Transcription-facilitating histone chaperons interact with genomic and synthetic G4 structures.

Affinity for G-quadruplex (G4) structures may be a common feature of transcription-facilitating histone chaperons (HCs). This assumption is based on previous unmatched studies of HCs FACT, nucleolin (NCL), BRD3, and ATRX. We verified this assumption and considered its implications for the therapeutic applications of synthetic (exogenous) G4s and the biological significance of genomic G4s. First, we questioned whether exogenous G4s that recognize cell-surface NCL and could trap other HCs in the nucleus are usable as anticancer agents. We performed in vitro binding assays and selected leading multi-targeted G4s. They exhibited minor effects on cell viability. The presumed NCL-regulated intracellular transport of G4s was inefficient or insufficient for tumor-specific G4 delivery. Next, to clarify whether G4s in the human genome could recruit HCs, we compared available HC ChIP-seq data with G4-seq/G4-ChIP-seq data. Several G4s, including the well-known c-Myc quadruplex structure, were found to be colocalized with HC occupancy sites in cancer cell lines. As evidenced by our molecular modeling data, c-Myc G4 might interfere with the HC function of BRD3 but is unlikely to prevent the BRD3-driven assembly of the chromatin remodeling complex. The c-Myc case illustrates the intricate role of genomic G4s in chromatin remodeling, nucleosome remodeling, and transcription.

ATRX Contributes to MeCP2-Mediated Pericentric Heterochromatin Organization during Neural Differentiation.

Methyl-CpG binding protein 2 (MeCP2) is a multi-function factor involved in locus-specific transcriptional modulation and the regulation of genome architecture, e.g., pericentric heterochromatin (PCH) organization. MECP2 mutations are responsible for Rett syndrome (RTT), a devastating postnatal neurodevelopmental disorder, the pathogenetic mechanisms of which are still unknown. MeCP2, together with Alpha-thalassemia/mental retardation syndrome X-linked protein (ATRX), accumulates at chromocenters, which are repressive PCH domains. As with MECP2, mutations in ATRX cause ATR-X syndrome which is associated with severe intellectual disability. We exploited two murine embryonic stem cell lines, in which the expression of MeCP2 or ATRX is abolished. Through immunostaining, chromatin immunoprecipitation and western blot, we show that MeCP2 and ATRX are reciprocally dependent both for their expression and targeting to chromocenters. Moreover, ATRX plays a role in the accumulation of members of the heterochromatin protein 1 (HP1) family at PCH and, as MeCP2, modulates their expression. Furthermore, ATRX and HP1 targeting to chromocenters depends on an RNA component. 3D-DNA fluorescence in situ hybridization (FISH) highlighted, for the first time, a contribution of ATRX in MeCP2-mediated chromocenter clustering during neural differentiation. Overall, we provide a detailed dissection of the functional interplay between MeCP2 and ATRX in higher-order PCH organization in neurons. Our findings suggest molecular defects common to RTT and ATR-X syndrome, including an alteration in PCH.

A metadynamic approach to understand the recognition mechanism of the histone H3 tail with the ATRXADD domain.

The binding affinity between the histone 3 (H3) tail and the ADD domain of ATRX (ATRXADD) increases with the subsequent addition of methyl groups on lysine 9 on H3. To improve our understanding of how the difference in methylation state affects binding between H3 and the ATRXADD, we adopted a metadynamic approach to explore the recognition mechanism between the two proteins and identify the key intermolecular interactions that mediate this protein-peptide interaction (PPI). The non-methylated H3 peptide is recognized only by the PHD finger of ATRXADD while mono-, di-, and trimethylated H3 is recognized by both the PHD and GATA-like zinc finger of the domain. Furthermore, water molecules play an important role in orienting the lysine 9 anchor towards the GATA-like zinc finger, which results in stabilizing the lysine 9 binding pocket on ATRXADD. We compared our computational results against experimentally determined NMR and X-ray structures by demonstrating the RMSD, order parameter S2 and hydration number of the complex. The metadynamics data provide new insight into roles of water-bridges and the mechanisms through which K9 hydration stabilizes the H3K9me3:ATRXADD PPI, providing context for the high affinity demonstrated between this protein and peptide.

Structural and mechanistic insights into ATRX-dependent and -independent functions of the histone chaperone DAXX.

The ATRX-DAXX histone chaperone complex incorporates the histone variant H3.3 at heterochromatic regions in a replication-independent manner. Here, we present a high-resolution x-ray crystal structure of an interaction surface between ATRX and DAXX. We use single amino acid substitutions in DAXX that abrogate formation of the complex to explore ATRX-dependent and ATRX-independent functions of DAXX. We find that the repression of specific murine endogenous retroviruses is dependent on DAXX, but not on ATRX. In support, we reveal the existence of two biochemically distinct DAXX-containing complexes: the ATRX-DAXX complex involved in gene repression and telomere chromatin structure, and a DAXX-SETDB1-KAP1-HDAC1 complex that represses endogenous retroviruses independently of ATRX and H3.3 incorporation into chromatin. We find that histone H3.3 stabilizes DAXX protein levels and can affect DAXX-regulated gene expression without incorporation into nucleosomes. Our study demonstrates a nucleosome-independent function for the H3.3 histone variant.

Deciphering the Molecular Effects of Mutations on ATRX Cause ATRX Syndrome: A Molecular Dynamics Study.

α-thalassemia mental retardation X-linked (ATRX) syndrome is caused by the dysfunction of ATRFfigX protein. The present study explored the structural consequences influenced by two observed mutations V194I and C220R on ADD domain of ATRX protein by applying all atom molecular dynamics (MD) simulation. MD result showed that both the mutants exhibited wide variations in their backbone dynamics, as a result, mutant V210I showed complete distortion on α3 and the mutant C220R displayed a biased disruption on α2-3. The interference in the local folding of α-helices in both the mutants resulted by the loss of hydrogen bonds mediated by the backbone atoms. Principle component analysis (PCA) elucidated that both the mutants endured a diverse conformational dynamics, consequently adopted thermodynamically different conformational state. Besides, binding residues in both the mutants showed more structural disorder, thereby unable to recognize the hallmark modification, K9me3 (tri-methylated lysine at position 9) of histone H3 peptide and it was not conducive for the wild type ADD domain like functionality. Altogether, our findings provide knowledge to understand the structural and functional relationship of disease-associated mutations, V194I and C220R on ADD domain as well as gain further insights into the molecular pathogenesis of ATRX syndrome. J. Cell. Biochem. 118: 3318-3327, 2017. © 2017 Wiley Periodicals, Inc.