transcriptional repression. Previous studies suggested that HDAC recruitment to cell-cycle gene promoters via the retinoblastoma (RB) protein or the DREAM complex through SIN3B is essential for G1/S and G2/M gene repression during cell-cycle arrest and exit. Here we investigate the interplay among DREAM, RB, SIN3 proteins, and HDACs in the context of cell-cycle gene repression. Knockout of SIN3B does not globally derepress cell-cycle genes in non-proliferating HCT116 and C2C12 cells. Loss of SIN3A/B moderately upregulates several cell-cycle genes in HCT116 cells but does so independently of DREAM/RB. HDAC inhibition does not induce general upregulation of RB/DREAM target genes in arrested transformed or non-transformed cells. Our findings suggest that E2F:RB and DREAM complexes can repress cell-cycle genes without relying on HDAC activity.

Cancer takes many paths through G1/S

In the commonly accepted paradigm for control of the mammalian cell cycle, sequential cyclin-dependent kinase (CDK) and cyclin activities drive the orderly transition from G1 to S phase. However, recent studies using different technological approaches and examining a broad range of cancer cell types are challenging this established paradigm. An alternative model is evolving in which cell cycles utilize different drivers and take different trajectories through the G1/S transition. We are discovering that cancer cells in particular can adapt their drivers and trajectories, which has important implications for antiproliferative therapies. These studies have helped to refine an understanding of how CDK inhibition impinges on proliferation and have significance for understanding fundamental features of cell biology and cancer.

HDAC activity is dispensable for repression of cell-cycle genes by DREAM and E2F:RB complexes

Histone deacetylases (HDACs) are pivotal in transcriptional regulation, and their dysregulation has been associated with various diseases including cancer. One of the critical roles of HDAC-containing complexes is the deacetylation of histone tails, which is canonically linked to transcriptional repression. Previous research has indicated that HDACs are recruited to cell-cycle gene promoters through the RB protein or the DREAM complex via SIN3B and that HDAC activity is essential for repressing G1/S and G2/M cell-cycle genes during cell-cycle arrest and exit. In this study, we sought to explore the interdependence of DREAM, RB, SIN3 proteins, and HDACs in the context of cell-cycle gene repression. We found that genetic knockout of SIN3B did not lead to derepression of cell-cycle genes in non-proliferating HCT116 and C2C12 cells. A combined loss of SIN3A and SIN3B resulted in a moderate upregulation in mRNA expression of several cell-cycle genes in arrested HCT116 cells, however, these effects appeared to be independent of DREAM or RB. Furthermore, HDAC inhibition did not induce a general upregulation of RB and DREAM target gene expression in arrested transformed or non-transformed cells. Our findings provide evidence that E2F:RB and DREAM complexes can repress cell-cycle genes without reliance on HDAC activity.

PROTAC-mediated CDK degradation differentially impacts cancer cell cycles due to heterogeneity in kinase dependencies

BACKGROUND

Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibition yields differential cellular responses in multiple tumor models due to redundancy in cell cycle. We investigate whether the differential requirements of CDKs in multiple cell lines function as determinant of response to pharmacological agents that target these kinases.

METHODS

We utilized proteolysis-targeted chimeras (PROTACs) that are conjugated with palbociclib (Palbo-PROTAC) to degrade both CDK4 and CDK6. FN-POM was synthesized by chemically conjugating pomalidomide moiety with a multi-kinase inhibitor, FN-1501. Patient derived PDAC organoids and PDX model were utilized to investigate the effect of FN-POM in combination with palbociclib.

RESULTS

Palbo-PROTAC mediates differential impact on cell cycle in different tumor models, indicating that the dependencies to CDK4 and 6 kinases are heterogenous. Cyclin E overexpression uncouples cell cycle from CDK4/6 and drives resistance to palbo-PROTAC. Elevated expression of P16INK4A antagonizes PROTAC-mediated degradation of CDK4 and 6. FN-POM degrades cyclin E and CDK2 and inhibits cell cycle progression in P16INK4A-high tumor models. Combination of palbociclib and FN-POM cooperatively inhibit tumor cell proliferation via RB activation.

CONCLUSION

Resistance to CDK4/6 inhibition could be overcome by pharmacologically limiting Cyclin E/CDK2 complex and proves to be a potential therapeutic approach.

DNA-Encoded Macrocyclic Peptide Libraries Enable the Discovery of a Neutral MDM2-p53 Inhibitor

Synthetic macrocyclic peptides are an emerging molecular class for both targeting intracellular protein-protein interactions (PPIs) and providing an oral modality for drug targets typically addressed by biologics. Display technologies, such as mRNA and phage display, often yield peptides that are too large and too polar to achieve passive permeability or oral bioavailability without substantial off-platform medicinal chemistry. Herein, we use DNA-encoded cyclic peptide libraries to discover a neutral nonapeptide, UNP-6457, that inhibits MDM2-p53 interaction with an IC₅₀ of 8.9 nM. X-ray structural analysis of the MDM2-UNP-6457 complex revealed mutual binding interactions and identified key ligand modification points which may be tuned to enhance its pharmacokinetic profile. These studies showcase how tailored DEL libraries can directly yield macrocyclic peptides benefiting from low MW, TPSA, and HBD/HBA counts that are capable of potently inhibiting therapeutically relevant protein-protein interactions.

