Asymmetry is defined during meiosis in the oocyte of the parthenogenetic nematode Diploscapter pachys

Asymmetric cell division is an essential feature of normal development and certain pathologies. The process and its regulation have been studied extensively in the Caenorhabditis elegans embryo, particularly how symmetry of the actomyosin cortical cytoskeleton is broken by a sperm-derived signal at fertilization, upstream of polarity establishment. Diploscapter pachys is the closest parthenogenetic relative to C. elegans, and D. pachys one-cell embryos also divide asymmetrically. However how polarity is triggered in the absence of sperm remains unknown. In post-meiotic embryos, we find that the nucleus inhabits principally one embryo hemisphere, the future posterior pole. When forced to one pole by centrifugation, the nucleus returns to its preferred pole, although poles appear identical as concerns cortical ruffling and actin cytoskeleton. The location of the meiotic spindle also correlates with the future posterior pole and slight actin enrichment is observed at that pole in some early embryos along with microtubule structures emanating from the meiotic spindle. Polarized location of the nucleus is not observed in pre-meiotic D. pachys oocytes. All together our results are consistent with the idea that polarity of the D. pachys embryo is attained during meiosis, seemingly based on the location of the meiotic spindle, by a mechanism that may be present but suppressed in C. elegans.

Capping protein is dispensable for polarized actin network growth and actin-based motility

Heterodimeric capping protein (CP) binds the rapidly growing barbed ends of actin filaments and prevents the addition (or loss) of subunits. Capping activity is generally considered to be essential for actin-based motility induced by Arp2/3 complex nucleation. By stopping barbed end growth, CP favors nucleation of daughter filaments at the functionalized surface where the Arp2/3 complex is activated, thus creating polarized network growth, which is necessary for movement. However, here using an in vitro assay where Arp2/3 complex-based actin polymerization is induced on bead surfaces in the absence of CP, we produce robust polarized actin growth and motility. This is achieved either by adding the actin polymerase Ena/VASP or by boosting Arp2/3 complex activity at the surface. Another actin polymerase, the formin FMNL2, cannot substitute for CP, showing that polymerase activity alone is not enough to override the need for CP. Interfering with the polymerase activity of Ena/VASP, its surface recruitment or its bundling activity all reduce Ena/VASP's ability to maintain polarized network growth in the absence of CP. Taken together, our findings show that CP is dispensable for polarized actin growth and motility in situations where surface-directed polymerization is favored by whatever means over the growth of barbed ends in the network.

Roles of Actin in the Morphogenesis of the Early Caenorhabditis elegans Embryo

The cell shape changes that ensure asymmetric cell divisions are crucial for correct development, as asymmetric divisions allow for the formation of different cell types and therefore different tissues. The first division of the Caenorhabditis elegans embryo has emerged as a powerful model for understanding asymmetric cell division. The dynamics of microtubules, polarity proteins, and the actin cytoskeleton are all key for this process. In this review, we highlight studies from the last five years revealing new insights about the role of actin dynamics in the first asymmetric cell division of the early C. elegans embryo. Recent results concerning the roles of actin and actin binding proteins in symmetry breaking, cortical flows, cortical integrity, and cleavage furrow formation are described.

Nesprin-2 accumulates at the front of the nucleus during confined cell migration

The mechanisms by which cells exert forces on their nuclei to migrate through openings smaller than the nuclear diameter remain unclear. We use CRISPR/Cas9 to fluorescently label nesprin-2 giant, which links the cytoskeleton to the nuclear interior. We demonstrate that nesprin-2 accumulates at the front of the nucleus during nuclear deformation through narrow constrictions, independently of the nuclear lamina. We find that nesprins are mobile at time scales similar to the accumulation. Using artificial constructs, we show that the actin-binding domain of nesprin-2 is necessary and sufficient for this accumulation. Actin filaments are organized in a barrel structure around the nucleus in the direction of movement. Using two-photon ablation and cytoskeleton-inhibiting drugs, we demonstrate an actomyosin-dependent pulling force on the nucleus from the front of the cell. The elastic recoil upon ablation is dampened when nesprins are reduced at the nuclear envelope. We thus show that actin redistributes nesprin-2 giant toward the front of the nucleus and contributes to pulling the nucleus through narrow constrictions, in concert with myosin.

