Rheological interfacial properties of plant protein-arabic gum coacervates at the oil-water interface

This study concerns the interfacial properties of the plant proteins-arabic gum coacervates, which are involved in encapsulation processes based on complex coacervation. The results make it possible to deduce the prerequisite characteristics of the protein, which are involved in the coacervate interfacial properties. The influence of pH and concentration on protein interfacial properties was also studied so as to enable us to predict the best conditions to achieve encapsulation. It has been established that, to obtain a good encapsulation yield, the coacervate must show high surface-active properties and its adsorption on the oil droplets must be favored compared to the free protein adsorption. On the other hand, mechanical properties of the interfacial film made of the coacervate, appear to be a key parameter, as reflected by the dilational viscoelasticity measurements. When compared to the properties of the proteins films, an increase of the rigidity of the interfacial film was shown with the coacervates. It was also observed that viscoelastic properties of the coacervate film were strongly reduced, as well as the associated relaxation times. In acidic conditions, the coacervates containing alpha-gliadin are characterized by an interfacial viscoelastic behavior. This behavior reflects the softness of the interfacial film. This viscoelasticity allows also the formation of a continuous layer around the oil droplets to be encapsulated. Drop tensiometry is shown to be a method that could allow the most adapted protein to be selected and the conditions of the coacervation process to be optimized with regard to concentration and pH.     

CASK and protein 4.1 support F-actin nucleation on neurexins

Rearrangements of the actin cytoskeleton are involved in a variety of cellular processes from locomotion of cells to morphological alterations of the cell surface. One important question is how local interactions of cells with the extracellular space are translated into alterations of their membrane organization. To address this problem, we studied CASK, a member of the membrane-associated guanylate kinase homologues family of adaptor proteins. CASK has been shown to bind the erythrocyte isoform of protein 4.1, a class of proteins that promote formation of actin/spectrin microfilaments. In neurons, CASK also interacts via its PDZ domain with the cytosolic C termini of neurexins, neuron-specific cell-surface proteins. We now show that CASK binds a brain-enriched isoform of protein 4.1, and nucleates local assembly of actin/spectrin filaments. These interactions can be reconstituted on the cytosolic tail of neurexins. Furthermore, CASK can be recovered with actin filaments prepared from rat brain extracts, and neurexins are recruited together with CASK and protein 4.1 into these actin filaments. Thus, analogous to the PDZ-domain protein p55 and glycophorin C at the erythrocyte membrane, a similar complex comprising CASK and neurexins exists in neurons. Our data suggest that intercellular junctions formed by neurexins, such as junctions initiated by beta-neurexins with neuroligins, are at least partially coupled to the actin cytoskeleton via an interaction with CASK and protein 4.1.     

Characterization of Flaxseed Gum/Rice Bran Protein Complex Coacervates

Effect of protein to polysaccharide ratio (3:1, 6:1 & 9:1) and total biopolymer concentration (0.1, 0.2 & 0.4) on ζ-potential, particle size and particle distribution index (PDI) of complex coacervates were investigated. Furthermore, the physical, thermal and morphological characteristics of FG, RBP, RBP-FG coacervates and cross-linked RBP-FG coacervates by sodium tripolyphosphate were surveyed. Results showed that at low concentrations of FG (9:1 ratio) and a total concentration of 0.4, the ζ-potential of coacervate was close to zero and the coacervates had the largest size revealing the greatest interaction between biopolymers. SEM results showed a porous network structure which was varied from the RBP and FG. In contrast, the cross-linked coacervates showed a fine, uniform structure with less number of pores. FTIR findings revealed that the coacervate, due to the non-covalent interaction forces, was successfully developed. The fading of the pure peaks of protein and polysaccharide in XRD diffractogram indicated the interactions between the RBP and FG, as well as the structural changes of the complex. NaTPP cross-linked coacervate was indicated a reflection of slightly increased crystallinity. However, the dried powder of coacervates was generally amorphous. According to TGA and DSC results, cross-linked coacervates exhibited the highest thermal stability amongthe single biopolymers and non cross-linked coacervate.     

