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PubMed1997

Genghis Khan (Gek) as a putative effector for Drosophila Cdc42 and regulator of actin polymerization.

The small GTPases Cdc42 and Rac regulate a variety of biological processes, including actin polymerization, cell proliferation, and JNK/mitogen-activated protein kinase activation, conceivably via distinct effectors. Whereas the effector for mitogen-activated protein kinase activation appears to be p65PAK, the identity of effector(s) for actin polymerization remains unclear. We have found a putative effector for Drosophila Cdc42, Genghis Khan (Gek), which binds to Dcdc42 in a GTP-dependent and effector domain-dependent manner. Gek contains a predicted serine/threonine kinase catalytic domain that is 63% identical to human myotonic dystrophy protein kinase and has protein kinase activities. It also possesses a large coiled-coil domain, a putative phorbol ester binding domain, a pleckstrin homology domain, and a Cdc42 binding consensus sequence that is required for its binding to Dcdc42. To study the in vivo function of gek, we generated mutations in the Drosophila gek locus. Egg chambers homozygous for gek mutations exhibit abnormal accumulation of F-actin and are defective in producing fertilized eggs. These phenotypes can be rescued by a wild-type gek transgene. Our results suggest that this multidomain protein kinase is an effector for the regulation of actin polymerization by Cdc42.

Actins↗

PubMed1996

Insulin and ATP stimulate actin polymerization in U937 cells by a wortmannin-sensitive mechanism.

ATP and insulin stimulate increases in phosphatidylinositol (3,4,5)-trisphosphate levels in myeloid-derived U937 cells. Quantification of FITC-phalloidin binding by fluorescence-activated cell sorting reveals that both ATP and insulin stimulate actin polymerization with distinctive kinetics in U937 cells. The response to ATP is rapid and dose-dependent with an EC50 of 200 nM, and is abolished by pre-incubation with the Ca2+ chelator BAPTA-AM. At 800 nM concentration, wortmannin, a potent inhibitor of phosphoinositide 3-kinase (PI3K), blocks the late, but not the early phase of actin polymerization stimulated by 100 nM ATP. Responses elicited by 10 micrograms/ml insulin are slower, smaller and more transient than responses to ATP, and are inhibited by preincubation with 100 nM wortmannin. Actin polymerization can also be stimulated by thapsigargin, but not by phorbol ester, providing further evidence for a role for Ca2+ in actin polymerization. These data implicate distinct Ca2+ and PI3K-mediated pathways in the regulation of actin polymerization.

Actins↗

PubMed1999

Inducible recruitment of Cdc42 or WASP to a cell-surface receptor triggers actin polymerization and filopodium formation.

BACKGROUND: Cdc42, a GTP-binding protein of the Rho family, controls actin cytoskeletal organization and helps to generate actin-based protruding structures, such as filopodia. In vitro, Cdc42 regulates actin polymerization by facilitating the creation of free barbed ends - the more rapidly growing ends of actin filaments - and subsequent elongation at these ends. The Wiskott- Aldrich syndrome protein, WASP, which has a pleckstrin-homology domain and a Cdc42/Rac-binding motif, has been implicated in cell signaling and cytoskeleton reorganization. We have investigated the consequences of local recruitment of activated Cdc42 or WASP to the plasma membrane. RESULTS: We used an activated Cdc42 protein that could be recruited to an engineered membrane receptor by adding rapamycin as a bridge, and added antibody-coupled beads to aggregate these receptors. Inducible recruitment of Cdc42 to clusters of receptors stimulated actin polymerization, resulting in the formation of membrane protrusions. Cdc42-induced protrusions were enriched in the vasodilator-stimulated phosphoprotein VASP and the focal-adhesion-associated proteins zyxin and ezrin. The Cdc42 effector WASP could also induce the formation of protrusions, albeit of different morphology. CONCLUSIONS: This is the first demonstration that the local recruitment of activated Cdc42 or its downstream effector, WASP, to a membrane receptor in whole cells is sufficient to trigger actin polymerization that results in the formation of membrane protrusions. Our data suggest that Cdc42-induced actin-based protrusions result from the local and serial recruitment of cytoskeletal proteins including zyxin, VASP, and ezrin.

