Biology subjects

Dalal, Y.

Publications and source records attributed to Dalal, Y..

4 recordsLinked to original sources

bioRxiv2018

The Oligomerization Landscape of Histones

In eukaryotes, DNA is packaged through nucleosomes. Each nucleosome is typically centered around an octameric histone protein core: one central tetramer plus two separate dimers. Studying the assembly mechanisms of histones is essential for understanding the dynamics of entire nucleosomes and higher-order DNA packaging. Here we investigate the canonical histone assembly and that of the centromere-specific histone variant CENP-A using molecular dynamics simulations. We quantitatively characterize their thermodynamical and dynamical features, showing that the canonical H3 tetramer exhibits large instability around the central interface manifested via a swiveling motion of two halves, supporting the recently observed DNA handedness flipping of the tetrasome. In contrast, the variant CENP-A encodes a distinctive stability to its tetramer with a rigid but twisted interface compared to the crystal structure, implying the diverse structural possibilities of the histone variant. Interestingly, the observed tetramer dynamics alter significantly and appear to reach a new dynamics balance when H2A/H2B dimers are present. In all, these data reveal key mechanistic insights and structural details for the assembly of canonical and variant CENP-A histone tetramers and octamers, providing theoretical quantifications and physical interpretations for longstanding and recent experimental observations.

biophysics↗

bioRxiv2018

Elastic and Rigidified CENP-A Nucleosomes govern Centromeric Chromatin Plasticity

Histone variants fine-tune transcription, replication, DNA damage repair, and faithful chromosome segregation. Whether and how nucleosome variants encode unique mechanical properties to their cognate chromatin structures remains elusive. Here, using novel in silico and in vitro nanoindentation methods, extending to in vivo dissections, we report that histone variant nucleosomes are intrinsically more elastic than their canonical counterparts. Furthermore, binding proteins which discriminate between histone variant nucleosomes suppress this innate elasticity and also compact chromatin. Interestingly, when we overexpress the binding proteins in vivo, we also observe increased compaction of chromatin enriched for histone variant nucleosomes, correlating with diminished access. Together, these data suggest a plausible link between innate mechanical properties possessed by histone variant nucleosomes, the adaptability of chromatin states in vivo, and the epigenetic plasticity of the underlying locus.\n\nSignificanceNucleosomes are the base unit which organize eukaryotic genomes. Besides the canonical histone, histone variants create unique local chromatin domains that fine-tune transcription, replication, DNA damage repair, and faithful chromosome segregation. We developed computational and single-molecule nanoindentation tools to determine mechanical properties of histone variant nucleosomes. We found that the CENP-A nucleosome variant is more elastic than the canonical H3 nucleosome but becomes stiffer when bound to its partner CENP-C. In addition, CENP-C induces cross-array clustering, creating a chromatin state that less accessible. These data suggest that innate material properties of nucleosomes can influence the ultimate chromatin state, thereby influence biological outcomes.

biochemistry↗

bioRxiv2017

Internal modifications in the CENP-A nucleosome modulate centromeric dynamics

Post-translational modifications (PTMs) of core histones have studied for over 2 decades, and are correlated with changes in transcriptional status, chromatin fiber folding, and nucleosome dynamics. However, within the centromere-specific histone H3 variant CENP-A, few modifications have been reported, and their functions remain largely unexplored. In this multidisciplinary report, we utilize in silico computational and in vivo approaches to dissect lysine 124 of human CENP-A, which was previously reported to be acetylated in advance of replication. Computational modeling demonstrates that acetylation of K124 causes tightening of the histone core, and hinders accessibility to its C-terminus, which in turn diminishes CENP-C binding. Additionally, CENP-A K124ac/H4 K79ac containing nucleosomes are prone to DNA sliding. In vivo experiments using an acetyl or unacetylatable mimic (CENP-A K124Q and K124A respectively) reveal alterations in CENP-C levels, and a modest increase in mitotic errors. Furthermore, mutation of K124 results in alterations in centromeric replication timing, with the permanently acetylated form replicating centromeres early, and the unacetylable form replicating centromeres late. Purification of native CENP-A proteins followed by mass spectrometry analysis reveal that while CENP-A K124 is acetylated at G1/S, it switches to monomethylation during early and mid-S phase. Finally, we provide evidence that the HAT p300 is involved in this cycle. Taken together, our data suggest that cyclical modifications within the CENP-A nucleosome can influence the binding of key kinetochore proteins, the integrity of mitosis and centromeric replication. These data support the emerging paradigm that core modifications in histone variant nucleosomes transduce defined changes to key biological processes.

biochemistry↗

bioRxiv2017

CENP-A associated lncRNAs influence chromosome segregation in human cells

Transcription occurs ubiquitously throughout non-coding parts of the genome, including at repetitive -satellite DNA elements which comprise the majority of human centromeres. The function of temporally regulated centromeric transcription, and transcripts, is consequently a topic of intense investigation. In this study, we use high throughput approaches to identify and describe lncRNAs associated with the centromere specific histone variant CENP-A that arise from the transcription of specific centromeres at early G1, which we then show are physically associated with centromeres, and which are functionally necessary for accurate chromosome segregation. Targeted depletion of one such centromeric RNA, which originates from a single centromere, is sufficient to increase the frequency of chromosome segregation defects. These data support the emerging paradigm of the necessity of centromere-specific lncRNAs in the integrity of faithful chromosome segregation.

cell biology↗