Actin-filament stochastic dynamics mediated by ADF/cofilin

BACKGROUND: The rapid dynamics of actin filaments is a fundamental process that powers a large number of cellular functions. However, the basic mechanisms that control and coordinate such dynamics remain a central question in cell biology. To reach beyond simply defining the inventory of molecules that control actin dynamics and to understand how these proteins act synergistically to modulate filament turnover, we combined evanescent-wave microscopy with a biomimetic system and followed the behavior of single actin filaments in the presence of a physiologically relevant mixture of accessory proteins. This approach allows for the real-time visualization of actin polymerization and age-dependent filament severing. RESULTS: In the presence of actin-depolymerizing factor (ADF)/cofilin and profilin, actin filaments with a processive formin attached at their barbed ends were observed to oscillate between stochastic growth and shrinkage phases. Fragmentation of continuously growing actin filaments by ADF/cofilin is the key mechanism modulating the prominent and frequent shortening events. The net effect of continuous actin polymerization, driven by a processive formin that uses profilin-actin, and of ADF/cofilin-mediating severing that trims the aged ends of the growing filaments is an up to 155-fold increase in the rate of actin-filament turnover in vitro in comparison to that of actin alone. Lateral contact between actin filaments dampens the dynamics and favors actin-cable formation. A kinetic simulation accurately validates these observations. CONCLUSIONS: Our proposed mechanism for the control of actin dynamics is dominated by ADF/cofilin-mediated filament severing that induces a stochastic behavior upon individual actin filaments. When combined with a selection process that stabilizes filaments in bundles, this mechanism could account for the emergence and extension of actin-based structures in cells.     

Filamin A, the Arp2/3 complex, and the morphology and function of cortical actin filaments in human melanoma cells.

The Arp2/3 complex and filamin A (FLNa) branch actin filaments. To define the role of these actin-binding proteins in cellular actin architecture, we compared the morphology of FLNa-deficient human melanoma (M2) cells and three stable derivatives of these cells expressing normal FLNa concentrations. All the cell lines contain similar amounts of the Arp2/3 complex. Serum addition causes serum-starved M2 cells to extend flat protrusions transiently; thereafter, the protrusions turn into spherical blebs and the cells do not crawl. The short-lived lamellae of M2 cells contain a dense mat of long actin filaments in contrast to a more three-dimensional orthogonal network of shorter actin filaments in lamellae of identically treated FLNa-expressing cells capable of translational locomotion. FLNa-specific antibodies localize throughout the leading lamellae of these cells at junctions between orthogonally intersecting actin filaments. Arp2/3 complex-specific antibodies stain diffusely and label a few, although not the same, actin filament overlap sites as FLNa antibody. We conclude that FLNa is essential in cells that express it for stabilizing orthogonal actin networks suitable for locomotion. Contrary to some proposals, Arp2/3 complex-mediated branching of actin alone is insufficient for establishing an orthogonal actin organization or maintaining mechanical stability at the leading edge.     

Probing the mechanism of incorporation of fluorescently labeled actin into stress fibers

