Ubiquity and Origins of Structural Maintenance of Chromosomes (SMC) Proteins in Eukaryotes

Structural Maintenance of Chromosomes Complexes

The Structural Maintenance of Chromosomes (SMC) protein complexes are DNA-binding molecular machines required to shape chromosomes into functional units and to safeguard the genome through cell division. These ring-shaped multi-subunit protein complexes, which are present in all kingdoms of life, achieve this by organizing chromosomes in three-dimensional space. Mechanistically, the SMC complexes hydrolyze ATP to either stably entrap DNA molecules within their lumen, or rapidly reel DNA into large loops, which allow them to link two stretches of DNA in cis or trans. In this chapter, the canonical structure of the SMC complexes is first introduced, followed by a description of the composition and general functions of the main types of eukaryotic and prokaryotic SMC complexes. Thereafter, the current model for how SMC complexes perform in vitro DNA loop extrusion is presented. Lastly, chromosome loop formation by SMC complexes is introduced, and how the DNA loop extrusion mechanism contributes to chromosome looping by SMC complexes in cells is discussed.

Genetic conflicts in budding yeast: The 2μ plasmid as a model selfish element

Antagonistic coevolution, arising from genetic conflict, can drive rapid evolution and biological innovation. Conflict can arise both between organisms and within genomes. This review focuses on budding yeasts as a model system for exploring intra- and inter-genomic genetic conflict, highlighting in particular the 2-micron (2μ) plasmid as a model selfish element. The 2μ is found widely in laboratory strains and industrial isolates of Saccharomyces cerevisiae and has long been known to cause host fitness defects. Nevertheless, the plasmid is frequently ignored in the context of genetic, fitness, and evolution studies. Here, I make a case for further exploring the evolutionary impact of the 2μ plasmid as well as other selfish elements of budding yeasts, discuss recent advances, and, finally, future directions for the field.

Intermolecular Gene Conversion for the Equalization of Genome Copies in the Polyploid Haloarchaeon Haloferax volcanii: Identification of Important Proteins

The model haloarchaeon Haloferax volcanii is polyploid with about 20 copies of its major chromosome. Recently it has been described that highly efficient intermolecular gene conversion operates in H. volcanii to equalize the chromosomal copies. In the current study, 24 genes were selected that encode proteins with orthologs involved in gene conversion or homologous recombination in archaea, bacteria, or eukaryotes. Single gene deletion strains of 22 genes and a control gene were constructed in two parent strains for a gene conversion assay; only radA and radB were shown to be essential. Protoplast fusions were used to generate strains that were heterozygous for the gene HVO_2528, encoding an enzyme for carotinoid biosynthesis. It was revealed that a lack of six of the proteins did not influence the efficiency of gene conversion, while sixteen mutants had severe gene conversion defects. Notably, lack of paralogous proteins of gene families had very different effects, e.g., mutant Δrad25b had no phenotype, while mutants Δrad25a, Δrad25c, and Δrad25d were highly compromised. Generation of a quadruple rad25 and a triple sph deletion strain also indicated that the paralogs have different functions, in contrast to sph2 and sph4, which cannot be deleted simultaneously. There was no correlation between the severity of the phenotypes and the respective transcript levels under non-stressed conditions, indicating that gene expression has to be induced at the onset of gene conversion. Phylogenetic trees of the protein families Rad3/25, MutL/S, and Sph/SMC/Rad50 were generated to unravel the history of the paralogous proteins of H. volcanii. Taken together, unselected intermolecular gene conversion in H. volcanii involves at least 16 different proteins, the molecular roles of which can be studied in detail in future projects.

SMC-mediated dosage compensation in C. elegans evolved in the presence of an ancestral nematode mechanism

