Structural mechanism of HP1⍺-dependent transcriptional repression and chromatin compaction.

Heterochromatin protein 1 (HP1) plays a central role in establishing and maintaining constitutive heterochromatin. However, the mechanisms underlying HP1-nucleosome interactions and their contributions to heterochromatin functions remain elusive. Here, we present the cryoelectron microscopy (cryo-EM) structure of an HP1α dimer bound to an H2A.Z-nucleosome, revealing two distinct HP1α-nucleosome interfaces. The primary HP1α binding site is located at the N terminus of histone H3, specifically at the trimethylated lysine 9 (K9me3) region, while a secondary binding site is situated near histone H2B, close to nucleosome superhelical location 4 (SHL4). Our biochemical data further demonstrates that HP1α binding influences the dynamics of DNA on the nucleosome. It promotes DNA unwrapping near the nucleosome entry and exit sites while concurrently restricting DNA accessibility in the vicinity of SHL4. Our study offers a model for HP1α-mediated heterochromatin maintenance and gene silencing. It also sheds light on the H3K9me-independent role of HP1 in responding to DNA damage.

Structural and Biochemical Characterization of the Nucleosome Containing Variants H3.3 and H2A.Z.

Variant H3.3, along with H2A.Z, is notably enriched at promoter regions and is commonly associated with transcriptional activation. However, the specific molecular mechanisms through which H3.3 influences chromatin dynamics at transcription start sites, and its role in gene regulation, remain elusive. Using a combination of biochemistry and cryo-electron microscopy (cryo-EM), we show that the inclusion of H3.3 alone does not induce discernible changes in nucleosome DNA dynamics. Conversely, the presence of both H3.3 and H2A.Z enhances DNA's flexibility similarly to H2A.Z alone. Interestingly, our findings suggest that the presence of H3.3 in the H2A.Z nucleosome provides slightly increased protection to DNA at internal sites within the nucleosome. These results imply that while H2A.Z at active promoters promotes the formation of more accessible nucleosomes with increased DNA accessibility to facilitate transcription, the simultaneous presence of H3.3 offers an additional mechanism to fine-tune nucleosome accessibility and the chromatin environment.

Structural mechanism of HP1α-dependent transcriptional repression and chromatin compaction.

Heterochromatin protein 1 (HP1) plays a central role in establishing and maintaining constitutive heterochromatin. However, the mechanisms underlying HP1-nucleosome interactions and their contributions to heterochromatin functions remain elusive. In this study, we employed a multidisciplinary approach to unravel the interactions between human HP1α and nucleosomes. We have elucidated the cryo-EM structure of an HP1α dimer bound to an H2A.Z nucleosome, revealing that the HP1α dimer interfaces with nucleosomes at two distinct sites. The primary binding site is located at the N-terminus of histone H3, specifically at the trimethylated K9 (K9me3) region, while a novel secondary binding site is situated near histone H2B, close to nucleosome superhelical location 4 (SHL4). Our biochemical data further demonstrates that HP1α binding influences the dynamics of DNA on the nucleosome. It promotes DNA unwrapping near the nucleosome entry and exit sites while concurrently restricting DNA accessibility in the vicinity of SHL4. This study offers a model that explains how HP1α functions in heterochromatin maintenance and gene silencing, particularly in the context of H3K9me-dependent mechanisms. Additionally, it sheds light on the H3K9me-independent role of HP1 in responding to DNA damage.

DNA-translocation-independent role of INO80 remodeler in DNA damage repairs.