The mechanism of actin incorporation into and association with stress fibers of 3T3 and WI38 fibroblasts was examined by fluorescent analog cytochemistry, fluorescence recovery after photobleaching (FRAP), image analysis, and immunoelectron microscopy. Microinjected, fluorescein-labeled actin (AF-actin) became associated with stress fibers as early as 5 min post-injection. There was no detectable cellular polarity in the association of AF-actin with pre-existing stress fibers relative to perinuclear or peripheral regions. The rate of incorporation was quantified by image analysis of images generated with a two-dimensional photon counting microchannel plate camera. After equilibration of up to 2 h post-injection, FRAP demonstrated that actin subunits exchanged rapidly between filaments in stress fibers and the surrounding cytoplasm. When co-injected with rhodamine-labeled bovine serum albumin as a control, only actin was detected in the phase-dense stress fibers. The control protein was excluded from fibers and any linear fluorescence of the control was demonstrated as a pathlength artifact. The incorporation of AF-actin into stress fibers was studied by immunoelectron microscopy using anti-fluorescein as the primary antibody and goat anti-rabbit IgG coupled to peroxidase as the secondary antibody. At 5 min post-injection, reaction product was localized periodically in some fibers with a periodicity of approximately 0.75 microns. In large diameter fibers at 5 min post-injection, the analog was seen first on the surface of fibers, with individual filaments resolvable within the core. In the same cell, thinner diameter fibers were labeled uniformly throughout the diameter. By 20 min post-injection, most fibers were uniformly labeled. We conclude that the rate of actin subunit exchange in vivo is extremely rapid with molecular incorporation into actin filaments of stress fibers occurring as early as a few minutes post-injection. Exchange appears to first occur in filaments along the surface of stress fibers and then into more central regions in a periodic manner. We suggest that the periodic localization of actin at very early time points is due to a local microheterogeneity in which microdomains of fast vs. slower incorporation result from the periodic localization of actin-binding protein, such as alpha-actinin, along the length of the fiber.     

Aip1 Promotes Actin Filament Severing by Cofilin and Regulates Constriction of the Cytokinetic Contractile Ring

Aip1 (actin interacting protein 1) is ubiquitous in eukaryotic organisms, where it cooperates with cofilin to disassemble actin filaments, but neither its mechanism of action nor its biological functions have been clear. We purified both fission yeast and human Aip1 and investigated their biochemical activities with or without cofilin. Both types of Aip1 bind actin filaments with micromolar affinities and weakly nucleate actin polymerization. Aip1 increases up to 12-fold the rate that high concentrations of yeast or human cofilin sever actin filaments, most likely by competing with cofilin for binding to the side of actin filaments, reducing the occupancy of the filaments by cofilin to a range favorable for severing. Aip1 does not cap the barbed ends of filaments severed by cofilin. Fission yeast lacking Aip1 are viable and assemble cytokinetic contractile rings normally, but rings in these Δaip1 cells accumulate 30% less myosin II. Further, these mutant cells initiate the ingression of cleavage furrows earlier than normal, shortening the stage of cytokinetic ring maturation by 50%. The Δaip1 mutation has negative genetic interactions with deletion mutations of both capping protein subunits and cofilin mutations with severing defects, but no genetic interaction with deletion of coronin.     

Local accumulations of B-50/GAP-43 evoke excessive bleb formation in PC12 cells

B-50 (GAP-43) is an axonal, plasma membrane-associated protein involved in growth cone morphology and function. We have conducted immunocytochemical, electron microscopic, and time-lapse experiments to visualize morphological consequences of local accumulations of B-50 at the plasma membrane of B-50-transfected PC-B2 cells, a clonal PC12 cell line with very low expression of endogenous B-50. The distribution of the transfected B-50 within these cells was inhomogeneous. At sites where the B-50 concentration was locally increased up to twofold, numerous filopodia were present in growth cone-like, substrate-attached regions. When local B-50 concentrations were even higher (up to 6.2-fold), blebs were formed, often containing vesicular structures, heavily decorated with B-50 immunoreactivity. Double labeling with f-actin binding phalloidin revealed that local B-50 accumulations were accompanied by increased actin filament concentrations. Colocalization of B-50 with actin filaments was prominent in filopodia, but was virtually absent in blebs, suggesting a disconnection of the bleb plasma membrane from the actin cytoskeleton. We conclude that B-50 evokes distinct effects on cell-surface activity in PC12 cells depending on its local concentration.     