Biolayer Interferometry Assay for Cyclin-Dependent Kinase-Cyclin Association Reveals Diverse Effects of Cdk2 Inhibitors on Cyclin Binding Kinetics

Cyclin-dependent kinases (CDKs) are key mediators of cell proliferation and have been a subject of oncology drug discovery efforts for over two decades. Several CDK and activator cyclin family members have been implicated in regulating the cell division cycle. While it is thought that there are canonical CDK-cyclin pairing preferences, the extent of selectivity is unclear, and increasing evidence suggests that the cell-cycle CDKs can be activated by a pool of available cyclins. The molecular details of CDK-cyclin specificity are not completely understood despite their importance for understanding cancer cell cycles and for pharmacological inhibition of cancer proliferation. We report here a biolayer interferometry assay that allows for facile quantification of CDK binding interactions with their cyclin activators. We applied this assay to measure the impact of Cdk2 inhibitors on Cyclin A (CycA) association and dissociation kinetics. We found that Type I inhibitors increase the affinity between Cdk2 and CycA by virtue of a slowed cyclin dissociation rate. In contrast, Type II inhibitors and other small-molecule Cdk2 binders have distinct effects on the CycA association and dissociation processes to decrease affinity. We propose that the differential impact of small molecules on the cyclin binding kinetics arises from the plasticity of the Cdk2 active site as the kinase transitions between active, intermediate, and inactive states.

Structural basis for tunable affinity and specificity of LxCxE-dependent protein interactions with the retinoblastoma protein family

The retinoblastoma protein (Rb) and its homologs p107 and p130 are critical regulators of gene expression during the cell cycle and are commonly inactivated in cancer. Rb proteins use their "pocket domain" to bind an LxCxE sequence motif in other proteins, many of which function with Rb proteins to co-regulate transcription. Here, we present binding data and crystal structures of the p107 pocket domain in complex with LxCxE peptides from the transcriptional co-repressor proteins HDAC1, ARID4A, and EID1. Our results explain why Rb and p107 have weaker affinity for cellular LxCxE proteins compared with the E7 protein from human papillomavirus, which has been used as the primary model for understanding LxCxE motif interactions. Our structural and mutagenesis data also identify and explain differences in Rb and p107 affinities for some LxCxE-containing sequences. Our study provides new insights into how Rb proteins bind their cell partners with varying affinity and specificity.

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

B-Myb is a highly conserved member of the vertebrate Myb family of transcription factors that plays a critical role in cell-cycle progression and proliferation. Myb proteins activate Myb-dependent promoters by interacting specifically with Myb-binding site (MBS) sequences using their DNA-binding domain (DBD). Transactivation of MBS promoters by B-Myb is repressed by its negative regulatory domain (NRD), and phosphorylation of the NRD by Cdk2-CyclinA relieves the repression to activate B-Myb–dependent promoters. However, the structural mechanisms underlying autoinhibition and activation of B-Myb–mediated transcription have been poorly characterized. Here, we determined that a region in the B-Myb NRD (residues 510–600) directly associates with the DBD and inhibits binding of the DBD to the MBS DNA sequence. We demonstrate using biophysical assays that phosphorylation of the NRD at T515, T518, and T520 is sufficient to disrupt the interaction between the NRD and the DBD, which results in increased affinity for MBS DNA and increased B-Myb–dependent promoter activation in cell assays. Our biochemical characterization of B-Myb autoregulation and the activating effects of phosphorylation provide insight into how B-Myb functions as a site-specific transcription factor.

CDK/cyclin dependencies define extreme cancer cell-cycle heterogeneity and collateral vulnerabilities

Progression through G1/S phase of the cell cycle is coordinated by cyclin-dependent kinase (CDK) activities. Here, we find that the requirement for different CDK activities and cyclins in driving cancer cell cycles is highly heterogeneous. The differential gene requirements associate with tumor origin and genetic alterations. We define multiple mechanisms for G1/S progression in RB-proficient models, which are CDK4/6 independent and elicit resistance to FDA-approved inhibitors. Conversely, RB-deficient models are intrinsically CDK4/6 independent, but exhibit differential requirements for cyclin E. These dependencies for CDK and cyclins associate with gene expression programs that denote intrinsically different cell-cycle states. Mining therapeutic sensitivities shows that there are reciprocal vulnerabilities associated with RB1 or CCND1 expression versus CCNE1 or CDKN2A. Together, these findings illustrate the complex nature of cancer cell cycles and the relevance for precision therapeutic intervention.

Knudsen et al. find that there is extensive heterogeneity in the requirement for CDK and cyclins across cancer models. Multiple biochemically distinct mechanisms drive cell division. Divergent cell-cycle states harbor distinct genetic and pharmacological vulnerabilities, suggesting that cell-cycle diversity could be exploited for a precision approach to cancer therapy.

The MuvB complex binds and stabilizes nucleosomes downstream of the transcription start site of cell-cycle dependent genes

The chromatin architecture in promoters is thought to regulate gene expression, but it remains uncertain how most transcription factors (TFs) impact nucleosome position. The MuvB TF complex regulates cell-cycle dependent gene-expression and is critical for differentiation and proliferation during development and cancer. MuvB can both positively and negatively regulate expression, but the structure of MuvB and its biochemical function are poorly understood. Here we determine the overall architecture of MuvB assembly and the crystal structure of a subcomplex critical for MuvB function in gene repression. We find that the MuvB subunits LIN9 and LIN37 function as scaffolding proteins that arrange the other subunits LIN52, LIN54 and RBAP48 for TF, DNA, and histone binding, respectively. Biochemical and structural data demonstrate that MuvB binds nucleosomes through an interface that is distinct from LIN54-DNA consensus site recognition and that MuvB increases nucleosome occupancy in a reconstituted promoter. We find in arrested cells that MuvB primarily associates with a tightly positioned +1 nucleosome near the transcription start site (TSS) of MuvB-regulated genes. These results support a model that MuvB binds and stabilizes nucleosomes just downstream of the TSS on its target promoters to repress gene expression.

The MuvB protein complex regulates genes that are differentially expressed through the cell cycle, yet its precise molecular function has remained unclear. Here the authors reveal MuvB associates with the nucleosome adjacent to the transcription start site of cell-cycle genes and that the tight positioning of this nucleosome correlates with MuvB-dependent gene repression.