Actin modulates shape and mechanics of tubular membranes

The actin cytoskeleton shapes cells and also organizes internal membranous compartments. In particular, it interacts with membranes for intracellular transport of material in mammalian cells, yeast, or plant cells. Tubular membrane intermediates, pulled along microtubule tracks, are formed during this process and destabilize into vesicles. While the role of actin in tubule destabilization through scission is suggested, literature also provides examples of actin-mediated stabilization of membranous structures. To directly address this apparent contradiction, we mimic the geometry of tubular intermediates with preformed membrane tubes. The growth of an actin sleeve at the tube surface is monitored spatiotemporally. Depending on network cohesiveness, actin is able to entirely stabilize or locally maintain membrane tubes under pulling. On a single tube, thicker portions correlate with the presence of actin. These structures relax over several minutes and may provide enough time and curvature geometries for other proteins to act on tube stability.

Adaptive F-Actin Polymerization and Localized ATP Production Drive Basement Membrane Invasion in the Absence of MMPs

Matrix metalloproteinases (MMPs) are associated with decreased patient prognosis but have failed as anti-invasive drug targets despite promoting cancer cell invasion. Through time-lapse imaging, optical highlighting, and combined genetic removal of the five MMPs expressed during anchor cell (AC) invasion in C. elegans, we find that MMPs hasten invasion by degrading basement membrane (BM). Though irregular and delayed, AC invasion persists in MMP- animals via adaptive enrichment of the Arp2/3 complex at the invasive cell membrane, which drives formation of an F-actin-rich protrusion that physically breaches and displaces BM. Using a large-scale RNAi synergistic screen and a genetically encoded ATP FRET sensor, we discover that mitochondria enrich within the protrusion and provide localized ATP that fuels F-actin network growth. Thus, without MMPs, an invasive cell can alter its BM-breaching tactics, suggesting that targeting adaptive mechanisms will be necessary to mitigate BM invasion in human pathologies.

Forces drive basement membrane invasion in Caenorhabditis elegans

During invasion, cells breach basement membrane (BM) barriers with actin-rich protrusions. It remains unclear, however, whether actin polymerization applies pushing forces to help break through BM, or whether actin filaments play a passive role as scaffolding for targeting invasive machinery. Here, using the developmental event of anchor cell (AC) invasion in Caenorhabditis elegans, we observe that the AC deforms the BM and underlying tissue just before invasion, exerting forces in the tens of nanonewtons range. Deformation is driven by actin polymerization nucleated by the Arp2/3 complex and its activators, whereas formins and cross-linkers are dispensable. Delays in invasion upon actin regulator loss are not caused by defects in AC polarity, trafficking, or secretion, as appropriate markers are correctly localized in the AC even when actin is reduced and invasion is disrupted. Overall force production emerges from this study as one of the main tools that invading cells use to promote BM disruption in C. elegans.

Dynamic stability of the actin ecosystem

In cells, actin filaments continuously assemble and disassemble while maintaining an apparently constant network structure. This suggests a perfect balance between dynamic processes. Such behavior, operating far out of equilibrium by the hydrolysis of ATP, is called a dynamic steady state. This dynamic steady state confers a high degree of plasticity to cytoskeleton networks that allows them to adapt and optimize their architecture in response to external changes on short time-scales, thus permitting cells to adjust to their environment. In this Review, we summarize what is known about the cellular actin steady state, and what gaps remain in our understanding of this fundamental dynamic process that balances the different forms of actin organization in a cell. We focus on the minimal steps to achieve a steady state, discuss the potential feedback mechanisms at play to balance this steady state and conclude with an outlook on what is needed to fully understand its molecular nature.

Invading, Leading and Navigating Cells in Caenorhabditis elegans: Insights into Cell Movement in Vivo

Highly regulated cell migration events are crucial during animal tissue formation and the trafficking of cells to sites of infection and injury. Misregulation of cell movement underlies numerous human diseases, including cancer. Although originally studied primarily in two-dimensional in vitro assays, most cell migrations in vivo occur in complex three-dimensional tissue environments that are difficult to recapitulate in cell culture or ex vivo Further, it is now known that cells can mobilize a diverse repertoire of migration modes and subcellular structures to move through and around tissues. This review provides an overview of three distinct cellular movement events in Caenorhabditis elegans-cell invasion through basement membrane, leader cell migration during organ formation, and individual cell migration around tissues-which together illustrate powerful experimental models of diverse modes of movement in vivo We discuss new insights into migration that are emerging from these in vivo studies and important future directions toward understanding the remarkable and assorted ways that cells move in animals.

