An SMC-like protein binds and regulates Caenorhabditis elegans condensins

Structural Maintenance of Chromosomes (SMC) family proteins participate in multisubunit complexes that govern chromosome structure and dynamics. SMC-containing condensin complexes create chromosome topologies essential for mitosis/meiosis, gene expression, recombination, and repair. Many eukaryotes have two condensin complexes (I and II); C. elegans has three (I, II, and the X-chromosome specialized condensin IDC) and their regulation is poorly understood. Here we identify a novel SMC-like protein, SMCL-1, that binds to C. elegans condensin SMC subunits, and modulates condensin functions. Consistent with a possible role as a negative regulator, loss of SMCL-1 partially rescued the lethal and sterile phenotypes of a hypomorphic condensin mutant, while over-expression of SMCL-1 caused lethality, chromosome mis-segregation, and disruption of condensin IDC localization on X chromosomes. Unlike canonical SMC proteins, SMCL-1 lacks hinge and coil domains, and its ATPase domain lacks conserved amino acids required for ATP hydrolysis, leading to the speculation that it may inhibit condensin ATPase activity. SMCL-1 homologs are apparent only in the subset of Caenorhabditis species in which the condensin I and II subunit SMC-4 duplicated to create the condensin IDC- specific subunit DPY-27, suggesting that SMCL-1 helps this lineage cope with the regulatory challenges imposed by evolution of a third condensin complex. Our findings uncover a new regulator of condensins and highlight how the duplication and divergence of SMC complex components in various lineages has created new proteins with diverse functions in chromosome dynamics

Chromosome-Biased Binding and Gene Regulation by the Caenorhabditis elegans DRM Complex

DRM is a conserved transcription factor complex that includes E2F/DP and pRB family proteins and plays important roles in development and cancer. Here we describe new aspects of DRM binding and function revealed through genome-wide analyses of the Caenorhabditis elegans DRM subunit LIN-54. We show that LIN-54 DNA-binding activity recruits DRM to promoters enriched for adjacent putative E2F/DP and LIN-54 binding sites, suggesting that these two DNA–binding moieties together direct DRM to its target genes. Chromatin immunoprecipitation and gene expression profiling reveals conserved roles for DRM in regulating genes involved in cell division, development, and reproduction. We find that LIN-54 promotes expression of reproduction genes in the germline, but prevents ectopic activation of germline-specific genes in embryonic soma. Strikingly, C. elegans DRM does not act uniformly throughout the genome: the DRM recruitment motif, DRM binding, and DRM-regulated embryonic genes are all under-represented on the X chromosome. However, germline genes down-regulated in lin-54 mutants are over-represented on the X chromosome. We discuss models for how loss of autosome-bound DRM may enhance germline X chromosome silencing. We propose that autosome-enriched binding of DRM arose in C. elegans as a consequence of germline X chromosome silencing and the evolutionary redistribution of germline-expressed and essential target genes to autosomes. Sex chromosome gene regulation may thus have profound evolutionary effects on genome organization and transcriptional regulatory networks.National Institutes of Health (U.S.) (grant GM24663)National Institutes of Health (U.S.) (grant DK068429)National Institutes of Health (U.S.) (grant GM082971)National Institutes of Health (U.S.) (grant GM076378

Opposing activities of DRM and MES-4 tune gene expression and X-chromosome repression in Caenorhabditis elegans germ cells.