Chromatin remodelers utilize ATP hydrolysis to reposition histone octamers on DNA, facilitating transcription by promoting histone displacements. Although their actions on chromatin with damaged DNA are assumed to be similar, the precise mechanisms by which they modulate damaged nucleosomes and their specific roles in DNA damage response (DDR) remain unclear. INO80-C, a versatile chromatin remodeler, plays a crucial role in the efficient repair of various types of damage. In this study, we have demonstrated that both abasic sites and UV-irradiation damage abolish the DNA translocation activity of INO80-C. Additionally, we have identified compromised ATP hydrolysis within the Ino80 catalytic subunit as the primary cause of the inhibition of DNA translocation, while its binding to damaged nucleosomes remains unaffected. Moreover, we have uncovered a novel function of INO80-C that operates independently of its DNA translocation activity, namely, its facilitation of apurinic/apyrimidinic (AP) site cleavage by the AP-endonuclease 1 (APE1). Our findings provide valuable insights into the role of the INO80-C chromatin remodeler in DDR, thereby advancing our understanding of chromatin remodeling during DNA damage repairs.

Histone variants and chromatin structure, update of advances.

Histone proteins are highly conserved among all eukaryotes. They have two important functions in the cell: to package the genomic DNA and to regulate gene accessibility. Fundamental to these functions is the ability of histone proteins to interact with DNA and to form the nucleoprotein complex called chromatin. One of the mechanisms the cells use to regulate chromatin and gene expression is through replacing canonical histones with their variants at specific loci to achieve functional consequence. Recent cryo-electron microscope (cryo-EM) studies of chromatin containing histone variants reveal new details that shed light on how variant-specific features influence the structures and functions of chromatin. In this article, we review the current state of knowledge on histone variants biochemistry and discuss the implication of these new structural information on histone variant biology and their functions in transcription.

Structural basis of chromatin regulation by histone variant H2A.Z.

The importance of histone variant H2A.Z in transcription regulation has been well established, yet its mechanism-of-action remains enigmatic. Conflicting evidence exists in support of both an activating and a repressive role of H2A.Z in transcription. Here we report cryo-electron microscopy (cryo-EM) structures of nucleosomes and chromatin fibers containing H2A.Z and those containing canonical H2A. The structures show that H2A.Z incorporation results in substantial structural changes in both nucleosome and chromatin fiber. While H2A.Z increases the mobility of DNA terminus in nucleosomes, it simultaneously enables nucleosome arrays to form a more regular and condensed chromatin fiber. We also demonstrated that H2A.Z's ability to enhance nucleosomal DNA mobility is largely attributed to its characteristic shorter C-terminus. Our study provides the structural basis for H2A.Z-mediated chromatin regulation, showing that the increase flexibility of the DNA termini in H2A.Z nucleosomes is central to its dual-functions in chromatin regulation and in transcription.

Structures of monomeric and dimeric PRC2:EZH1 reveal flexible modules involved in chromatin compaction.

Polycomb repressive complex 2 (PRC2) is a histone methyltransferase critical for maintaining gene silencing during eukaryotic development. In mammals, PRC2 activity is regulated in part by the selective incorporation of one of two paralogs of the catalytic subunit, EZH1 or EZH2. Each of these enzymes has specialized biological functions that may be partially explained by differences in the multivalent interactions they mediate with chromatin. Here, we present two cryo-EM structures of PRC2:EZH1, one as a monomer and a second one as a dimer bound to a nucleosome. When bound to nucleosome substrate, the PRC2:EZH1 dimer undergoes a dramatic conformational change. We demonstrate that mutation of a divergent EZH1/2 loop abrogates the nucleosome-binding and methyltransferase activities of PRC2:EZH1. Finally, we show that PRC2:EZH1 dimers are more effective than monomers at promoting chromatin compaction, and the divergent EZH1/2 loop is essential for this function, thereby tying together the methyltransferase, nucleosome-binding, and chromatin-compaction activities of PRC2:EZH1. We speculate that the conformational flexibility and the ability to dimerize enable PRC2 to act on the varied chromatin substrates it encounters in the cell.

Structure of the Centromere Binding Factor 3 Complex from Kluyveromyces lactis.

