Biology subjects

Heun, P.

Publications and source records attributed to Heun, P..

7 recordsLinked to original sources

bioRxiv2026

Dosage compensation defects due to roX RNA deletion are rescued by recalibration of X/autosome stoichiometry

Metazoa evolved regulatory networks to balance the expression of their sex chromosomes. In Drosophila, males have a single gene-rich X chromosome, whereas females have two. Balanced X/autosome expression is essential for viability, and in male flies is achieved by activation of genes on the X through the male-specific-lethal (MSL) dosage compensation complex (DCC). This ribonucleoprotein assembly contains long, non-coding roX RNAs. To dissect the functional requirements of roX in a cell-based system, we deleted the roX2 gene in male S2 cells and selected two independent lines lacking detectable roX RNA. In the absence of roX, the remaining MSL protein complex was unable to associate with known or newly identified binding sites and thus failed to activate transcription. Surprisingly, the X/autosome expression ratio appeared nevertheless compensated. Cytogenetic and genomic analyses revealed that both roX-deficient cell populations had acquired additional X chromosomes. Apparently, chromosome gains due to mis-segregation made up for the loss of DCC-mediated dosage compensation. Interestingly, ectopic expression of full-length roX2, but not of shortened derivatives, fully restored DCC binding and normalized the karyotype. These findings illustrate that X chromosome dosage compensation is critical for viability even in cultured cells and provide a striking example of rapid evolution under stringent selection. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=83 SRC="FIGDIR/small/707606v1_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@1bbf907org.highwire.dtl.DTLVardef@1da43a9org.highwire.dtl.DTLVardef@903326org.highwire.dtl.DTLVardef@10e3865_HPS_FORMAT_FIGEXP M_FIG C_FIG

molecular biology↗

bioRxiv2023

Efficient Formation of Single-copy Human Artificial Chromosomes

Large DNA assembly methodologies underlie milestone achievements in synthetic prokaryotic and budding yeast chromosomes. While budding yeast control chromosome inheritance through [~]125 bp DNA sequence-defined centromeres, mammals and many other eukaryotes use large, epigenetic centromeres. Harnessing centromere epigenetics permits human artificial chromosome (HAC) formation but is not sufficient to avoid rampant multimerization of the initial DNA molecule upon introduction to cells. Here, we describe an approach that efficiently forms single-copy HACs. It employs a [~]750 kb construct that is sufficiently large to house the distinct chromatin types present at the inner and outer centromere, obviating the need to multimerize. Delivery to mammalian cells is streamlined by employing yeast spheroplast fusion. These developments permit faithful chromosome engineering in the context of metazoan cells. One-Sentence SummaryA quarter century after the first human artificial chromosomes, a solution to their uncontrolled multimerization is achieved.

molecular biology↗

bioRxiv2021

Structural Basis for Mis18 Complex Assembly: Implications for Centromere Maintenance

The centromere, defined by the enrichment of CENP-A (a Histone H3 variant) containing nucleosomes, is a specialised chromosomal locus that acts as a microtubule attachment site. To preserve centromere identity, CENP-A levels must be maintained through active CENP-A loading during the cell cycle. A central player mediating this process is the Mis18 complex (Mis18, Mis18y and Mis18BP1), which recruits the CENP-A specific chaperone HJURP to centromeres for CENP-A deposition. Here, using a multi-pronged approach, we characterise the structure of the Mis18 complex and show that multiple hetero- and homo-oligomeric interfaces facilitate the hetero-octameric Mis18 complex assembly composed of 4 Mis18, 2 Mis18y and 2 Mis18BP1. Evaluation of structure-guided/separation-of-function mutants reveals structural determinants essential for Mis18 complex assembly and centromere maintenance. Our results provide new mechanistic insights on centromere maintenance, highlighting that while Mis18 can associate with centromeres and deposit CENP-A independently of Mis18y, the latter is indispensable for the optimal level of CENP-A loading required for preserving the centromere identity.