Actins↗

PubMed1994

Induction of actin polymerization in permeabilized neutrophils. Role of ATP.

We have used streptolysin-O (SO)-permeabilized neutrophils to investigate the signal transduction pathway through which chemoattractants induce actin polymerization. Chemoattractants stimulate phosphorylation of various proteins and lipids but whether these phosphorylations are required for actin polymerization is not known. Addition of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) to SO-permeabilized neutrophils induced a doubling of the F-actin. This induction of F-actin, assayed by TRITC-labeled phalloidin binding, did not require the addition of ATP. Neither addition of apyrase to deplete residual ATP nor addition of ADP or UDP to compete with residual endogenous ATP inhibited significantly the GTP gamma S-induced polymerization. Addition of ATP on its own caused no increase in F-actin and did not affect the time course or concentration dependence of GTP gamma S-induced F-actin. Addition of ATP did increase the maximal amount of F-actin induced by GTP gamma S by about 20%. N-Formylnorleucylleucylphenalanine (formyl-peptide) in the presence of GTP, but not in its absence, also stimulated an increase in F-actin in SO-permeabilized cells. The F-actin induced by formyl-peptide plus GTP was inhibited by pertussis toxin. The induction did not require addition of ATP and addition of ADP to compete with residual ATP only slightly decreased the level of actin. However, addition of UDP significantly reduced the response to formyl-peptide plus GTP. Addition of ATP enhanced the increase in F-actin induced by optimal concentrations of GTP with formyl-peptide. ATP also lowered the apparent Km for GTP, but not for N-formyl peptide. The non-hydrolyzable ATP analog, adenosine 5'-(beta, gamma-imino)triphosphate, did not enhance the actin polymerization. Rather its presence inhibited the response induced by formyl-peptide plus GTP. The data suggest that actin polymerization can be induced by GTP gamma S in an manner that is largely ATP-independent. A role for ATP cannot be ruled out in the induction of actin polymerization by formyl-peptide plus GTP.

Actins↗

PubMed1981

Isolation of an inhibitor of actin polymerization from human polymorphonuclear leukocytes.

A large proportion of actin in extracts of human polymorphonuclear leukocytes is unpolymerized in the presence of 0.1 M KCl, a condition expected to promote maximal assembly of purified actin. Raising of the KCl concentration increases the extent of actin polymerization in leukocyte extracts, and, at KCl concentrations greater than or equal to 0.3 M, the polymer concentration is maximal and approaches that of purified muscle actin in 0.1 M KCl solution. After removal of the actin polymers from leukocyte extracts made 0.6 M in KCl, the supernatant fluid contains an activity which inhibits the extent of assembly of purified skeletal muscle actin in 0.1 M KCl solution. We have purified this activity by ion exchange and gel filtration chromatography. The purified inhibitor contains polypeptides of 65,000 and 62,000 daltons in a molar ratio of 1:2 as determined by polyacrylamide gel electrophoresis in dodecyl sulfate. The Stokes radius of 32 A, and S20,w of 4.8 of the native inhibitor suggest that the purified protein is a mixture of monomers of slightly different molecular weight. Substoichiometric concentrations of the inhibitor rapidly prevent the assembly of purified actin in 0.1 m KCl solution in a concentration-dependent manner, and the reduction in final viscosity remains constant over time. Raising the KCl concentration reverses the inhibition, as observed for actin polymerization in unfractionated extracts. The findings are consistent with the conclusion that this inhibitor protein is responsible for the large proportion of unpolymerized actin in extracts of polymorphonuclear leukocytes.

Actin Depolymerizing Factors↗

PubMed2000

Membrane raft association of CD47 is necessary for actin polymerization and protein kinase C theta translocation in its synergistic activation of T cells.