Cyclin F drives proliferation through SCF-dependent degradation of the retinoblastoma-like tumor suppressor p130/RBL2

Cell cycle gene expression programs fuel proliferation and are universally dysregulated in cancer. The retinoblastoma (RB)-family of proteins, RB1, RBL1/p107, and RBL2/p130, coordinately represses cell cycle gene expression, inhibiting proliferation, and suppressing tumorigenesis. Phosphorylation of RB-family proteins by cyclin-dependent kinases is firmly established. Like phosphorylation, ubiquitination is essential to cell cycle control, and numerous proliferative regulators, tumor suppressors, and oncoproteins are ubiquitinated. However, little is known about the role of ubiquitin signaling in controlling RB-family proteins. A systems genetics analysis of CRISPR/Cas9 screens suggested the potential regulation of the RB-network by cyclin F, a substrate recognition receptor for the SCF family of E3 ligases. We demonstrate that RBL2/p130 is a direct substrate of SCF^{cyclin F}. We map a cyclin F regulatory site to a flexible linker in the p130 pocket domain, and show that this site mediates binding, stability, and ubiquitination. Expression of a mutant version of p130, which cannot be ubiquitinated, severely impaired proliferative capacity and cell cycle progression. Consistently, we observed reduced expression of cell cycle gene transcripts, as well a reduced abundance of cell cycle proteins, analyzed by quantitative, iterative immunofluorescent imaging. These data suggest a key role for SCF^{cyclin F} in the CDK-RB network and raise the possibility that aberrant p130 degradation could dysregulate the cell cycle in human cancers.

Molecular mimicry: HUWE1 binds an atypical site in MIZ1 by adopting a typical BTB fold

In this issue of Structure, Orth et al. describe an exposed binding site in the MIZ1 BTB domain due to an atypical flexible region within the BTB core. This site allows HECT-type ubiquitin ligase 1 to bind the MIZ1 homodimer by forming β strands and completing the canonical BTB fold.

E2F4's cytoplasmic role in multiciliogenesis is mediated via an N-terminal domain that binds two components of the centriole replication machinery, Deup1 and SAS6

Multiciliated cells play critical roles in the airway, reproductive organs, and brain. Generation of multiple cilia requires both activation of a specialized transcriptional program and subsequent massive amplification of centrioles within the cytoplasm. The E2F4 transcription factor is required for both roles and consequently for multiciliogenesis. Here we establish that E2F4 associates with two distinct components of the centriole replication machinery, Deup1 and SAS6, targeting nonhomologous domains in these proteins. We map Deup1 and SAS6 binding to E2F4’s N-terminus and show that this domain is sufficient to mediate E2F4’s cytoplasmic role in multiciliogenesis. This sequence is highly conserved across the E2F family, but the ability to bind Deup1 and SAS6 is specific to E2F4 and E2F5, consistent with their shared roles in multiciliogenesis. By generating E2F4/E2F1 chimeras, we identify a six-residue motif that is critical for Deup1 and SAS6 binding. We propose that the ability of E2F4 and E2F5 to recruit Deup1 and/or SAS6, and enable centriole replication, contributes to their cytoplasmic roles in multiciliogenesis.

A tyrosine phosphoregulatory system controls exopolysaccharide biosynthesis and biofilm formation in Vibrio cholerae

Production of an extracellular matrix is essential for biofilm formation, as this matrix both secures and protects the cells it encases. Mechanisms underlying production and assembly of matrices are poorly understood. Vibrio cholerae, relies heavily on biofilm formation for survival, infectivity, and transmission. Biofilm formation requires Vibrio polysaccharide (VPS), which is produced by vps gene-products, yet the function of these products remains unknown. Here, we demonstrate that the vps gene-products vpsO and vpsU encode respectively for a tyrosine kinase and a cognate tyrosine phosphatase. Collectively, VpsO and VpsU act as a tyrosine phosphoregulatory system to modulate VPS production. We present structures of VpsU and the kinase domain of VpsO, and we report observed autocatalytic tyrosine phosphorylation of the VpsO C-terminal tail. The position and amount of tyrosine phosphorylation in the VpsO C-terminal tail represses VPS production and biofilm formation through a mechanism involving the modulation of VpsO oligomerization. We found that tyrosine phosphorylation enhances stability of VpsO. Regulation of VpsO phosphorylation by the phosphatase VpsU is vital for maintaining native VPS levels. This study provides new insights into the mechanism and regulation of VPS production and establishes general principles of biofilm matrix production and its inhibition.

The biofilm life style protects microbes from a plethora of harm, to increase their survival and pathogenicity. Exopolysaccharides are the essential glue of the microbial biofilm matrix, and loss of this glue negates biofilm formation and renders cells more sensitive to antimicrobial agents. Here, we show that a tyrosine phosphoregulatory system controls the biosynthesis and abundance of Vibrio exopolysaccharide (VPS), an essential biofilm component of the pathogen Vibrio cholerae. The phosphorylation state of the tyrosine autokinase VpsO, mediated by the tyrosine phosphatase VpsU, directly modulates VPS production and also affects the kinase’s own degradation, to regulate VPS production. This study provides new insights into the mechanisms of V. cholerae biofilm formation and consequently ways to combat pathogens more broadly, due to conservation of tyrosine phosphoregulatory systems among exopolysaccharide producing bacteria.

