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PubMed1993

Diminished actin polymerization by neutrophils from newborn infants.

During the newborn period, abnormalities of neutrophil (PMN) function predispose infants to serious bacterial disease. Actin is a major contributor to PMN shape change and motile behavior. To determine the mechanism underlying defects in newborn granulocyte polarity and chemotaxis, we investigated actin polymerization by cord blood PMN from healthy term infants and adult controls. F-actin (filamentous) content was quantified in the resting state and after stimulation by fluorescence-activated cell-sorter analysis of nitrobenzoxadiazole-phallacidin-stained cells. PMN from newborn infants demonstrated similar basal F-actin levels when compared with adults. N-formyl methionyl leucyl phenylalanine induced a marked increase in actin polymerization that was maximal at 30 s in both neonates and adults and that then declined slowly (depolymerization) over the following 10 min. However, the F-actin content of PMN from newborn infants was significantly diminished when compared with adults at 30 and 60 s after N-formyl methionyl leucyl phenylalanine stimulation (p < 0.05). Both the rate and dose response of N-formyl methionyl leucyl phenylalanine-induced actin polymerization were similar for adult and neonatal PMN. PMN from newborn infants also demonstrated significantly diminished actin polymerization when compared with adults 60 s after stimulation with platelet-activating factor (p < 0.05). Decreased concentrations of F-actin may help explain the observed abnormalities of PMN polarity and chemotaxis in healthy newborn infants.

Actins↗

PubMed1992

Effects of the neuronal phosphoprotein synapsin I on actin polymerization. II. Analytical interpretation of kinetic curves.

The general features of the kinetics of actin polymerization are investigated by mathematical models, with the aim of identifying the kinetically relevant parameters in the process and detecting and interpreting the alterations occurring in actin polymerization under various experimental conditions. Polymerization curves, obtained by following the increase in fluorescence of actin derivatized with N-(1-pyrenyl) iodoacetamide, are fitted using analytical equations derived from biochemical models of the actin polymerization process. Particular attention is given to the evaluation of the effects of the neuronal phosphoprotein synapsin I. The models obtained under various ionic conditions reveal that synapsin I interacts with actin in a very complex fashion, sharing some of the properties of classical nucleating proteins but displaying also actions not described previously for other actin-binding proteins. Synapsin I appears to bind G-actin with a very high stoichiometry (1:2-4), and the complex behaves as an F-actin nucleus, producing actin filaments under conditions where spontaneous polymerization is negligible. These actions of synapsin I are markedly affected by site-specific phosphorylation of the protein. An original transformation of the fluorescence data, which estimates the disappearance rate of actin monomer toward the critical concentration, is presented and shown to be of general usefulness for the study of actin-binding proteins.

Actins↗

PubMed2003

Force generation by actin polymerization II: the elastic ratchet and tethered filaments.

The motion of many intracellular pathogens is driven by the polymerization of actin filaments. The propulsive force developed by the polymerization process is thought to arise from the thermal motions of the polymerizing filament tips. Recent experiments suggest that the nucleation of actin filaments involves a phase when the filaments are attached to the pathogen surface by a protein complex. Here we extend the "elastic ratchet model" of Mogilner and Oster to incorporate these new findings. We apply this "tethered ratchet" model to derive the force-velocity relation for Listeria and discuss relations of our theoretical predictions to experimental measurements. We also discuss "symmetry breaking" dynamics observed in ActA-coated bead experiments, and the implications of the model for lamellipodial protrusion in migrating cells.

Actins↗

PubMed1998

Actin polymerization is required for negative feedback regulation of epidermal growth factor-induced signal transduction.

