A backbone-centred energy function of neural networks for protein design.

A protein backbone structure is designable if a substantial number of amino acid sequences exist that autonomously fold into it1,2. It has been suggested that the designability of backbones is governed mainly by side chain-independent or side chain type-insensitive molecular interactions3-5, indicating an approach for designing new backbones (ready for amino acid selection) based on continuous sampling and optimization of the backbone-centred energy surface. However, a sufficiently comprehensive and precise energy function has yet to be established for this purpose. Here we show that this goal is met by a statistical model named SCUBA (for Side Chain-Unknown Backbone Arrangement) that uses neural network-form energy terms. These terms are learned with a two-step approach that comprises kernel density estimation followed by neural network training and can analytically represent multidimensional, high-order correlations in known protein structures. We report the crystal structures of nine de novo proteins whose backbones were designed to high precision using SCUBA, four of which have novel, non-natural overall architectures. By eschewing use of fragments from existing protein structures, SCUBA-driven structure design facilitates far-reaching exploration of the designable backbone space, thus extending the novelty and diversity of the proteins amenable to de novo design.

Insight into the interaction between the RNA helicase CGH-1 and EDC-3 and its implications.

Previous studies indicated that the P-body components, CGH-1 and EDC-3 may play a crucial role in the regulation of lifespan in Caenorhabditis elegans. Homo sapiens DDX6 or Saccharomyces cerevisiae Dhh1p (CGH-1 in C. elegans) could form complexes with EDC3 (Edc3p in yeast), respectively, which is significant for translation inhibition and mRNA decay. However, it is currently unclear how CGH-1 can be recognized by EDC-3 in C. elegans. Here, we provided structural and biochemical insights into the interaction between CGH-1 and EDC-3. Combined with homology modeling, mutation, and ITC assays, we uncovered an interface between CGH-1 RecA2 domain and EDC-3 FDF-FEK. Additionally, GST-pulldown and co-localization experiments confirmed the interaction between CGH-1 and EDC-3 in vitro and in vivo. We also analyzed PATR-1-binding interface on CGH-1 RecA2 by ITC assays. Moreover, we unveiled the similarity and differences of the binding mode between EDC-3 and CAR-1 or PATR-1. Taken together, these findings provide insights into the recognition of DEAD-box protein CGH-1 by EDC-3 FDF-FEK motif, suggesting important functional implications.

Structural and biochemical insights into the recognition of RNA helicase CGH-1 by CAR-1 in C. elegans.

A protein-RNA complex containing the RNA helicase CGH-1 and a germline specific RNA-binding protein CAR-1 is involved in various aspects of function in C. elegans. However, the structural basis for the assembly of this protein complex remains unclear. Here, we elucidate the molecular basis of the recognition of CGH-1 by CAR-1. Additionally, we found that the ATPase activity of CGH-1 is stimulated by NTL-1a MIF4G domain in vitro. Furthermore, we determined the structures of the two RecA-like domains of CGH-1 by X-ray crystallography at resolutions of 1.85 and 2.40 Å, respectively. Structural and biochemical approaches revealed a bipartite interface between CGH-1 RecA2 and the FDF-TFG motif of CAR-1. NMR and structure-based mutations in CGH-1 RecA2 or CAR-1 attenuated or disrupted CGH-1 binding to CAR-1, assessed by ITC and GST-pulldown in vitro. These findings provide insights into a conserved mechanism in the recognition of CGH-1 by CAR-1. Together, our data provide the missing physical links in understanding the assembly and function of CGH-1 and CAR-1 in C. elegans.

Structural Insights into ceNAP1 Chaperoning Activity toward ceH2A-H2B.

In eukaryotes, nucleosome assembly is crucial for genome integrity. The histone chaperone NAP1 plays an important role in histone folding, storage, and transport, as well as histone exchange and nucleosome assembly. At present, the molecular basis of these activities is not fully understood. We have solved high-resolution crystal structures of Caenorhabditis elegans NAP1 (ceNAP1) in complex with its cognate substrates: the C. elegans H2A-H2B dimer (ceH2A-H2B) and the H2A.Z-H2B dimer (ceH2A.Z-H2B). Our structural and biochemical data reveals the acidic concave surface is relevant to tetramerization, and uncovers how a ceNAP1 homodimer uses its concave surface to asymmetrically recognize a ceH2A-H2B or ceH2A.Z-H2B heterodimer. Intriguingly, an "acidic strip" within the concave surface of ceNAP1 is crucial for binding histones, including H2A-H2B, H3-H4, and histone variants. Thus, our results provide insight into the molecular mechanisms of NAP1 histone chaperone activity.

Structure of the primed state of the ATPase domain of chromatin remodeling factor ISWI bound to the nucleosome.