A long lost key opens an ancient lock: Drosophila Myb causes a synthetic multivulval phenotype in nematodes

The five-protein MuvB core complex is highly conserved in animals. This nuclear complex interacts with RB-family tumor suppressor proteins and E2F-DP transcription factors to form DREAM complexes that repress genes that regulate cell cycle progression and cell fate. The MuvB core complex also interacts with Myb family oncoproteins to form the Myb-MuvB complexes that activate many of the same genes. We show that animal-type Myb genes are present in Bilateria, Cnidaria and Placozoa, the latter including the simplest known animal species. However, bilaterian nematode worms lost their animal-type Myb genes hundreds of millions of years ago. Nevertheless, amino acids in the LIN9 and LIN52 proteins that directly interact with the MuvB-binding domains of human B-Myb and Drosophila Myb are conserved in Caenorhabditiselegans Here, we show that, despite greater than 500 million years since their last common ancestor, the Drosophila melanogaster Myb protein can bind to the nematode LIN9-LIN52 proteins in vitro and can cause a synthetic multivulval (synMuv) phenotype in vivo This phenotype is similar to that caused by loss-of-function mutations in C. elegans synMuvB-class genes including those that encode homologs of the MuvB core, RB, E2F and DP. Furthermore, amino acid substitutions in the MuvB-binding domain of Drosophila Myb that disrupt its functions in vitro and in vivo also disrupt these activities in C. elegans We speculate that nematodes and other animals may contain another protein that can bind to LIN9 and LIN52 in order to activate transcription of genes repressed by DREAM complexes.

p27 allosterically activates cyclin-dependent kinase 4 and antagonizes palbociclib inhibition

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.

Structural Basis of BRCC36 Function in DNA Repair and Immune Regulation

In mammals, ∼100 deubiquitinases act on ∼20,000 intracellular ubiquitination sites. Deubiquitinases are commonly regarded as constitutively active, with limited regulatory and targeting capacity. The BRCA1-A and BRISC complexes serve in DNA double-strand break repair and immune signaling and contain the lysine-63 linkage-specific BRCC36 subunit that is functionalized by scaffold subunits ABRAXAS and ABRO1, respectively. The molecular basis underlying BRCA1-A and BRISC function is currently unknown. Here we show that in the BRCA1-A complex structure, ABRAXAS integrates the DNA repair protein RAP80 and provides a high-affinity binding site that sequesters the tumor suppressor BRCA1 away from the break site. In the BRISC structure, ABRO1 binds SHMT2α, a metabolic enzyme enabling cancer growth in hypoxic environments, which we find prevents BRCC36 from binding and cleaving ubiquitin chains. Our work explains modularity in the BRCC36 DUB family, with different adaptor subunits conferring diversified targeting and regulatory functions.

E2F4 regulates transcriptional activation in mouse embryonic stem cells independently of the RB family

E2F transcription factors are central regulators of cell division and cell fate decisions. E2F4 often represents the predominant E2F activity in cells. E2F4 is a transcriptional repressor implicated in cell cycle arrest and whose repressive activity depends on its interaction with members of the RB family. Here we show that E2F4 is important for the proliferation and the survival of mouse embryonic stem cells. In these cells, E2F4 acts in part as a transcriptional activator that promotes the expression of cell cycle genes. This role for E2F4 is independent of the RB family. Furthermore, E2F4 functionally interacts with chromatin regulators associated with gene activation and we observed decreased histone acetylation at the promoters of cell cycle genes and E2F targets upon loss of E2F4 in RB family-mutant cells. Taken together, our findings uncover a non-canonical role for E2F4 that provide insights into the biology of rapidly dividing cells.

E2F transcription factors are regulators of cell division and cell fate decisions. Here the authors show that E2F4 is important for proliferation and survival of mouse ESCs, independent of the RB family, and that E2F4 interacts with chromatin regulators associated with gene activation.

An order-to-disorder structural switch activates the FoxM1 transcription factor

Intrinsically disordered transcription factor transactivation domains (TADs) function through structural plasticity, adopting ordered conformations when bound to transcriptional co-regulators. Many transcription factors contain a negative regulatory domain (NRD) that suppresses recruitment of transcriptional machinery through autoregulation of the TAD. We report the solution structure of an autoinhibited NRD-TAD complex within FoxM1, a critical activator of mitotic gene expression. We observe that while both the FoxM1 NRD and TAD are primarily intrinsically disordered domains, they associate and adopt a structured conformation. We identify how Plk1 and Cdk kinases cooperate to phosphorylate FoxM1, which releases the TAD into a disordered conformation that then associates with the TAZ2 or KIX domains of the transcriptional co-activator CBP. Our results support a mechanism of FoxM1 regulation in which the TAD undergoes switching between disordered and different ordered structures.

Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix

The cyclin-dependent kinases Cdk4 and Cdk6 form complexes with D-type cyclins to drive cell proliferation. A well-known target of cyclin D-Cdk4,6 is the retinoblastoma protein Rb, which inhibits cell-cycle progression until its inactivation by phosphorylation. However, the role of Rb phosphorylation by cyclin D-Cdk4,6 in cell-cycle progression is unclear because Rb can be phosphorylated by other cyclin-Cdks, and cyclin D-Cdk4,6 has other targets involved in cell division. Here, we show that cyclin D-Cdk4,6 docks one side of an alpha-helix in the Rb C terminus, which is not recognized by cyclins E, A, and B. This helix-based docking mechanism is shared by the p107 and p130 Rb-family members across metazoans. Mutation of the Rb C-terminal helix prevents its phosphorylation, promotes G1 arrest, and enhances Rb's tumor suppressive function. Our work conclusively demonstrates that the cyclin D-Rb interaction drives cell division and expands the diversity of known cyclin-based protein docking mechanisms.