Mechanisms of X chromosome dosage compensation have been studied extensively in a few model species representing clades of shared sex chromosome ancestry. However, the diversity within each clade as a function of sex chromosome evolution is largely unknown. Here, we anchor ourselves to the nematode Caenorhabditis elegans, for which a well-studied mechanism of dosage compensation occurs through a specialized structural maintenance of chromosomes (SMC) complex, and explore the diversity of dosage compensation in the surrounding phylogeny of nematodes. Through phylogenetic analysis of the C. elegans dosage compensation complex and a survey of its epigenetic signatures, including X-specific topologically associating domains (TADs) and X-enrichment of H4K20me1, we found that the condensin-mediated mechanism evolved recently in the lineage leading to Caenorhabditis through an SMC-4 duplication. Intriguingly, an independent duplication of SMC-4 and the presence of X-specific TADs in Pristionchus pacificus suggest that condensin-mediated dosage compensation arose more than once. mRNA-seq analyses of gene expression in several nematode species indicate that dosage compensation itself is ancestral, as expected from the ancient XO sex determination system. Indicative of the ancestral mechanism, H4K20me1 is enriched on the X chromosomes in Oscheius tipulae, which does not contain X-specific TADs or SMC-4 paralogs. Together, our results indicate that the dosage compensation system in C. elegans is surprisingly new, and condensin may have been co-opted repeatedly in nematodes, suggesting that the process of evolving a chromosome-wide gene regulatory mechanism for dosage compensation is constrained.

Significance statement

X chromosome dosage compensation mechanisms evolved in response to Y chromosome degeneration during sex chromosome evolution. However, establishment of dosage compensation is not an endpoint. As sex chromosomes change, dosage compensation strategies may have also changed. In this study, we performed phylogenetic and epigenomic analyses surrounding Caenorhabditis elegans and found that the condensin-mediated dosage compensation mechanism in C. elegans is surprisingly new, and has evolved in the presence of an ancestral mechanism. Intriguingly, condensin-based dosage compensation may have evolved more than once in the nematode lineage, the other time in Pristionchus. Together, our work highlights a previously unappreciated diversity of dosage compensation mechanisms within a clade, and suggests constraints in evolving new mechanisms in the presence of an existing one.

Cohesin-Dependent Loop Extrusion: Molecular Mechanics and Role in Cell Physiology

The most prominent representatives of multisubunit SMC complexes, cohesin and condensin, are best known as structural components of mitotic chromosomes. It turned out that these complexes, as well as their bacterial homologues, are molecular motors, the ATP-dependent movement of these complexes along DNA threads leads to the formation of DNA loops. In recent years, we have witnessed an avalanche-like accumulation of data on the process of SMC dependent DNA looping, also known as loop extrusion. This review briefly summarizes the current understanding of the place and role of cohesin-dependent extrusion in cell physiology and presents a number of models describing the potential molecular mechanism of extrusion in a most compelling way. We conclude the review with a discussion of how the capacity of cohesin to extrude DNA loops may be mechanistically linked to its involvement in sister chromatid cohesion.

Identification of structural and regulatory cell-shape determinants in Haloferax volcanii

Archaea play indispensable roles in global biogeochemical cycles, yet many crucial cellular processes, including cell-shape determination, are poorly understood. Haloferax volcanii, a model haloarchaeon, forms rods and disks, depending on growth conditions. Here, we used a combination of iterative proteomics, genetics, and live-cell imaging to identify mutants that only form rods or disks. We compared the proteomes of the mutants with wild-type cells across growth phases, thereby distinguishing between protein abundance changes specific to cell shape and those related to growth phases. The results identified a diverse set of proteins, including predicted transporters, transducers, signaling components, and transcriptional regulators, as important for cell-shape determination. Through phenotypic characterization of deletion strains, we established that rod-determining factor A (RdfA) and disk-determining factor A (DdfA) are required for the formation of rods and disks, respectively. We also identified structural proteins, including an actin homolog that plays a role in disk-shape morphogenesis, which we named volactin. Using live-cell imaging, we determined volactin's cellular localization and showed its dynamic polymerization and depolymerization. Our results provide insights into archaeal cell-shape determination, with possible implications for understanding the evolution of cell morphology regulation across domains.

Activity of MukBEF for chromosome management in E. coli and its inhibition by MatP

Structural maintenance of chromosomes (SMC) complexes share conserved structures and serve a common role in maintaining chromosome architecture. In the bacterium Escherichia coli, the SMC complex MukBEF is necessary for rapid growth and the accurate segregation and positioning of the chromosome, although the specific molecular mechanisms involved are still unknown. Here, we used a number of in vivo assays to reveal how MukBEF controls chromosome conformation and how the MatP/matS system prevents MukBEF activity. Our results indicate that the loading of MukBEF occurs preferentially on newly replicated DNA, at multiple loci on the chromosome where it can promote long-range contacts in cis even though MukBEF can promote long-range contacts in the absence of replication. Using Hi-C and ChIP-seq analyses in strains with rearranged chromosomes, the prevention of MukBEF activity increases with the number of matS sites and this effect likely results from the unloading of MukBEF by MatP. Altogether, our results reveal how MukBEF operates to control chromosome folding and segregation in E. coli.

