For more papers, visit a faculty member's page from the listing on Whitehead Faculty and access the PubMed link.
A mitotic SKAP isoform regulates spindle positioning at astral microtubule plus ends.
J Cell Biol. 2016 May 9;213(3):315-28
Kern, D.M.*, Nicholls, P.K.*, Page, D.C.*, and Cheeseman, I.M.*
The Astrin/SKAP complex plays important roles in mitotic chromosome alignment and centrosome integrity, but previous work found conflicting results for SKAP function. Here, we demonstrate that SKAP is expressed as two distinct isoforms in mammals: a longer, testis-specific isoform that was used for the previous studies in mitotic cells and a novel, shorter mitotic isoform. Unlike the long isoform, short SKAP rescues SKAP depletion in mitosis and displays robust microtubule plus-end tracking, including localization to astral microtubules. Eliminating SKAP microtubule binding results in severe chromosome segregation defects. In contrast, SKAP mutants specifically defective for plus-end tracking facilitate proper chromosome segregation but display spindle positioning defects. Cells lacking SKAP plus-end tracking have reduced Clasp1 localization at microtubule plus ends and display increased lateral microtubule contacts with the cell cortex, which we propose results in unbalanced dynein-dependent cortical pulling forces. Our work reveals an unappreciated role for the Astrin/SKAP complex as an astral microtubule mediator of mitotic spindle positioning.
Ubiquitin-mediated fluctuations in MHC class II facilitate efficient germinal center B cell responses.
J Exp Med. 2016 May 9.
Bannard, O., McGowan, S.J., Ersching, J.*, Ishido, S., Victora, G.D.*, Shin, J.S., and Cyster, J.G.
Antibody affinity maturation occurs in germinal centers (GCs) through iterative rounds of somatic hypermutation and selection. Selection involves B cells competing for T cell help based on the amount of antigen they capture and present on their MHC class II (MHCII) proteins. How GC B cells are able to rapidly and repeatedly transition between mutating their B cell receptor genes and then being selected shortly after is not known. We report that MHCII surface levels and degradation are dynamically regulated in GC B cells. Through ectopic expression of a photoconvertible MHCII-mKikGR chimeric gene, we found that individual GC B cells differed in the rates of MHCII protein turnover. Fluctuations in surface MHCII levels were dependent on ubiquitination and the E3 ligase March1. Increases in March1 expression in centroblasts correlated with decreases in surface MHCII levels, whereas CD83 expression in centrocytes helped to stabilize MHCII at that stage. Defects in MHCII ubiquitination caused GC B cells to accumulate greater amounts of a specific peptide-MHCII (pMHCII), suggesting that MHCII turnover facilitates the replacement of old complexes. We propose that pMHCII complexes are periodically targeted for degradation in centroblasts to favor the presentation of recently acquired antigens, thereby promoting the fidelity and efficiency of selection.
Comparative transcriptomics across the prokaryotic tree of life.
Nucleic Acids Res. 2016 May 6.
Cohen, O., Doron, S., Wurtzel, O.*, Dar, D., Edelheit, S., Karunker, I., Mick, E., and Sorek, R.
Whole-transcriptome sequencing studies from recent years revealed an unexpected complexity in transcriptomes of bacteria and archaea, including abundant non-coding RNAs, cis-antisense transcription and regulatory untranslated regions (UTRs). Understanding the functional relevance of the plethora of non-coding RNAs in a given organism is challenging, especially since some of these RNAs were attributed to 'transcriptional noise'. To allow the search for conserved transcriptomic elements we produced comparative transcriptome maps for multiple species across the microbial tree of life. These transcriptome maps are detailed in annotations, comparable by gene families, and BLAST-searchable by user provided sequences. Our transcriptome collection includes 18 model organisms spanning 10 phyla/subphyla of bacteria and archaea that were sequenced using standardized RNA-seq methods. The utility of the comparative approach, as implemented in our web server, is demonstrated by highlighting genes with exceptionally long 5'UTRs across species, which correspond to many known riboswitches and further suggest novel putative regulatory elements. Our study provides a standardized reference transcriptome to major clinically and environmentally important microbial phyla. The viewer is available at http://exploration.weizmann.ac.il/TCOL, setting a framework for comparative studies of the microbial non-coding genome.
Population-scale sequencing data enable precise estimates of Y-STR mutation rates.
Am J Hum Genet. 2016 May 5;98(5):919-33.