Adaptive Actin Networks

Despite their fundamental importance in the regulation of cell physiology, the mechanisms that confer cell adaptability to changes in the microenvironment are poorly understood. A recent study in Cell (Mueller et al., 2017) examines the capability of branched actin networks to respond and adapt to mechanical load in vivo.

Cell-sized liposome doublets reveal active tension build-up driven by acto-myosin dynamics

Cells modulate their shape to fulfill specific functions, mediated by the cell cortex, a thin actin shell bound to the plasma membrane. Myosin motor activity, together with actin dynamics, contributes to cortical tension. Here, we examine the individual contributions of actin polymerization and myosin activity to tension increase with a non-invasive method. Cell-sized liposome doublets are covered with either a stabilized actin cortex of preformed actin filaments, or a dynamic branched actin network polymerizing at the membrane. The addition of myosin II minifilaments in both cases triggers a change in doublet shape that is unambiguously related to a tension increase. Preformed actin filaments allow us to evaluate the effect of myosin alone while, with dynamic actin cortices, we examine the synergy of actin polymerization and myosin motors in driving shape changes. Our assay paves the way for a quantification of tension changes triggered by various actin-associated proteins in a cell-sized system.

Membrane tension and cytoskeleton organization in cell motility

Cell membrane shape changes are important for many aspects of normal biological function, such as tissue development, wound healing and cell division and motility. Various disease states are associated with deregulation of how cells move and change shape, including notably tumor initiation and cancer cell metastasis. Cell motility is powered, in large part, by the controlled assembly and disassembly of the actin cytoskeleton. Much of this dynamic happens in close proximity to the plasma membrane due to the fact that actin assembly factors are membrane-bound, and thus actin filaments are generally oriented such that their growth occurs against or near the membrane. For a long time, the membrane was viewed as a relatively passive scaffold for signaling. However, results from the last five years show that this is not the whole picture, and that the dynamics of the actin cytoskeleton are intimately linked to the mechanics of the cell membrane. In this review, we summarize recent findings concerning the role of plasma membrane mechanics in cell cytoskeleton dynamics and architecture, showing that the cell membrane is not just an envelope or a barrier for actin assembly, but is a master regulator controlling cytoskeleton dynamics and cell polarity.

WAVE binds Ena/VASP for enhanced Arp2/3 complex-based actin assembly

A dual in vitro/in vivo approach is used to show that WAVE directly binds Ena/VASP, coordinating its activity with that of the Arp2/3 complex for enhanced actin assembly.

The WAVE complex is the main activator of the Arp2/3 complex for actin filament nucleation and assembly in the lamellipodia of moving cells. Other important players in lamellipodial protrusion are Ena/VASP proteins, which enhance actin filament elongation. Here we examine the molecular coordination between the nucleating activity of the Arp2/3 complex and the elongating activity of Ena/VASP proteins for the formation of actin networks. Using an in vitro bead motility assay, we show that WAVE directly binds VASP, resulting in an increase in Arp2/3 complex–based actin assembly. We show that this interaction is important in vivo as well, for the formation of lamellipodia during the ventral enclosure event of Caenorhabditis elegans embryogenesis. Ena/VASP's ability to bind F-actin and profilin-complexed G-actin are important for its effect, whereas Ena/VASP tetramerization is not necessary. Our data are consistent with the idea that binding of Ena/VASP to WAVE potentiates Arp2/3 complex activity and lamellipodial actin assembly.

