In higher eukaryotes, the centromere is epigenetically specified by the histone H3 variant Centromere Protein-A (CENP-A). Deposition of CENP-A to the centromere requires histone chaperone HJURP (Holliday junction recognition protein). The crystal structure of an HJURP-CENP-A-histone H4 complex shows that HJURP binds a CENP-A-H4 heterodimer. The C-terminal b-sheet domain of HJURP caps the DNA-binding region of the histone heterodimer, preventing it from spontaneous association with DNA. Our analysis also revealed a novel site in CENP-A that distinguishes it from histone H3 in its ability to bind HJURP. These findings provide key information for specific recognition of CENP-A and mechanistic insights into the process of centromeric chromatin assembly.

The H3 histone variant CENP-A is an epigenetic marker critical for the centromere identity and function. However, the precise regulation of the spatiotemporal deposition and propagation of CENP-A at centromeres during the cell cycle is still poorly understood. Here, we show that CENP-A is phosphorylated at Ser68 during early mitosis by Cdk1. Our results demonstrate that phosphorylation of Ser68 eliminates the binding of CENP-A to the assembly factor HJURP, thus preventing the premature loading of CENP-A to the centromere prior to mitotic exit. Because Cdk1 activity is at its minimum at the mitotic exit, the ratio of Cdk1/PP1α activity changes in favor of Ser68 dephosphorylation, thus making CENP-A available for centromeric deposition by HJURP. Thus, we reveal that dynamic phosphorylation of CENP-A Ser68 orchestrates the spatiotemporal assembly of newly synthesized CENP-A at active centromeres during the cell cycle.

Specific recognition of centromere-specific histone variant CENP-A-containing chromatin by CENP-N is an essential process in the assembly of the kinetochore complex at centromeres prior to mammalian cell division. However, the mechanisms of CENP-N recruitment to centromeres/kinetochores remain unknown. Here, we show that a CENP-A-specific RG loop (Arg80/Gly81) plays an essential and dual regulatory role in this process. The RG loop assists the formation of a compact "ladder-like" structure of CENP-A chromatin, concealing the loop and thus impairing its role in recruiting CENP-N. Upon G1/S-phase transition, however, centromeric chromatin switches from the compact to an open state, enabling the now exposed RG loop to recruit CENP-N prior to cell division. Our results provide the first insights into the mechanisms by which the recruitment of CENP-N is regulated by the structural transitions between compaction and relaxation of centromeric chromatin during the cell cycle.

Liquid crystalline polymers (LCPs) combine the attributes of liquid crystals and polymers, while discotic LCPs have been less developed in sharp contrast to their calamitic counterparts mainly due to lack of suitable discotic LCP materials. Here we successfully prepared a series of welldefined triphenylene (TP) based discotic LC polyacrylates via reversible addition−fragmentation chain-transfer (RAFT) polymerization for the first time, and through a combination of multiple analysis techniques and phase transition kinetics study, a remarkable molecular weight effect or polymer effect at a critical degree of polymerization (DP) around 20 has been disclosed. Moreover, the first proposed discrete columnar stacks (DCS) based hierarchical self-organization model accounts well for the formation and transformation of ordered hexagonal columnar lattice Col ho dominated by side-chain TP stacking and oblique columnar superlattice Col ob-s induced by compaction and ordering of polymer backbones. The in-depth understanding of their superstructures and readily achieved uniaxial alignment pave the way for the rational design and preparation of such kind of solution processable cutting-edge polymeric semiconducting materials and may boost various fascinating optoelectronic applications.

Large numbers of studies have focused on the posttranslational regulation of p53 activity. One of the best-known negative regulators for p53 is MDM2, an E3 ubiquitin ligase that promotes p53 degradation through proteasome degradation pathways. Additional E3 ligases have also been reported to negatively regulate p53. However, whether these E3 ligases have distinct/overlapping roles in the regulation of p53 is largely unknown. In this study, we identify RNF2 (ring finger protein 2) as an E3 ligase that targets p53 for degradation. The E3 ligase activity of RNF2 requires Bmi1 protein, a component of the polycomb group (PcG) complex. The up-regulation of p53 does not affect RNF2 expression. Unlike Mdm2, RNF2 only degrades p53 in selective cell lines, such as those from germ-cell tumors. The knockdown of RNF2 induces apoptosis, which can be rescued through the reduction of p53 expression. Moreover, the down-regulation of RNF2 expression in germ-cell tumors significantly reduces tumor cell growth, while the simultaneous down-regulation of both genes restores tumor cell growth in vitro and in tumor xenograft models. Furthermore, a reverse correlation between RNF2 and p53 expression was detected in human ovarian cancer tissues. Together, these results indicate that RNF2 is an E3 ligase for p53 degradation in selective cells, implicating RNF2 as a therapeutic target to restore tumor suppression through p53 in certain tumor cells.

Centrosomes are microtubule-organizing centers required for error-free mitosis and embryonic development. The microtubule-nucleating activity of centrosomes is conferred by the pericentriolar material (PCM), a composite of numerous proteins subject to cell cycle–dependent oscillations in levels and organization. In diverse cell types, mRNAs localize to centrosomes and may contribute to changes in PCM abundance. Here, we investigate the regulation of mRNA localization to centrosomes in the rapidly cycling Drosophila melanogaster embryo. We find that RNA localization to centrosomes is regulated during the cell cycle and developmentally. We identify a novel role for the fragile-X mental retardation protein in the posttranscriptional regulation of a model centrosomal mRNA, centrocortin (cen). Further, mistargeting cen mRNA is sufficient to alter cognate protein localization to centrosomes and impair spindle morphogenesis and genome stability.

A large amount of maternal RNA is deposited in oocytes and is reserved for later development. Control of maternal RNA translation during oocyte maturation has been extensively investigated and its regulatory mechanisms are well documented. However, translational regulation of maternal RNA in early oogenesis is largely unexplored. In this study, we generated zebrafish zar1 mutants that result in early oocyte apoptosis and fully penetrant male development. Loss of p53 suppresses the apoptosis in zar1 mutants and restores oocyte development. zar1 immature ovaries show upregulation of proteins implicated in endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). More importantly, loss of Zar1 causes marked upregulation of zona pellucida (ZP) family proteins, while overexpression of ZP proteins in oocytes causes upregulation of stress-related activating transcription factor 3 (atf3), arguing that tightly controlled translation of ZP proteins is essential for ER homeostasis during early oogenesis. Furthermore, Zar1 binds to ZP gene mRNAs and represses their translation. Together, our results indicate that regulation of translational repression and de-repression are essential for precisely controlling protein expression during early oogenesis.

Human cytomegalovirus (hCMV) immediate early 1 (IE1) protein associates with condensed chromatin of the host cell during mitosis. We have determined the structure of the chromatin-tethering domain (CTD) of IE1 bound to the nucleosome core particle, and discovered that the specific interaction between IE1-CTD and the H2A-H2B acidic patch impairs the compaction of higher-order chromatin structure. Our results suggest that IE1 loosens up the folding of host chromatin during hCMV infections.DOI: http://dx.doi.org/10.7554/eLife.11911.001