CD47 is a ubiquitously expressed membrane protein with an extracellular Ig domain and a multiple membrane-spanning domain that can synergize with antigen to induce interleukin (IL)-2 secretion by T lymphocytes. Ligation of CD47 induced actin polymerization and increased protein kinase Ctheta (PKCtheta) association with the cytoskeleton independent of antigen receptor ligation, but ligation of mutant forms of the molecule missing either the Ig domain or the multiple membrane-spanning domain did not. Simultaneous ligation of CD47 and CD3 led to additive effects on F-actin and synergistic effects on PKCtheta cytoskeletal association. Disruption of membrane rafts by removal of cholesterol with cyclodextrin blocked CD47-induced actin polymerization, and mutant forms of CD47 that localized poorly to rafts failed to effect cytoskeletal rearrangement. However, raft association alone was not sufficient, because a raft-localized CD47 Ig domain bound to the membrane by a glycan phosphoinositol anchor was unable to induce actin polymerization. A mutant form of CD47 without its Ig domain that did not induce actin polymerization or localize to rafts still enhanced T cell receptor (TCR)-dependent tyrosine phosphorylation of PLCgamma and associated Ca(2+) signaling but did not augment IL-2 secretion. Thus, CD47 synergy with TCR to increase [Ca(2+)](i) is independent of actin and rafts but is insufficient to explain CD47 cooperation with TCR in IL-2 synthesis. Full synergy with TCR requires CD47 localization to membrane rafts where ligation leads to TCR-independent signals causing actin polymerization and PKCtheta translocation.

Actins↗

PubMed2004

Inducible clustering of membrane-targeted SH3 domains of the adaptor protein Nck triggers localized actin polymerization.

BACKGROUND: SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood. RESULTS: We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42. CONCLUSIONS: We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

Actins↗

PubMed1987

Facilitation of actin polymerization by interfilamentous factors.

The polymerization of actin in low ionic strength buffer at 0 degrees C in the presence of 0.25 mM Mg2+ was studied by viscometry, turbidity and absorbance at 232 nm. Under these conditions, significant polymerization occurred only in the viscometer and not in isotropic mixtures. The polymerization rate with 0.25 mM MgCl2, as judged from shear viscosity, was equal to or greater than that observed with 0.1 M KCl and 0.25 mM MgCl2 at 0 degrees C, and was characterized by a longer nucleation period. Measurements of the turbidity at 350 nm (detecting filament formation and aggregation) and the absorbance at 232 nm (detecting conformational changes of the G-F transition) showed no evidence for polymerization or nucleation in a bulk solution at 0 degrees C when Mg2+ was added to 0.25 mM and, furthermore, F-actin nuclei were ineffective as seeds under these conditions. However, nucleation and polymerization by these criteria could be induced by raising the temperature to 20 degrees C. These results demonstrate the existence of narrow conditions when elongation of F-actin is dissociated from nucleation of oligomeric acceptor nuclei, even if monitored on the sub-molecular level (absorbance at 232 nm). Under these conditions, elongation appears to require anisotropic F-actin, i.e. that filaments are ordered by laminar flow.

Actins↗

PubMed1986

The role of the cytosolic free Ca2+ transient for fMet-Leu-Phe induced actin polymerization in human neutrophils.

We have addressed the important question as to if and how the cytosolic free Ca2+ concentration, [Ca2+]i, is involved in fMet-Leu-Phe induced actin polymerization in human neutrophils. Stimulation of human neutrophils with the chemotactic peptide (10(-7) M), known to result in a prompt rise of the [Ca2+]i to above 500 nM, also induced a rapid decrease of monomeric actin, G-actin, content (to 35% of basal) and increase of filamentous actin, F-actin, content (to 320% of basal). A reduction of the fMet-Leu-Phe induced [Ca2+]i transient to about 250 nM, resulted in a less pronounced decrease of G-actin content (to 80% of basal) and increase of F-actin content (to 235% of basal). A total abolishment of the chemotactic peptide induced [Ca2+]i rise, still led to a decrease of the G-actin content (to 85% of basal) and increase of F-actin (to 200% of basal). These results indicate that the [Ca2+]i rise is not an absolute requirement, but has a modulating role for the fMet-Leu-Phe induced actin polymerization. Another possible intracellular candidate for fMet-Leu-Phe induced actin polymerization is protein kinase C. However, direct activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) only resulted in a minor increase of F-actin content. The recent hypothesis that a metabolite of the polyphosphoinositide cycle, independently of [Ca2+]i and protein kinase C, is responsible for actin polymerization agrees well with these results and by the fact that preexposure to pertussis toxin totally abolished a subsequent increase of F-actin content induced by fMet-Leu-Phe.

