The Drosophila DAXX-Like Protein (DLP) Cooperates with ASF1 for H3.3 Deposition and Heterochromatin Formation

Abstract

Histone variants are nonallelic isoforms of canonical histones, and they are deposited, in contrast to canonical histones, in a replication-independent (RI) manner. RI deposition of H3.3, a histone variant from the H3.3 family, is mediated in mammals by distinct pathways involving either the histone regulator A (HIRA) complex or the death-associated protein (DAXX)/␣-thalassemia X-linked mental retardation protein (ATRX) complex. Here, we investigated the function of the Drosophila DAXX-like protein (DLP) by using both fly genetic approaches and protein biochemistry. DLP specifically interacts with H3.3 and shows a prominent localization on the base of the X chromosome, where it appears to act in concert with XNP, the Drosophila homolog of ATRX, in heterochromatin assembly and maintenance. The functional association between DLP and XNP is further supported by a series of experiments that illustrate genetic interactions and the DLP-XNP-dependent localization of specific chromosomal proteins. In addition, DLP both participates in the RI deposition of H3.3 and associates with anti-silencing factor 1 (ASF1). We suggest, in agreement with a recently proposed model, that DLP and ASF1 are part of a predeposition complex, which is recruited by XNP and is necessary to prevent DNA exposure in the nucleus.KEYWORDS DLP, heterochromatin, histone variant, H3.3, XNP, ASF1 I n the nucleus, DNA is packaged under the form of chromatin. The basic repeating unit of chromatin is the nucleosome, and the nucleosome core particle consists of approximately 146 bp of DNA wrapped around a histone octamer composed of two copies of each of the histones H2A, H2B, H3, and H4. (1, 2). Gene-rich domains are packaged into euchromatin, which is decondensed in interphase nuclei and is enriched for histone modifications characteristic of transcriptionally active regions. Moreover, euchromatin often exhibits high DNA accessibility. In contrast, gene-poor domains and repetitive sequences are packaged into condensed heterochromatin that carries histone modifications associated with transcriptional repression.In addition to the canonical core histones, cells express nonallelic-isoform histone variants (3). Histone variants have emerged as essential contributors to the regulation of chromatin structure, and they are involved in multiple processes, including chromatin stability, DNA repair, and transcriptional regulation. Canonical histones are almost exclusively expressed during the S phase of the cell cycle and are incorporated into chromatin in a DNA replication-dependent fashion, whereas replication-independent (RI) histone variants are expressed throughout the cell cycle.One of the best-studied histone variants is H3.3, which can replace the major species H3 (4). H3.3 is expressed and incorporated at all phases of the cell cycle (5). The available data suggest that H3.3 is a marker of active chromatin and associated with the

The histone replacement gene His4r is involved in heat stress induced chromatin rearrangement

“…However, histones are also deposited to chromatin during the interphase. The histone H3.3 variant is known to be incorporated into chromatin in a replication independent manner and to replace H3 on transcriptionally active genes 33 , 34 and also at silenced heterochromatic regions 35 by the histone regulator A (HIRA) 36 and death-associated protein (DAXX)/α-thalassemia X-linked mental retardation protein (ATRX) 35 histone chaperon complexes, respectively. In mice, this process leads to the accumulation of H3.3 at near saturation levels (~ 99% of total H3) in liver, kidney and brain cells by late adulthood 37 .…”

ASF1 is required to load histones on the HIRA complex in preparation of paternal chromatin assembly at fertilization

“…How the partitioning of H3.2–H4 and H3.3–H4 histone dimers on ASF1 is established before zygote formation remains, however, poorly understood. A recent study has proposed that the higher abundance of ASF1 in H3.3 complexes purified from early Drosophila embryos could indicate a higher affinity of ASF1 for H3.3 over H3.2 [ 52 ]. If true, this difference could fit well with the role of ASF1 in providing H3.3–H4 dimers to the HIRA complex at fertilization.…”

Molecular effects of dADD1 misexpression in chromatin organization and transcription

“…Vertebrate ATRX is capable of interacting with the histone chaperone DAXX and exchange the H3.3 variant at different heterochromatic regions such as the telomeres and pericentric heterochromatin [25] also, the H3.3 that is deposited helps to maintain the levels H3K9me3 necessary for proper heterochromatin maintenance [25,50]. In Drosophila, the DAXX like protein has been shown to cooperate with ASF1 for the deposition of H3.3 and also with dXNP at certain heterochromatic regions [51]. Thus, it is possible that also dADD1 proteins may be cooperating with this complex to maintain heterochromatin, however to date, there are no studies on histone variant H3.3 in a null dadd1 background or overexpression condition, but it would be a critical and interest aspect for future research to thoroughly understand the cooperative roles of these proteins.…”