Actin

Bioinformatics Analysis of Actin Interactome: Characterization of the Nuclear and Cytoplasmic Actin-Binding Proteins

Actin is present in the cytoplasm and nucleus of every eukaryotic cell. In the cytoplasm, framework and motor functions of actin are associated with its ability to polymerize to form F-actin. In the nucleus, globular actin plays a significant functional role. For a globular protein, actin has a uniquely large number of proteins with which it interacts. Bioinformatics analysis of the actin interactome showed that only a part of actin-binding proteins are both cytoplasmic and nuclear. There are proteins that interact only with cytoplasmic, or only with nuclear actin. The first pool includes proteins associated with the formation, regulation, and functioning of the actin cytoskeleton predominate, while nuclear actin-binding proteins are involved in the majority of key nuclear processes, from regulation of transcription to DNA damage response. Bioinformatics analysis of the structure of actin-binding proteins showed that these are mainly intrinsically disordered proteins, many of which are part of membrane-less organelles. Interestingly, although the number of intrinsically disordered actin-binding proteins in the nucleus is greater than in the cytoplasm, the drivers for the formation of the membrane-less organelles in the cytoplasm are significantly (four times) greater than in the nucleus.

Toxoplasma gondii actin filaments are tuned for rapid disassembly and turnover

The cytoskeletal protein actin plays a critical role in the pathogenicity of the intracellular parasite, Toxoplasma gondii, mediating invasion and egress, cargo transport, and organelle inheritance. Advances in live cell imaging have revealed extensive filamentous actin networks in the Apicomplexan parasite, but there are conflicting data regarding the biochemical and biophysical properties of Toxoplasma actin. Here, we imaged the in vitro assembly of individual Toxoplasma actin filaments in real time, showing that native, unstabilized filaments grow tens of microns in length. Unlike skeletal muscle actin, Toxoplasma filaments intrinsically undergo rapid treadmilling due to a high critical concentration, fast monomer dissociation, and rapid nucleotide exchange. Cryo-EM structures of jasplakinolide-stabilized and native (i.e. unstabilized) filaments show an architecture like skeletal actin, with differences in assembly contacts in the D-loop that explain the dynamic nature of the filament, likely a conserved feature of Apicomplexan actin. This work demonstrates that evolutionary changes at assembly interfaces can tune the dynamic properties of actin filaments without disrupting their conserved structure.

On the role of nucleotides and lipids in the polymerization of the actin homolog MreB from a Gram-positive bacterium

In vivo, bacterial actin MreB assembles into dynamic membrane-associated filamentous structures that exhibit circumferential motion around the cell. Current knowledge of MreB biochemical and polymerization properties in vitro remains limited and is mostly based on MreB proteins from Gram-negative species. In this study, we report the first observation of organized protofilaments by electron microscopy and the first 3D-structure of MreB from a Gram-positive bacterium. We show that Geobacillus stearothermophilus MreB forms straight pairs of protofilaments on lipid surfaces in the presence of ATP or GTP, but not in the presence of ADP, GDP or non-hydrolysable ATP analogs. We demonstrate that membrane anchoring is mediated by two spatially close short hydrophobic sequences while electrostatic interactions also contribute to lipid binding, and show that the population of membrane-bound protofilament doublets is in steady-state. In solution, protofilament doublets were not detected in any condition tested. Instead, MreB formed large sheets regardless of the bound nucleotide, albeit at a higher critical concentration. Altogether, our results indicate that both lipids and ATP are facilitators of MreB polymerization, and are consistent with a dual effect of ATP hydrolysis, in promoting both membrane binding and filaments assembly/disassembly.

Toxoplasma gondii actin filaments are tuned for rapid disassembly and turnover

The cytoskeletal protein actin plays a critical role in the pathogenicity of Toxoplasma gondii, mediating invasion and egress, cargo transport, and organelle inheritance. Advances in live cell imaging have revealed extensive filamentous actin networks in the Apicomplexan parasite, but there is conflicting data regarding the biochemical and biophysical properties of Toxoplasma actin. Here, we imaged the in vitro assembly of individual Toxoplasma actin filaments in real time, showing that native, unstabilized filaments grow tens of microns in length. Unlike skeletal muscle actin, Toxoplasma filaments intrinsically undergo rapid treadmilling due to a high critical concentration, fast monomer dissociation, and rapid nucleotide exchange. Cryo-EM structures of stabilized and unstabilized filaments show an architecture like skeletal actin, with differences in assembly contacts in the D-loop that explain the dynamic nature of the filament, likely a conserved feature of Apicomplexan actin. This work demonstrates that evolutionary changes at assembly interfaces can tune dynamic properties of actin filaments without disrupting their conserved structure.

Membrane Tension and the Role of Ezrin During Phagocytosis

During phagocytosis, there is an apparent expansion of the plasma membrane to accommodate the target within a phagosome. This is accompanied (or driven by) a change in membrane tension. It is proposed that the wrinkled topography of the phagocyte surface, by un-wrinkling, provides the additional available membrane and that this explains the changes in membrane tension. There is no agreement as to the mechanism by which unfolding of cell surface wrinkles occurs during phagocytosis, but there is a good case building for the involvement of the actin-plasma membrane crosslinking protein ezrin. Not only have direct measurements of membrane tension strongly implicated ezrin as the key component in establishing membrane tension, but the cortical location of ezrin changes at the phagocytic cup, suggesting that it is locally signalled. This chapter therefore attempts to synthesise our current state of knowledge about ezrin and membrane tension with phagocytosis to provide a coherent hypothesis.

