Assembly of Acanthamoeba Myosin-II Minifilaments. Model of Anti-parallel Dimers Based on EM and X-ray Diffraction of 2D and 3D Crystals

The Drosophila melanogaster Rab GAP RN-tre cross-talks with the Rho1 signaling pathway to regulate nonmuscle myosin II localization and function

To identify novel regulators of nonmuscle myosin II (NMII) we performed an image-based RNA interference screen using stable Drosophila melanogaster S2 cells expressing the enhanced green fluorescent protein (EGFP)-tagged regulatory light chain (RLC) of NMII and mCherry-Actin. We identified the Rab-specific GTPase-activating protein (GAP) RN-tre as necessary for the assembly of NMII RLC into contractile actin networks. Depletion of RN-tre led to a punctate NMII phenotype, similar to what is observed following depletion of proteins in the Rho1 pathway. Depletion of RN-tre also led to a decrease in active Rho1 and a decrease in phosphomyosin-positive cells by immunostaining, while expression of constitutively active Rho or Rho-kinase (Rok) rescues the punctate phenotype. Functionally, RN-tre depletion led to an increase in actin retrograde flow rate and cellular contractility in S2 and S2R+ cells, respectively. Regulation of NMII by RN-tre is only partially dependent on its GAP activity as overexpression of constitutively active Rabs inactivated by RN-tre failed to alter NMII RLC localization, while a GAP-dead version of RN-tre partially restored phosphomyosin staining. Collectively, our results suggest that RN-tre plays an important regulatory role in NMII RLC distribution, phosphorylation, and function, likely through Rho1 signaling and putatively serving as a link between the secretion machinery and actomyosin contractility.

Non-muscle myosin II in disease: mechanisms and therapeutic opportunities

The actin motor protein non-muscle myosin II (NMII) acts as a master regulator of cell morphology, with a role in several essential cellular processes, including cell migration and post-synaptic dendritic spine plasticity in neurons. NMII also generates forces that alter biochemical signaling, by driving changes in interactions between actin-associated proteins that can ultimately regulate gene transcription. In addition to its roles in normal cellular physiology, NMII has recently emerged as a critical regulator of diverse, genetically complex diseases, including neuronal disorders, cancers and vascular disease. In the context of these disorders, NMII regulatory pathways can be directly mutated or indirectly altered by disease-causing mutations. NMII regulatory pathway genes are also increasingly found in disease-associated copy-number variants, particularly in neuronal disorders such as autism and schizophrenia. Furthermore, manipulation of NMII-mediated contractility regulates stem cell pluripotency and differentiation, thus highlighting the key role of NMII-based pharmaceuticals in the clinical success of stem cell therapies. In this Review, we discuss the emerging role of NMII activity and its regulation by kinases and microRNAs in the pathogenesis and prognosis of a diverse range of diseases, including neuronal disorders, cancer and vascular disease. We also address promising clinical applications and limitations of NMII-based inhibitors in the treatment of these diseases and the development of stem-cell-based therapies.

Structural basis for the assembly of the mitotic motor Kinesin-5 into bipolar tetramers

Chromosome segregation during mitosis depends upon Kinesin-5 motors, which display a conserved, bipolar homotetrameric organization consisting of two motor dimers at opposite ends of a central rod. Kinesin-5 motors crosslink adjacent microtubules to drive or constrain their sliding apart, but the structural basis of their organization is unknown. In this study, we report the atomic structure of the bipolar assembly (BASS) domain that directs four Kinesin-5 subunits to form a bipolar minifilament. BASS is a novel 26-nm four-helix bundle, consisting of two anti-parallel coiled-coils at its center, stabilized by alternating hydrophobic and ionic four-helical interfaces, which based on mutagenesis experiments, are critical for tetramerization. Strikingly, N-terminal BASS helices bend as they emerge from the central bundle, swapping partner helices, to form dimeric parallel coiled-coils at both ends, which are offset by 90°. We propose that BASS is a mechanically stable, plectonemically-coiled junction, transmitting forces between Kinesin-5 motor dimers during microtubule sliding. DOI: http://dx.doi.org/10.7554/eLife.02217.001.

Lipid monolayer and sparse matrix screening for growing two-dimensional crystals for electron crystallography: methods and examples

Electron microscopy provides an efficient method for rapidly assessing whether a solution of macromolecules is homogeneous and monodisperse. If the macromolecules can be induced to form two-dimensional crystals that are a single layer in thickness, then electron crystallography of frozen-hydrated crystals has the potential of achieving three-dimensional density maps at sub-nanometer or even atomic resolution. Here we describe the lipid monolayer and sparse matrix screening methods for growing two-dimensional crystals and present successful applications to soluble macromolecular complexes: carboxysome shell proteins and HIV CA, respectively. Since it is common to express recombinant proteins with poly-His tags for purification by metal affinity chromatography, the monolayer technique using bulk lipids doped with Ni(2+) lipids has the potential for broad application. Likewise, the sparse matrix method uses screening conditions for three-dimensional crystallization and is therefore of broad applicability.

Drug target identification, validation, characterisation and exploitation for treatment of Acanthamoeba (species) infections

New more efficacious antimicrobials as required for the treatment of Acanthamoeba infections as those currently available require arduous treatment regimes, are not always effective and are poorly active against the cystic stages. Herein, we review potential drug targets including tubulin, alternative oxidase, amino acid biosynthesis and myosin. In addition, we review the literature for current missing tools and resources for the identification, validation and development of new antimicrobials for this organism. Additional targets should come to light through a concerted genome sequencing effort.

Structure and function of myosin filaments

Myosin filaments interact with actin to generate muscle contraction and many forms of cell motility. X-ray and electron microscopy (EM) studies have revealed the general organization of myosin molecules in relaxed filaments, but technical difficulties have prevented a detailed description. Recent studies using improved ultrastructural and image analysis techniques are overcoming these problems. Three-dimensional reconstructions using single-particle methods have provided many new insights into the organization of the myosin heads and tails. Docking of atomic structures into cryo-EM density maps suggests how regulated myosin filaments are 'switched off', bringing about muscle relaxation. Additionally, sequence analysis suggests probable interactions between myosin tails in the backbone, whereas crystallographic and EM studies are starting to reveal tail interactions directly in three dimensions.

Assembly of Acanthamoeba myosin-II minifilaments. Definition of C-terminal residues required to form coiled-coils, dimers, and octamers

Acanthamoeba myosin-II forms bipolar octamers by three successive steps of dimerization of the C-terminal, coiled-coil tail. In this study, we generated N-terminal and C-terminal truncation constructs and point mutants of the Acanthamoeba myosin-II tail to delineate the structural requirements for assembly of bipolar mini-filaments. By the use of light-scattering, CD spectroscopy, analytical ultracentrifugation, and tryptophan fluorescence experiments, we determined that: (1) the C-terminal 14 heptad repeats plus most of the tailpiece (residues 1381-1509) are required to form antiparallel dimers of coiled-coils; (2) amino acid residues within heptads 23-32 (residues 1254-1325) are required to form tetramers; (3) the C-terminal 32 heptad repeats suffice to assemble octameric minifilaments; (4) A1378 is outside of the interaction interface; (5) the mutation L1475W inhibits dimerization; and (6) F1443 is involved in the dimerization interface but is exposed to the solvent. We propose that the tailpiece (residues 1483-1509) interacts with two heptads (13 and 14, residues 1381-1393), which are important for dimerization and coiled-coil formation. These results support a model in which hydrophobic as well as electrostatic interactions control the register between myosin-II coiled-coils and guide sequential steps of dimerization that generate stable, octameric mini-filaments.