ATP-dependent chromatin remodeling factors of SWI/SNF2 family including ISWI, SNF2, CHD1 and INO80 subfamilies share a conserved but functionally non-interchangeable ATPase domain. Here we report cryo-electron microscopy (cryo-EM) structures of the nucleosome bound to an ISWI fragment with deletion of the AutoN and HSS regions in nucleotide-free conditions and the free nucleosome at ∼ 4 Å resolution. In the bound conformation, the ATPase domain interacts with the super helical location 2 (SHL 2) of the nucleosomal DNA, with the N-terminal tail of H4 and with the α1 helix of H3. Density for other regions of ISWI is not observed, presumably due to disorder. Comparison with the structure of the free nucleosome reveals that although the histone core remains largely unchanged, remodeler binding causes perturbations in the nucleosomal DNA resulting in a bulge near the SHL2 site. Overall, the structure of the nucleotide-free ISWI-nucleosome complex is similar to the corresponding regions of the recently reported ADP bound ISWI-nucleosome structures, which are significantly different from that observed for the ADP-BeFx bound structure. Our findings are relevant to the initial step of ISWI binding to the nucleosome and provide additional insights into the nucleosome remodeling process driven by ISWI.

Atomic resolution cryo-EM structure of a native-like CENP-A nucleosome aided by an antibody fragment.

Genomic DNA in eukaryotes is organized into chromatin through association with core histones to form nucleosomes, each distinguished by their DNA sequences and histone variants. Here, we used a single-chain antibody fragment (scFv) derived from the anti-nucleosome antibody mAb PL2-6 to stabilize human CENP-A nucleosome containing a native α-satellite DNA and solved its structure by the cryo-electron microscopy (cryo-EM) to 2.6 Å resolution. In comparison, the corresponding cryo-EM structure of the free CENP-A nucleosome could only reach 3.4 Å resolution. We find that scFv binds to a conserved acidic patch on the histone H2A-H2B dimer without perturbing the nucleosome structure. Our results provide an atomic resolution cryo-EM structure of a nucleosome and insight into the structure and function of the CENP-A nucleosome. The scFv approach is applicable to the structural determination of other native-like nucleosomes with distinct DNA sequences.

Dynamic acetylation of the kinetochore-associated protein HEC1 ensures accurate microtubule-kinetochore attachment.

Faithful chromosome segregation during mitosis is critical for maintaining genome integrity in cell progeny and relies on accurate and robust kinetochore-microtubule attachments. The NDC80 complex, a tetramer comprising kinetochore protein HEC1 (HEC1), NDC80 kinetochore complex component NUF2 (NUF2), NDC80 kinetochore complex component SPC24 (SPC24), and SPC25, plays a critical role in kinetochore-microtubule attachment. Mounting evidence indicates that phosphorylation of HEC1 is important for regulating the binding of the NDC80 complex to microtubules. However, it remains unclear whether other post-translational modifications, such as acetylation, regulate NDC80-microtubule attachment during mitosis. Here, using pulldown assays with HeLa cell lysates and site-directed mutagenesis, we show that HEC1 is a bona fide substrate of the lysine acetyltransferase Tat-interacting protein, 60 kDa (TIP60) and that TIP60-mediated acetylation of HEC1 is essential for accurate chromosome segregation in mitosis. We demonstrate that TIP60 regulates the dynamic interactions between NDC80 and spindle microtubules during mitosis and observed that TIP60 acetylates HEC1 at two evolutionarily conserved residues, Lys-53 and Lys-59. Importantly, this acetylation weakened the phosphorylation of the N-terminal HEC1(1-80) region at Ser-55 and Ser-62, which is governed by Aurora B and regulates NDC80-microtubule dynamics, indicating functional cross-talk between these two post-translation modifications of HEC1. Moreover, the TIP60-mediated acetylation was specifically reversed by sirtuin 1 (SIRT1). Taken together, our results define a conserved signaling hierarchy, involving HEC1, TIP60, Aurora B, and SIRT1, that integrates dynamic HEC1 acetylation and phosphorylation for accurate kinetochore-microtubule attachment in the maintenance of genomic stability during mitosis.

Holliday junction recognition protein interacts with and specifies the centromeric assembly of CENP-T.

The centromere is an evolutionarily conserved eukaryotic protein machinery essential for precision segregation of the parental genome into two daughter cells during mitosis. Centromere protein A (CENP-A) organizes the functional centromere via a constitutive centromere-associated network composing the CENP-T complex. However, how CENP-T assembles onto the centromere remains elusive. Here we show that CENP-T binds directly to Holliday junction recognition protein (HJURP), an evolutionarily conserved chaperone involved in loading CENP-A. The binding interface of HJURP was mapped to the C terminus of CENP-T. Depletion of HJURP by CRISPR-elicited knockout minimized recruitment of CENP-T to the centromere, indicating the importance of HJURP in CEPN-T loading. Our immunofluorescence analyses indicate that HJURP recruits CENP-T to the centromere in S/G2 phase during the cell division cycle. Significantly, the HJURP binding-deficient mutant CENP-T6L failed to locate to the centromere. Importantly, CENP-T insufficiency resulted in chromosome misalignment, in particular chromosomes 15 and 18. Taken together, these data define a novel molecular mechanism underlying the assembly of CENP-T onto the centromere by a temporally regulated HJURP-CENP-T interaction.