Neutrophil Cell Shape Change: Mechanism and Signalling during Cell Spreading and Phagocytosis

Perhaps the most important feature of neutrophils is their ability to rapidly change shape. In the bloodstream, the neutrophils circulate as almost spherical cells, with the ability to deform in order to pass along narrower capillaries. Upon receiving the signal to extravasate, they are able to transform their morphology and flatten onto the endothelium surface. This transition, from a spherical to a flattened morphology, is the first key step which neutrophils undergo before moving out of the blood and into the extravascular tissue space. Once they have migrated through tissues towards sites of infection, neutrophils carry out their primary role-killing infecting microbes by performing phagocytosis and producing toxic reactive oxygen species within the microbe-containing phagosome. Phagocytosis involves the second key morphology change that neutrophils undergo, with the formation of pseudopodia which capture the microbe within an internal vesicle. Both the spherical to flattened stage and the phagocytic capture stage are rapid, each being completed within 100 s. Knowing how these rapid cell shape changes occur in neutrophils is thus fundamental to understanding neutrophil behaviour. This article will discuss advances in our current knowledge of this process, and also identify an important regulated molecular event which may represent an important target for anti-inflammatory therapy.

Posttranscriptional and Posttranslational Regulation of Actin

Actin is one of the most abundant intracellular proteins, essential in every eukaryotic cell type. Actin plays key roles in tissue morphogenesis, cell adhesion, muscle contraction, and developmental reprogramming. Most actin studies have focused on its regulation at the protein level, either directly or through differential interactions with over a hundred intracellular binding partners. However, numerous studies emerging in recent years demonstrate specific types of nucleotide-level regulation that strongly affect non-muscle actins during cell migration and adhesion and are potentially applicable to other members of the actin family. This regulation involves zipcode-mediated actin mRNA targeting to the cell periphery, proposed to mediate local synthesis of actin at the cell leading edge, as well as the recently discovered N-terminal arginylation that specifically targets non-muscle β-actin via a nucleotide-dependent mechanism. Moreover, a study published this year suggests that actin's essential roles at the organismal level may be entirely nucleotide-dependent. This review summarizes the emerging data on actin's nucleotide-level regulation. Anat Rec, 301:1991-1998, 2018. © 2018 Wiley Periodicals, Inc.

Covalent and non-covalent chemical engineering of actin for biotechnological applications

The cytoskeletal filaments are self-assembled protein polymers with 8-25nm diameters and up to several tens of micrometres length. They have a range of pivotal roles in eukaryotic cells, including transportation of intracellular cargoes (primarily microtubules with dynein and kinesin motors) and cell motility (primarily actin and myosin) where muscle contraction is one example. For two decades, the cytoskeletal filaments and their associated motor systems have been explored for nanotechnological applications including miniaturized sensor systems and lab-on-a-chip devices. Several developments have also revolved around possible exploitation of the filaments alone without their motor partners. Efforts to use the cytoskeletal filaments for applications often require chemical or genetic engineering of the filaments such as specific conjugation with fluorophores, antibodies, oligonucleotides or various macromolecular complexes e.g. nanoparticles. Similar conjugation methods are also instrumental for a range of fundamental biophysical studies. Here we review methods for non-covalent and covalent chemical modifications of actin filaments with focus on critical advantages and challenges of different methods as well as critical steps in the conjugation procedures. We also review potential uses of the engineered actin filaments in nanotechnological applications and in some key fundamental studies of actin and myosin function. Finally, we consider possible future lines of investigation that may be addressed by applying chemical conjugation of actin in new ways.

Bridging length scales to measure polymer assembly

Time-resolvable quantitative measurements of polymer concentration are very useful to elucidate protein polymerization pathways. There are numerous techniques to measure polymer concentrations in purified protein solutions, but few are applicable in vivo. Here we develop a methodology combining microscopy and spectroscopy to overcome the limitations of both approaches for measuring polymer concentration in cells and cell extracts. This technique is based on quantifying the relationship between microscopy and spectroscopy measurements at many locations. We apply this methodology to measure microtubule assembly in tissue culture cells and Xenopus egg extracts using two-photon microscopy with FLIM measurements of FRET. We find that the relationship between FRET and two-photon intensity quantitatively agrees with predictions. Furthermore, FRET and intensity measurements change as expected with changes in acquisition time, labeling ratios, and polymer concentration. Taken together, these results demonstrate that this approach can quantitatively measure microtubule assembly in complex environments. This methodology should be broadly useful for studying microtubule nucleation and assembly pathways of other polymers.

Steered molecular dynamics analysis of the role of cofilin in increasing the flexibility of actin filaments

Cofilin is one of the most essential regulatory proteins and participates in the process of disassembling actin filaments. Cofilin induces conformational changes to actin filaments, and both the bending and torsional rigidity of the filament. In this study, we investigate the effects of cofilin on the mechanical properties of actin filaments using computational methods. Three models defined by their number of bound cofilins are constructed using coarse-grained MARTINI force field, and they are then extended with steered molecular dynamics simulation. After obtaining the stress-strain curves of the models, we calculate their Young's moduli and other mechanical properties that have not yet been determined for actin filaments. We analyze the cause of the different behaviors of the three models based on their atomistic geometrical differences. Finally, it is demonstrated that cofilin binding causes changes in the distances, angles, and stabilities of the residues in actin filaments.

Cofilin reduces the mechanical properties of actin filaments: approach with coarse-grained methods

An actin filament is an essential cytoskeleton protein in a cell. Various proteins bind to actin for cell functions such as migration, division, and shape control. ADF/cofilin is a protein that severs actin filaments and is related to their dynamics. Actin is known to have excellent mechanical properties. Binding cofilin reduces its mechanical properties, and is related to the severing process. In this research, we applied a coarse-grained molecular dynamics simulation (CGMD) method to obtain actin filaments and cofilin-bound actin (cofilactin) filaments. Using these two obtained models, we constructed an elastic network model-based structure and conducted a normal mode analysis. Based on the low-frequency normal modes of the filament structure, we applied the continuum beam theory to calculate the mechanical properties of the actin and cofilactin filaments. The CGMD method provided structurally accurate actin and cofilactin filaments in relation to the mechanical properties, which showed good agreement with the established experimental results.

