A SUMO-Dependent Protein Network Regulates Chromosome Congression during Oocyte Meiosis.

During Caenorhabditis elegans oocyte meiosis, a multi-protein ring complex (RC) localized between homologous chromosomes, promotes chromosome congression through the action of the chromokinesin KLP-19. While some RC components are known, the mechanism of RC assembly has remained obscure. We show that SUMO E3 ligase GEI-17/PIAS is required for KLP-19 recruitment to the RC, and proteomic analysis identified KLP-19 as a SUMO substrate in vivo. In vitro analysis revealed that KLP-19 is efficiently sumoylated in a GEI-17-dependent manner, while GEI-17 undergoes extensive auto-sumoylation. GEI-17 and another RC component, the kinase BUB-1, contain functional SUMO interaction motifs (SIMs), allowing them to recruit SUMO modified proteins, including KLP-19, into the RC. Thus, dynamic SUMO modification and the presence of SIMs in RC components generate a SUMO-SIM network that facilitates assembly of the RC. Our results highlight the importance of SUMO-SIM networks in regulating the assembly of dynamic protein complexes.

Controlling and co-ordinating chitinase secretion in a Serratia marcescens population.

Serratia marcescens is a γ-Proteobacterium and an opportunistic animal and insect pathogen. The bacterium exhibits a complex extracellular protein 'secretome' comprising numerous enzymes, toxins and effector molecules. One component of the secretome is the 'chitinolytic machinery', which is a set of at least four chitinases that allow the use of insoluble extracellular chitin as sole carbon source. Secretion of the chitinases across the outer membrane is governed by the chiWXYZ operon encoding a holin/endopeptidase pair. Expression of the chiWXYZ operon is co-ordinated with the chitinase genes and is also bimodal, as normally only 1% of the population expresses the chitinolytic machinery. In this study, the role of the ChiR protein in chitinase production has been explored. Using live cell imaging and flow cytometry, ChiR was shown to govern the co-ordinated regulation of chiWXYZ with both chiA and chiC. Moreover, overexpression of chiR alone was able to increase the proportion of the cell population expressing chitinase genes to >60 %. In addition, quantitative label-free proteomic analysis of cells overexpressing chiR established that ChiR regulates the entire chitinolytic machinery. The proteomic experiments also revealed a surprising link between the regulation of the chitinolytic machinery and the production of proteins involved in the metabolism of nitrogen compounds such as nitrate and nitrite. The research demonstrates for the first time that ChiR plays a critical role in controlling bimodal gene expression in S. marcescens, and provides new evidence of a clear link between chitin breakdown and nitrogen metabolism.

Gene expression profiling of placentae from women with obesity and obstructive sleep apnoea.

INTRODUCTION: Obstructive sleep apnoea (OSA), a condition characterised by intermittent hypoxia and reoxygenation during sleep, is associated with an increased risk of adverse pregnancy outcomes including gestational diabetes and hypertensive disorders of pregnancy. The biological mechanisms of these associations are poorly understood. The impact of OSA on placental function has not been well characterised. METHODS: We performed 3' mRNA sequencing on placenta from women with obesity and OSA (n = 11) and women with obesity and no OSA (n = 9). RESULTS: After correcting for multiple testing, there were no statistically significant differences in gene expression between OSA and no OSA groups (adjusted p < 0.05). In unadjusted analyses, 101 genes were differentially expressed in OSA compared to no OSA placentae (p < 0.01). In Reactome pathway and GO term analysis, this included downregulation of genes involved in O-linked glycosylation (B3GNT5 and B3GNT8) and Wnt signalling (TRABD2B and FRZB) pathways. In gene set enrichment analysis, genes within 24 pathways had a non-random distribution in OSA compared to no OSA placentae (adjusted p < 0.05). This included an increase in genes relating to the reversible hydration of carbon dioxide in OSA placentae, a potential novel mechanism contributing to the development of adverse pregnancy outcomes in women with OSA. DISCUSSION: There is overall similarity in the placental transcriptome of women with obesity who do and do not have OSA during pregnancy. Alterations in the reversible hydration of carbon dioxide are a potential mechanism contributing to the development of adverse pregnancy outcomes in maternal OSA, however this finding requires validation in larger cohorts.

Bacterial ribosome heterogeneity: Changes in ribosomal protein composition during transition into stationary growth phase.

Ribosomes consist of many small proteins and few large RNA molecules. Both components are necessary for ribosome functioning during translation. According to widely accepted view, bacterial ribosomes contain always the same complement of ribosomal proteins. Comparative bacterial genomics data indicates that several ribosomal proteins are encoded by multiple paralogous genes suggesting structural heterogeneity of ribosomes. In E. coli, two r-proteins bL31 and bL36 are encoded by two genes: rpmE and ykgM encode bL31 protein paralogs bL31A and bL31B, and rpmJ and ykgO encode bL36 protein paralogs bL36A and bL36B respectively. We have found several similarities and differences between ribosomes of exponential and stationary growth phases by using quantitative mass spectrometry and X-ray crystallography. First, composition of ribosome associating proteins changes profoundly as cells transition from exponential to stationary growth phase. Ribosomal core proteins bL31A and bL36A are replaced by bL31B and bL36B, respectively. Second, our X-ray structure of the 70S ribosome demonstrates that bL31B and bL36B proteins have similar ribosome binding sites to their A counterparts. Third, ribosome subpopulations containing A or B paralogs existed simultaneously demonstrating that E. coli ribosomes are heterogeneous with respect to their paralogous ribosomal protein composition that changes via protein exchange.