Structural basis of divergent cyclin-dependent kinase activation by Spy1/RINGO proteins

Cyclin-dependent kinases (Cdks) are principal drivers of cell division and are an important therapeutic target to inhibit aberrant proliferation. Cdk enzymatic activity is tightly controlled through cyclin interactions, posttranslational modifications, and binding of inhibitors such as the p27 tumor suppressor protein. Spy1/RINGO (Spy1) proteins bind and activate Cdk but are resistant to canonical regulatory mechanisms that establish cell-cycle checkpoints. Cancer cells exploit Spy1 to stimulate proliferation through inappropriate activation of Cdks, yet the mechanism is unknown. We have determined crystal structures of the Cdk2-Spy1 and p27-Cdk2-Spy1 complexes that reveal how Spy1 activates Cdk. We find that Spy1 confers structural changes to Cdk2 that obviate the requirement of Cdk activation loop phosphorylation. Spy1 lacks the cyclin-binding site that mediates p27 and substrate affinity, explaining why Cdk-Spy1 is poorly inhibited by p27 and lacks specificity for substrates with cyclin-docking sites. We identify mutations in Spy1 that ablate its ability to activate Cdk2 and to proliferate cells. Our structural description of Spy1 provides important mechanistic insights that may be utilized for targeting upregulated Spy1 in cancer.

Conservation and divergence of C-terminal domain structure in the retinoblastoma protein family

The retinoblastoma protein (Rb) and the homologous pocket proteins p107 and p130 negatively regulate cell proliferation by binding and inhibiting members of the E2F transcription factor family. The structural features that distinguish Rb from other pocket proteins have been unclear but are critical for understanding their functional diversity and determining why Rb has unique tumor suppressor activities. We describe here important differences in how the Rb and p107 C-terminal domains (CTDs) associate with the coiled-coil and marked-box domains (CMs) of E2Fs. We find that although CTD-CM binding is conserved across protein families, Rb and p107 CTDs show clear preferences for different E2Fs. A crystal structure of the p107 CTD bound to E2F5 and its dimer partner DP1 reveals the molecular basis for pocket protein-E2F binding specificity and how cyclin-dependent kinases differentially regulate pocket proteins through CTD phosphorylation. Our structural and biochemical data together with phylogenetic analyses of Rb and E2F proteins support the conclusion that Rb evolved specific structural motifs that confer its unique capacity to bind with high affinity those E2Fs that are the most potent activators of the cell cycle.

An RB-EZH2 Complex Mediates Silencing of Repetitive DNA Sequences

Repetitive genomic regions include tandem sequence repeats and interspersed repeats, such as endogenous retroviruses and LINE-1 elements. Repressive heterochromatin domains silence expression of these sequences through mechanisms that remain poorly understood. Here, we present evidence that the retinoblastoma protein (pRB) utilizes a cell-cycle-independent interaction with E2F1 to recruit enhancer of zeste homolog 2 (EZH2) to diverse repeat sequences. These include simple repeats, satellites, LINEs, and endogenous retroviruses as well as transposon fragments. We generated a mutant mouse strain carrying an F832A mutation in Rb1 that is defective for recruitment to repetitive sequences. Loss of pRB-EZH2 complexes from repeats disperses H3K27me3 from these genomic locations and permits repeat expression. Consistent with maintenance of H3K27me3 at the Hox clusters, these mice are developmentally normal. However, susceptibility to lymphoma suggests that pRB-EZH2 recruitment to repetitive elements may be cancer relevant.

Abstract A17: Cell cycle control through DREAM and MMB complexes

The loss of cell cycle control is the most recognized trait of cancer cells. The ability of cells to exit the cell cycle has recently been shown to be dependent upon a large multi-protein repressor complex called DREAM. This complex represses the expression of all cell cycle genes during reversible cell cycle exit. In cycling cells, DREAM is disassembled and some of DREAM associated proteins form a new complex to promote cell cycle gene expression. These proteins that can act together both as repressors and activators of cell cycle gene expression are called the MuvB core. The MuvB core interacts with B-Myb during S phase to promote late cell cycle gene expression. This activator complex is called MMB and its integrity is required for proper cell division. The purpose of this project is to determine how the MuvB core interacts with B-Myb, and if MuvB can be targeted by therapeutics to restore proper cell cycle control in cancer. We have structurally and functionally characterized the mechanism of DREAM assembly and regulation, however MMB regulation remains poorly understood. DREAM has been suggested as a therapeutic target in cancer cells that arrest in response to drug treatment to evade apoptosis. Disruption of DREAM could force evasive cancer cells back into the cell cycle and elicit a response to chemotherapeutic treatment. Until recently, a DREAM interface to target was unavailable. I solved the x-ray crystal structure of MuvB core protein LIN52 complexed with RB tumor suppressor protein p107 and functionally characterized how this interaction is regulated in DREAM assembly. This is a suitable DREAM interface to target given disruption of this interface drives cell cycle entry. I have found that LIN52 also interacts with B-Myb and have obtained crystals of this subcomplex that diffract to 2.4 Å resolution. Therefore, I hypothesize that the MuvB core protein LIN52 mediates interactions with various transcription factors throughout the cell cycle to drive gene specificity through cooperative binding. I will test this hypothesis with two aims: 1) solve the x-ray crystal structure of B-Myb complexed with the MuvB core and investigate the cooperative DNA binding of MuvB and B-Myb; 2) Disrupt LIN52 interactions to disassemble DREAM in drug resistant cancer cells. Completion of these aims will unveil the mechanism of B-Myb activation in cancer and determine if LIN52 is a viable cancer therapeutic target.

Citation Format: Keelan Guiley, Seth M. Rubin. Cell cycle control through DREAM and MMB complexes. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr A17.