Sexual dimorphism of motorneurons: Timbal muscle innervation in male periodical cicadas and homologous structures in females

Summary Treatment of cultured goldfish xanthophores by hormone (ACTH) or c-AMP induces not only pigment dispersion, but subsequent outgrowth of processes, and pigment translocation into these processes. These latter effects are shown to proceed as follows: First the edge of the cytoplasmic lamellae takes on a scalloped contour with numerous protrusions. These presumably serve as nucleation centers where short microfilament bundles are assembled, Later, the microfilament bundles elongate (grow), often resulting in an extension of the protrusions to become filopodia while the proximal end of the microfilaments penetrates into the thicker portion of the cellular process which now houses the pigment, i.e., the carotenoid droplets. Carotenoid droplets appear to migrate along the microfilament bundles, or cytoplasmic channels associated with them, into the filopodia. Finally, some of the filopodia become broader, thicker and laden with carotenoid droplets and are then recognized by light microscopy as pigmented cellular processes. The microfilaments have been shown to be actin filaments by their thickness, the size of their subunits, and decoration by heavy meromyosin. Evidence is presented which suggests that the growth of these actin filaments may come about by recruitment from short F-actin strands found in random orientation in adjacent areas.     

Specification of actin filament function and molecular composition by tropomyosin isoforms

The specific functions of greater than 40 vertebrate nonmuscle tropomyosins (Tms) are poorly understood. In this article we have tested the ability of two Tm isoforms, TmBr3 and the human homologue of Tm5 (hTM5(NM1)), to regulate actin filament function. We found that these Tms can differentially alter actin filament organization, cell size, and shape. hTm5(NM1) was able to recruit myosin II into stress fibers, which resulted in decreased lamellipodia and cellular migration. In contrast, TmBr3 transfection induced lamellipodial formation, increased cellular migration, and reduced stress fibers. Based on coimmunoprecipitation and colocalization studies, TmBr3 appeared to be associated with actin-depolymerizing factor/cofilin (ADF)-bound actin filaments. Additionally, the Tms can specifically regulate the incorporation of other Tms into actin filaments, suggesting that selective dimerization may also be involved in the control of actin filament organization. We conclude that Tm isoforms can be used to specify the functional properties and molecular composition of actin filaments and that spatial segregation of isoforms may lead to localized specialization of actin filament function.     

A cytoskeletal demolition worker: myosin II acts as an actin depolymerization agent

Myosin II motors play several important roles in a variety of cellular processes, some of which involve active assembly/disassembly of cytoskeletal substructures. Myosin II motors have been shown to function in actin bundle turnover in neuronal growth cones and in the recycling of actin filaments during cytokinesis. Close examination had shown an intimate relationship between myosin II motor adenosine triphosphatase activity and actin turnover rate. However, the direct implication of myosin II in actin turnover is still not understood. Herein, we show, using high-resolution cryo-transmission electron microscopy, that myosin II motors control the turnover of actin bundles in a concentration-dependent manner in vitro. We demonstrate that disassembly of actin bundles occurs through two main stages: the first stage involves unbundling into individual filaments, and the second involves their subsequent depolymerization. These evidence suggest that, in addition to their “classical” contractile abilities, myosin II motors may be directly implicated in active actin depolymerization. We believe that myosin II motors may function similarly in vivo (e.g., in the disassembly of the contractile ring by fine tuning the local concentration/activity of myosin II motors).     