Actin Cytoskeleton↗

PubMed2003

Mechanical constraint imposed on plasma membrane through transverse phospholipid imbalance induces reversible actin polymerization via phosphoinositide 3-kinase activation.

Platelets were used to explore the effect of membrane curvature induced by phospholipid excess on cell shape and on organization of the actin cytoskeleton. We showed that the addition of short chain analogues of phospholipids to the outer leaflet of plasma membrane of resting platelets immediately induced a shape change with long filopodia formation containing newly polymerized actin. Cells recovered rapidly their discoid shape and their initial F-actin content only with the phosphatidylserine analogue, which was transported to the inner leaflet by aminophospholipid translocase. Filopodia formation and actin polymerization were inhibited in platelets pre-incubated with cytochalasin D. Both wortmannin and LY294002, two unrelated inhibitors of phosphoinositide 3-kinase, considerably reduced actin polymerization and filopodia formation. Phospholipid imbalance was accompanied by a reversible translocation of phosphoinositide 3-kinase from cytoplasm to plasma membrane. In agreement with a role for PI 3-kinase, when phospholipids were added to platelets, PtdIns(3,4)P2 increased two-fold and Akt protein was partly phosphorylated. A similar shape change was also observed in nocodazole-treated L929 fibroblasts which were incubated with the similar phospholipid analogues. In those nucleated cells, where the microtubule cytoskeleton was disrupted, a major actin-dependent membrane extension was induced by addition of short chain phospholipids that required the functional integrity of PI 3-kinase. We conclude that any physical constraint acting on plasma membrane and resulting on local changes in membrane curvature is sufficient to initiate transient actin polymerization via phosphoinositide 3-kinase activation.

Actin Cytoskeleton↗

PubMed1986

Actin polymerization. The mechanism of action of cytochalasin D.

Fluorescence changes using actin covalently labeled with N-(1-pyrenyl)iodoacetamide have been used to determine the effect of cytochalasin D on actin polymerization. A mechanism for the effect of cytochalasin D on actin polymerization is presented, which explains the experimental observation of a cytochalasin D-induced increase in the initial rate of polymerization and a decrease in the final extent of the reaction. Central to this mechanism is the Mg2+-dependent formation of cytochalasin D-induced dimers. The dimers serve as nuclei to enhance the polymerization rate. Binding of Mg2+ to a low affinity site on the dimer induces a conformational change which can be observed as a rapid fluorescence increase. A subsequent time-dependent fluorescence decrease observed prior to polymerization appears to represent ATP hydrolysis resulting in dissociation of the dimer and release of actin monomers containing ADP. We postulate that a slow rate of exchange of ATP for bound ADP relative to hydrolysis results in the accumulation of monomers containing ADP. As these monomers have a high critical concentration, the final extent of polymerization is reduced dramatically. The Mg2+ dependence of the final extent of polymerization in the presence of cytochalasin D is also explained in the context of this mechanism.

Actins↗

PubMed2005

Actin polymerization kinetics, cap structure, and fluctuations.