During animal development, gene transcription is tuned to tissue-appropriate levels. Here we uncover antagonistic regulation of transcript levels in the germline of Caenorhabditis elegans hermaphrodites. The histone methyltransferase MES-4 (Maternal Effect Sterile-4) marks genes expressed in the germline with methylated lysine on histone H3 (H3K36me) and promotes their transcription; MES-4 also represses genes normally expressed in somatic cells and genes on the X chromosome. The DRM transcription factor complex, named for its Dp/E2F, Retinoblastoma-like, and MuvB subunits, affects germline gene expression and prevents excessive repression of X-chromosome genes. Using genome-scale analyses of germline tissue, we show that common germline-expressed genes are activated by MES-4 and repressed by DRM, and that MES-4 and DRM co-bind many germline-expressed genes. Reciprocally, MES-4 represses and DRM activates a set of autosomal soma-expressed genes and overall X-chromosome gene expression. Mutations in mes-4 and the DRM subunit lin-54 oppositely skew the transcript levels of their common targets and cause sterility. A double mutant restores target gene transcript levels closer to wild type, and the concomitant loss of lin-54 suppresses the severe germline proliferation defect observed in mes-4 single mutants. Together, "yin-yang" regulation by MES-4 and DRM ensures transcript levels appropriate for germ-cell function, elicits robust but not excessive dampening of X-chromosome-wide transcription, and may poise genes for future expression changes. Our study reveals that conserved transcriptional regulators implicated in development and cancer counteract each other to fine-tune transcript dosage

Opposing activities of DRM and MES-4 tune gene expression and X-chromosome repression in Caenorhabditis elegans germ cells.

During animal development, gene transcription is tuned to tissue-appropriate levels. Here we uncover antagonistic regulation of transcript levels in the germline of Caenorhabditis elegans hermaphrodites. The histone methyltransferase MES-4 (Maternal Effect Sterile-4) marks genes expressed in the germline with methylated lysine on histone H3 (H3K36me) and promotes their transcription; MES-4 also represses genes normally expressed in somatic cells and genes on the X chromosome. The DRM transcription factor complex, named for its Dp/E2F, Retinoblastoma-like, and MuvB subunits, affects germline gene expression and prevents excessive repression of X-chromosome genes. Using genome-scale analyses of germline tissue, we show that common germline-expressed genes are activated by MES-4 and repressed by DRM, and that MES-4 and DRM co-bind many germline-expressed genes. Reciprocally, MES-4 represses and DRM activates a set of autosomal soma-expressed genes and overall X-chromosome gene expression. Mutations in mes-4 and the DRM subunit lin-54 oppositely skew the transcript levels of their common targets and cause sterility. A double mutant restores target gene transcript levels closer to wild type, and the concomitant loss of lin-54 suppresses the severe germline proliferation defect observed in mes-4 single mutants. Together, "yin-yang" regulation by MES-4 and DRM ensures transcript levels appropriate for germ-cell function, elicits robust but not excessive dampening of X-chromosome-wide transcription, and may poise genes for future expression changes. Our study reveals that conserved transcriptional regulators implicated in development and cancer counteract each other to fine-tune transcript dosage

C. elegans condensin promotes mitotic chromosome architecture, centromere organization, and sister chromatid segregation during mitosis and meiosis

Chromosome segregation and X-chromosome gene regulation in Caenorhabditis elegans share the component MIX-1, a mitotic protein that also represses X-linked genes during dosage compensation. MIX-1 achieves its dual roles through interactions with different protein partners. To repress gene expression, MIX-1 acts in an X-chromosome complex that resembles the mitotic condensin complex yet lacks chromosome segregation function. Here we show that MIX-1 interacts with a mitotic condensin subunit, SMC-4, to achieve chromosome segregation. The SMC-4/MIX-1 complex positively supercoils DNA in vitro and is required for mitotic chromosome structure and segregation in vivo. Thus, C. elegans has two condensin complexes, one conserved for mitosis and another specialized for gene regulation. SMC-4 and MIX-1 colocalize with centromere proteins on condensed mitotic chromosomes and are required for the restricted orientation of centromeres toward spindle poles. This cell cycle-dependent localization requires AIR-2/AuroraB kinase. Depletion of SMC-4/MIX-1 causes aberrant mitotic chromosome structure and segregation, but not dramatic decondensation at metaphase. Moreover, SMC-4/MIX-1 depletion disrupts sister chromatid segregation during meiosis II but not homologous chromosome segregation during meiosis I, although both processes require chromosome condensation. These results imply that condensin is not simply required for compaction, but plays a more complex role in chromosome architecture that is essential for mitotic and meiotic sister chromatid segregation

SMCL-1 expression and protein features.