Yeast Translation Elongation Factor-1A Binds Vacuole-localized Rho1p to Facilitate Membrane Integrity through F-actin Remodeling

Rho GTPases are molecular switches that modulate a variety of cellular processes, most notably those involving actin dynamics. We have previously shown that yeast vacuolar membrane fusion requires re-organization of actin filaments mediated by two Rho GTPases, Rho1p and Cdc42p. Cdc42p initiates actin polymerization to facilitate membrane tethering; Rho1p has a role in the late stages of vacuolar fusion, but its mode of action is unknown. Here, we identified eEF1A as a vacuolar Rho1p-interacting protein. eEF1A (encoded by the TEF1 and TEF2 genes in yeast) is an aminoacyl-tRNA transferase needed during protein translation. eEF1A also has a second function that is independent of translation; it binds and organizes actin filaments into ordered cable structures. Here, we report that eEF1A interacts with Rho1p via a C-terminal subdomain. This interaction occurs predominantly when both proteins are in the GDP-bound state. Therefore, eEF1A is an atypical downstream effector of Rho1p. eEF1A does not promote vacuolar fusion; however, overexpression of the Rho1p-interacting subdomain affects vacuolar morphology. Vacuoles were destabilized and prone to leakage when treated with the eEF1A inhibitor narciclasine. We propose a model whereby eEF1A binds to Rho1p-GDP on the vacuolar membrane; it is released upon Rho1p activation and then bundles actin filaments to stabilize fused vacuoles. Therefore, the Rho1p-eEF1A complex acts to spatially localize a pool of eEF1A to vacuoles where it can readily organize F-actin.     

Mechanical Heterogeneity Favors Fragmentation of Strained Actin Filaments

We present a general model of actin filament deformation and fragmentation in response to compressive forces. The elastic free energy density along filaments is determined by their shape and mechanical properties, which were modeled in terms of bending, twisting, and twist-bend coupling elasticities. The elastic energy stored in filament deformation (i.e., strain) tilts the fragmentation-annealing reaction free-energy profile to favor fragmentation. The energy gradient introduces a local shear force that accelerates filament intersubunit bond rupture. The severing protein, cofilin, renders filaments more compliant in bending and twisting. As a result, filaments that are partially decorated with cofilin are mechanically heterogeneous (i.e., nonuniform) and display asymmetric shape deformations and energy profiles distinct from mechanically homogenous (i.e., uniform), bare actin, or saturated cofilactin filaments. The local buckling strain depends on the relative size of the compliant segment as well as the bending and twisting rigidities of flanking regions. Filaments with a single bare/cofilin-decorated boundary localize energy and force adjacent to the boundary, within the compliant cofilactin segment. Filaments with small cofilin clusters were predicted to fragment within the compliant cofilactin rather than at boundaries. Neglecting contributions from twist-bend coupling elasticity underestimates the energy density and gradients along filaments, and thus the net effects of filament strain to fragmentation. Spatial confinement causes compliant cofilactin segments and filaments to adopt higher deformation modes and store more elastic energy, thereby promoting fragmentation. The theory and simulations presented here establish a quantitative relationship between actin filament fragmentation thermodynamics and elasticity, and reveal how local discontinuities in filament mechanical properties introduced by regulatory proteins can modulate both the severing efficiency and location along filaments. The emergent behavior of mechanically heterogeneous filaments, particularly under confinement, emphasizes that severing in cells is likely to be influenced by multiple physical and chemical factors.     

Analysis of the formation mechanism for thermoresponsive-type coacervate with functional copolymers consisting of N-isopropylacrylamide and 2-hydroxyisopropylacrylamide