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of a variety of cells. To investigate the significance of this process for signal transduction, actin polymerization is inhibited by dihydrocytochalasin B (CB). CB almost completely abolishes EGF-induced actin polymerization, as assessed by quantitative confocal laser scanning microscopy. Under these conditions, EGF induces enhanced EGF receptor (EGFR) tyrosine kinase activity, as well as superinduction of the c-fos proto-oncogene. These data suggest that EGF-induced actin polymerization may be important for negative feedback regulation of signal transduction by the EGFR. The phosphorylation of Thr654 by protein kinase C (PKC) is a well-characterized negative feedback control mechanism for signal transduction by the EGFR tyrosine kinase. A synthetic peptide, corresponding to the regions flanking Thr654 of the EGFR, is used to analyze EGF stimulated PKC activity by incorporation of 32P into the peptide. Cotreatment of cells with CB and EGF results in a complete loss of EGF-induced phosphorylation of the peptide. These data suggest that actin polymerization is obligatory for negative feedback regulation of the EGFR tyrosine kinase through the C-kinase pathway.

Actins↗

PubMed2006

Syndapin oligomers interconnect the machineries for endocytic vesicle formation and actin polymerization.

Syndapins were proposed to interconnect the machineries for vesicle formation and actin polymerization, as they interact with dynamin and the Arp2/3 complex activator N-WASP (neural Wiskott-Aldrich syndrome protein). Syndapins, however, have only one Src homology 3 domain mediating both interactions. Here we show that syndapins self-associate via direct syndapin/syndapin interactions, providing a molecular mechanism for the coordinating role of syndapin. Cross-link studies with overexpressed and endogenous syndapins suggest that predominantly dimers form in vivo. Our analyses show that the N-terminal Fes/Cip4 homology domain but not the central coiled-coil domain is sufficient for oligomerization. Additionally, a second interface located further C-terminally mediated interactions with the N terminus. The Src homology 3 domain and the NPF region are not involved and thus available for further interactions interconnecting different syndapin binding partners. Our analyses showed that self-association is crucial for syndapin function. Both syndapin-mediated cytoskeletal rearrangements and endocytosis were disrupted by a self-association-deficient mutant. Consistent with a role of syndapins in linking actin polymerization bursts with endocytic vesicle formation, syndapin-containing complexes had a size of 300-500 kDa in gel filtration analysis and contained both dynamin and N-WASP. The existence of an interconnection of the GTPase dynamin with N-WASP via syndapin oligomers was demonstrated both by coimmunoprecipitations and by reconstitution at membranes in intact cells. The interconnection was disrupted by coexpression of syndapin mutants incapable of self-association. Syndapin oligomers may thus act as multivalent organizers spatially and temporally coordinating vesicle fission with local actin polymerization.

Actins↗

PubMed2002

Collagen I initiates endothelial cell morphogenesis by inducing actin polymerization through suppression of cyclic AMP and protein kinase A.

Collagen I provokes endothelial cells to assume a spindle-shaped morphology and to align into solid cord-like assemblies. These cords closely imitate the solid pre-capillary cords of embryonic angiogenesis, raising interesting questions about underlying mechanisms. Studies described here identify a critical mechanism beginning with collagen I ligation of integrins alpha(1)beta(1) and alpha(2)beta(1), followed by suppression of cyclic AMP and cyclic AMP (cAMP)-dependent protein kinase A, and marked induction of actin polymerization to form prominent stress fibers. In contrast to collagen I, laminin-1 neither suppressed cAMP nor protein kinase A activity nor induced actin polymerization or changes in cell shape. Moreover, fibroblasts did not respond to collagen I with changes in cAMP, actin polymerization, or cell shape, thus indicating that collagen signaling, as observed in endothelial cells, does not extend to all cell types. Pharmacological elevation of cAMP blocked collagen-induced actin polymerization and formation of cords by endothelial cells; conversely, pharmacological suppression of either cAMP or protein kinase A induced actin polymerization. Collectively, these studies identify a previously unrecognized and critical mechanism, involving suppression of cAMP-dependent protein kinase A and induction of actin polymerization, through which collagen I drives endothelial cell organization into multicellular pre-capillary cords.

Actins↗

PubMed2005

Differential regulation of cortactin and N-WASP-mediated actin polymerization by missing in metastasis (MIM) protein.