cell biology↗

bioRxiv2020

Drosophila SWR1 and NuA4 complexes are defined by DOMINO isoforms

Histone acetylation and deposition of H2A.Z variant are integral aspects of active transcription. In Drosophila, the single DOMINO chromatin regulator complex is thought to combine both activities via an unknown mechanism. Here we show that alternative isoforms of the DOMINO nucleosome remodeling ATPase, DOM-A and DOM-B, directly specify two distinct multi-subunit complexes. Both complexes are necessary for transcriptional regulation but through different mechanisms. The DOM-B complex incorporates H2A.V (the fly ortholog of H2A.Z) genome-wide in an ATP-dependent manner, like the yeast SWR1 complex. The DOM-A complex, instead, functions as an ATP-independent histone acetyltransferase complex similar to the yeast NuA4, targeting lysine 12 of histone H4. Our work provides an instructive example of how different evolutionary strategies lead to similar functional separation. In yeast and humans, nucleosome remodeling and histone acetyltransferase complexes originate from gene duplication and paralog specification. Drosophila generates the same diversity by alternative splicing of a single gene. HighlightsO_LIIsoforms of DOMINO dictate the formation of distinct complexes C_LIO_LIDOM-B complex is the Drosophila SWR1 and incorporates H2A.V genome-wide C_LIO_LIDOM-A complex acetylates H4K12 and is the Drosophila NuA4 C_LI

molecular biology↗

bioRxiv2019

Structural Basis for CAL1-Mediated Centromere Maintenance

Centromeres are microtubule attachment sites on chromosomes defined by the enrichment of CENP-A-containing nucleosomes. To preserve centromere identity, CENP-A must be escorted to centromeres by a CENP-A-specific chaperone for deposition. Despite this essential requirement, many eukaryotes differ in the composition of players involved in centromere maintenance highlighting the plasticity of this process. In humans, CENP-A recognition and centromere targeting is achieved by HJURP and the Mis18 complex, respectively. Here, using crystal structures, we show how Drosophila CAL1, an evolutionarily distinct CENP-A chaperone, targets CENP-A to the centromere receptor CENP-C without the requirement of the Mis18 complex: while the N-terminal CAL1 fragment (CAL11-160) wraps around CENP-A/H4 through multiple physical contacts, the C-terminal CAL1 fragment (CAL1893-914) directly binds CENP-C cupin dimer. Our work shows CAL1, though divergent at the primary structure, employs evolutionarily conserved and adaptive structural principles to recognise CENP-A/H4 and CENP-C providing insights into the minimalistic principles underlying centromere maintenance.

biochemistry↗

bioRxiv2019

Spt6 is a maintenance factor for centromeric CENP-A

Replication and transcription of genomic DNA requires partial disassembly of nucleosomes to allow progression of polymerases. This constitutes both an opportunity to remodel the underlying chromatin as well as the potential danger of losing epigenetic information. Centromeric transcription has been shown to be required for stable incorporation of the centromere-specific histone dCENP-A in M/G1-phase, which depends on the eviction of previously deposited H3/H3.3-placeholder nucleosomes. Here we demonstrate that the histone chaperone and transcription elongation factor Spt6 spatially and temporarily coincides with centromeric transcription and prevents the loss of old CENP-A nucleosomes in both Drosophila and human cells. Spt6 binds directly to dCENP-A and shows enhanced association with non-phosphorylatable dCENP-A mutants compared to histone H3, while phosphomimetic residues alleviate association with Spt6. We conclude that Spt6 acts as a conserved CENP-A maintenance factor, which is required during transcription-mediated chromatin remodelling at the centromere to ensure long-term stability of epigenetic centromere identity.

cell biology↗

bioRxiv2019

Epigenetic inheritance of centromere identity in a heterologous system

The centromere is an essential chromosomal region required for accurate chromosome segregation. Most eukaryotic centromeres are defined epigenetically by the histone H3 variant, CENP-A, yet how its self-propagation is achieved remains poorly understood. Here we developed a heterologous system to reconstitute epigenetic inheritance of centromeric chromatin by ectopically targeting the Drosophila centromere proteins dCENP-A, dCENP-C and CAL1 to LacO arrays in human cells. Dissecting the function of these three components uncovers the key role of self-association of dCENP-C and CAL1 for their mutual interaction and dCENP-A deposition. Importantly, we identify the components required for dCENP-C loading onto chromatin, involving a cooperation between CAL1 and dCENP-A nucleosomes, thus closing the epigenetic loop to ensure dCENP-C and dCENP-A replenishment during the cell division cycle. Finally, we show that all three Drosophila factors are sufficient for dCENP-A propagation and propose a model for the epigenetic inheritance of centromere identity.

cell biology↗