Extending the molecular clutch beyond actin-based cell motility

Many cell movements occur via polymerization of the actin cytoskeleton beneath the plasma membrane at the front of the cell, forming a protrusion called a lamellipodium, while myosin contraction squeezes forward the back of the cell. In what is known as the "molecular clutch" description of cell motility, forward movement results from the engagement of the acto-myosin motor with cell-matrix adhesions, thus transmitting force to the substrate and producing movement. However during cell translocation, clutch engagement is not perfect, and as a result, the cytoskeleton slips with respect to the substrate, undergoing backward (retrograde) flow in the direction of the cell body. Retrograde flow is therefore inversely proportional to cell speed and depends on adhesion and acto-myosin dynamics. Here we asked whether the molecular clutch was a general mechanism by measuring motility and retrograde flow for the Caenorhabditis elegans sperm cell in different adhesive conditions. These cells move by adhering to the substrate and emitting a dynamic lamellipodium, but the sperm cell does not contain an acto-myosin cytoskeleton. Instead the lamellipodium is formed by the assembly of Major Sperm Protein (MSP), which has no biochemical or structural similarity to actin. We find that these cells display the same molecular clutch characteristics as acto-myosin containing cells. We further show that retrograde flow is produced both by cytoskeletal assembly and contractility in these cells. Overall this study shows that the molecular clutch hypothesis of how polymerization is transduced into motility via adhesions is a general description of cell movement regardless of the composition of the cytoskeleton.

Actin dynamics, architecture, and mechanics in cell motility

Tight coupling between biochemical and mechanical properties of the actin cytoskeleton drives a large range of cellular processes including polarity establishment, morphogenesis, and motility. This is possible because actin filaments are semi-flexible polymers that, in conjunction with the molecular motor myosin, can act as biological active springs or "dashpots" (in laymen's terms, shock absorbers or fluidizers) able to exert or resist against force in a cellular environment. To modulate their mechanical properties, actin filaments can organize into a variety of architectures generating a diversity of cellular organizations including branched or crosslinked networks in the lamellipodium, parallel bundles in filopodia, and antiparallel structures in contractile fibers. In this review we describe the feedback loop between biochemical and mechanical properties of actin organization at the molecular level in vitro, then we integrate this knowledge into our current understanding of cellular actin organization and its physiological roles.

Actin polymerization or myosin contraction: two ways to build up cortical tension for symmetry breaking

Cells use complex biochemical pathways to drive shape changes for polarization and movement. One of these pathways is the self-assembly of actin filaments and myosin motors that together produce the forces and tensions that drive cell shape changes. Whereas the role of actin and myosin motors in cell polarization is clear, the exact mechanism of how the cortex, a thin shell of actin that is underneath the plasma membrane, can drive cell shape changes is still an open question. Here, we address this issue using biomimetic systems: the actin cortex is reconstituted on liposome membranes, in an 'outside geometry'. The actin shell is either grown from an activator of actin polymerization immobilized at the membrane by a biotin-streptavidin link, or built by simple adsorption of biotinylated actin filaments to the membrane, in the presence or absence of myosin motors. We show that tension in the actin network can be induced either by active actin polymerization on the membrane via the Arp2/3 complex or by myosin II filament pulling activity. Symmetry breaking and spontaneous polarization occur above a critical tension that opens up a crack in the actin shell. We show that this critical tension is reached by growing branched networks, nucleated by the Arp2/3 complex, in a concentration window of capping protein that limits actin filament growth and by a sufficient number of motors that pull on actin filaments. Our study provides the groundwork to understanding the physical mechanisms at work during polarization prior to cell shape modifications.

Membrane tension regulates motility by controlling lamellipodium organization

Many cell movements proceed via a crawling mechanism, where polymerization of the cytoskeletal protein actin pushes out the leading edge membrane. In this model, membrane tension has been seen as an impediment to filament growth and cell motility. Here we use a simple model of cell motility, the Caenorhabditis elegans sperm cell, to test how membrane tension affects movement and cytoskeleton dynamics. To enable these analyses, we create transgenic worm strains carrying sperm with a fluorescently labeled cytoskeleton. Via osmotic shock and deoxycholate treatments, we relax or tense the cell membrane and quantify apparent membrane tension changes by the membrane tether technique. Surprisingly, we find that membrane tension reduction is correlated with a decrease in cell displacement speed, whereas an increase in membrane tension enhances motility. We further demonstrate that apparent polymerization rates follow the same trends. We observe that membrane tension reduction leads to an unorganized, rough lamellipodium, composed of short filaments angled away from the direction of movement. On the other hand, an increase in tension reduces lateral membrane protrusions in the lamellipodium, and filaments are longer and more oriented toward the direction of movement. Overall we propose that membrane tension optimizes motility by streamlining polymerization in the direction of movement, thus adding a layer of complexity to our current understanding of how membrane tension enters into the motility equation.