Polymerization of actin proteins into dynamic structures is essential to eukaryotic cell life, motivating many in vitro experiments measuring polymerization kinetics of individual filaments. Here, we model these kinetics, accounting for all relevant steps revealed by experiment: polymerization, depolymerization, random ATP hydrolysis, and release of phosphate (P(i)). We relate filament growth rates to the dynamics of ATP-actin and ADP-P(i)-actin caps that develop at filament ends. At the critical concentration of the barbed end, c(crit), we find a short ATP cap and a long fluctuation-stabilized ADP-P(i) cap. We show that growth rates and the critical concentration at the barbed end are intimately related to cap structure and dynamics. Fluctuations in filament lengths are described by the length diffusion coefficient, D. Recently Fujiwara et al. [Fujiwara, I., Takahashi, S., Takaduma, H., Funatsu, T. & Ishiwata, S. (2002) Nat. Cell Biol. 4, 666-673] and Kuhn and Pollard [Kuhn, J. & Pollard, T. D. (2005) Biophys. J. 88, 1387-1402] observed large length fluctuations slightly above c(crit), provoking speculation that growth may proceed by oligomeric rather than monomeric on-off events. For the single-monomer growth process, we find that D exhibits a pronounced peak below c(crit), due to filaments alternating between capped and uncapped states, a mild version of the dynamic instability of microtubules. Fluctuations just above c(crit) are enhanced but much smaller than those reported experimentally. Future measurements of D as a function of concentration can help identify the origin of the observed fluctuations.

Actin Cytoskeleton↗

PubMed2001

A bacterial type III secretion system inhibits actin polymerization to prevent pore formation in host cell membranes.

The bacterial pathogen Yersinia pseudotuberculosis uses type III secretion machinery to translocate Yop effector proteins through host cell plasma membranes. A current model suggests that a type III translocation channel is inserted into the plasma membrane, and if Yops are not present to fill the channel, the channel will form a pore. We examined the possibility that Yops act within the host cell to prevent pore formation. Yop- mutants of Y.pseudotuberculosis were assayed for pore-forming activity in HeLa cells. A YopE- mutant exhibited high levels of pore-forming activity. The GTPase-downregulating function of YopE was required to prevent pore formation. YopE+ bacteria had increased pore-forming activity when HeLa cells expressed activated Rho GTPases. Pore formation by YopE- bacteria required actin polymerization. F-actin was concentrated at sites of contact between HeLa cells and YopE- bacteria. The data suggest that localized actin polymerization, triggered by the type III machinery, results in pore formation in cells infected with YopE- bacteria. Thus, translocated YopE inhibits actin polymerization to prevent membane damage to cells infected with wild-type bacteria.

Actins↗

PubMed2006

Actin polymerization in neutrophils from donors of peripheral blood stem cells: divergent effects of glycosylated and nonglycosylated recombinant human granulocyte colony-stimulating factor.

Neutrophil functions can be modified by Recombinant human G-CSF (rhG-CSF) treatment, with divergent effects on phagocytosis, motility, bactericidal activity, and surface molecule expression. Neutrophil morphology is modified by treatment with filgrastim (the nonglycosylated form of rhG-CSF), while it is not affected by lenograstim (the glycosylated type of rhG-CSF). Little information is available about actin polymerization in neutrophils from subjects treated with the two types of rhG-CSF. In the current paper we evaluated two groups of donors of peripheral blood stem cells (PBSC) for allogeneic transplantation. Ten subjects were treated with filgrastim and 10 with lenograstim to mobilize PBSC; 15 blood donors were evaluated as a control group. Actin polymerization (both spontaneous and fMLP-stimulated) was studied by a flow cytometric assay. A microscopic fluorescent assay was also carried out to evaluate F-actin distribution in neutrophils. We found that filgrastim induced an increased F-actin content in resting neutrophils, along with morphologic evidence for increased actin polymerization distributed principally at the cell membrane and frequently polarized in focal areas; in addition, fMLP was not able to induce further actin polymerization. On the contrary, treatment with lenograstim was associated with F-actin content, distribution, and polymerization kinetics indistinguishable from those displayed by control neutrophils. Such experimental results show that filgrastim and lenograstim display divergent effects also on neutrophil actin polymerization and provide further explanation for previous experimental findings.

Actins↗

PubMed2000

A complex of N-WASP and WIP integrates signalling cascades that lead to actin polymerization.