Ascomycete fungal actins differentially support plant spatial cell and organ development

Actin interacts with a wide variety of cytoplasmic and nuclear proteins to support spatial development in nearly all eukaryotes. Null mutations in plant vegetative actins produce dramatically altered cell, tissue, and organ morphologies. Animal cytoplasmic actins (e.g., human HsACTB, HsACTG1) and some ancestral protist actins fully suppress these mutant phenotypes suggesting that some animal, plant, and protist actins share functional competence for spatial development. Considering that fungi have a phylogenetic origin closer to animals than plants, we were interested to explore whether the fungal actins may have this same capacity to function in plants and support development. We ectopically expressed actins from four highly divergent ascomycete fungi in two different Arabidopsis double vegetative actin null mutants. We found that expression of actin from the earliest diverging ascomycete subphyla, the archiascomycete Schizosaccharomyces pombe, qualitatively and quantitatively suppressed the root cell polarity and root organ developmental defects of act8/act7 mutants and the root-hairless cell elongation phenotype of act2/act8 mutants. Interestingly, the actin from the pyrenomycete Neurospora crassa was modestly effective in the suppression of vegetative actin mutant phenotypes. In contrast, actins from the saccharomycetes Saccharomyces cerevisiae and Candida albicans were unable to support any aspect of plant development, and moreover induced severe dwarfism and sterility. These data imply that basal fungi inherited an actin with full competence for spatial development from their protist ancestor and maintained it via non-progressive sequence evolution, while the later more derived fungal species lost these activities.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Mechanisms of nuclear actin in chromatin-remodeling complexes

The mystery of nuclear actin has puzzled biologists for decades largely due to the lack of defined experimental systems. However, the development of actin-containing chromatin-modifying complexes as a defined genetic and biochemical system in the past decade has provided an unprecedented opportunity to dissect the mechanism of actin in the nucleus. Although the established functions of actin mostly rely on its dynamic polymerization, the novel finding of the mechanism of action of actin in the INO80 chromatin-remodeling complex suggests a conceptually distinct mode of actin that functions as a monomer. In this review we highlight the new paradigm and discuss how actin interaction with chromatin suggests a fundamental divergence between conventional cytoplasmic actin and nuclear actin. Furthermore, we provide how this framework could be applied to investigations of nuclear actin in other actin-containing chromatin-modifying complexes.

Multi-level control of actin dynamics by protein kinase D

Dynamic actin remodeling is fundamental to processes such as cell motility, vesicle trafficking, and cytokinesis. Protein kinase D (PKD) is a serine-threonine kinase known to be involved in diverse biological functions ranging from vesicle fission at the Golgi complex to regulation of cell motility and invasion. This review addresses the role of PKD in the organization of the actin cytoskeleton with a particular emphasis on the substrates associated with this function. We further highlight the multi-level control of actin dynamics by PKD and suggest that the tight spatio-temporal control of PKD activity is critical for the coordination of directed cell migration.

Cloning, overexpression, purification and crystallization of the CRN12 coiled-coil domain from Leishmania donovani

Leishmania donovani coronin CRN12 is an actin-binding protein which consists of two domains: an N-terminal WD repeat domain and a C-terminal coiled-coil domain. The coiled-coil domain is 53 residues in length. Helix-helix interactions in general and coiled coils in particular are ubiquitous in the structure of proteins and play a significant role in the association among proteins, including supramolecular assemblies and transmembrane receptors that mediate cellular signalling, transport and actin dynamics. The L. donovani coronin CRN12 coiled-coil domain (5.8 kDa) was cloned, overexpressed, purified to homogeneity and the N-terminal 6×His tag was successfully removed by thrombin cleavage. Crystals of recombinant L. donovani coronin CRN12 coiled-coil domain were grown by vapour diffusion using a hanging-drop setup. Diffraction-quality crystals were obtained and data extending to 2.46 Å resolution were collected at 100 K on BM14, ESRF, Grenoble, France. The crystal belonged to the monoclinic space group C2, with unit-cell parameters a = 118.0, b = 50.6, c = 46.0 Å, β = 111.0°. Matthews coefficient (VM) calculations suggested the presence of 4-6 molecules in the asymmetric unit, corresponding to a solvent content of ∼33-55%, and are consistent with self-rotation function calculations.

Exon-primed, intron-crossing (EPIC) loci for five nuclear genes in deep-sea protobranch bivalves: primer design, PCR protocols and locus utility

We describe PCR primers and amplification protocols developed to obtain introns from conserved nuclear genes in deep-sea protobranch bivalves. Because almost no sequence data for protobranchs are publically available, mollusk and other protostome sequences from GenBank were used to design degenerate primers, making these loci potentially useful in other invertebrate taxa. Amplification and sequencing success varied across the test group of 30 species, and we present five loci spanning this range of outcomes. Intron presence in the targeted regions also varied across genes and species, often within single genera; for instance, the calmodulin and β-tubulin loci contained introns with high frequency, whereas the triose phosphate isomerase locus never contained an intron. In introns for which we were able to obtain preliminary estimates of polymorphism levels in single species, polymorphism was greater than traditional mitochondrial loci. These markers will greatly increase the ability to assess population structure in the ecologically important protobranchs, and may prove useful in other taxa as well.