Structural Conservation and E2F Binding Specificity within the Retinoblastoma Pocket Protein Family

The human pocket proteins retinoblastoma (Rb), p107, and p130 are critical negative regulators of the cell cycle and contribute to tumor suppression. While strong structural conservation within the pocket protein family provides for some functional redundancy, important differences have been observed and may underlie the reason that Rb is a uniquely potent tumor suppressor. It has been proposed that distinct pocket protein activities are mediated by their different E2F transcription factor binding partners. In humans, Rb binds E2F1-E2F5, whereas p107 and p130 almost exclusively associate with E2F4 and E2F5. To identify the molecular determinants of this specificity, we compared the crystal structures of Rb and p107 pocket domains and identified several key residues that contribute to E2F selectivity in the pocket family. Mutation of these residues in p107 to match the analogous residue in Rb results in an increase in affinity for E2F1 and E2F2 and an increase in the ability of p107 to inhibit E2F2 transactivation. Additionally, we investigated how phosphorylation by Cyclin-dependent kinase on distinct residues regulates p107 affinity for the E2F4 transactivation domain. We found that phosphorylation of residues S650 and S975 weakens the E2F4 transactivation domain binding. Our data reveal molecular features of pocket proteins that are responsible for their similarities and differences in function and regulation.

A Strategy for Direct Chemical Activation of the Retinoblastoma Protein

The retinoblastoma (Rb) tumor suppressor protein negatively regulates cell proliferation by binding and inhibiting E2F transcription factors. Rb inactivation occurs in cancer cells upon cyclin-dependent kinase (Cdk) phosphorylation, which induces E2F release and activation of cell cycle genes. We present a strategy for activating phosphorylated Rb with molecules that bind Rb directly and enhance affinity for E2F. We developed a fluorescence polarization assay that can detect the effect of exogenous compounds on modulating affinity of Rb for the E2F transactivation domain. We found that a peptide capable of disrupting the compact inactive Rb conformation increases affinity of the repressive Rb-E2F complex. Our results demonstrate the feasibility of discovering novel molecules that target the cell cycle and proliferation through directly targeting Rb rather than upstream kinase activity.

Structural basis of template-boundary definition in Tetrahymena telomerase

Telomerase is required to maintain repetitive G-rich telomeric DNA sequences at chromosome ends. To do so, the telomerase reverse transcriptase (TERT) subunit reiteratively uses a small region of the integral telomerase RNA (TER) as a template. An essential feature of telomerase catalysis is the strict definition of the template boundary to determine the precise TER nucleotides to be reverse transcribed by TERT. We report the 3-Å crystal structure of the Tetrahymena TERT RNA-binding domain (tTRBD) bound to the template boundary element (TBE) of TER. tTRBD is wedged into the base of the TBE RNA stem-loop, and each of the flanking RNA strands wraps around opposite sides of the protein domain. The structure illustrates how the tTRBD establishes the template boundary by positioning the TBE at the correct distance from the TERT active site to prohibit copying of nontemplate nucleotides.

Structural mechanisms of DREAM complex assembly and regulation

To understand the biochemical mechanisms underpinning DREAM function and regulation, Guiley et al. investigated the structural basis for DREAM assembly. Together, the data inform a novel target interface for studying MuvB and p130 function and the design of inhibitors that prevent tumor escape in quiescence.

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.

Defining a new vision for the retinoblastoma gene: report from the 3rd International Rb Meeting

The retinoblastoma tumor suppressor (Rb) pathway is mutated in most, if not all human tumors. In the G0/G1 phase, Rb and its family members p107 and p130 inhibit the E2F family of transcription factors. In response to mitogenic signals, Cyclin-dependent kinases (CDKs) phosphorylate Rb family members, which results in the disruption of complexes between Rb and E2F family members and in the transcription of genes essential for S phase progression. Beyond this role in early cell cycle decisions, Rb family members regulate DNA replication and mitosis, chromatin structure, metabolism, cellular differentiation, and cell death. While the RB pathway has been extensively studied in the past three decades, new investigations continue to provide novel insights into basic mechanisms of cancer development and, beyond cancer, help better understand fundamental cellular processes, from plants to mammals. This meeting report summarizes research presented at the recently held 3rd International Rb Meeting.

Multisite phosphorylation networks as signal processors for Cdk1

The order and timing of cell cycle events is controlled by changing substrate specificity and different activity thresholds of cyclin-dependent kinases (CDK). However, it is not understood how a single protein kinase can trigger hundreds of switches in a sufficiently time-resolved fashion. We show that the cyclin-Cdk1-Cks1-dependent phosphorylation of multisite targets in Saccharomyces cerevisiae is controlled by key substrate parameters including distances between phosphorylation sites, the distribution of serines and threonines as phospho-acceptors, and the positioning of cyclin-docking motifs. The component mediating the key interactions in this process is Cks1, the phospho-adaptor subunit of the cyclin-Cdk1-Cks1 complex. We propose that variation of these parameters within the networks of phosphorylation sites in different targets provides a wide range of possibilities for the differential amplification of Cdk1 signals, providing a mechanism to generate a wide range of thresholds in the cell cycle.

Multiple mechanisms for E2F binding inhibition by phosphorylation of the retinoblastoma protein C-terminal domain

The retinoblastoma protein C-terminal domain (RbC) is necessary for the tumor suppressor protein's activities in growth suppression and E2F transcription factor inhibition. Cyclin-dependent kinase phosphorylation of RbC contributes to Rb inactivation and weakens the Rb-E2F inhibitory complex. Here we demonstrate two mechanisms for how RbC phosphorylation inhibits E2F binding. We find that phosphorylation of S788 and S795 weakens the direct association between the N-terminal portion of RbC (RbC(N)) and the marked-box domains of E2F and its heterodimerization partner DP. Phosphorylation of these sites and S807/S811 also induces an intramolecular association between RbC and the pocket domain, which overlaps with the site of E2F transactivation domain binding. A reduction in E2F binding affinity occurs with S788/S795 phosphorylation that is additive with the effects of phosphorylation at other sites, and we propose a structural mechanism that explains this additivity. We find that different Rb phosphorylation events have distinct effects on activating E2F family members, which suggests a novel mechanism for how Rb may differentially regulate E2F activities.