Kinetochores are the multiprotein complexes that link chromosomal centromeres to mitotic-spindle microtubules. Budding yeast centromeres comprise three sequential "centromere-determining elements", CDEI, II, and III. CDEI (8 bp) and CDEIII (∼25 bp) are conserved between Kluyveromyces lactis and Saccharomyces cerevisiae, but CDEII in the former is twice as long (160 bp) as CDEII in the latter (80 bp). The CBF3 complex recognizes CDEIII and is required for assembly of a centromeric nucleosome, which in turn recruits other kinetochore components. To understand differences in centromeric nucleosome assembly between K. lactis and S. cerevisiae, we determined the structure of a K. lactis CBF3 complex by electron cryomicroscopy at ∼4 Å resolution and compared it with published structures of S. cerevisiae CBF3. We show differences in the pose of Ndc10 and discuss potential models of the K. lactis centromeric nucleosome that account for the extended CDEII length.

UtpA and UtpB chaperone nascent pre-ribosomal RNA and U3 snoRNA to initiate eukaryotic ribosome assembly.

Early eukaryotic ribosome biogenesis involves large multi-protein complexes, which co-transcriptionally associate with pre-ribosomal RNA to form the small subunit processome. The precise mechanisms by which two of the largest multi-protein complexes-UtpA and UtpB-interact with nascent pre-ribosomal RNA are poorly understood. Here, we combined biochemical and structural biology approaches with ensembles of RNA-protein cross-linking data to elucidate the essential functions of both complexes. We show that UtpA contains a large composite RNA-binding site and captures the 5' end of pre-ribosomal RNA. UtpB forms an extended structure that binds early pre-ribosomal intermediates in close proximity to architectural sites such as an RNA duplex formed by the 5' ETS and U3 snoRNA as well as the 3' boundary of the 18S rRNA. Both complexes therefore act as vital RNA chaperones to initiate eukaryotic ribosome assembly.

Structures of the double-ring AAA ATPase Pex1-Pex6 involved in peroxisome biogenesis.

The Pex1 and Pex6 proteins are members of the AAA family of ATPases and are involved in peroxisome biogenesis. Recently, cryo-electron microscopy structures of the Pex1-Pex6 complex in different nucleotide states have been determined. This Structural Snapshot describes the structural features of the complex and their implications for its function, as well as questions that still await answers.

Unique double-ring structure of the peroxisomal Pex1/Pex6 ATPase complex revealed by cryo-electron microscopy.

Members of the AAA family of ATPases assemble into hexameric double rings and perform vital functions, yet their molecular mechanisms remain poorly understood. Here, we report structures of the Pex1/Pex6 complex; mutations in these proteins frequently cause peroxisomal diseases. The structures were determined in the presence of different nucleotides by cryo-electron microscopy. Models were generated using a computational approach that combines Monte Carlo placement of structurally homologous domains into density maps with energy minimization and refinement protocols. Pex1 and Pex6 alternate in an unprecedented hexameric double ring. Each protein has two N-terminal domains, N1 and N2, structurally related to the single N domains in p97 and N-ethylmaleimide sensitive factor (NSF); N1 of Pex1 is mobile, but the others are packed against the double ring. The N-terminal ATPase domains are inactive, forming a symmetric D1 ring, whereas the C-terminal domains are active, likely in different nucleotide states, and form an asymmetric D2 ring. These results suggest how subunit activity is coordinated and indicate striking similarities between Pex1/Pex6 and p97, supporting the hypothesis that the Pex1/Pex6 complex has a role in peroxisomal protein import analogous to p97 in ER-associated protein degradation.

Structural analyses of the chromatin remodelling enzymes INO80-C and SWR-C.

