High mobility group proteins 14 and 17 can space nucleosomal particles deficient in histones H2A and H2B creating a template that is transcriptionally active

Pioneer Factors and Architectural Proteins Mediating Embryonic Expression Signatures in Cancer

Accumulation of mutations causing aberrant changes in the genome promotes cancer. However, mutations do not occur in every cancer subtype, suggesting additional events that trigger cancer. Chromatin rearrangements initiated by pioneer factors and architectural proteins are key events occurring before cancer-related genes are expressed. Both protein groups are also master regulators of important processes during embryogenesis. Several publications demonstrated that embryonic gene expression signatures are reactivated during cancer. This review article highlights current knowledge on pioneer factors and architectural proteins mediating chromatin rearrangements, which are the backbone of embryonic expression signatures promoting malignant transformation. Understanding chromatin rearrangements inducing embryonic expression signatures in adult cells might be the key to novel therapeutic approaches against cancers subtypes that arise without genomic mutations.

CAF-1 and the inheritance of chromatin states: at the crossroads of DNA replication and repair

Chromatin is no longer considered to be a static structural framework for packaging DNA within the nucleus but is instead believed to be an interactive component of DNA metabolism. The ordered assembly of chromatin produces a nucleoprotein template capable of epigenetically regulating the expression and maintenance of the genome. Factors have been isolated from cell extracts that stimulate early steps in chromatin assembly in vitro. The function of one such factor, chromatin-assembly factor 1 (CAF-1), might extend beyond simply facilitating the progression through an individual assembly reaction to its active participation in a marking system. This marking system could be exploited at the crossroads of DNA replication and repair to monitor genome integrity and to define particular epigenetic states.

Retinol-induced changes in the phosphorylation levels of histones and high mobility group proteins from Sertoli cells

Chromatin proteins play a role in the organization and functions of DNA. Covalent modifications of nuclear proteins modulate their interactions with DNA sequences and are probably one of the multiple factors involved in the process of switch on/off transcriptionally active regions of DNA. Histones and high mobility group proteins (HMG) are subject to many covalent modifications that may modulate their capacity to bind to DNA. We investigated the changes induced in the phosphorylation pattern of cultured Wistar rat Sertoli cell histones and high mobility group protein subfamilies exposed to 7 microM retinol for up to 48 h. In each experiment, 6 h before the end of the retinol treatment each culture flask received 370 KBq/ml [32P]-phosphate. The histone and HMGs were isolated as previously described [Moreira et al. Medical Science Research (1994) 22: 783-784]. The total protein obtained by either method was quantified and electrophoresed as described by Spiker [Analytical Biochemistry (1980) 108: 263-265]. The gels were stained with Coomassie brilliant blue R-250 and the stained bands were cut and dissolved in 0.5 ml 30% H2O2 at 60oC for 12 h. The vials were chilled and 5.0 ml scintillation liquid was added. The radioactivity in each vial was determined with a liquid scintillation counter. Retinol treatment significantly changed the pattern of each subfamily of histone and high mobility group proteins.

Distinct importin recognition properties of histones and chromatin assembly factors

Synthesis of the protein components of nuclear chromatin occurs in the cytoplasm, necessitating specific import into the nucleus. Here, we report the binding affinities of the nuclear localisation sequence (NLS)-binding importin subunits for a range of histones and chromatin assembly factors. The results suggest that import of histones to the nucleus may be mediated predominantly by importin beta1, whereas the import of the other components probably relies on the conventional alpha/beta1 import pathway. Differences in recognition by importin beta1 were observed between histone H2A and the variant H2AZ, as well as between histone H3/4 with or without acetylation. The results imply that different histone variants may possess distinct nuclear import properties, with acetylation possibly playing an inhibitory role through NLS masking.

Phosphorylation and subcellular redistribution of high mobility group proteins 14 and 17, analyzed by mass spectrometry

High mobility group (HMG) proteins 14 and 17 are nonhistone nuclear proteins that have been implicated in control of transcription and chromatin structure. To examine the posttranslational modifications of HMG-14 and -17 in vivo, HMG proteins were prepared from nuclear vs. cytosolic fractions of human K562 cells treated with 12-O-tetradecanoylphorbol 13-acetate (TPA) or okadaic acid (OA) and examined by electrospray mass spectrometry. Analysis of full-length masses demonstrated mono-, di-, and triphosphorylation of HMG-14 and mono- and diphosphorylation of HMG-17 from OA treated cells, whereas HMG-14 and -17 from TPA treated cells were monophosphorylated. Peptide mass and sequence analysis showed major and minor phosphorylation sites, respectively, at Ser24 and Ser28 in HMG-17, and Ser20 and Ser24 in HMG-14. These sites were found in the consensus sequence RRSARLSAK, within the nucleosomal binding domain of each protein. A third phosphorylation site in HMG-14 was located at either Ser6 or Ser7. Interestingly, the proportion of HMG-14 and -17 found in cytosolic pools increased significantly after 1 h of treatment compared to control cells and showed preferential phosphorylation compared with proteins from nuclear fractions. These results suggest that phosphorylation of HMG-14 and -7 interferes with nuclear localization mechanisms in a manner favoring release from nuclei.