Relaxing the actin cytoskeleton for adhesion and movement with Ena/VASP

At cell–cell contacts, as well as at the leading edge of motile cells, the plasticity of actin structures is maintained, in part, through labile connections to the plasma membrane. Here we explain how and why Drosophila enabled/vasodilator stimulated phosphoprotein (Ena/VASP) proteins are candidates for driving this cytoskeleton modulation under the membrane.

Phosphorylation on Ser5 increases the F-actin-binding activity of L-plastin and promotes its targeting to sites of actin assembly in cells

L-plastin, a malignant transformation-associated protein, is a member of a large family of actin filament cross-linkers. Here, we analysed how phosphorylation of L-plastin on Ser5 of the headpiece domain regulates its intracellular distribution and its interaction with F-actin in transfected cells and in in vitro assays. Phosphorylated wild-type L-plastin localised to the actin cytoskeleton in transfected Vero cells. Ser5Ala substitution reduced the capacity of L-plastin to localise with peripheral actin-rich membrane protrusions. Conversely, a Ser5Glu variant mimicking a constitutively phosphorylated state, accumulated in actin-rich regions and promoted the formation of F-actin microspikes in two cell lines. Similar to phosphorylated wild-type L-plastin, this variant remained associated with cellular F-actin in detergent-treated cells, whereas the Ser5Ala variant was almost completely extracted. When compared with non-phosphorylated protein, phosphorylated L-plastin and the Ser5Glu variant bound F-actin more efficiently in an in vitro assay. Importantly, expression of L-plastin elicited collagen invasion in HEK293T cells, in a manner dependent on Ser5 phosphorylation. Based on our findings, we propose that conversely to other calponin homology (CH)-domain family members, phosphorylation of L-plastin switches the protein from a low-activity to a high-activity state. Phosphorylated L-plastin might act as an integrator of signals controlling the assembly of the actin cytoskeleton and cell motility in a 3D-space.

VASP governs actin dynamics by modulating filament anchoring

Actin filament dynamics at the cell membrane are important for cell-matrix and cell-cell adhesions and the protrusion of the leading edge. Since actin filaments must be connected to the cell membrane to exert forces but must also detach from the membrane to allow it to move and evolve, the balance between actin filament tethering and detachment at adhesion sites and the leading edge is key for cell shape changes and motility. How this fine tuning is performed in cells remains an open question, but possible candidates are the Drosophila enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family of proteins, which localize to dynamic actin structures in the cell. Here we study VASP-mediated actin-related proteins 2/3 (Arp2/3) complex-dependent actin dynamics using a substrate that mimics the fluid properties of the cell membrane: an oil-water interface. We show evidence that polymerization activators undergo diffusion and convection on the fluid surface, due to continual attachment and detachment to the actin network. These dynamics are enhanced in the presence of VASP, and we observe cycles of catastrophic detachment of the actin network from the surface, resulting in stop-and-go motion. These results point to a role for VASP in the modulation of filament anchoring, with implications for actin dynamics at cell adhesions and at the leading edge of the cell.

The actin slingshot

Actin polymerization generates the force that deforms the cell membrane, pulls the cell forward and propels endosomes and bacteria within the cell. The mechanism of force generation has been probed using experimental biomimetic systems where force generation and movement occur by the same actin-polymerization processes observed in cells. The advantage of such systems over living cells is that their physical properties can be changed, such as the size of the load, its composition and its deformability, in order to respond to specific questions. Recent experimental developments and associated theoretical models have provided us with a better understanding of motility based on actin polymerization. This paves the way towards a better comprehension of cell motility.

Stress release drives symmetry breaking for actin-based movement

By using a simple assay composed of purified proteins, we studied the spontaneous polarization of actin networks polymerizing on spherical beads, which subsequently undergo movement. We show evidence that this symmetry breaking is based on the release of elastic energy, analogous to the fracture of polymer gels. The dynamics of this process and the thickness at which it occurs depend on the growth rate and mechanical properties of the actin gel. We explain our experimental results with a model based on elasticity theory and fracture mechanics.

