Core histones and HIRIP3, a novel histone-binding protein, directly interact with WD repeat protein HIRA

Autor: Yann Lécluse, Stéphanie Lorain, Jean-Pierre Quivy, Christine Scamps, Marc Lipinski, Frédérique Monier-Gavelle, Geneviève Almouzni
Rok vydání: 1998
Předmět:
Zdroj: Molecular and cellular biology. 18(9)
ISSN: 0270-7306
Popis: Developmental anomalies are frequently observed in humans in association with deletions affecting the proximal region of the long arm of chromosome 22. These 22q deletion disorders (22DD) include the DiGeorge syndrome (Mendelion inheritance in man [MIM] 188400) and the velocardiofacial syndrome (MIM 192430), whose phenotypes overlap partially. Main clinical features associated with 22DD comprise abnormalities of the face and palate, hypoplastic parathyroid glands, and conotruncal malformations (38), all probably resulting from anomalies of neural crest cells in the embryological region of the pharyngeal arches and pouches (26). Genetically, 90% of all patients have a large (approximately 3-Mb-long) 22q deletion. Although most deletions occur de novo, up to 28% could be inherited (38). In these familial cases whose transmission is autosomal dominant, the phenotypic expression of the same chromosomal defect is largely variable. The additional lack of correlation between the extent of the deletion and the intensity of the phenotype seems to argue against different contiguous genes being each responsible for distinct clinical features. Recently, however, the hypothesis that two causative genes each mapping to the same 22q region may together be responsible for the disorders has been reconsidered (5, 9). In the absence of any mutation identified in the minority of patients without a confirmed 22q deletion, none of the genes cloned from the large commonly deleted region has been definitely linked to 22DD, making it necessary to study each plausible candidate in detail in order to evaluate its possible implication. This task will be facilitated by recent reports of a few patients whose unusually small 22q deletions with variant proximal and distal chromosomal breakpoints have reduced the critical region to less than 500 kb (6, 15, 28). Of the five genes characterized within the region, neither CTP, which codes for a mitochondrial citrate transport protein (20), CLTD (21), a clathrin heavy chain-like gene, nor the ubiquitously expressed TMVCF gene, which encodes a putative transmembrane protein (41), appears to be a plausible candidate. GSCL, a small (less than 4-kb) gene identified by systematic genomic sequencing, is more intriguing since it contains regions of homology to Goosecoid, a homeodomain gene whose specific expression pattern in the mouse suggests a role in the development of neural tissues (15). The HIRA gene (27), first reported as TUPLE1 (18) for its partial similarity to the yeast general transcriptional repressor TUP1 (51), also appears to be an interesting candidate. It consists of 25 exons scattered over 100 kb of genomic DNA which is entirely reduced to single copy in 22DD patients (30). In situ hybridization experiments have demonstrated high levels of transcripts in the heart, cranial neural folds, pharyngeal arches, and circumpharyngeal neural crest of murine embryos (53) and in the neuroepithelium, neural crest-derived regions of the head, branchial arches, and pharyngeal pouches of chicken embryos (37). These evolutionarily conserved spatiotemporal expression patterns suggest that haploinsufficiency of HIRA could play an important part in the genesis of 22DD. HIRA was named for its sequence similarity to two yeast proteins, Hir1p and Hir2p (40). In the HIR family, which also comprises HIRA homologs subsequently identified in mice, chickens, and the fish Fugu rubripes, all protein sequences can be aligned over their entire lengths (29, 37, 39, 53). Shared features include basic nuclear localization signals, absence of an identifiable DNA- or RNA-binding motif, and seven characteristic WD repeats conserved at the amino terminus of all family members but Hir2p. WD repeats are ancient motifs that have been detected throughout the eukaryotic kingdom (33). They are present in a set of functionally diverse proteins that are part of macromolecular complexes (33) and in several instances have been shown to provide interfaces for protein interactions (14, 24, 25, 42, 50). Contrasting with the single HIRA gene present in higher eukaryotes, the two budding yeast genes HIR1 and HIR2 likely have diverged and specialized functionally following an ancestral duplication event. HIR1 and HIR2 had been identified in the course of a genetic screen for Saccharomyces cerevisiae mutants with deregulated core histone gene transcription (34). In a wild-type strain, transcription of core histone genes is repressed outside late G1 and S phases, while in either a hir1 or hir2 mutant, transcription becomes constitutive throughout the cell cycle. To perform their cyclic repressive function, Hir1p and Hir2p, which can be coimmunoprecipitated (43), require the presence of other proteins, including the products of the SPT4, SPT5, and SPT6 genes, which are known for their impact on chromatin (3, 44) and transcription elongation by RNA polymerase II (19, 49). A regulatory protein complex targeted to the negative element identified in the promoter region of regulated histone gene loci by a DNA-binding factor that has been detected but not characterized would provoke transcriptional repression, probably in association with a local remodeling of chromatin structure (43). In humans, where the number and complexity of core histone genes (1) far exceed those in yeast, it appeared that the main function of HIRA proteins would not necessarily involve the regulation of histone gene transcription. Rather, we hypothesized that during evolution, HIRA proteins could have built upon ancient biochemical properties to acquire the developmental role that is suggested by their regulated expression pattern during vertebrate embryogenesis. To explore the function of HIRA in multicellular species, we have looked for HIRA-interacting proteins (HIRIPs). We here report that human HIRA is a primarily nuclear protein that directly binds core histones H2B and H4, using overlapping but distinguishable domains outside its WD repeat region. HIRIP3, one of the other HIRA interactors identified, is a novel protein which also displays binding to two core histones, H2B and H3, suggesting that HIRA and HIRIP3 can assemble into a protein complex with a role at the level of chromatin components or structure.
Databáze: OpenAIRE
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