<p>(A) Adult hermaphrodites from wild-type (WT) and a strain carrying the <i>map</i>::<i>smcl-1</i> transgene driven by endogenous <i>smcl-1</i> 5’ and 3’ elements. A section of the germline is shown, imaged by DIC to show structures and fluorescent microscopy to detect mVenus expression from the MAP tag. Arrowheads denote the first four oocytes. (B) A typical SMC protein folds back on itself at a hinge domain, bringing coil regions together and creating a “head domain” (yellow) from ATPase domains in the N- and C-termini. SMCL-1 lacks predicted coil and hinge domains, but has N- and C-terminal ATPase domains that may be capable of forming a head domain (purple). (C) SMC head domain and the ATPase cycle, showing binding of ATP (red circle), ATP-dependent engagement of heads from two SMC proteins, and disengagement upon ATP hydrolysis. (D) SMCL-1 amino acid sequence aligned to <i>C</i>. <i>elegans</i> condensin SMC proteins. Shown are regions surrounding three conserved motifs found in SMCs and related ATPases: the Walker A motif, ABC transporter signature motif, and Walker B motifs, and their consensus sequences. SMCL-1 shares a conserved Walker A motif, but differs from consensus signature motif and Walker B motif at residues shown in red. Asterisk denotes catalytic amino acid required for ATP hydrolysis. x = any amino acid and h = hydrophobic amino acid.</p

Condensin subunits co-purify with MAP::SMCL-1.

<p>(A-B) Proteins that co-purified with MAP::SMCL-1 but not untagged control adult extracts, identified by tandem affinity purification and MudPIT mass spectrometry. Numbers represent average NSAF values from two replicas. Co-purified proteins with the highest NSAF values are shown in (A), values for other condensin subunits are shown in (B), and all other proteins are shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006614#pgen.1006614.s011" target="_blank">S5 Table</a>. Condensin SMC subunits are highlighted.</p

Presence of predicted orthologs of SMCL-1, DPY-27 (I^DC), and SMC-4 (I & II) in various species.

<p>Phylogenetic tree built from all available <i>Caenorhabditis</i> species with sequenced and well-assembled genomes, other selected nematode species, and other selected model organisms. “<i>+”</i> symbol denotes the presence of SMCL-1-like protein based on similarity in a BLAST search and the additional criteria of short length, imperfect signature motif, and a Walker B motif lacking the catalytic glutamate (see <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006614#sec014" target="_blank">Methods</a> and <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006614#pgen.1006614.s003" target="_blank">S3 Fig</a>). “1” denotes orthologs detected using a high-confidence Ensemble-COMPARA method and “2” denotes orthologs detected using BLAST-neighbor-joining tree methods.</p

SMCL-1 overexpression in the gut disrupts condensin I^DC localization on the X chromosomes.

<p>(A) Heat shock regimen for data in (B-E). Bolt represents the single heat-shock pulse given to young adult hermaphrodites from the wild-type or inducible <i>hs</i>:<i>smcl-1(+)</i> transgenic strain. (B-E) Adult hermaphrodite gut tissue of the indicated strain and treatment was stained with DAPI to image DNA (green in merge) and immuno-stained with antibody against CAPG-1(B-C), DPY-27 (D) and DPY-28 (E), (red in merges). Antibody against SMCL-1 was also included in (B and C), showing overexpression upon heat shock (blue in merge). The foci of staining created by condensin I^DC subunit association with the X chromosomes are lost when SMCL-1 is overexpressed. (C) A mosaic animal in which anti-SMCL-1 staining indicates that one cell lacks the transgene (left) and shows foci of anti-CAPG-1, while a neighboring cells has the <i>smcl-1</i> overexpression transgene (right) and CAPG-1 staining is weak and not localized to foci (also see <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006614#pgen.1006614.s006" target="_blank">S6C and S6D Fig</a>). HS = heat shock.</p