INO80-C and SWR-C are conserved members of a subfamily of ATP-dependent chromatin remodelling enzymes that function in transcription and genome-maintenance pathways. A crucial role for these enzymes is to control chromosomal distribution of the H2A.Z histone variant. Here we use electron microscopy (EM) and two-dimensional class averaging to demonstrate that these remodelling enzymes have similar overall architectures. Each enzyme is characterized by a dynamic 'tail' domain and a compact 'head' that contains Rvb1/Rvb2 subunits organized as hexameric rings. EM class averages and mass spectrometry support the existence of single heterohexameric rings in both SWR-C and INO80-C. EM studies define the position of the Arp8/Arp4/Act1 module within INO80-C, and we find that this module enhances nucleosome-binding affinity but is largely dispensable for remodelling activities. In contrast, the Ies6/Arp5 module is essential for INO80-C remodelling, and furthermore this module controls conformational changes that may couple nucleosome binding to remodelling.

A requirement for ER-derived COPII vesicles in phagophore initiation.

A major unanswered question in the field of autophagy is how the double-membrane phagophore is formed. As this membrane expands, it engulfs proteins and organelles that are destined for degradation and then seals to form an autophagosome. A growing consensus in the field is that a subdomain of the ER initiates formation of the phagophore. We show that ER-derived COPII-coated vesicles, which bud from a specialized domain of the ER called the ER exit site (ERES), are a source of this membrane. This finding will now pave the way for a biochemical description of the early steps of phagophore initiation.

The EM structure of the TRAPPIII complex leads to the identification of a requirement for COPII vesicles on the macroautophagy pathway.

The transport protein particle (TRAPP) III complex, comprising the TRAPPI complex and additional subunit Trs85, is an autophagy-specific guanine nucleotide exchange factor for the Rab GTPase Ypt1 that is recruited to the phagophore assembly site when macroautophagy is induced. We present the single-particle electron microscopy structure of TRAPPIII, which reveals that the dome-shaped Trs85 subunit associates primarily with the Trs20 subunit of TRAPPI. We further demonstrate that TRAPPIII binds the coat protein complex (COP) II coat subunit Sec23. The COPII coat facilitates the budding and targeting of ER-derived vesicles with their acceptor compartment. We provide evidence that COPII-coated vesicles and the ER-Golgi fusion machinery are needed for macroautophagy. Our results imply that TRAPPIII binds to COPII vesicles at the phagophore assembly site and that COPII vesicles may provide one of the membrane sources used in autophagosome formation. These events are conserved in yeast to mammals.

Structural model for tubulin recognition and deformation by kinesin-13 microtubule depolymerases.

To elucidate the structural basis of the mechanism of microtubule depolymerization by kinesin-13s, we analyzed complexes of tubulin and the Drosophila melanogaster kinesin-13 KLP10A by electron microscopy (EM) and fluorescence polarization microscopy. We report a nanometer-resolution (1.1 nm) cryo-EM three-dimensional structure of the KLP10A head domain (KLP10AHD) bound to curved tubulin. We found that binding of KLP10AHD induces a distinct tubulin configuration with displacement (shear) between tubulin subunits in addition to curvature. In this configuration, the kinesin-binding site differs from that in straight tubulin, providing an explanation for the distinct interaction modes of kinesin-13s with the microtubule lattice or its ends. The KLP10AHD-tubulin interface comprises three areas of interaction, suggesting a crossbow-type tubulin-bending mechanism. These areas include the kinesin-13 family conserved KVD residues, and as predicted from the crossbow model, mutating these residues changes the orientation and mobility of KLP10AHDs interacting with the microtubule.

The Drosophila kinesin-13, KLP59D, impacts Pacman- and Flux-based chromosome movement.

Chromosome movements are linked to the active depolymerization of spindle microtubule (MT) ends. Here we identify the kinesin-13 family member, KLP59D, as a novel and uniquely important regulator of spindle MT dynamics and chromosome motility in Drosophila somatic cells. During prometaphase and metaphase, depletion of KLP59D, which targets to centrosomes and outer kinetochores, suppresses the depolymerization of spindle pole-associated MT minus ends, thereby inhibiting poleward tubulin Flux. Subsequently, during anaphase, loss of KLP59D strongly attenuates chromatid-to-pole motion by suppressing the depolymerization of both minus and plus ends of kinetochore-associated MTs. The mechanism of KLP59D's impact on spindle MT plus and minus ends appears to differ. Our data support a model in which KLP59D directly depolymerizes kinetochore-associated plus ends during anaphase, but influences minus ends indirectly by localizing the pole-associated MT depolymerase KLP10A. Finally, electron microscopy indicates that, unlike the other Drosophila kinesin-13s, KLP59D is largely incapable of oligomerizing into MT-associated rings in vitro, suggesting that such structures are not a requisite feature of kinetochore-based MT disassembly and chromosome movements.

