Rickettsia Sca2 has evolved formin-like activity through a different molecular mechanism
Yadaiah Madasu, Cristian Suarez, David J. Kast, David R. Kovar, and Roberto Dominguez
Rickettsia Sca2 mimics eukaryotic formins by promoting actin filament nucleation and elongation to assemble actin comet tails for Rickettsia motility. We show (pp. E2677–E2686) that unlike formins, Sca2 is monomeric, but has N- and C-terminal repeat domains (NRD and CRD) that interact with each other. The structure of NRD reveals a new crescent-like fold. CRD is predicted to share this fold, and might form together with NRD a doughnut-shaped formin-like structure for processive elongation. Between NRD and CRD, proline-rich sequences incorporate profilin-actin for elongation, and WASP-homology 2 (WH2) domains recruit actin monomers for nucleation. Rickettsia has therefore “rediscovered” formin-like actin nucleation and elongation.
Far upstream element-binding protein 1 and RNA secondary structure both mediate second-step splicing repression
Huang Li, Zhijia Wang, Xuexia Zhou, Yuanming Cheng, Zhiqin Xie, James L. Manley, and Ying Feng
Splicing of mRNA precursors occurs in two sequential transesterification steps. We characterize a highly unusual inhibition of splicing in which the reaction is blocked between the two steps. We demonstrate that RNA secondary structure and an exonic splicing silencer element (ESS) can independently mediate second-step splicing repression in vitro and cause exon exclusion in vivo. Importantly, we provide evidence (pp. E2687–E2695) that far upstream element-binding protein 1, a single-stranded DNA- and RNA-binding protein initially identified as a regulator of MYC transcription and recently implicated in several cancers, binds the ESS and functions as a splicing regulator in vitro and in vivo.
Severing and end-to-end annealing of neurofilaments in neurons
Atsuko Uchida, Gülsen Çolakoğlu, Lina Wang, Paula C. Monsma, and Anthony Brown
Neurofilaments, which are the intermediate filaments of neurons, are major components of nerve cells, but their assembly dynamics have not been determined. Here (pp. E2696–E2705) we demonstrate efficient end-to-end annealing of neurofilaments in nerve cells, and we show that there also is a mechanism that severs these polymers. Our efforts thus identify severing of neuronal intermediate filaments in vivo and suggest a mechanism for regulating intermediate filament length involving a dynamic cycle of severing and end-to-end annealing.
Controlled insertional mutagenesis using a LINE-1 (ORFeus) gene-trap mouse model
Kathryn A. O’Donnell, Wenfeng An, Christina T. Schrum, Sarah J. Wheelan, and Jef D. Boeke
Transposons are powerful tools widely used for insertional mutagenesis screens because of the straightforward identification of transposon-induced mutations. We developed a conditionally regulated long interspersed element 1 [tetracycline (tet)-ORFeus] retrotransposon mouse model that may be used for random mutagenesis and mechanistic studies. High-level expression of the retrotransposon during development results in embryonic lethality due to a significant burden of insertions. At lower levels, transgene induction results in a somatic spotting phenotype due to mutations that likely disrupt melanocyte development, suggesting high activity of tet-ORFeus in melanocyte precursors. This model (pp. E2706–E2713) may be used to identify genes and pathways involved in diverse biological processes.
Axon position within the corpus callosum determines contralateral cortical projection
Jing Zhou, Yunqing Wen, Liang She, Ya-nan Sui, Lu Liu, Linda J. Richards, and Mu-ming Poo
Two hemispheres of the neocortex are connected via a large axon bundle, the corpus callosum (CC). Axons from one side of the cortex project primarily to the equivalent cortical area on the contralateral side. How this homotopic axon projection is achieved during development remains unclear. Quantitative analysis of the cortical axons' positions within CC and their projection pattern after crossing the midline showed (pp. E2714–E2723) that axon position within CC is critical for homotopic projection. Further genetic perturbations of semaphorin/neuropilin-1 signaling disrupted the axon order in CC, resulting in an ectopic contralateral axon projection that could not be corrected by developmental refinement.
Voltage sensor interaction site for selective small molecule inhibitors of voltage-gated sodium channels
Ken McCormack, Sonia Santos, Mark L. Chapman, Douglas S. Krafte, Brian E. Marron, Christopher W. West, Michael J. Krambis, Brett M. Antonio, Shannon G. Zellmer, David Printzenhoff, Karen M. Padilla, Zhixin Lin, P. Kay Wagoner, Nigel A. Swain, Paul A. Stupple, Marcel de Groot, Richard P. Butt, and Neil A. Castle
Voltage-gated sodium (Nav) channels contribute to physiological and pathophysiological electrical signaling in nerve and muscle cells. Because Nav channel isoforms exhibit tissue-specific expression, subtype selective modulation of this channel family provides important drug development opportunities. However, most available Nav channel modulators are unable to distinguish between Nav channel subtypes, which limits their therapeutic utility because of cardiac or nervous system toxicity. This study (pp. E2724–E2732) describes a new class of subtype selective Nav channel inhibitors that interact with a region of the channel that controls voltage sensitivity. This interaction site may enable development of selective therapeutic interventions with reduced potential for toxicity.
A thioredoxin-like/β-propeller protein maintains the efficiency of light harvesting in Arabidopsis
Matthew D. Brooks, Emily J. Sylak-Glassman, Graham R. Fleming, and Krishna K. Niyogi
Plants need to regulate light harvesting to match incoming light energy to photosynthetic capacity. Excess excitation can generate reactive oxygen species and damage the photosynthetic machinery. To dissipate excess absorbed light energy, plants have evolved several quenching pathways, each of which acts on a different timescale. Here (pp. E2733–E2740) we identify a chloroplast protein involved in a previously uncharacterized, slowly reversible form of energy dissipation in the photosynthetic antenna. This mutant will be critical in characterizing this quenching pathway and will further our understanding of how plants are able to balance light harvesting and photoprotection to maximize photosynthetic efficiency.
Characterization and comparison of human nuclear and cytosolic editomes
Liang Chen
RNA editing can alter the information content of genome sequences and increase transcriptome diversity. Global analyses of RNA editing remain challenging, largely due to the analytical barriers to separating editing events from errors in sequencing and alignment. Our computational statistical framework addresses these challenges and will also benefit other single-nucleotide level analyses of RNA-Seq data. Our results provide guidance for detecting nucleotide variants in RNA-Seq. We characterized (pp. E2741–E2747) human nuclear and cytosolic editomes and revealed relationships between editing and nucleocytoplasmic compartmentalization, microRNA regulation, and coding capacity. We also discovered that A-to-G editing mediates RNA memory of ancestral DNA alleles.