Rif1 and Rif2 shape telomere function and architecture through multivalent Rap1 interactions

Yeast telomeres comprise irregular TG₁₋₃ DNA repeats bound by the general transcription factor Rap1. Rif1 and Rif2, along with Rap1, form the telosome, a protective cap that inhibits telomerase, counteracts SIR-mediated transcriptional silencing, and prevents inadvertent recognition of telomeres as DNA double-strand breaks. We provide a molecular, biochemical, and functional dissection of the protein backbone at the core of the yeast telosome. The X-ray structures of Rif1 and Rif2 bound to the Rap1 C-terminal domain and that of the Rif1 C terminus are presented. Both Rif1 and Rif2 have separable and independent Rap1-binding epitopes, allowing Rap1 binding over large distances (42-110 Å). We identify tetramerization (Rif1) and polymerization (Rif2) modules that, in conjunction with the long-range binding, give rise to a higher-order architecture that interlinks Rap1 units. This molecular Velcro relies on Rif1 and Rif2 to recruit and stabilize Rap1 on telomeric arrays and is required for telomere homeostasis in vivo.

Structures of inactive retinoblastoma protein reveal multiple mechanisms for cell cycle control

Cyclin-dependent kinase (Cdk) phosphorylation of the Retinoblastoma protein (Rb) drives cell proliferation through inhibition of Rb complexes with E2F transcription factors and other regulatory proteins. We present the first structures of phosphorylated Rb that reveal the mechanism of its inactivation. S608 phosphorylation orders a flexible "pocket" domain loop such that it mimics and directly blocks E2F transactivation domain (E2F(TD)) binding. T373 phosphorylation induces a global conformational change that associates the pocket and N-terminal domains (RbN). This first multidomain Rb structure demonstrates a novel role for RbN in allosterically inhibiting the E2F(TD)-pocket association and protein binding to the pocket "LxCxE" site. Together, these structures detail the regulatory mechanism for a canonical growth-repressive complex and provide a novel example of how multisite Cdk phosphorylation induces diverse structural changes to influence cell cycle signaling.

Methylation of the retinoblastoma tumor suppressor by SMYD2

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 methyl-binding 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.

Phosphorylation-induced conformational changes in the retinoblastoma protein inhibit E2F transactivation domain binding

Inactivation of the retinoblastoma protein (Rb) through phosphorylation is an important step in promoting cell cycle progression, and hyperphosphorylated Rb is commonly found in tumors. Rb phosphorylation prevents its association with the E2F transcription factor; however, the molecular basis for complex inhibition has not been established. We identify here the key phosphorylation events and conformational changes that occur in Rb to inhibit the specific association between the E2F transactivation domain (E2F(TD)) and the Rb pocket domain. Calorimetry assays demonstrate that phosphorylation of Rb reduces the affinity of E2F(TD) binding approximately 250-fold and that phosphorylation at Ser(608)/Ser(612) and Thr(356)/Thr(373) is necessary and sufficient for this effect. An NMR assay identifies phosphorylation-driven conformational changes in Rb that directly inhibit E2F(TD) binding. We find that phosphorylation at Ser(608)/Ser(612) promotes an intramolecular association between a conserved sequence in the flexible pocket linker and the pocket domain of Rb that occludes the E2F(TD) binding site. We also find that phosphorylation of Thr(356)/Thr(373) inhibits E2F(TD) binding in a manner that requires the Rb N-terminal domain. Taken together, our results suggest two distinct mechanisms for how phosphorylation of Rb modulates association between E2F(TD) and the Rb pocket and describe for the first time a function for the structured N-terminal domain in Rb inactivation.

Predictors of maintaining cognitive function in older adults: the Health ABC study

BACKGROUND

Although several risk factors for cognitive decline have been identified, much less is known about factors that predict maintenance of cognitive function in advanced age.

METHODS

We studied 2,509 well-functioning black and white elders enrolled in a prospective study. Cognitive function was measured using the Modified Mini-Mental State Examination at baseline and years 3, 5, and 8. Random effects models were used to classify participants as cognitive maintainers (cognitive change slope > or = 0), minor decliners (slope < 0 and > 1 SD below mean), or major decliners (slope < or = 1 SD below mean). Logistic regression was used to identify domain-specific factors associated with being a maintainer vs a minor decliner.

RESULTS

Over 8 years, 30% of the participants maintained cognitive function, 53% showed minor decline, and 16% had major cognitive decline. In the multivariate model, baseline variables significantly associated with being a maintainer vs a minor decliner were age (odds ratio [OR] = 0.65, 95% confidence interval [CI] 0.55-0.77 per 5 years), white race (OR = 1.72, 95% CI 1.30-2.28), high school education level or greater (OR = 2.75, 95% CI 1.78-4.26), ninth grade literacy level or greater (OR = 4.85, 95% CI 3.00-7.87), weekly moderate/vigorous exercise (OR = 1.31, 95% CI 1.06-1.62), and not smoking (OR = 1.84, 95% CI 1.14-2.97). Variables associated with major cognitive decline compared to minor cognitive decline are reported.

CONCLUSION

Elders who maintain cognitive function have a unique profile that differentiates them from those with minor decline. Importantly, some of these factors are modifiable and thus may be implemented in prevention programs to promote successful cognitive aging. Further, factors associated with maintenance may differ from factors associated with major cognitive decline, which may impact prevention vs treatment strategies.

Dietary factors in relation to daily activity energy expenditure and mortality among older adults

OBJECTIVES

To examine the association between dietary factors to daily activity energy expenditure (DAEE) and mortality among older adults.

DESIGN AND SETTING

A sub-study of Health, Aging, and Body Composition study.

PARTICIPANTS

298 older participants (aged 70-82 years) in the Health, Aging, and Body Composition Energy Expenditure sub-study.