Actin-filament cross-linking protein T-plastin increases Arp2/3-mediated actin-based movement

Increasing evidence suggests that actin cross-linking or bundling proteins might not only structure the cortical actin cytoskeleton but also control actin dynamics. Here, we analyse the effects of T-plastin/T-fimbrin, a representative member of an important actin-filament cross-linking protein by combining a quantitative biomimetic motility assay with biochemical and cell-based approaches. Beads coated with the VCA domain of the Wiskott/Aldrich-syndrome protein (WASP) recruit the actin-nucleating Arp2/3 complex, polymerize actin at their surface and undergo movement when placed in cell-free extracts. T-Plastin increased the velocity of VCA beads 1.5 times, stabilized actin comets and concomitantly displaced cofilin, an actin-depolymerizing protein. T-Plastin also decreased the F-actin disassembly rate and inhibited cofilin-mediated depolymerization of actin filaments in vitro. Importantly, a bundling-incompetent variant comprising the first actin-binding domain (ABD1) had similar effects. In cells, this domain induced the formation of long actin cables to which other actin-regulating proteins were recruited. Altogether, these results favor a mechanism in which binding of ABD1 controls actin turnover independently of cross-link formation. In vivo, this activity might contribute to the assembly and maintenance of the actin cytoskeleton of plasma-membrane protrusions.

Actin Filaments Align into Hollow Comets for Rapid VASP-Mediated Propulsion

For cells, the growth of a dense array of branched actin filaments organized by the actin-related proteins 2 and 3 (Arp2/3) complex at the plasma membrane offers an explanation as to how movement is produced, and this arrangement is considered to be optimal for motility. Here, we challenged this assumption by using an in vitro system of polystyrene beads in cell extracts that contained a complex mix of actin polymerization proteins as in vivo. We employed the surface of the bead as a reactor where we mixed two different actin polymerization-activating factors, the Arp2/3 complex and the vasodilator-stimulated phosphoprotein (VASP), to examine their contribution to actin-based movement and filament organization. We varied the coating of the bead surface but left the extracts identical for all assays. We found that the degree of filament alignment in the actin comet tails depended on the surface ratio of VASP to Arp2/3. Alignment of actin filaments parallel to the direction of bead movement in the presence of VASP was accompanied by an abrupt 7-fold increase in velocity that was independent of bead size and by hollowing out of the comets. The actin filament-bundling proteins fimbrin and fascin did not appear to play a role in this transformation. Together with the idea that VASP enhances filament detachment and with the presence of pulling forces at the rear of the bead, a mesoscopic analysis of movement provides a possible explanation for our results.

The effect of diffusion, depolymerization and nucleation promoting factors on actin gel growth

In eukaryotic cells, localized actin polymerization is able to deform the plasma membrane and push the cell forward. Depolymerization of actin filaments and diffusion of actin monomers ensure the availability of monomers at sites of polymerization, and therefore these processes must play an active role in cellular actin dynamics. Here we reveal experimental evidence that actin gel growth can be limited by monomer diffusion, consistent with theoretical predictions. We study actin gels formed on beads coated with ActA (and ActA fragments), the bacterial factor responsible for actin-based movement of Listeria monocytogenes. We observe a saturation of gel thickness with increasing bead radius, the signature of diffusion control. Data analysis using an elastic model of actin gel growth gives an estimate of 2x10(-8) cm(-2) s(-1) for the diffusion coefficient of actin monomers through the gel, ten times less than in buffer, and in agreement with literature values in bulk cytoskeleton, providing corroboration of our model. The depolymerization rate of actin filaments and the elastic modulus of the gel are also evaluated. Furthermore, we qualitatively examine the different actin gels produced when ActA fragments interact with either VASP or the Arp2/3 complex.