Wiskott-Aldrich syndrome protein (WASP) and N-WASP have emerged as key proteins connecting signalling cascades to actin polymerization. Here we show that the amino-terminal WH1 domain, and not the polyproline-rich region, of N-WASP is responsible for its recruitment to sites of actin polymerization during Cdc42-independent, actin-based motility of vaccinia virus. Recruitment of N-WASP to vaccinia is mediated by WASP-interacting protein (WIP), whereas in Shigella WIP is recruited by N-WASP. Our observations show that vaccinia and Shigella activate the Arp2/3 complex to achieve actin-based motility, by mimicking either the SH2/SH3-containing adaptor or Cdc42 signalling pathways to recruit the N-WASP-WIP complex. We propose that the N-WASP-WIP complex has a pivotal function in integrating signalling cascades that lead to actin polymerization.

Actin Cytoskeleton↗

PubMed1981

Spectrin-4.1-actin complex of the human erythrocyte: molecular basis of its ability to bind cytochalasins with high-affinity and to accelerate actin polymerization in vitro.

The spectrin-4.1-actin complex isolated from the cytoskeleton of human erythrocyte was found to be similar to muscle F-actin in several aspects: Both the complex and F-actin nucleate cytochalasin-sensitive actin polymerization; both bind dihydrocytochalasin B with similar binding contrasts; both can be depolymerized by DNase I with loss of cytochalasin binding activity. From these results, we conclude that the actin in the complex is in an oligomeric form. However, the presence of spectrin and band 4.1 in the complex not only stabilized the actin in the complex as evidenced by its resistance to depolymerization in low-ionic-strength conditions and to DNase I as compared with F-actin, but also altered the characteristics of the binding site(s) for cytochalasins believed to be located at the "barbed" (polymerizing) end of the oligomeric actin.

Actins↗

PubMed1987

Actin polymerization modifies stimulus-oxidase coupling in rat neutrophils.

Oxidase activity in rat neutrophils was monitored by oxygen consumption rate and luminol-dependent chemiluminescence. Two agents which inhibit actin polymerization, cytochalasin B and dihydrocytochalasin B, produced a marked enhancement (up to 10-fold) of oxidase activation induced by two Ca2+-dependent stimuli, chemotactic peptide and ionophore A23187. In contrast, activation by the calcium-independent stimulus, phorbol myristate acetate, was unaffected by these agents. Other agents that interact with the cytoskeleton, phalloidin and colchicine have no effect on activation by any stimulus tested. The effect of cytochalasin B, when added after stimulation by chemotactic peptide, was transient with t0.5 approx. 10 s. Similarly, the degree of actin polymerization following stimulation by chemotactic peptide was transient, decaying with a t0.5 of approx. 10 s. The half-maximal concentration of cytochalasin B for inhibition of actin polymerization was similar to that for enhancement of oxidase activation. It was concluded, therefore, that the intracellular Ca2+ rise in rat neutrophils that accompanies stimulation by chemotactic peptide affects actin polymerization in a manner that modifies oxidase activation.

Actins↗

PubMed1984

Actin polymerization. The effect of brevin on filament size and rate of polymerization.

Fluorescent probes covalently bound to actin or to the actin binding protein, brevin, have been utilized to provide information about actin filaments formed in the presence of brevin as well as about the effect of brevin on the rate of polymerization. At actin to brevin ratios of 10:1 to 100:1, the observed diffusion coefficients of filaments, as measured by fluorescence photobleaching recovery using rhodamine-labeled actin or fluorescein-labeled brevin, are similar to those calculated from theoretical considerations for rigid rods. At lower brevin concentrations, the observed diffusion coefficients for actin filaments are lower than predicted, indicating that the filament structure is closer to that observed in the absence of brevin where filaments are immobilized due to interactions between them. The fluorescein-labeled brevin was found to be about as effective in influencing actin polymerization as unlabeled brevin. Using pyrene-labeled actin, we show that brevin binds 2 mol of monomeric actin. We conclude that at sufficiently high brevin concentration there is one brevin molecule per actin filament. From measurements of the initial rate of polymerization at 5.9 microM actin in the presence of brevin, we calculate both the apparent elongation rate constant and dissociation rate constant from one end (presumably the slow-growing end) of the actin filament. The former is highly dependent on Mg2+ concentration while the latter is not.

Actins↗