Motor domain phosphorylation and regulation of the Drosophila kinesin 13, KLP10A.

Microtubule (MT)-destabilizing kinesin 13s perform fundamental roles throughout the cell cycle. In this study, we show that the Drosophila melanogaster kinesin 13, KLP10A, is phosphorylated in vivo at a conserved serine (S573) positioned within the alpha-helix 5 of the motor domain. In vitro, a phosphomimic KLP10A S573E mutant displays a reduced capacity to depolymerize MTs but normal affinity for the MT lattice. In cells, replacement of endogenous KLP10A with KLP10A S573E dampens MT plus end dynamics throughout the cell cycle, whereas a nonphosphorylatable S573A mutant apparently enhances activity during mitosis. Electron microscopy suggests that KLP10A S573 phosphorylation alters its association with the MT lattice, whereas molecular dynamics simulations reveal how KLP10A phosphorylation can alter the kinesin-MT interface without changing important structural features within the motor's core. Finally, we identify casein kinase 1alpha as a possible candidate for KLP10A phosphorylation. We propose a model in which phosphorylation of the KLP10A motor domain provides a regulatory switch controlling the time and place of MT depolymerization.

Structure of the kinesin13-microtubule ring complex.

To investigate the mechanism of kinesin13-induced microtubule depolymerization, we have calculated a three-dimensional (3D) map of the kinesin13-microtubule ring complex, using cryo-electron microscopy (cryo-EM) and image analysis. An atomic model of the complex was produced by docking the crystal structures of tubulin and a kinesin13 motor domain (MD) into the 3D map. The model reveals a snapshot of the depolymerization mechanism by providing a 3D view of the complex formed between the kinesin13 MD and a curved tubulin protofilament (pf). It suggests that contacts mediated by kinesin13 class-specific residues in the putative microtubule-binding site stabilize intra-dimer tubulin curvature. In addition, a tubulin-binding site on the kinesin13 MD was identified. Mutations at this class-conserved site selectively disrupt the formation of microtubule-associated ring complexes.

Kinesin-13s form rings around microtubules.

Kinesin is a superfamily of motor proteins that uses the energy of adenosine triphosphate hydrolysis to move and generate force along microtubules. A notable exception to this general description is found in the kinesin-13 family that actively depolymerizes microtubules rather than actively moving along them. This depolymerization activity is important in mitosis during chromosome segregation. It is still not fully clear by which mechanism kinesin-13s depolymerize microtubules. To address this issue, we used electron microscopy to investigate the interaction of kinesin-13s with microtubules. Surprisingly, we found that proteins of the kinesin-13 family form rings and spirals around microtubules. This is the first report of this type of oligomeric structure for any kinesin protein. These rings may allow kinesin-13s to stay at the ends of microtubules during depolymerization.

Morphology and morphogenesis of severe acute respiratory syndrome (SARS)-associated virus.

After infecting the Vero E6 cells by nasal/throat swabs collected from SARS patients, we studied the SARS-associated virus by electron microscopy and molecular biological technique. The results show that the diameter of newly isolated virus is about 50 nm without envelope or 100 nm with envelope. The virus was proved to be a new coronavirus by RT-PCR and it responded positively to convalescent-phase serum specimen from SARS patients, which is the evidence that this new virus is etiologically linked to the outbreak of SARS. The morphogenesis and distribution of the virus are also discussed in this article.