Phalloidin perturbs the interaction of human non-muscle myosin isoforms 2A and 2C1 with F-actin

Phalloidin and fluorescently labeled phalloidin analogs are established reagents to stabilize and mark actin filaments for the investigation of acto-myosin interactions. In the present study, we employed transient and steady-state kinetic measurements as well as in vitro motility assays to show that phalloidin perturbs the productive interaction of human non-muscle myosin-2A and -2C1 with filamentous actin. Phalloidin binding to F-actin results in faster dissociation of the complex formed with non-muscle myosin-2A and -2C1, reduced actin-activated ATP turnover, and slower velocity of actin filaments in the in vitro motility assay. In contrast, phalloidin binding to F-actin does not affect the interaction with human non-muscle myosin isoform 2B and Dictyostelium myosin-2 and myosin-5b.

Proteome analysis of brain in murine experimental autoimmune encephalomyelitis

Multiple sclerosis is considered a prototype inflammatory autoimmune disorder of the CNS. Experimental autoimmune encephalomyelitis (EAE) induced by myelin oligodendrocyte glycoprotein is one of the best-characterized animal models of multiple sclerosis. Comprehensive understanding of gene expression in EAE can help identify genes that are important in drug response and pathogenesis. We applied a 2-DE-based proteomics approach to analyze the protein expression pattern of the brain in healthy and EAE samples. Of more than 1000 protein spots we analyzed, 70 showed reproducible and significant changes in EAE compared to controls. Of these, 42 protein spots could be identified using MALDI TOF-TOF-MS. They included mitochondrial and structural proteins as well as proteins involved in ionic and neurotransmitter release, blood barriers, apoptosis, and signal transduction. The possible role of these proteins in the responses of mice to animal models of multiple sclerosis is discussed.

Evolutionary selection of new breast cancer cell-targeting peptides and phages with the cell-targeting peptides fully displayed on the major coat and their effects on actin dynamics during cell internalization

Filamentous phage as a bacteria-specific virus can be conjugated with an anticancer drug and has been proposed to serve as a carrier to deliver drugs to cancer cells for targeted therapy. However, how cell-targeting filamentous phage alone affects cancer cell biology is unclear. Phage libraries provide an inexhaustible reservoir of new ligands against tumor cells and tissues that have potential therapeutic and diagnostic applications in cancer treatment. Some of these identified ligands might stimulate various cell responses. Here we identified new cell internalizing peptides (and the phages with such peptides fused to each of ~3900 copies of their major coat protein) using landscape phage libraries and for the first time investigated the actin dynamics when selected phages are internalized into the SKBR-3 breast cancer cells. Our results show that phages harboring VSSTQDFP and DGSIPWST peptides could selectively internalize into the SKBR-3 breast cancer cells with high affinity, and also show rapid involvement of membrane ruffling and rearrangements of actin cytoskeleton during the phage entry. The actin dynamics was studied by using live cell and fluorescence imaging. The cell-targeting phages were found to enter breast cancer cells through energy dependent mechanism and phage entry interferes with actin dynamics, resulting in reorganization of actin filaments and increased membrane rufflings in SKBR-3 cells. These results suggest that, when phage enters epithelial cells, it triggers transient changes in the host cell actin cytoskeleton. This study also shows that using multivalent phage libraries considerably increases the repertoire of available cell-internalizing ligands with potential applications in targeted drug delivery, imaging, molecular monitoring and profiling of breast cancer cells.

Nucleotide-dependence of G-actin conformation from multiple molecular dynamics simulations and observation of a putatively polymerization-competent superclosed state

The assembly of monomeric G-actin into filamentous F-actin is nucleotide dependent: ATP-G-actin is favored for filament growth at the "barbed end" of F-actin, whereas ADP-G-actin tends to dissociate from the "pointed end." Structural differences between ATP- and ADP-G-actin are examined here using multiple molecular dynamics simulations. The "open" and "closed" conformational states of G-actin in aqueous solution are characterized, with either ATP or ADP in the nucleotide binding pocket. With both ATP and ADP bound, the open state closes in the absence of actin-bound profilin. The position of the nucleotide in the protein is found to be correlated with the degree of opening of the active site cleft. Further, the simulations reveal the existence of a structurally well-defined, compact, "superclosed" state of ATP-G-actin, as yet unseen crystallographically and absent in the ADP-G-actin simulations. The superclosed state resembles structurally the actin monomer in filament models derived from fiber diffraction and is putatively the polymerization competent conformation of ATP-G-actin.

[Cloning and expression analysis of porcine ACTA2 gene and its association with production traits]

To identify new DNA markers which have significant impact on pig production traits, the full coding sequence and partial genomic sequence of porcine ACTA2(Actin alpha 2)gene were isolated using in silico cloning and PCR. PCR-Hinf-RFLP was developed to detect C1554T substitution in intron 2. The frequency of allele C is higher than that of allele T in all the seven detected pig populations except for Large White and MeishanxLarge White. Association analysis of markers and production traits showed that the relation between ACTA2 gene and shoulder fat thickness, buttock fat thickness, fat meat percentage, lean meat percentage, meat pH (m.Biceps Femoris, BF), and intramuscular fat were significant or highly significant. Compared with CC genotype, TT had a higher lean meat percentage, a lower fat meat percentage and backfat thickness. Real-time RT-PCR analysis showed that the expression level of ACTA2 gene in the skeletal muscle of Large White and Meishan pigs decreased with the increasing of days. And during each period, the expression level was higher in Meishan pigs than in Large White pigs.

Vimentin intermediate filament formation: in vitro measurement and mathematical modeling of the filament length distribution during assembly

The salt-induced in vitro assembly of cytoplasmic intermediate filament (IF) proteins such as vimentin is characterized by a very rapid lateral association of soluble tetrameric subunits into 60-nm-long full-width "unit-length" filaments (ULFs). We have demonstrated for this prototype IF protein that filament elongation occurs by the longitudinal annealing of ULFs into short IFs. These IFs further longitudinally anneal and thus constitute a progressively elongating filament population that over time yields filaments of several microm in length. Previously, we provided a mathematical model for the kinetics of the assembly process based on the average length distribution of filaments as determined by time-lapse electron and atomic force microscopy. Thereby, we were able to substantiate the concept that end-to-end-annealing of both ULFs and short filaments is obligatory for the formation of long IFs (Kirmse, R.; Portet, S.; Mücke, N. Aebi, U.; Herrmann, H.; Langowski, J. J. Biol. Chem. 2007, 282, 18563-18572). As the next step in understanding the mechanics of IF formation, we have expanded our mathematical model to describe the quantitative aspects of IF assembly by taking into account geometry constraints as well as the diffusion properties of rodlike linear aggregates. Thereby, we have developed a robust model for the time-dependent filament length distribution of IFs under standard conditions.

