Histone H2A‐H2B binding by Pol α in the eukaryotic replisome contributes to the maintenance of repressive chromatin

The eukaryotic replisome disassembles parental chromatin at DNA replication forks, but then plays a poorly understood role in the re‐deposition of the displaced histone complexes onto nascent DNA. Here, we show that yeast DNA polymerase α contains a histone‐binding motif that is conserved in human Pol α and is specific for histones H2A and H2B. Mutation of this motif in budding yeast cells does not affect DNA synthesis, but instead abrogates gene silencing at telomeres and mating‐type loci. Similar phenotypes are produced not only by mutations that displace Pol α from the replisome, but also by mutation of the previously identified histone‐binding motif in the CMG helicase subunit Mcm2, the human orthologue of which was shown to bind to histones H3 and H4. We show that chromatin‐derived histone complexes can be bound simultaneously by Mcm2, Pol α and the histone chaperone FACT that is also a replisome component. These findings indicate that replisome assembly unites multiple histone‐binding activities, which jointly process parental histones to help preserve silent chromatin during the process of chromosome duplication.     

Common features of analogous replacement histone H3 genes in animals and plants

Phylogenetic analysis of histone H3 protein sequences demonstrates the independent origin of the replacement histone H3 genes in animals and in plants. Multiple introns in the replacement histone H3 genes of animals in a pattern distinct from that in plant replacement H3 genes supports this conclusion. It is suggested that replacement H3 genes arose at the same time that, independently, multicellular forms of animals and of plants evolved. Judged by the degree of invariant and functionally constrained amino acid positions, histones H3 and H4, which form together the tetramer kernel of the nucleosome, have co-evolved with equal rates of sequence divergence. Residues 31 and 87 in histone H3 are the only residues that consistently changed across each gene duplication event that created functional replacement histone H3 variant forms. Once changed, these residues have remained invariant across divergent speciation. This suggests that they are required to allow replacement histone H3 to participate in the assembly of nucleosomes in non–S-phase cells. The abundant occurrence of polypyrimidine sequences in the introns of all replacement H3 genes, and the replacement of an intron by a polypyrimidine motif upstream of the alfalfa replacement H3 gene, suggests a function. It is speculated that they may contribute to the characteristic cell-cycle-independent pattern of replacement histone H3 genes by binding nucleosome-excluding proteins.     

Identification of a novel histone H3 specific protease activity in nuclei of chicken liver

Evolutionary conserved histone proteins play a very important role in the regulation of eukaryotic gene expression by undergoing post translational modifications within the tail regions. However, their role in tissue-specific gene expression and development remains unclear. In this study, we provide evidence for in vivo tissue-specific proteolytic cleavage of histone H3 in the liver of adult white Leghorn chickens, which we believe to be regulated by tissue-specific protease activity and epigenetic markers. The cleavage of histone H3 in the liver of adult chickens is very unique, and can serve as a model for studying tissue-specific changes in chromatin organization and gene expression. For the first time, we have identified and partially purified histone H3-specific protease activity that is distinct from histone H3 protease activities recently reported. Together, our data provide evidence of proteolytic processing and identification of protease activity that is specific to histone H3 in the liver of adult chickens, which may be involved in the regulation of gene expression during development, aging, and age-associated diseases.     

Structure of a 14-3-3 coordinated hexamer of the plant plasma membrane H+ -ATPase by combining X-ray crystallography and electron cryomicroscopy

Regulatory 14-3-3 proteins activate the plant plasma membrane H(+)-ATPase by binding to its C-terminal autoinhibitory domain. This interaction requires phosphorylation of a C-terminal, mode III, recognition motif as well as an adjacent span of approximately 50 amino acids. Here we report the X-ray crystal structure of 14-3-3 in complex with the entire binding motif, revealing a previously unidentified mode of interaction. A 14-3-3 dimer simultaneously binds two H(+)-ATPase peptides, each of which forms a loop within the typical 14-3-3 binding groove and therefore exits from the center of the dimer. Several H(+)-ATPase mutants support this structure determination. Accordingly, 14-3-3 binding could result in H(+)-ATPase oligomerization. Indeed, by using single-particle electron cryomicroscopy, the 3D reconstruction of the purified H(+)-ATPase/14-3-3 complex demonstrates a hexameric arrangement. Fitting of 14-3-3 and H(+)-ATPase atomic structures into the 3D reconstruction map suggests the spatial arrangement of the holocomplex.