Missing in metastasis (MIM) gene encodes an actin binding protein that is expressed at low levels in a subset of malignant cell lines. MIM protein tagged by green fluorescent protein (GFP) colocalizes with cortactin, an Arp2/3 complex activator, and interacts directly with the SH3 domain of cortactin. Recombinant full-length MIM promotes markedly cortactin and Arp2/3 complex-mediated actin polymerization in an SH3 dependent manner. In contrast, MIM-CT, a short splicing variant of MIM, binds poorly to cortactin in vitro and is unable to enhance actin polymerization. Full-length MIM binds to G-actin with a similar affinity as N-WASP-VCA, a constitutively active form of N-WASP, and inhibits N-WASP-VCA-mediated actin polymerization as analysed in vitro. The significance of the association of MIM with cortactin and G-actin was evaluated in NIH3T3 cells expressing several MIM constructs. Overexpression of full-length wild-type MIM-GFP inhibited markedly the motility of NIH3T3 cells induced by PDGF and that of human vein umbilical endothelial cells induced by sphingosine 1 phosphate. However, an MIM mutant with deletion of the WH2 domain, which is responsible for G-actin binding, enhanced cell motility. The motility inhibition imposed by MIM was compromised in the cells overexpressing N-WASP. In contrast, deletion of an MIM proline-rich domain, which is required for an optimal binding to cortactin, substantiated the MIM-mediated inhibition of cell motility. These data imply that MIM regulates cell motility by modulating different Arp2/3 activators in a distinguished manner.

3T3 Cells↗

PubMed1997

Phosphatidic acid elicits calcium mobilization and actin polymerization through a tyrosine kinase-dependent process in human neutrophils: a mechanism for induction of chemotaxis.

Phospholipids mediate important effects as extracellular messengers in diverse biological systems. We investigated the effects of phosphatidic acid, a biologically active phospholipid potentially involved in the inflammatory process, on calcium mobilization and actin polymerization in human neutrophils and correlated these effects with induction of chemotactic migration. Intermediate-chain length phosphatidic acid (DiC10-PA) induced a biphasic increase in intracellular Ca2+ characterized by a rapid rise commencing immediately upon addition of stimulus followed by a secondary increase which, unlike the initial response, was eliminated by chelation of extracellular Ca2+. Neither of these responses were induced by C10-lysophosphatidic acid or diacylglycerol. The tyrosine kinase inhibitor herbimycin-A (5-10 microg/ml) completely blunted the initial but not the delayed response effected by DiC10-PA. Long-chain phosphatidic acid (DiC18:1) induced only an initial rapid increase in intracellular Ca2+ and this response was similarly markedly attenuated by herbimycin-A. Among several physiologically relevant phospholipids, only phosphatidic acid was able to induce Ca2+ mobilization; phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol -- used individually or in mixed liposomes -- were without effect. Phosphatidic acid conferred calcium-mobilizing activity upon inactive liposome preparations and phosphatidic acid-enriched cellular plasma membranes possessed similar calcium-mobilizing activity. Both DiC10-PA and DiC18:1-PA induced actin polymerization in neutrophils at rates which mirrored the influence of each agent on Ca2+ mobilization. Herbimycin-A blunted the initial increase in actin polymerization effected by phosphatidic acid but had no effect on the delayed, EGTA-sensitive phase. DiC10-PA and DiC18:1-PA also induced neutrophil migration along a concentration gradient. Phospholipids that failed to induce a calcium transient, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol, likewise failed to induce either actin polymerization or chemotactic migration. Unlike chemotaxis induced by zymosan-activated human serum, phosphatidate-induced chemotaxis was strongly inhibited by pretreatment of cells with herbimycin-A. Consistent with these observations, phosphatidic acid induced the tyrosine phosphorylation of several proteins as early as 10 s after stimulation. Phosphorylation of two distinct proteins with approximate molecular sizes of 72 and 82 kDa was inhibited by levels of herbimycin A used to effectively inhibit calcium mobilization, actin polymerization and chemotaxis. Thus, in neutrophilic leukocytes, extracellular phosphatidic acid induces a unique tyrosine kinase-based signalling pathway that results in calcium mobilization and actin polymerization. These processes may promote directed cellular migration as a consequence of the interaction of phosphatidic acid with neutrophil plasma membranes.