Cysteine engineering of actin self-assembly interfaces

The Holmes model of filamentous actin (F-actin) and recent structural studies suggest specific atomic interactions between F-actin subunits. We tested these interactions through a cysteine-engineering approach with the goal of inhibiting filament formation by introducing chemical groups at sites important for polymerization. We substituted surface amino acids on the actin molecule with cysteine residues and tested the effect of producing these actin mutant proteins in a yeast expression system. The intrinsic folding and polymerization characteristics of the cysteine-engineered actin proteins were measured. The effect of chemical modification of the introduced cysteine residues on the polymerization of the actin mutant proteins was also examined. Modification of cysteine residues with large hydrophobic reagents resulted in polymerization inhibition. We examined the finding that the D288C actin protein does not polymerize under oxidizing conditions and forms protein aggregates when magnesium and EGTA are present. Chemical crosslinking experiments revealed the presence of a lower dimer when only D288C actin was present. When both D288C and A204C actin were present, crosslinking experiments support the proximity of Asp288 on the barbed end of one subunit to Ala204 on the pointed end of a neighboring subunit in the Holmes model of F-actin.

Overexpression of cardiac actin with baculovirus is promoter dependent

The influence of the promoter and an N-terminal hexahistidine tag on human cardiac actin (ACTC) expression and function was investigated using four baculovirus constructs. It was found that both non-tagged ACTC and hisACTC expression from the p10 promoter was higher than from the polh promoter. Characterization showed that an N-terminal hexahistidine tag has a negative effect on ACTC. Recombinant ACTC inhibits DNase-I and binds myosin S1, indicative of proper folding. Our data support the hypothesis that the actin protein down-regulates the polh promoter.

Yeast actin with a subdomain 4 mutation (A204C) exhibits increased pointed-end critical concentration

Characterizing mutants of actin that do not polymerize will advance our understanding of the mechanism of actin polymerization and will be invaluable for the production of short F-actin structures for structural studies. To circumvent the problem of expressing dominant lethal nonpolymerizing actin in yeast, we adopted a cysteine engineering strategy. Here we report the characterization of a mutant of yeast actin, AC-actin, possessing a single pointed-end mutation, A204C. Expression of this mutant in yeast results in actin-polymerization-deficient phenotypes. When copolymerized with wild-type actin, ATP-AC-actin is incorporated into filaments. ADP-AC-actin participates in the nucleation and elongation of wild-type filaments only at very high concentrations. At low concentrations, ADP-AC-actin appears to participate only in the nucleation of wild-type filaments, suggesting that Ala-204 is involved in modulating the critical concentration of the pointed end of actin.

Expression patterns of ubiquitin, heat shock protein 70, alpha-actin and beta-actin over the molt cycle in the abdominal muscle of marine shrimp Litopenaeus vannamei

Crustacean muscle growth is discontinuous due to molt cycle. To characterize molt-related gene expression patterns, we studied the mRNA levels of molecular chaperone-ubiquitin and heat shock protein 70 (Hsp 70) in comparison with muscle protein alpha-actin and beta-actin in marine shrimp Litopenaeus vannamei. Total RNA from abdominal muscle was isolated from 3-month-old animals in six different molt stages. The mRNA levels of target genes were detected by reverse-transcriptase-multiplex PCR and expressed as the ratio to elongation factor-1alpha. Ubiquitin mRNA levels were relatively steady over all stages of the molt cycle. Hsp70 levels were not detectable in early postmolt and late premolt stages, but showed a progressive increase from late postmolt to intermolt stages. Expression levels of alpha-actin gene were lower during postmolt, reached a plateau in intermolt and remained relatively high in premolt stage. Levels of beta-actin increased progressively from postmolt to intermolt, reaching a maximum value in premolt. Therefore, the mRNAs encoding for ubiquitin and Hsp 70 in abdominal muscle did not increase significantly in premolt stages, which is typically associated with claw muscle degradation. Muscle structural alpha-actin and cytoskeletal beta-actin were increased during intermolt and premolt stages, suggesting high muscle growth during these stages in the abdominal muscle of the L. vannamei.

Comparative analysis of differentially expressed genes in normal and white spot syndrome virus infected Penaeus monodon

Background

White spot syndrome (WSS) is a viral disease that affects most of the commercially important shrimps and causes serious economic losses to the shrimp farming industry worldwide. However, little information is available in terms of the molecular mechanisms of the host-virus interaction. In this study, we used an expressed sequence tag (EST) approach to observe global gene expression changes in white spot syndrome virus (WSSV)-infected postlarvae of Penaeus monodon.

Results

Sequencing of the complementary DNA clones of two libraries constructed from normal and WSSV-infected postlarvae produced a total of 15,981 high-quality ESTs. Of these ESTs, 46% were successfully matched against annotated genes in National Center of Biotechnology Information (NCBI) non-redundant (nr) database and 44% were functionally classified using the Gene Ontology (GO) scheme. Comparative EST analyses suggested that, in postlarval shrimp, WSSV infection strongly modulates the gene expression patterns in several organs or tissues, including the hepatopancreas, muscle, eyestalk and cuticle. Our data suggest that several basic cellular metabolic processes are likely to be affected, including oxidative phosphorylation, protein synthesis, the glycolytic pathway, and calcium ion balance. A group of immune-related chitin-binding protein genes is also likely to be strongly up regulated after WSSV infection. A database containing all the sequence data and analysis results is accessible at .