Enhancement of the transcription potential of nascent chromatin by chromosomal proteins HMG-14/-17 is coupled to nucleosome assembly and not DNA synthesis

We have previously demonstrated that in Xenopus egg extracts, which support DNA strand synthesis and chromatin assembly, incorporation of chromosomal proteins HMG-14/-17 into nascent nucleosomes increases the transcriptional potential of a chromatin template carrying the Xenopus 5S RNA gene. Here we use the single-stranded and double-stranded forms of a plasmid carrying a 5S RNA maxigene, to test whether the effect of HMG-14/-17 on transcription requires DNA synthesis and whether these proteins will affect transcription through a region containing nucleosomes. We find that most of the transcripts were about 350 nucleotides long, suggesting that HMG-14/-17 enhance transcription through a region that could contain nucleosomes. HMG-14/-17 enhance transcription of chromatin templates assembled onto double-stranded DNA, in the absence of DNA synthesis. Single-round transcription assays suggest that HMG-14/-17 increase transcription from templates assembled onto both single- and double-stranded DNA by increasing the specific activity, and not the number, of transcriptionally active templates. We conclude that the effect of HMG-14/-17 on the transcriptional potential of chromatin is linked to nucleosome assembly and is not linked to DNA synthesis.

High mobility group protein 14 and 17 can prevent the close packing of nucleosomes by increasing the strength of protein contacts in the linker DNA

High mobility group (HMG) proteins 14 and 17 are abundant chromatin-associated proteins found in all higher eukaryotic nuclei. This observation demonstrates that HMGs 14 and 17 must have an important and universal function with regard to the structure and function of chromatin. What this function is, including how they interact with a nucleosomal array in vivo, is not known. Recently, we have demonstrated that HMGs 14 and 17 can organize nucleosomes into a regular array and increase the repeat length from 145 to about 160-165 base pairs in vitro. In addition, they can increase the apparent repeat length of chromatin deficient in histones H2A/H2B from 125 to approximately 145 base pairs. Importantly, this template was transcriptionally active. In this study, we report five new observations that begin to address the mechanism by which HMGs 14 and 17 space nucleosomal particles. First, we demonstrate that both human placenta HMG 14 and HMG 17 can space nucleosomes to produce a chromatin template with a repeat length around 160 base pairs. This result further highlights the similarity between these proteins in terms of protein structure and perhaps function. Second, we show that digestion of HMG containing chromatin with micrococcal nuclease produces DNA fragments that were approximately 10 and 20 base pairs longer than nucleosome core-particle DNA. This suggests that HMG 14 or HMG 17 can protect, directly or indirectly, at least an additional 10 base pairs of linker DNA from micrococcal digestion. However, this HMG-containing particle does not produce a strong kinetic block, and further digestion results in the eventual accumulation of DNA fragments 145 base pairs in length. Third, by comparing the full-length protein with different domains, we demonstrate that the acidic carboxyl-terminal domain is absolutely required for nucleosome spacing, neither the nucleosome binding domain of HMG 14 or HMG 17 nor the amino-terminal domain plus the nucleosome binding domain of HMG 14 could space nucleosomes. Fourth, we demonstrate that extensive micrococcal nuclease digestion of chromatin deficient in histones H2A/H2B led to the accumulation of DNA fragments about 110 base pairs in length, which is presumably the length of DNA associated with a nucleosomal particle deficient in one H2A/H2B dimer. Incorporation of either HMG 14 or HMG 17 into this chromatin results in the disappearance of this band and increase in the accumulation of fragments around 140-150 base pairs in length. Finally, in contrast to spacing of complete nucleosomes, we find that the nucleosome binding domain of HMG 17 (but not the nucleosome binding of HMG 14) is the only domain required for spacing of H2A/H2B-deficient chromatin.

HMG17 is a chromatin-specific transcriptional coactivator that increases the efficiency of transcription initiation

We have examined the effect of HMG17 on transcription by RNA polymerase II by the assembly and analysis of HMG17-containing chromatin templates consisting of regularly spaced nucleosomal arrays. Structural analysis of the chromatin indicated that HMG17 is incorporated into chromatin in a physiological manner with the full complement of core histones. The transcriptional studies revealed that HMG17 stimulates transcription in conjunction with the sequence-specific activator GAL4-VP16. This effect was observed with chromatin, but not with non-nucleosomal templates, and required the presence of HMG17 during chromatin assembly. The incorporation of HMG17 into chromatin resulted in a 7- to 40-fold stimulation of GAL4-VP16-activated transcription to levels that were comparable to those observed with histone-free DNA templates. In contrast, transcription from HMG17-containing chromatin was not detectable in the absence of GAL4-VP16 or with a GAL4 derivative [GAL4(1-147)] lacking the VP16 activation domain. Finally, the incorporation of HMG17 into chromatin was found to increase the efficiency of transcription initiation, but not the extent of transcriptional elongation. Thus, HMG17 is a chromatin-specific transcriptional coactivator that increases the efficiency of initiation of transcription by RNA polymerase II.