We now report the formation mechanism of the thermoresponsive-type coacervate with the novel functional temperature-sensitive polymer, poly(N-isopropylacrylamide-co-2-hydroxyisopropylacrylamide) (poly(NIPAAm-co-HIPAAm)), synthesized in our laboratory. The effects of introducing the hydrophilic comonomer (HIPAAm) into the copolymer chains and adding salts on the behaviors of the coacervate droplets induced in the poly(NIPAAm-co-HIPAAm) aqueous solutions were investigated. Not only the particle sizes of the coacervate droplets but also the cloud points of the copolymer solutions could be modulated by the HIPAAm content incorporated in the copolymers. Moreover, the particle sizes of the coacervate droplets were also changed by adding salts. Namely, the particle sizes increased with the decreasing HIPAAm composition and increasing NaCl concentration. In addition, the 1H NMR and differential scanning calorimetric measurements suggested that as the HIPAAm content decreased or NaCl concentration increased, dehydration of the copolymers induced in the phase transition and/or separation became much easier. Therefore, on the basis of the findings obtained from these measurements, we determined that the particle sizes of the coacervate droplets induced in the temperature-sensitive polymers increased as the number of the water molecules, which are dissociated from the polymeric chains during the phase transition and/or separation, increased. Besides, to examine the separation of the model solutes, the aqueous two-phase separation with the coacervate droplets of poly(NIPAAm-co-HIPAAm) was carried out. The partitions of Methyl Orange as a model solute under both acidic (pH 2) and basic (pH 12) conditions were performed. The amount of Methyl Orange partitioned into the coacervate droplets at pH 12 is much greater than that at pH 2, which indicated that the coacervate droplets could recognize a slight difference in the polarity or structure between the model solutes.     

Direct observation of actin filament severing by gelsolin and binding by gCap39 and CapZ

Dynamic behavior of actin filaments in cells is the basis of many different cellular activities. Remodeling of the actin filament network involves polymerization and depolymerization of the filaments. Proteins that regulate these behaviors include proteins that sever and/or cap actin filaments. This report presents direct observation of severing of fluorescently-labeled actin filaments. Coverslips coated with gelsolin, a multi-domain, calcium-dependent capping and severing protein, bound rhodamine-phalloidin-saturated filaments along their length in the presence of EGTA. Upon addition of calcium, attached filaments bent as they broke. Actophorin, a low molecular weight, monomer sequestering, calcium-independent severing protein did not sever phalloidin-saturated filaments. Both gCap 39, a gelsolin-like, calcium-dependent capping protein that does not sever filaments, and CapZ, a heterodimeric, non-calcium-dependent capping protein, bound the filaments by one end to the coverslip. Visualization of individual filaments also revealed severing activity present in mixtures of actin-binding proteins isolated by filamentous actin affinity chromatography from early Drosophila embryos. This activity was different from either gelsolin or actophorin because it was not inhibited by phalloidin, but was calcium independent. The results of these studies provide new information about the molecular mechanisms of severing and capping by well-characterized proteins as well as definition of a novel type of severing activity.     

Reactivities of Actin as a Contractile Protein

The molecular basis for the mechanism of contraction in striated muscle, with primary emphasis on the interaction between the thick and thin filaments and the role of the thin (actin) filaments, is the theme presented. Recent information relating to actin-myosin interaction points up the fact that definitive statements cannot be made regarding the molecular interaction(s) that lead to contraction. Nevertheless, the properties of actin indicate that (a) actin in the monomeric state has properties differing markedly from actin in the polymer (filament) state; (b) these property differences may be significant in the contractile process, for they include changes in the reactivity of the bound nucleotide and actin-myosin complex formation; (c) the bound nucleotide seems to be required in the contraction process. For these, and other, reasons discussed, the tentative hypothesis is advanced that the contraction reaction involves local changes in the actin filament providing local monomer or monomer-like actin units in the reaction with myosin.     

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.     

Cytoskeletal rearrangements and the functional role of T-plastin during entry of Shigella flexneri into HeLa cells