Conclusion

This study suggests that WSSV infection modulates expression of various kinds of genes. The predicted gene expression pattern changes not only reflect the possible responses of shrimp to the virus infection but also suggest how WSSV subverts cellular functions for virus multiplication. In addition, the ESTs reported in this study provide a rich source for identification of novel genes in shrimp.

Mapping the interaction of cofilin with subdomain 2 on actin

Cofilin, a member of the actin-depolymerizing factor (ADF)/cofilin family of proteins, is a key regulator of actin dynamics. Cofilin binds to monomer (G-) and filamentous (F-) actin, severs the filaments, and increases their turnover rate. Electron microscopy studies suggested cofilin interactions with subdomains 2 and 1/3 on adjacent actin protomers in F-actin. To probe for the presence of a cryptic cofilin binding site in subdomain 2 in G-actin, we used transglutaminase-mediated cross-linking, which targets Gln41 in subdomain 2. The cross-linking proceeded with up to 85% efficiency with skeletal alpha-actin and WT yeast actin, yielding a single product corresponding to a 1:1 actin-cofilin complex but was strongly inhibited in Q41C yeast actin (in which Q41 was substituted with cysteine). LC-MS/MS analysis of the proteolytic fragments of this complex mapped the cross-linking to Gln41 on actin and Gly1 on recombinant yeast cofilin. The actin-cofilin (AC) heterodimer was purified on FPLC for analytical ultracentrifugation and electron microscopy analysis. Sedimentation equilibrium and velocity runs revealed oligomers of AC in G-actin buffer. In the presence of excess cofilin, the covalent AC heterodimer bound a second cofilin, forming a 2:1 cofilin/actin complex, as revealed by sedimentation results. Under polymerizing conditions the cross-linked AC formed mostly short filaments, which according to image reconstruction were similar to uncross-linked actin-cofilin filaments. Although a majority of the cross-linking occurs at Gln41, a small fraction of the AC cross-linked complex forms in the Q41C yeast actin mutant. This secondary cross-linking site was sequenced by MALDI-MS/MS as linking Gln360 in actin to Lys98 on cofilin. Overall, these results demonstrate that the region around Gln41 (subdomain 2) is involved in a weak binding of cofilin to G-actin.

Actin Cys374 as a nucleophilic target of alpha,beta-unsaturated aldehydes

We have recently shown that actin can be modified by the Michael addition of 4-hydroxynonenal to Cys374. Here, we have exposed purified actin at increasing acrolein concentrations and have identified the sites of acrolein addition using LC-ESI-MS/MS. Acrolein reacted with Cys374, His87, His173, and, minimally, His40. Cys374 adduction by both 4-hydroxynonenal and acrolein negligibly affected the polymerization of aldehyde-modified (carbonylated) actin, as shown by fluorescence measurements. Differently, acrolein binding at histidine residues, when Cys374 was completely saturated, inhibited polymerization in a dose-dependent manner. Molecular modeling analyses indicated that structural distortions of the ATP-binding site, induced by four acrolein-Michael adducts, could explain the changes in the polymerization process. Aldehyde binding to Cys374 does not alter significantly actin polymerization because this residue is located in a very flexible region, whose covalent modifications do not alter the protein folding. These data demonstrate that Cys374 represents the primary target site of alpha,beta-unsaturated aldehyde addition to actin in vitro. As Cys374 is a preferential target for various oxidative/nitrosative modifications, and actin is one of the main carbonylated proteins in vivo, these findings also suggest that the highly reactive Cys374 could serve as a carbonyl scavenger of reactive alpha,beta-unsaturated aldehydes and other electrophilic lipids.

Actin gene in prawn, Macrobrachium rosenbergii: characteristics and differential tissue expression during embryonic development

An actin gene (named Mar-actin) from the commercial prawn, Macrobrachium rosenbergii, was isolated, sequenced and gene expression was characterized. The cDNA sequence was 1281 bp in length and contained 1131 bp open reading frame encoding 376 amino acids. The amino acid sequence deduced from the nucleotide sequence showed high identity (70.3% to 98.1%) with other known actins of various organisms, highest with that of the European flounder (98.1%). The 3' untranslated region (3' UTR) of the Mar-actin mRNA has a high A+U content (approximately 78%) and contains one AUUUUUA and two repeats of the AUUUA motifs, that may function in regulating mRNA decay. Northern blot analysis revealed that the Mar-actin gene was expressed predominantly in muscle tissues. Transcripts in hepatopancreas were barely detectable. Expression of the Mar-actin gene varied during embryonic development and reached the maximal level at the zoea stage. This is the first report describing the complete sequence and expression pattern of the actin gene in prawns.

Actin Activation of Myosin Heavy Chain Kinase A in Dictyostelium

Studies in Dictyostelium discoideum have established that the cycle of myosin II bipolar filament assembly and disassembly controls the temporal and spatial localization of myosin II during critical cellular processes, such as cytokinesis and cell locomotion. Myosin heavy chain kinase A (MHCK A) is a key enzyme regulating myosin II filament disassembly through myosin heavy chain phosphorylation in Dictyostelium. Under various cellular conditions, MHCK A is recruited to actin-rich cortical sites and is preferentially enriched at sites of pseudopod formation, and thus MHCK A is proposed to play a role in regulating localized disassembly of myosin II filaments in the cell. MHCK A possesses an aminoterminal coiled-coil domain that participates in the oligomerization, cellular localization, and actin binding activities of the kinase. In the current study, we show that the interaction between the coiled-coil domain of MHCK A and filamentous actin leads to an approximately 40-fold increase in the initial rate of kinase catalytic activity. Actin-mediated activation of MHCK A involves increased rates of kinase autophosphorylation and requires the presence of the coiled-coil domain. Structure-function analyses revealed that the coiled-coil domain alone binds to actin filaments (apparent K(D) = 0.9 microm) and thus mediates the direct interaction with F-actin required for MHCK A activation. Collectively, these results indicate that MHCK A recruitment to actin-rich sites could lead to localized activation of the kinase via direct interaction with actin filaments, and thus this mode of kinase regulation may represent an important mechanism by which the cell achieves localized disassembly of myosin II filaments required for specific changes in cell shape.