Characterisation of the biflavonoid hinokiflavone as a pre-mRNA splicing modulator that inhibits SENP.

We have identified the plant biflavonoid hinokiflavone as an inhibitor of splicing in vitro and modulator of alternative splicing in cells. Chemical synthesis confirms hinokiflavone is the active molecule. Hinokiflavone inhibits splicing in vitro by blocking spliceosome assembly, preventing formation of the B complex. Cells treated with hinokiflavone show altered subnuclear organization specifically of splicing factors required for A complex formation, which relocalize together with SUMO1 and SUMO2 into enlarged nuclear speckles containing polyadenylated RNA. Hinokiflavone increases protein SUMOylation levels, both in in vitro splicing reactions and in cells. Hinokiflavone also inhibited a purified, E. coli expressed SUMO protease, SENP1, in vitro, indicating the increase in SUMOylated proteins results primarily from inhibition of de-SUMOylation. Using a quantitative proteomics assay we identified many SUMO2 sites whose levels increased in cells following hinokiflavone treatment, with the major targets including six proteins that are components of the U2 snRNP and required for A complex formation.

Targeting of SUMO substrates to a Cdc48-Ufd1-Npl4 segregase and STUbL pathway in fission yeast.

In eukaryotes, the conjugation of proteins to the small ubiquitin-like modifier (SUMO) regulates numerous cellular functions. A proportion of SUMO conjugates are targeted for degradation by SUMO-targeted ubiquitin ligases (STUbLs) and it has been proposed that the ubiquitin-selective chaperone Cdc48/p97-Ufd1-Npl4 facilitates this process. However, the extent to which the two pathways overlap, and how substrates are selected, remains unknown. Here we address these questions in fission yeast through proteome-wide analyses of SUMO modification sites. We identify over a thousand sumoylated lysines in a total of 468 proteins and quantify changes occurring in the SUMO modification status when the STUbL or Ufd1 pathways are compromised by mutations. The data suggest the coordinated processing of several classes of SUMO conjugates, many dynamically associated with centromeres or telomeres. They provide new insights into subnuclear organization and chromosome biology, and, altogether, constitute an extensive resource for the molecular characterization of SUMO function and dynamics.

Proteome-wide identification of SUMO modification sites by mass spectrometry.

The protein called 'small ubiquitin-like modifier' (SUMO) is post-translationally linked to target proteins at the ɛ-amino group of lysine residues. This 'SUMOylation' alters the behavior of the target protein, a change that is utilized to regulate diverse cellular processes. Understanding the target-specific consequences of SUMO modification requires knowledge of the location of conjugation sites, and we have developed a straightforward protocol for the proteome-wide identification of SUMO modification sites using mass spectrometry (MS). The approach described herein requires the expression of a mutant form of SUMO, in which the residue preceding the C-terminal Gly-Gly (diGly) is replaced with a lysine (SUMO(KGG)). Digestion of SUMO(KGG) protein conjugates with endoproteinase Lys-C yields a diGly motif attached to target lysines. Peptides containing this adduct are enriched using a diGly-Lys (K-ɛ-GG)-specific antibody and identified by MS. This diGly signature is characteristic of SUMO(KGG) conjugation alone, as no other ubiquitin-like protein (Ubl) yields this adduct upon Lys-C digestion. We have demonstrated the utility of the approach in SUMOylation studies, but, in principle, it may be adapted for the site-specific identification of proteins modified by any Ubl. Starting from cell lysis, this protocol can be completed in ∼5 d.

Stepwise splitting of ribosomal proteins from yeast ribosomes by LiCl.

Structural studies have revealed that the core of the ribosome structure is conserved among ribosomes of all kingdoms. Kingdom-specific ribosomal proteins (r-proteins) are located in peripheral parts of the ribosome. In this work, the interactions between rRNA and r-proteins of eukaryote Saccharomyces cerevisiae ribosome were investigated applying LiCl induced splitting and quantitative mass spectrometry. R-proteins were divided into four groups according to their binding properties to the rRNA. Most yeast r-proteins are removed from rRNA by 0.5-1 M LiCl. Eukaryote-specific r-proteins are among the first to dissociate. The majority of the strong binders are known to be required for the early ribosome assembly events. As compared to the bacterial ribosome, yeast r-proteins are dissociated from rRNA at lower ionic strength. Our results demonstrate that the nature of protein-RNA interactions in the ribosome is not conserved between different kingdoms.

Proteome-wide identification of SUMO2 modification sites.

Posttranslational modification with small ubiquitin-like modifiers (SUMOs) alters the function of proteins involved in diverse cellular processes. SUMO-specific enzymes conjugate SUMOs to lysine residues in target proteins. Although proteomic studies have identified hundreds of sumoylated substrates, methods to identify the modified lysines on a proteomic scale are lacking. We developed a method that enabled proteome-wide identification of sumoylated lysines that involves the expression of polyhistidine (6His)-tagged SUMO2 with Thr(90) mutated to Lys. Endoproteinase cleavage with Lys-C of 6His-SUMO2(T90K)-modified proteins from human cell lysates produced a diGly remnant on SUMO2(T90K)-conjugated lysines, enabling immunoprecipitation of SUMO2(T90K)-modified peptides and producing a unique mass-to-charge signature. Mass spectrometry analysis of SUMO-enriched peptides revealed more than 1000 sumoylated lysines in 539 proteins, including many functionally related proteins involved in cell cycle, transcription, and DNA repair. Not only can this strategy be used to study the dynamics of sumoylation and other potentially similar posttranslational modifications, but also, these data provide an unprecedented resource for future research on the role of sumoylation in cellular physiology and disease.