Shigella flexneri is an enteroinvasive bacterium which causes bacillary dysentery in humans. A major feature of its pathogenic potential is the capacity to invade epithelial cells. Shigella entry into epithelial cells is considered a parasite-induced internalization process requiring polymerization of actin. Here we describe the cytoskeletal rearrangements during S. flexneri invasion of HeLa cells. After an initial contact of the bacterium with the cell surface, distinct nucleation zones of heavy chain actin polymerization appear in close proximity to the contact site underneath the parasite with long filaments being polymerized. These structures then push cellular protrusions that rise beside the entering bacterium, being sustained by tightly bundled long actin filaments organized in parallel orientation with their positive ends pointing to the cytoplasmic membrane. Finally, the cellular projections coalesce above the bacterial body, leading to its internalization. In addition, we found the actin-bundling protein plastin to be concentrated in these protrusions. Since plastin is known to bundle actin filaments in parallel orientation, colocalization of parallel actin filaments and plastin in the cellular protrusions strongly suggested a functional role of this protein in the architecture of parasite-induced cellular projections. Using transfection experiments, we show the differential recruitment of the two plastin isoforms (T- and L-) into Shigella entry zones. By transient expression of a truncated T-plastin which is deprived of one of its actin-binding sites, we also demonstrate the functional role of T-plastin in Shigella entry into HeLa cells.     

Differential effects of actin cytoskeleton dynamics on equine infectious anemia virus particle production

Retrovirus assembly and budding involve a highly dynamic and concerted interaction of viral and cellular proteins. Previous studies have shown that retroviral Gag proteins interact with actin filaments, but the significance of these interactions remains to be defined. Using equine infectious anemia virus (EIAV), we now demonstrate differential effects of cellular actin dynamics at distinct stages of retrovirus assembly and budding. First, virion production was reduced when EIAV-infected cells were treated with phallacidin, a cell-permeable reagent that stabilizes actin filaments by slowing down their depolymerization. Confocal microscopy confirmed that the inhibition of EIAV production correlated temporally over several days with the incorporation dynamics of phallacidin into the actin cytoskeleton. Although the overall structure of the actin cytoskeleton and expression of viral protein appeared to be unaffected, phallacidin treatment dramatically reduced the amount of full-length Gag protein associated with the actin cytoskeleton. These data suggest that an association of full-length Gag proteins with de novo actin filaments might contribute to Gag assembly and budding. On the other hand, virion production was enhanced when EIAV-infected cells were incubated briefly (2 h) with the actin-depolymerizing drugs cytochalasin D and latrunculin B. Interestingly, the enhanced virion production induced by cytochalasin D required a functional late (L) domain, either the EIAV YPDL L-domain or the proline-rich L domains derived from human immunodeficiency virus type 1 or Rous sarcoma virus, respectively. Thus, depolymerization of actin filaments may be a common function mediated by retrovirus L domains during late stages of viral budding. Taken together, these observations indicate that dynamic actin polymerization and depolymerization may be associated with different stages of viral production.     

The cytokinesis formins from the nematode worm and fission yeast differentially mediate actin filament assembly

Formins drive actin filament assembly for diverse cellular processes including motility, establishing polarity, and cell division. To investigate the mechanism of contractile ring assembly in animal cells, we directly compared the actin assembly properties of formins required for cytokinesis in the nematode worm early embryo (CYK-1) and fission yeast (Cdc12p). Like Cdc12p and most other formins, CYK-1 nucleates actin filament assembly and remains processively associated with the elongating barbed end while facilitating the addition of profilin-actin above the theoretical diffusion-limited rate. However, specific properties differ significantly between Cdc12p and CYK-1. Cdc12p efficiently nucleates filaments that in the presence of profilin elongate at approximately the same rate as control filaments without formin (approximately 10.0 subunits/s). CYK-1 is an inefficient nucleator but allows filaments to elongate profilin-actin 6-fold faster than Cdc12p (approximately 60 subunits/s). Both Cdc12p and CYK-1 bind to pre-assembled actin filaments with low nanomolar affinity, but CYK-1 dissociates 2 orders of magnitude more quickly. However, CYK-1 rapidly re-associates with free barbed ends. Cdc12p allows barbed ends to elongate in the presence of excess capping protein, whereas capping protein inhibits CYK-1-mediated actin assembly. Therefore, these evolutionarily diverse formins can drive contractile ring assembly by a generally similar mechanism, but cells with unique dimensions and physical parameters might require proteins with carefully tuned actin assembly properties.