Sertoli-Sertoli and Sertoli-germ cell interactions and their significance in germ cell movement in the seminiferous epithelium during spermatogenesis

Spermatogenesis is the process by which a single spermatogonium develops into 256 spermatozoa, one of which will fertilize the ovum. Since the 1950s when the stages of the epithelial cycle were first described, reproductive biologists have been in pursuit of one question: How can a spermatogonium traverse the epithelium, while at the same time differentiating into elongate spermatids that remain attached to the Sertoli cell throughout their development? Although it was generally agreed upon that junction restructuring was involved, at that time the types of junctions present in the testis were not even discerned. Today, it is known that tight, anchoring, and gap junctions are found in the testis. The testis also has two unique anchoring junction types, the ectoplasmic specialization and tubulobulbar complex. However, attention has recently shifted on identifying the regulatory molecules that "open" and "close" junctions, because this information will be useful in elucidating the mechanism of germ cell movement. For instance, cytokines have been shown to induce Sertoli cell tight junction disassembly by shutting down the production of tight junction proteins. Other factors such as proteases, protease inhibitors, GTPases, kinases, and phosphatases also come into play. In this review, we focus on this cellular phenomenon, recapping recent developments in the field.

Actin crystal dynamics: structural implications for F-actin nucleation, polymerization, and branching mediated by the anti-parallel dimer

Actin filament nucleation, polymerization, and branching are crucial steps in many forms of cell motility, cell shape, and intracellular organelle movements in a wide range of organisms. Previous biochemical data suggests that an anti-parallel actin dimer can incorporate itself into growing filamentous actin (F-actin) and has a role in branching. Furthermore, it is a widespread belief that nucleation is spawned from an actin trimer complex. Here we present the structures of actin dimers and trimers in two tetragonal crystal systems P4(3)2(1)2 and P4(3). Both crystal systems formed by an induced condensation transformation of a previously reported orthorhombic crystal system P2(1)2(1)2(1). Comparison between the three crystal systems demonstrates the dynamics and flexibility of actin-actin interactions. The dimer and trimer actin rearrangements observed between the three crystal systems may provide insight to in vivo actin-actin interactions that occur during the nucleation, polymerization, and branching of F-actin.

Intestinal injury after thoracic aortic cross-clamping in the pig

BACKGROUND

The mucosal surface epithelium is an essential part of the functional intestinal barrier, but its structural response to ischemia/reperfusion is only partly characterized. The purpose of this study was to provide a detailed morphological evaluation of intestinal surface epithelium after aortic cross-clamping.

MATERIAL AND METHODS

Pigs were subjected to thoracic aortic cross-clamping for 60 min and subsequent reperfusion for 120 min. Tissue blood flow and high-energy phosphates were measured with microspheres and HPLC, respectively. Urinary excretion of (14)C polyethylene glycol (MW 4000 Da) (PEG-4000), loaded into an intestinal loop, provided an index of intestinal permeability.

RESULTS

Jejunal blood flow was restored at 10 min after aortic declamping. Denudation of the basement membrane of the intestinal villi tips, as a consequence of epithelial shedding, increased markedly during the initial 60 min of reperfusion (P = 0.002). During the following 45 min, the denuded basement membrane was partly covered with low cuboidal and squamous-shaped cells extending lamellipodia over a wavy basement membrane. Restoration of ATP at 60 min after aortic declamping correlated inversely to the extent of denuded basement membrane (r = 0.75, P = 0.032). Permeability of PEG-4000 increased markedly after aortic declamping and was linearly correlated to the area of denuded basement membrane (r = 0.87, P = 0.01).

CONCLUSIONS

Reperfusion for 2 h after aortic cross-clamping is associated with initial aggravation of ischemia-induced injury in the porcine jejunum, but thereafter with restitution of the surface epithelium. Restoration of ATP may be important to avoid intestinal injury after ischemia. Increased permeability of a macromolecule in response to reperfusion is closely correlated to injury of the surface epithelium.

Synchrotron radiolysis and mass spectrometry: a new approach to research on the actin cytoskeleton

Hydroxyl radicals generated from millisecond exposure of aqueous solutions to synchrotron X-rays react with proteins to yield stable oxidative modifications of solvent-accessible amino acid side chains. Following proteolysis, HPLC/MS analysis is performed to quantitate the side chain reactivities, and MS/MS analysis is used to identify the modification site(s). Side chain reactivity is shown to be correlated with solvent accessibility; thus the method provides detailed site-specific information about protein structure. The application of these techniques to the study of the actin cytoskeleton is described in detail, including defining the binding sites of monomeric actin with gelsolin segment-1, the actin monomer binding surface on cofilin, the divalent cation-dependent structure changes of monomeric actin, and the conformational changes associated with actin filamentous assembly.

Chemical treatment of mica for atomic force microscopy can affect biological sample conformation

An important aspect in the preparation of substrate materials to use in atomic force microscopy lies in the question of interactions introduced by treatments designed to immobilize the sample over the substrate. Here we used a mica substrate that was chemically modified with cationic nickel to immobilize actin filaments (F-actin). Chemical modification could be followed quantitatively by measuring the interaction force between the scanning tip and the mica surface. This approach allowed us to observe polymeric F-actin in a structure that resembles an actin gel. It also improved sample throughput and conferred sample stability as well as repeatability from run to run.