MEASUREMENTS

Dietary factors, DAEE, and all-cause mortality were measured in 298 older participants. Dietary factors include dietary intake assessed by the Block Food Frequency Questionnaire (FFQ), Healthy Eating Index (HEI), and self-reported appetite and enjoyment of eating. DAEE was assessed using doubly labeled water. All-cause mortality was evaluated over a 9 year period.

RESULTS

Participants in the highest tertile of DAEE were more likely to be men and to report having a 'good' appetite; BMI among men, proportion married, IL-6 and CRP levels and energy intake were also higher. Fewer black participants were in the 'good' HEI category. Participants in the 'good' HEI category had higher cognitive scores and a higher education level. Participants who reported improvement in their appetite as well as participants who reported a 'good' appetite were at lower risk for mortality (HR (95% CI): 0.42 (0.24-0.74) and 0.50 (0.26-0.88), respectively) even after adjusting for DAEE, demographic, nutritional and health indices.

CONCLUSIONS

We showed an association between DAEE and appetite and mortality among well-functioning, community-dwelling older adults. These findings may have some practical use for the health providers. Inclusion of a question regarding appetite of an elderly patient may provide important information regarding risk for health deterioration and mortality.

The association between weight history and physical performance in the Health, Aging and Body Composition study

OBJECTIVE

Although the association between current obesity and physical disability is well known, the cumulative effect of obesity is unknown. Using data from the Health, Aging and Body Composition study, we examined the association between weight history in young and middle adulthood and weight status in late adulthood with physical performance in late adulthood.

DESIGN

Longitudinal cohort study.

SUBJECTS

White and black men and women aged 70-79 years at study baseline (n=2803).

MEASURES

Body mass index (BMI; kg/m(2)) was calculated using recalled height at age 25 and weight at age 25 and 50 and measured height and weight at ages 70-79. Physical performance at ages 70-79 was assessed using a short physical performance battery (SPPB) and a 400-m walk test.

RESULTS

In this well-functioning cohort, approximately 24% of men and 8% of women reported being overweight or obese (BMI > or =25 kg/m(2)) at age 25, 51% of men and 37% of women reported being overweight or obese at age 50, and 69% of men and 66% of women were overweight or obese at ages 70-79. Men and women who were obese (BMI > or =30 kg/m(2)) at ages 25, 50 and 70-79 had significantly worse SPPB scores and 400-m walk times than those who were normal weight. Women who were overweight (BMI 25-29.9 kg/m(2)) at ages 25, 50 and 70-79 also had significantly worse physical performance. Furthermore, men and women who had a history of being overweight or obese at ages 25 or 50 had worse physical performance compared to those who were normal weight throughout or who were overweight or obese at ages 70-79 but not in midlife or earlier.

CONCLUSIONS

Maintaining a healthy body weight throughout adulthood may play a role in preventing or delaying the onset of physical disability.

Cognitive ability is associated with suspected reporting errors on food frequency questionnaires

OBJECTIVE

To examine potential for bias in reported total energy intake on a Food Frequency Questionnaire (FFQ) among older adults.

DESIGN

Longitudinal cohort study.

SUBJECTS/SETTING

2,706 Community-dwelling Black and White older adults, aged 70-79 years, enrolled in the Health, Aging, and Body Composition study. Multivariate logistic regression analyses were conducted with potential errors on reported total energy intake on the Food Frequency Questionnaire (FFQ) as the outcome variable and with cognitive ability, measured by the Modified Mini Mental State Exam (3MS) as the primary independent variable. The regression model controlled for site, race, gender, age, body size, and physical activity. Separate models were fit using 3MS as a continuous variable and for multiple 3MS cutpoints. All models revealed similar findings.

RESULTS

Cognitive ability was inversely associated with potential errors in reporting total energy intake, whereby a five-point increase in 3MS scores was associated with a 14% decreased likelihood of reporting errors (Odds Ratio=0.86, 95% Confidence Interval: 0.77, 0.95). Additionally, compared to White women, White men were 2 times more likely, and Black women and Black men were 3 times more likely, to have errors in reporting total energy intake.

CONCLUSION

This study provides evidence that for older adults, lower cognition scores are associated with increased potential errors in reporting total energy intake.

APPLICATIONS

Dietary reporting from older adults may be inaccurate due to cognitive deficits. A brief assessment of cognitive function may assist clinicians in dietary evaluations and recommendation and may benefit studies using FFQ data where the measure of cognitive function could be utilized to stratify data analyses and conduct sensitivity analyses.

Structure of the Rb C-terminal domain bound to E2F1-DP1: a mechanism for phosphorylation-induced E2F release

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.

Solution structure of a putative ribosome binding protein from Mycoplasma pneumoniae and comparison to a distant homolog

The solution structure of MPN156, a ribosome-binding factor A (RBFA) protein family member from Mycoplasma pneumoniae, is presented. The structure, solved by nuclear magnetic resonance, has a type II KH fold typical of RNA binding proteins. Despite only approximately 20% sequence identity between MPN156 and another family member from Escherichia coli, the two proteins have high structural similarity. The comparison demonstrates that many of the conserved residues correspond to conserved elements in the structures. Compared to a structure based alignment, standard alignment methods based on sequence alone mispair a majority of amino acids in the two proteins. Implications of these discrepancies for sequence based structural modeling are discussed.

Distinguishing multiple chemotaxis Y protein conformations with laser-polarized 129Xe NMR

The chemical shift of the (129)Xe NMR signal has been shown to be extremely sensitive to the local environment around the atom and has been used to follow processes such as ligand binding by bacterial periplasmic binding proteins. Here we show that the (129)Xe shift can sense more subtle changes: magnesium binding, BeF(3)(-) activation, and peptide binding by the Escherichia coli chemotaxis Y protein. (1)H-(15)N correlation spectroscopy and X-ray crystallography were used to identify two xenon-binding cavities in CheY that are primarily responsible for the shift changes. One site is near the active site, and the other is near the peptide binding site.