Towards unraveling the origins of eukaryotic nuclear genome organization

With 3D genome mapping maturing over the past decade, studies exposed the differences between eukaryotic and prokaryotic genome organization. This raises the question of how the complex eukaryotic genome organization originated. Here, I explore potential pathways to answering this question, guided by our changing understanding of the origins of eukaryotes.

Characterization of the conserved features of the NSE6 subunit of the Physcomitrium patens SMC5/6 complex

Structural maintenance of chromosomes (SMC) complexes are molecular machines ensuring chromatin organization at higher levels. They play direct roles in cohesion, condensation, replication, transcription, and DNA repair. Their cores are composed of long-armed SMC, kleisin, and kleisin-associated subunits. Additional factors, like NSE6 within SMC5/6, bind to SMC core complexes and regulate their activities. In the human HsNSE6/SLF2, we recently identified a new CANIN domain. Here we tracked down its sequence homology to lower plants, selected the bryophyte Physcomitrium patens, and analyzed PpNSE6 protein-protein interactions to explore its conservation in detail. We identified a previously unrecognized core sequence motif conserved from yeasts to humans within the NSE6 CANIN domain. This motif mediates the interaction between NSE6 and its NSE5 partner in yeasts and plants. In addition, the CANIN domain and its preceding PpNSE6 sequences bind both PpSMC5 and PpSMC6 arms. Interestingly, we mapped the PpNSE6-binding site at the PpSMC5 arm right next to the PpNSE2-binding surface. The position of NSE6 at SMC arms suggests its role in the regulation of SMC5/6 dynamics. Consistent with the regulatory role of NSE6 subunits, Ppnse6 mutant lines were viable and sensitive to the DNA-damaging drug bleomycin and lost a large portion of rDNA copies. These moss mutants also exhibited reduced growth and developmental aberrations. Altogether, our data showed the conserved function of the NSE6 subunit and architecture of the SMC5/6 complex across species.

Function and Evolution of the Loop Extrusion Machinery in Animals

Structural maintenance of chromosomes (SMC) complexes are essential proteins found in genomes of all cellular organisms. Essential functions of these proteins, such as mitotic chromosome formation and sister chromatid cohesion, were discovered a long time ago. Recent advances in chromatin biology showed that SMC proteins are involved in many other genomic processes, acting as active motors extruding DNA, which leads to the formation of chromatin loops. Some loops formed by SMC proteins are highly cell type and developmental stage specific, such as SMC-mediated DNA loops required for VDJ recombination in B-cell progenitors, or dosage compensation in Caenorhabditis elegans and X-chromosome inactivation in mice. In this review, we focus on the extrusion-based mechanisms that are common for multiple cell types and species. We will first describe an anatomy of SMC complexes and their accessory proteins. Next, we provide biochemical details of the extrusion process. We follow this by the sections describing the role of SMC complexes in gene regulation, DNA repair, and chromatin topology.

Protein Profiling of Psittacanthus calyculatus during Mesquite Infection

Psittacanthus calyculatus is a hemiparasite mistletoe that represents an ecological problem due to the impacts caused to various tree species of ecological and commercial interest. Although the life cycle for the Psittacanthus genus is well established in the literature, the development stages and molecular mechanism implicated in P. calyculatus host infection are poorly understood. In this study, we used a manageable infestation of P. laevigata with P. calyculatus to clearly trace the infection, which allowed us to describe five phenological infective stages of mistletoe on host tree branches: mature seed (T1), holdfast formation (T2), haustorium activation (T3), haustorium penetration (T4), and haustorium connection (T5) with the host tree. Proteomic analyses revealed proteins with a different accumulation and cellular processes in infective stages. Activities of the cell wall-degrading enzymes cellulase and β-1,4-glucosidase were primarily active in haustorium development (T3), while xylanase, endo-glucanase, and peptidase were highly active in the haustorium penetration (T4) and xylem connection (T5). Patterns of auxins and cytokinin showed spatial concentrations in infective stages and moreover were involved in haustorium development. These results are the first evidence of proteins, cell wall-degrading enzymes, and phytohormones that are involved in early infection for the Psittacanthus genus, and thus represent a general infection mechanism for other mistletoe species. These results could help to understand the molecular dialogue in the establishment of P. calyculatus parasitism.