Willems, T.*, Gymrek, M.*, Poznik, G.D., Tyler-Smith, C., and Erlich, Y.*
Short tandem repeats (STRs) are mutation-prone loci that span nearly 1% of the human genome. Previous studies have estimated the mutation rates of highly polymorphic STRs by using capillary electrophoresis and pedigree-based designs. Although this work has provided insights into the mutational dynamics of highly mutable STRs, the mutation rates of most others remain unknown. Here, we harnessed whole-genome sequencing data to estimate the mutation rates of Y chromosome STRs (Y-STRs) with 2-6 bp repeat units that are accessible to Illumina sequencing. We genotyped 4,500 Y-STRs by using data from the 1000 Genomes Project and the Simons Genome Diversity Project. Next, we developed MUTEA, an algorithm that infers STR mutation rates from population-scale data by using a high-resolution SNP-based phylogeny. After extensive intrinsic and extrinsic validations, we harnessed MUTEA to derive mutation-rate estimates for 702 polymorphic STRs by tracing each locus over 222,000 meioses, resulting in the largest collection of Y-STR mutation rates to date. Using our estimates, we identified determinants of STR mutation rates and built a model to predict rates for STRs across the genome. These predictions indicate that the load of de novo STR mutations is at least 75 mutations per generation, rivaling the load of all other known variant types. Finally, we identified Y-STRs with potential applications in forensics and genetic genealogy, assessed the ability to differentiate between the Y chromosomes of father-son pairs, and imputed Y-STR genotypes.
Induced pluripotent stem cells meet genome editing.
Cell Stem Cell. 2016 May 5;18(5):573-86.
Hockemeyer, D., and Jaenisch, R.*
It is extremely rare for a single experiment to be so impactful and timely that it shapes and forecasts the experiments of the next decade. Here, we review how two such experiments-the generation of human induced pluripotent stem cells (iPSCs) and the development of CRISPR/Cas9 technology-have fundamentally reshaped our approach to biomedical research, stem cell biology, and human genetics. We will also highlight the previous knowledge that iPSC and CRISPR/Cas9 technologies were built on as this groundwork demonstrated the need for solutions and the benefits that these technologies provided and set the stage for their success.
The Caenorhabditis elegans protein FIC-1 is an AMPylase that covalently modifies heat-shock 70 family proteins, translation elongation factors and histones.
PLoS Genet. 2016 May 3;12(5):e1006023.
Truttmann, M.C.*, Cruz, V.E., Guo, X., Engert, C., Schwartz, T.U., and Ploegh, H.L.*
Protein AMPylation by Fic domain-containing proteins (Fic proteins) is an ancient and conserved post-translational modification of mostly unexplored significance. Here we characterize the Caenorhabditis elegans Fic protein FIC-1 in vitro and in vivo. FIC-1 is an AMPylase that localizes to the nuclear surface and modifies core histones H2 and H3 as well as heat shock protein 70 family members and translation elongation factors. The three-dimensional structure of FIC-1 is similar to that of its human ortholog, HYPE, with 38% sequence identity. We identify a link between FIC-1-mediated AMPylation and susceptibility to the pathogen Pseudomonas aeruginosa, establishing a connection between AMPylation and innate immunity in C. elegans.
Mutations in the substrate binding glycine-rich loop of the mitochondrial processing peptidase-alpha protein (PMPCA) cause a severe mitochondrial disease.
Cold Spring Harb Mol Case Stud. 2016 May;2(3):a000786.
Joshi, M., Anselm, I., Shi, J., Bale, T.A., Towne, M., Schmitz-Abe, K., Crowley, L., Giani, F.C., Kazerounian, S., Markianos, K., Lidov, H.G., Folkerth, R., Sankaran, V.G.*, Agrawal, P.B.
We describe a large Lebanese family with two affected members, a young female proband and her male cousin, who had multisystem involvement including profound global developmental delay, severe hypotonia and weakness, respiratory insufficiency, blindness, and lactic acidemia-findings consistent with an underlying mitochondrial disorder. Whole-exome sequencing was performed on DNA from the proband and both parents. The proband and her cousin carried compound heterozygous mutations in the PMPCA gene that encodes for alpha-mitochondrial processing peptidase (alpha-MPP), a protein likely involved in the processing of mitochondrial proteins. The variants were located close to and postulated to affect the substrate binding glycine-rich loop of the alpha-MPP protein. Functional assays including immunofluorescence and western blot analysis on patient's fibroblasts revealed that these variants reduced alpha-MPP levels and impaired frataxin production and processing. We further determined that those defects could be rescued through the expression of exogenous wild-type PMPCA cDNA. Our findings link defective alpha-MPP protein to a severe mitochondrial disease.