Intestinal epithelial hyperpermeability: update on the pathogenesis of gut mucosal barrier dysfunction in critical illness

PURPOSE OF REVIEW

Tight junctions between adjacent epithelial cells are essential for the maintenance of compositionally distinct fluid compartments in various organs, such as the liver, lungs, kidneys, and intestine. These epithelial organs are commonly affected in the condition known as multiple organ dysfunction syndrome, which can complicate the clinical course of patients with sepsis or other conditions associated with poorly controlled systemic inflammation. The gut serves as a useful model for this problem, and studies using reductionist in vitro models and experiments carried out using laboratory animals are starting to clarify the cellular and biochemical mechanisms that are responsible for intestinal epithelial hyperpermeability secondary to critical illness.

RECENT FINDINGS

One key factor that has been identified is excessive production of nitric oxide and related species, although other factors, such as increased expression of the cytokine interleukin 6, appear to be important as well. A newly described, cytokine-like molecule, high-mobility group B1, increases permeability of cultured epithelial monolayers in vitro and murine ileal mucosa in vivo.

SUMMARY

Epithelial dysfunction may be a common final pathway contributing to organ dysfunction in sepsis and other forms of critical illness.

Comparative genome- and proteome analysis of cerebral cortex from MK-801-treated rats

cDNA microarrays and two-dimensional gel-electrophoresis in combination with mass spectrometry, were used to screen alterations in mRNA and protein levels, respectively, in cerebral cortex of MK-801-treated rats. The rats were divided in two groups; group 1 (short-term treated) and group 2 (long-term treated). In group 1, four genes were up-regulated and five down-regulated. In group 2, seven genes were up-regulated and six down-regulated. In group 1, the levels of one protein was increased and eight proteins reduced. In group 2, the levels of two proteins were increased and four proteins reduced. Several of the altered genes (casein kinase 2, glutamic acid decarboxylase, synaptotagmin, gamma aminobutyric acid [GABA] transporter, creatine kinase, and cytochrome c oxidase) and proteins (superoxide dismutase, hsp 60, hsp 72 and gamma-enolase) have previously been connected to schizophrenia. Alterations of the genes (microglobulin, c-jun proto-oncogene, 40S ribosomal protein S19, adenosine diphosphate (ADP)-ribosylation factors, platelet-derived growth factor, fructose-bisphophate aldolase A, and myelin proteolipid) and the proteins (stathmin, H+-transp. Adenosine triphosphate (ATP) synthase, pyruvate dehydrogenase, beta-actin and alpha-enolase), have not, to our knowledge, earlier been implicated in schizophrenia pathology. Overall, these results with a combined approach of genomics and proteomics add to the validity of subchronic N-methyl-D-aspartate (NMDA)-receptor antagonist treatment as an animal model of schizophrenia.

Induction of fascin spikes in breast cancer cells by activation of the insulin-like growth factor-I receptor

Insulin-like growth factor-I receptor (IGF-IR) signaling contributes to the formation of mammary carcinomas and has chiefly been studied with regard to the proliferative and anti-apoptotic effects of IGF-IR signaling. However, IGF-IR activation also affects the actin cytoskeleton and alterations in cell migratory behavior are of known importance for the malignant conversion and metastasis of epithelial cells. The actin-binding protein fascin is found in cell projections and spikes that are involved in the locomotion of mesenchymal cells. Fascin expression is typically low in normal epithelial cells, but is markedly upregulated in several types of carcinomas. Here, we also demonstrate increased fascin expression in breast carcinoma cell lines and adopt MCF-7 human mammary carcinoma cells that over-express wild-type or kinase-inactivated forms of the IGF-IR as a model system to test the hypothesis that IGF-IR activation induces fascin projections. We show that the time-dependent dissociation of cell colonies that occurs upon receptor activation by IGF-I involves the formation of dynamic, fascin-containing lateral cell projections that co-localize with ruffling membranes in association with protrusive activity and cell migratory phenotype. The molecular mechanism of these effects is completely dependent on IGF-IR tyrosine kinase activity and is mediated by a phosphatidylinositol (PI) 3-kinase-dependent process. In demonstrating transduction of fascin spike assembly by activation of a peptide growth factor receptor, these novel data reveal a wide role for fascin spikes in cell motility and provide new insight into the complex effects of IGF-IR signaling on actin cytoskeletal organization.

Mapping the G-actin binding surface of cofilin using synchrotron protein footprinting

Cofilin is an actin regulatory protein that binds to both monomeric and filamentous actin, and has filament severing activity. Although crystal structures for the monomeric forms of both G-actin and cofilin have been described, the structure of the binary cofilin-G-actin complex is not available. Synchrotron protein footprinting is used to identify specific side chain residues on the cofilin surface that are buried in the formation of the cofilin-G-actin binary complex. Exposure to synchrotron X-rays results in stable oxidative modifications of aromatic, aliphatic, and sulfur-containing side chains, with the rate of modification for a particular residue being dependent on its intrinsic reactivity and solvent accessibility. The rates of modification were monitored for a number of peptides generated by digestion of oxidized cofilin, both in isolation and in its binary complex with G-actin. After binding to G-actin takes place, a significant decrease in modification rates, indicating protection of side chain groups, is seen for cofilin peptides corresponding to residues 4-20, 10-17, 83-96, 91-105, and 106-117. A number of other peptides show no change in reactivity, and are presumed to represent regions distal to the binding site. Tandem mass spectrometry demonstrates that residues Leu 13, Pro 94, Met 99, and Leu 108 and 112 directly participate in the binding interface. These results are generally consistent with, and complementary to, the results of previous site-directed mutagenesis studies and extend our understanding of the G-actin binding surface of cofilin.