The cohesin complex of yeasts: sister chromatid cohesion and beyond

Each time a cell divides, it needs to duplicate the genome and then separate the two copies. In eukaryotes, which usually have more than one linear chromosome, this entails tethering the two newly replicated DNA molecules, a phenomenon known as sister chromatid cohesion (SCC). Cohesion ensures proper chromosome segregation to separate poles during mitosis. SCC is achieved by the presence of the cohesin complex. Besides its canonical function, cohesin is essential for chromosome organization and DNA damage repair. Surprisingly, yeast cohesin is loaded in G1 before DNA replication starts but only acquires its binding activity during DNA replication. Work in microorganisms, such as Saccharomyces cerevisiae and Schizosaccharomyces pombe has greatly contributed to the understanding of cohesin composition and functions. In the last few years, much progress has been made in elucidating the role of cohesin in chromosome organization and compaction. Here, we discuss the different functions of cohesin to ensure faithful chromosome segregation and genome stability during the mitotic cell division in yeast. We describe what is known about its composition and how DNA replication is coupled with SCC establishment. We also discuss current models for the role of cohesin in chromatin loop extrusion and delineate unanswered questions about the activity of this important, conserved complex.

Form and function of archaeal genomes

A key maxim in modernist architecture is that 'form follows function'. While modernist buildings are hopefully the product of intelligent design, the architectures of chromosomes have been sculpted by the forces of evolution over many thousands of generations. In the following, I will describe recent advances in our understanding of chromosome architecture in the archaeal domain of life. Although much remains to be learned about the mechanistic details of archaeal chromosome organization, some general principles have emerged. At the 10-100 kb level, archaeal chromosomes have a conserved local organization reminiscent of bacterial genomes. In contrast, lineage-specific innovations appear to have imposed distinct large-scale architectural features. The ultimate functions of genomes are to store and to express genetic information. Gene expression profiles have been shown to influence chromosome architecture, thus their form follows function. However, local changes to chromosome conformation can also influence gene expression and therefore, in these instances, function follows form.

DNA-measuring Wadjet SMC ATPases restrict smaller circular plasmids by DNA cleavage

Structural maintenance of chromosome (SMC) complexes fold DNA by loop extrusion to support chromosome segregation and genome maintenance. Wadjet systems (JetABCD/MksBEFG/EptABCD) are derivative SMC complexes with roles in bacterial immunity against selfish DNA. Here, we show that JetABCD restricts circular plasmids with an upper size limit of about 100 kb, whereas a linear plasmid evades restriction. Purified JetABCD complexes cleave circular DNA molecules, regardless of the DNA helical topology; cleavage is DNA sequence nonspecific and depends on the SMC ATPase. A cryo-EM structure reveals a distinct JetABC dimer-of-dimers geometry, with the two SMC dimers facing in opposite direction-rather than the same as observed with MukBEF. We hypothesize that JetABCD is a DNA-shape-specific endonuclease and propose the "total extrusion model" for DNA cleavage exclusively when extrusion of an entire plasmid has been completed by a JetABCD complex. Total extrusion cannot be achieved on the larger chromosome, explaining how self-DNA may evade processing.

Evolutionary diversification of the autophagy-related ubiquitin-like conjugation systems

Two autophagy-related (ATG) ubiquitin-like conjugation systems, the ATG12 and ATG8 systems, play important roles in macroautophagy. While multiple duplications and losses of the ATG conjugation system proteins are found in different lineages, the extent to which the underlying systems diversified across eukaryotes is not fully understood. Here, in order to understand the evolution of the ATG conjugation systems, we constructed a transcriptome database consisting of 94 eukaryotic species covering major eukaryotic clades and systematically identified ATG conjugation system components. Both ATG10 and the C-terminal glycine of ATG12 are essential for the canonical ubiquitin-like conjugation of ATG12 and ATG5. However, loss of ATG10 or the C-terminal glycine of ATG12 occurred at least 16 times in a wide range of lineages, suggesting that possible covalent-to-non-covalent transition is not limited to the species that we previously reported such as Alveolata and some yeast species. Some species have only the ATG8 system (with conjugation enzymes) or only ATG8 (without conjugation enzymes). More than 10 species have ATG8 homologs without the conserved C-terminal glycine, and Tetrahymena has an ATG8 homolog with a predicted transmembrane domain, which may be able to anchor to the membrane independent of the ATG conjugation systems. We discuss the possibility that the ancestor of the ATG12 and ATG8 systems is more similar to ATG8. Overall, our study offers a whole picture of the evolution and diversity of the ATG conjugation systems among eukaryotes, and provides evidence that functional diversifications of the systems are more common than previously thought.Abbreviations: APEAR: ATG8-PE association region; ATG: autophagy-related; LIR: LC3-interacting region; NEDD8: neural precursor cell expressed, developmentally down-regulated gene 8; PE: phosphatidylethanolamine; SAMP: small archaeal modifier protein; SAR: Stramenopiles, Alveolata, and Rhizaria; SMC: structural maintenance of chromosomes; SUMO: small ubiquitin like modifier; TACK: Thaumarchaeota, Aigarchaeota, Crenarchaeota, and Korarchaeota; UBA: ubiquitin like modifier activating enzyme; UFM: ubiquitin fold modifier; URM: ubiquitin related modifier.