Luminidependens (LD) is an Arabidopsis protein with prion behavior.
Proc Natl Acad Sci U S A. 2016 Apr 25.
Chakrabortee, S.*, Kayatekin, C.*, Newby, G.A.*, Mendillo, M.L.*, Lancaster, A.*, and Lindquist, S.*
Prion proteins provide a unique mode of biochemical memory through self-perpetuating changes in protein conformation and function. They have been studied in fungi and mammals, but not yet identified in plants. Using a computational model, we identified candidate prion domains (PrDs) in nearly 500 plant proteins. Plant flowering is of particular interest with respect to biological memory, because its regulation involves remembering and integrating previously experienced environmental conditions. We investigated the prion-forming capacity of three prion candidates involved in flowering using a yeast model, where prion attributes are well defined and readily tested. In yeast, prions heritably change protein functions by templating monomers into higher-order assemblies. For most yeast prions, the capacity to convert into a prion resides in a distinct prion domain. Thus, new prion-forming domains can be identified by functional complementation of a known prion domain. The prion-like domains (PrDs) of all three of the tested proteins formed higher-order oligomers. Uniquely, the Luminidependens PrD (LDPrD) fully replaced the prion-domain functions of a well-characterized yeast prion, Sup35. Our results suggest that prion-like conformational switches are evolutionarily conserved and might function in a wide variety of normal biological processes.
A specialized flavone biosynthetic pathway has evolved in the medicinal plant, Scutellaria baicalensis.
Sci Adv. 2016 Apr 8;2(4):e1501780.
Zhao, Q., Zhang, Y., Wang, G., Hill, L., Weng, J.K*., Chen, X.Y., Xue, H., and Martin, C.
Wogonin and baicalein are bioactive flavones in the popular Chinese herbal remedy Huang-Qin (Scutellaria baicalensis Georgi). These specialized flavones lack a 4'-hydroxyl group on the B ring (4'-deoxyflavones) and induce apoptosis in a wide spectrum of human tumor cells in vitro and inhibit tumor growth in vivo in different mouse tumor models. Root-specific flavones (RSFs) from Scutellaria have a variety of reported additional beneficial effects including antioxidant and antiviral properties. We describe the characterization of a new pathway for the synthesis of these compounds, in which pinocembrin (a 4'-deoxyflavanone) serves as a key intermediate. Although two genes encoding flavone synthase II (FNSII) are expressed in the roots of S. baicalensis, FNSII-1 has broad specificity for flavanones as substrates, whereas FNSII-2 is specific for pinocembrin. FNSII-2 is responsible for the synthesis of 4'-deoxyRSFs, such as chrysin and wogonin, wogonoside, baicalein, and baicalin, which are synthesized from chrysin. A gene encoding a cinnamic acid-specific coenzyme A ligase (SbCLL-7), which is highly expressed in roots, is required for the synthesis of RSFs by FNSII-2, as demonstrated by gene silencing. A specific isoform of chalcone synthase (SbCHS-2) that is highly expressed in roots producing RSFs is also required for the synthesis of chrysin. Our studies reveal a recently evolved pathway for biosynthesis of specific, bioactive 4'-deoxyflavones in the roots of S. baicalensis.
Evolutionary inference across eukaryotes identifies specific pressures favoring mitochondrial gene retention.
Cell Syst. 2016 Feb 24;2(2):101-11.
Johnston, I.G., and Williams, B.P.*
Since their endosymbiotic origin, mitochondria have lost most of their genes. Although many selective mechanisms underlying the evolution of mitochondrial genomes have been proposed, a data-driven exploration of these hypotheses is lacking, and a quantitatively supported consensus remains absent. We developed HyperTraPS, a methodology coupling stochastic modeling with Bayesian inference, to identify the ordering of evolutionary events and suggest their causes. Using 2015 complete mitochondrial genomes, we inferred evolutionary trajectories of mtDNA gene loss across the eukaryotic tree of life. We find that proteins comprising the structural cores of the electron transport chain are preferentially encoded within mitochondrial genomes across eukaryotes. A combination of high GC content and high protein hydrophobicity is required to explain patterns of mtDNA gene retention; a model that accounts for these selective pressures can also predict the success of artificial gene transfer experiments in vivo. This work provides a general method for data-driven inference of the ordering of evolutionary and progressive events, here identifying the distinct features shaping mitochondrial genomes of present-day species.
*Author affiliated with Whitehead Institute for Biomedical Research