Chromosomal high mobility group (HMG) proteins of the HMGB-type occurring in the moss Physcomitrella patens

HBD1 protein with a tandem repeat of two HMG-box domains is a DNA clip to organize chloroplast nucleoids in Chlamydomonas reinhardtii

Compaction of bulky DNA is a universal issue for all DNA-based life forms. Chloroplast nucleoids (chloroplast DNA-protein complexes) are critical for chloroplast DNA maintenance and transcription, thereby supporting photosynthesis, but their detailed structure remains enigmatic. Our proteomic analysis of chloroplast nucleoids of the green alga Chlamydomonas reinhardtii identified a protein (HBD1) with a tandem repeat of two DNA-binding high mobility group box (HMG-box) domains, which is structurally similar to major mitochondrial nucleoid proteins transcription factor A, mitochondrial (TFAM), and ARS binding factor 2 protein (Abf2p). Disruption of the HBD1 gene by CRISPR-Cas9-mediated genome editing resulted in the scattering of chloroplast nucleoids. This phenotype was complemented when intact HBD1 was reintroduced, whereas a truncated HBD1 with a single HMG-box domain failed to complement the phenotype. Furthermore, ectopic expression of HBD1 in the mitochondria of yeast Δabf2 mutant successfully complemented the defects, suggesting functional similarity between HBD1 and Abf2p. Furthermore, in vitro assays of HBD1, including the electrophoretic mobility shift assay and DNA origami/atomic force microscopy, showed that HBD1 is capable of introducing U-turns and cross-strand bridges, indicating that proteins with two HMG-box domains would function as DNA clips to compact DNA in both chloroplast and mitochondrial nucleoids.

Isolation and expression analysis of a candidate gametophyte sex determination gene (sjhmg) of kelp (Saccharina japonica)

Saccharina japonica undergoes an alternating life cycle during which the diploid sporophyte generation alternates with the happloid gametophyte generation. Saccharina japonica uses the UV sex determination system to determine the sex of its haploid gametophytes. However, the sex-determining genes and the sex-determining mechanisms of kelp gametophytes have not been thoroughly elucidated to date. In this study, a kelp HMG-box-containing gene (SjHMG), which is located within the sex determination region of S. japonica, was isolated and characterized. SjHMG contained an open reading frame of 1,266 bp in length and encoded a deduced protein of 421 amino acid residues with two HMG-box domains. Phylogenetic analysis showed the strongest relationship between SjHMG and its orthologs in brown algae. An alternatively spliced transcript (SjHMG isoform-2) encoding a protein of 256 amino acid residues was also identified. The two isoforms were specific for male gametophytes. A real-time quantitative PCR analysis showed significantly higher abundances of two isoforms in immature male gametophytes than in mature ones. These findings suggested that the SjHMG gene is a candidate male gametophyte determination gene of kelp. In addition, the abundance of SjHMG isoform-2 transcripts was significantly lower than that of SjHMG isoform-1 transcripts, and only an HMG-box domain was conserved among species in the order Laminariales, which indicated that the gene is specifically involved in sex regulation in some species of the order Laminariales by alternative splicing.

The HMGB1-TLR4 axis contributes to myocardial ischemia/reperfusion injury via regulation of cardiomyocyte apoptosis

Toll-like receptor 4 (TLR4) and its ligand high mobility group box 1 (HMGB1), are known for playing central roles in ischemia-reperfusion injury in myocardium. However, the detailed mechanisms of TLR4 and HMGB1 are not fully understood. The aim of this study was to investigate the effects and possible mechanisms of the HMGB1-TLR4 axis and cardiomyocyte apoptosis on myocardial ischemic damage. Artificial oxygen ventilated anesthetized C3H/HeN mice and C3H/HeJ mice were subjected to 30 min of left anterior descending coronary artery occlusion followed by 6h of reperfusion. The myocardial infarct size, HMGB1 levels, apoptosis index, Bax, Bcl-2 and TNF-α mRNA levels were assessed. The results showed that a lowered amount of cardiomyocyte apoptosis and infarct size in the myocardium of TLR4-mutant mice after myocardial I/R and that TLR4 deficiency notably inhibited the expression of HMGB1 and TNF-a, both of which were up-regulated by ischemia/reperfusion. These findings suggest that the HMGB1-TLR4 axis plays a pathogenic role in triggering cardiomyocyte apoptosis during myocardial I/R injury and that the possible mechanism for this process is the result of released cytokines and inflammatory response involved in the HMGB1/TLR4-related pathway.