Comparative multiomics analysis of cell physiological state after culture in a basket bioreactor

Bioreactors are one of the most important, basic pieces of equipment in the biopharmaceutical industry. Understanding the effects of mechanical damage and other factors on the physiological state of cells during cell matrix culture is the basis for continuously achieving greater efficiency and higher product quality. In this study, Vero cells were used as a model and apoptosis, senescence, transcriptomics, proteomics, and metabolomics were carried out for analysis at the cellular and molecular levels. The results showed that compared with cells cultured in the simulated natural state, the cells cultured in the basket bioreactor displayed no obvious senescence. Additionally, the proportion of early apoptotic cells increased, but the proportions of damaged, late apoptotic and dead cells did not change significantly. The transcription levels of aminoacyl-tRNA synthetase and cyclin D1 and the expression levels of DNA replication licensing factor, methenyltetrahydrofolate cyclohydrolase, arachidonic acid and other metabolites of cells cultured in the basket bioreactor were significantly increased. These results suggest that DNA replication, protein translation and the metabolic activities in cells cultured in basket bioreactors are more active, which is more conducive to cell amplification and target product production. In this study, the growth and physiological state of cells in a basket bioreactor were characterized at the molecular level for the first time. Additionally, a tool to evaluate the physiological state of cells in a bioreactor was established, which can be used to guide the development and optimization of cell matrix culture conditions in industrial production and improve the production efficiency of the target products.

A joint-ParB interface promotes Smc DNA recruitment

Chromosomes readily unlink and segregate to daughter cells during cell division, highlighting a remarkable ability of cells to organize long DNA molecules. SMC complexes promote DNA organization by loop extrusion. In most bacteria, chromosome folding initiates at dedicated start sites marked by the ParB/parS partition complexes. Whether SMC complexes recognize a specific DNA structure in the partition complex or a protein component is unclear. By replacing genes in Bacillus subtilis with orthologous sequences from Streptococcus pneumoniae, we show that the three subunits of the bacterial Smc complex together with the ParB protein form a functional module that can organize and segregate foreign chromosomes. Using chimeric proteins and chemical cross-linking, we find that ParB directly binds the Smc subunit. We map an interface to the Smc joint and the ParB CTP-binding domain. Structure prediction indicates how the ParB clamp presents DNA to the Smc complex, presumably to initiate DNA loop extrusion.

A novel structural maintenance of chromosomes (SMC)-related protein family specific to Archaea

The ATPases belonging to the structural maintenance of chromosomes (SMC) superfamily are involved in the maintenance of chromosome organization and dynamics, as well as DNA repair. The major proteins in this superfamily recognized to date are either conserved among the three domains of Life (i.e., SMC and Rad50) or specific to Bacteria (i.e., RecF, RecN, and MukB). In Archaea, no protein related to SMC (SMC-related protein) with a broad taxonomic distribution has been reported. Nevertheless, two SMC-related proteins, namely coalescin and Sph, have been identified in crenarchaea Sulfolobus spp. and the euryarchaeon Halobacterium salinarum, respectively, hinting that the diversity of SMC-related proteins has been overlooked in Archaea. In this study, we report a novel SMC-related protein that is distributed among broad archaeal lineages and termed “Archaea-specific SMC-related proteins” or “ASRPs.” We further demonstrate that the ASRP family encloses both coalescin and Sph but the two proteins represent only a tip of the diversity of this family.