Mechanistic comparison of the cobalt-substituted and wild-type copper amine oxidase from Hansenula polymorpha

A recent report by Mills and Klinman [Mills, S. A., and Klinman, J. P. (2000) J. Am. Chem. Soc. 122, 9897-9904] described the preparation and initial characterization of a cobalt-substituted form of the copper amine oxidase from Hansenula polymorpha (HPAO). This enzyme was found to be fully catalytically active at saturating substrate concentrations, but with a K(m) for O(2) approximately 70-fold higher than that of the copper-containing, wild-type enzyme. Herein, we report a detailed analysis of the mechanism of catalysis for the wild-type and the cobalt-substituted forms of HPAO. Both forms of enzyme are concluded to utilize the same mechanism for oxygen reduction, involving initial, rate-limiting electron transfer from the reduced cofactor of the enzyme to prebound dioxygen. Superoxide formed in this manner is stabilized by the active site metal, facilitating the transfer of a second electron and two protons to form the product hydrogen peroxide. The elevated K(m) for O(2) at the dioxygen binding site in Co-substituted HPAO, relative to that of wild-type HPAO, is proposed to be due to a change in the net charge at the adjacent metal site from +1 (cupric hydroxide) in wild-type enzyme to +2 (cobaltous H(2)O) in cobalt-substituted HPAO.

ActA and human zyxin harbour Arp2/3-independent actin-polymerization activity

The actin cytoskeleton is a dynamic network that is composed of a variety of F-actin structures. To understand how these structures are produced, we tested the capacity of proteins to direct actin polymerization in a bead assay in vitro and in a mitochondrial-targeting assay in cells. We found that human zyxin and the related protein ActA of Listeria monocytogenes can generate new actin structures in a vasodilator-stimulated phosphoprotein-dependent (VASP) manner, but independently of the Arp2/3 complex. These results are consistent with the concept that there are multiple actin-polymerization machines in cells. With these simple tests it is possible to probe the specific function of proteins or identify novel molecules that act upon cellular actin polymerization.

An unexpected role for the active site base in cofactor orientation and flexibility in the copper amine oxidase from Hansenula polymorpha

The role of the active site aspartate base in the aminotransferase mechanism of the copper amine oxidase from the yeast Hansenula polymorpha has been probed by site-directed mutagenesis. The D319E mutant catalyzes the oxidation of methylamine and phenethylamine, but not that of benzylamine. kcat/Km for methylamine is found to be 80-fold reduced compared to that of the wild type. Viscosogen and substrate and solvent deuteration have no effect on this parameter for D319E, which is suggestive of limitation of kcat/Km by a conformational change. This conformational change is proposed to be the movement of the cofactor into a productive orientation upon the binding of substrate. In the absence of substrate, a flipped cofactor orientation is likely, on the basis of resonance Raman evidence that the C5 carbonyl of the cofactor is less solvent accessible than the C3 hydrogen. kcat for D319E methylamine oxidase is reduced 200-fold compared to that of the wild type and is unaffected by substrate deuteration, but displays a substantial solvent isotope effect. A 428 nm absorbance is evident under conditions of saturating methylamine and oxygen with D319E. The D319N mutant is observed to produce a similar absorbance at 430 nm when treated with ammonia despite the fact that this mutant has no amine oxidase activity. Resonance Raman spectroscopy indicates the formation of a covalent ammonia adduct and identifies it as the deprotonated iminoquinone. In contrast, when the D319E mutant is reacted with ammonia, it gives predominantly a 340-350 nm species. This absorbance is ascribed to a localization of the cofactor oxyanion induced by binding of the cation at the active site and not to covalent adduct formation. Resonance Raman spectroscopic examination of the steady state species of D319E methylamine oxidation, in combination with the kinetic data, indicates that the 428 nm species is the deprotonated iminoquinone produced upon reoxidation of the reduced cofactor. A model is proposed in which a central role of the active site base is to position the free cofactor and several enzyme intermediates for optimal activity.

Limited proteolysis of Hansenula polymorpha yeast amine oxidase: isolation of a C-terminal fragment containing both a copper and quino-cofactor

Limited proteolysis of recombinant Hansenula polymorpha yeast amino oxidase produces a 48 kDa fragment which corresponds to the C-terminal two-thirds of the protein. The fragment contains both TOPA (2,4,5-trihydroxyphenylalanine) and copper, as well as the histidine ligands implicated in copper binding. The fragment is proposed to be the domain responsible for cofactor production in yeast amine oxidase.