Structural mechanisms of centromeric nucleosome recognition by the kinetochore protein CENP-N.

Accurate chromosome segregation requires the proper assembly of kinetochore proteins. A key step in this process is the recognition of the histone H3 variant CENP-A in the centromeric nucleosome by the kinetochore protein CENP-N. We report cryo-electron microscopy (cryo-EM), biophysical, biochemical, and cell biological studies of the interaction between the CENP-A nucleosome and CENP-N. We show that human CENP-N confers binding specificity through interactions with the L1 loop of CENP-A, stabilized by electrostatic interactions with the nucleosomal DNA. Mutational analyses demonstrate analogous interactions in Xenopus, which are further supported by residue-swapping experiments involving the L1 loop of CENP-A. Our results are consistent with the coevolution of CENP-N and CENP-A and establish the structural basis for recognition of the CENP-A nucleosome to enable kinetochore assembly and centromeric chromatin organization.

The catalytic subunit of the SWR1 remodeler is a histone chaperone for the H2A.Z-H2B dimer.

Histone variant H2A.Z-containing nucleosomes exist at most eukaryotic promoters and play important roles in gene transcription and genome stability. The multisubunit nucleosome-remodeling enzyme complex SWR1, conserved from yeast to mammals, catalyzes the ATP-dependent replacement of histone H2A in canonical nucleosomes with H2A.Z. How SWR1 catalyzes the replacement reaction is largely unknown. Here, we determined the crystal structure of the N-terminal region (599-627) of the catalytic subunit Swr1, termed Swr1-Z domain, in complex with the H2A.Z-H2B dimer at 1.78 Å resolution. The Swr1-Z domain forms a 310 helix and an irregular chain. A conserved LxxLF motif in the Swr1-Z 310 helix specifically recognizes the αC helix of H2A.Z. Our results show that the Swr1-Z domain can deliver the H2A.Z-H2B dimer to the DNA-(H3-H4)2 tetrasome to form the nucleosome by a histone chaperone mechanism.

Identification of functionally conserved regions in the structure of the chaperone/CenH3/H4 complex.

In eukaryotes, a variant of conventional histone H3 termed CenH3 epigenetically marks the centromere. The conserved CenH3 chaperone specifically recognizes CenH3 and is required for CenH3 deposition at the centromere. Recently, the structures of the chaperone/CenH3/H4 complexes have been determined for Homo sapiens (Hs) and the budding yeasts Saccharomyces cerevisiae (Sc) and Kluyveromyces lactis (Kl). Surprisingly, the three structures are very different, leading to different proposed structural bases for chaperone function. The question of which structural region of CenH3 provides the specificity determinant for the chaperone recognition is not fully answered. Here, we investigated these issues using solution NMR and site-directed mutagenesis. We discovered that, in contrast to previous findings, the structures of the Kl and Sc chaperone/CenH3/H4 complexes are actually very similar. This new finding reveals that both budding yeast and human chaperones use a similar structural region to block DNA from binding to the histones. Our mutational analyses further indicate that the N-terminal region of the CenH3 α2 helix is sufficient for specific recognition by the chaperone for both budding yeast and human. Thus, our studies have identified conserved structural bases of how the chaperones recognize CenH3 and perform the chaperone function.

Solution structure of S. cerevisiae PDCD5-like protein and its promoting role in H(2)O(2)-induced apoptosis in yeast.

Human PDCD5 protein is a novel programmed cell death-promoting molecule. However, the function of Ymr074cP, a S. cerevisiae homologue of hPDCD5, is still unknown. Heteronuclear NMR methods were used to determine the solution structure of the N-terminal 116-residue fragment (N116) of Ymr074cP protein. N116 is shown to be a heterogeneous ensemble of flexibly folded conformations, adopting an extended triple-helix bundle fold that is connected to a mobile but structured alpha-helix in the N-terminus by means of a lengthy highly flexible linker. By the nitroxide spin label, attached to the mutant cysteine residue at position 7 or 11, significant transient interactions were probed between the N-terminal helical portion and the core moiety plus several residues in the C-terminal tail. The topology of the triple-helix bundle is encoded mainly by hydrophobic interactions, and the N-terminal helical structure has a unique electrostatic potential character. A comparison of the solution structures of PDCD5-related proteins indicates that the structure of the triple-helix bundle is significantly conserved during evolution. We are the first to demonstrate that YMR074c overexpression promotes H(2)O(2)-induced apoptosis in yeast, not only in a metacaspase Yca1-dependent manner but also in a Yca1-independent manner and that deletion of the N-terminal helical portion greatly attenuates the apoptosis-promoting activity of this protein.