High mobility group [corrected] box 1 mediates neutrophil recruitment in myocardial ischemia-reperfusion injury through toll like receptor 4-related pathway

This study aimed to explore the role of high mobility group [corrected] box 1 (HMGB1) and its receptor toll like receptor 4 (TLR4) on neutrophils in myocardial ischemia reperfusion (I/R) injury. We constructed TLR4-mutant (C3H/HeJ) and control (C3H/HeN) mouse models of myocardial I/R injury and subjected the mice to 30 min of ischemia and 6h of reperfusion. Light microscope was used to observe structural changes in the myocardium. HMGB1 levels were measured using quantitative real-time PCR and immunohistochemistry. Neutrophil accumulation, TNF-a expression and IL-8 levels were analyzed via myeloperoxidase (MPO) biochemical studies, quantitative real-time PCR and ELISA, respectively. The results demonstrated that fewer neutrophils infiltrated in the myocardium of TLR4-mutant mice after myocardial I/R and that TLR4 deficiency markedly decreased the ischemic injury caused by ischemia/reperfusion, and inhibited the expression of HMGB1, TNF-a, and IL-8, all of which were up-regulated by ischemia/reperfusion. These findings suggest that HMGB1 plays a central role in recruiting neutrophils during myocardial I/R leading to worsened myocardial I/R injury. This recruitment mechanism is possibly due to its inflammatory and chemokine functions based on the TLR4-dependent pathway.

The dengue vector Aedes aegypti contains a functional high mobility group box 1 (HMGB1) protein with a unique regulatory C-terminus

The mosquito Aedes aegypti can spread the dengue, chikungunya and yellow fever viruses. Thus, the search for key molecules involved in the mosquito survival represents today a promising vector control strategy. High Mobility Group Box (HMGB) proteins are essential nuclear factors that maintain the high-order structure of chromatin, keeping eukaryotic cells viable. Outside the nucleus, secreted HMGB proteins could alert the innate immune system to foreign antigens and trigger the initiation of host defenses. In this work, we cloned and functionally characterized the HMGB1 protein from Aedes aegypti (AaHMGB1). The AaHMGB1 protein typically consists of two HMG-box DNA binding domains and an acidic C-terminus. Interestingly, AaHMGB1 contains a unique alanine/glutamine-rich (AQ-rich) C-terminal region that seems to be exclusive of dipteran HMGB proteins. AaHMGB1 is localized to the cell nucleus, mainly associated with heterochromatin. Circular dichroism analyses of AaHMGB1 or the C-terminal truncated proteins revealed α-helical structures. We showed that AaHMGB1 can effectively bind and change the topology of DNA, and that the AQ-rich and the C-terminal acidic regions can modulate its ability to promote DNA supercoiling, as well as its preference to bind supercoiled DNA. AaHMGB1 is phosphorylated by PKA and PKC, but not by CK2. Importantly, phosphorylation of AaHMGB1 by PKA or PKC completely abolishes its DNA bending activity. Thus, our study shows that a functional HMGB1 protein occurs in Aedes aegypt and we provide the first description of a HMGB1 protein containing an AQ-rich regulatory C-terminus.

Transcript elongation factor TFIIS is involved in arabidopsis seed dormancy

Transcript elongation factor TFIIS promotes efficient transcription by RNA polymerase II, since it assists in bypassing blocks during mRNA synthesis. While yeast cells lacking TFIIS are viable, inactivation of mouse TFIIS causes embryonic lethality. Here, we have identified a protein encoded in the Arabidopsis genome that displays a marked sequence similarity to TFIIS of other organisms, primarily within domains II and III in the C-terminal part of the protein. TFIIS is widely expressed in Arabidopsis, and a green fluorescent protein-TFIIS fusion protein localises specifically to the cell nucleus. When expressed in yeast cells lacking the endogenous TFIIS, Arabidopsis TFIIS partially complements the sensitivity of mutant cells to the nucleotide analog 6-azauridine, which is a typical characteristic of transcript elongation factors. We have characterised Arabidopsis lines harbouring T-DNA insertions in the coding sequence of TFIIS. Plants homozygous for T-DNA insertions are viable, and genomewide transcript profiling revealed that compared to control plants, a relatively small number of genes are differentially expressed in mutant plants. TFIIS(-/-) plants display essentially normal development, but they flower slightly earlier than control plants and show clearly reduced seed dormancy. Plants with RNAi-mediated knockdown of TFIIS expression also are affected in seed dormancy. Therefore, TFIIS plays a critical role in Arabidopsis seed development.

Physcomitrella HMGA-type proteins display structural differences compared to their higher plant counterparts

High mobility group (HMG) proteins of the HMGA family are chromatin-associated proteins that act as architectural factors in nucleoprotein structures involved in gene transcription. To date, HMGA-type proteins have been studied in various higher plant species, but not in lower plants. We have identified two HMGA-type proteins, HMGA1 and HMGA2, encoded in the genome of the moss model Physcomitrella patens. Compared to higher plant HMGA proteins, the two Physcomitrella proteins display some structural differences. Thus, the moss HMGA proteins have six (rather than four) AT-hook DNA-binding motifs and their N-terminal domain lacks similarity to linker histone H1. HMGA2 is expressed in moss protonema and it localises to the cell nucleus. Typical of HMGA proteins, HMGA2 interacts preferentially with A/T-rich DNA, when compared with G/C-rich DNA. In cotransformation assays in Physcomitrella protoplasts, HMGA2 stimulated reporter gene expression. In summary, our data show that functional HMGA-type proteins occur in Physcomitrella.