Actins↗

PubMed2002

Pressure-induced actin polymerization in vascular smooth muscle as a mechanism underlying myogenic behavior.

We hypothesize that actin polymerization within vascular smooth muscle (VSM) in response to increased intravascular pressure is a novel and previously unrecognized mechanism underlying arterial myogenic behavior. This hypothesis is based on the following observations. 1) Unlike skeletal or cardiac muscle, VSM contains a substantial pool of unpolymerized globular (G) actin whose function is not known. 2) The cytosolic concentration of G-actin is significantly reduced by an elevation in intravascular pressure, demonstrating the dynamic nature of actin within VSM and implying a shift in the F:G equilibrium in favor of F-actin. 3) Agents that inhibit actin polymerization and stabilize the cytoskeleton (cytochalasins and latrunculin) inhibit the development of myogenic tone and decrease the effectiveness of myogenic reactivity. 4) Depolymerization of F-actin with cytochalasin D causes VSM relaxation and increased G-actin content, whereas polymerization of F-actin with jasplakinolide causes VSM contraction and decreased G-actin content. These results are consistent with observations in other cell types in which actin dynamics have been implicated in contractility and/or motility. Actin filament formation in VSM may therefore underlie mechanotransduction and, by providing additional sites for interaction with myosin, enhance force production in response to pressure. Although the mechanism by which actin polymerization is stimulated by pressure is not known, it likely occurs via integrin-mediated activation of signal transduction pathways previously associated with VSM contraction (e.g., PKC activation, Rho A, and tyrosine phosphorylation).

Actins↗

PubMed1992

Effects of the neuronal phosphoprotein synapsin I on actin polymerization. I. Evidence for a phosphorylation-dependent nucleating effect.

Synapsin I is a synaptic vesicle-specific phosphoprotein which is able to bind and bundle actin filaments in a phosphorylation-dependent fashion. In the present paper we have analyzed the effects of synapsin I on the kinetics of actin polymerization and their modulation by site-specific phosphorylation of synapsin I. We found that dephosphorylated synapsin I accelerates the initial rate of actin polymerization and decreases the rate of filament elongation. The effect was observed at both low and high ionic strength, was specific for synapsin I, and was still present when polymerization was triggered by F-actin seeds. Dephosphorylated synapsin I was also able to induce actin polymerization and bundle formation in the absence of KCl and MgCl2. The effects of synapsin I were strongly decreased after its phosphorylation by Ca2+/calmodulin-dependent protein kinase II. These observations suggest that synapsin I has a phosphorylation-dependent nucleating effect on actin polymerization. The data are compatible with the view that changes in the phosphorylation state of synapsin I play a functional role in regulating the interactions between the nerve terminal cytoskeleton and synaptic vesicles in various stages of the exoendocytotic cycle.

Actins↗

PubMed1997

Does actin polymerization status modulate Ca2+ storage in human neutrophils? Release and coalescence of Ca2+ stores by cytochalasins.

The aim of this paper was to establish whether actin polymerization modulated cytosolic Ca2+ storage in human neutrophils. Over the concentration ranges which inhibit actin polymerization, cytochalasins A, B, and D liberated Ca2+ from membrane-bound stores within neutrophils. Two Ca2+ storage sites were identified in neutrophils by the accumulation of the Ca2+ binding probe, chlortetracycline: one at the center of the cell and the other at the cell periphery. Confocal imaging demonstrated that cytochalasins released Ca2+ from the neutrophil periphery, but not from the central Ca2+ store. Ca2+ store release was coupled to Ca2+ influx, suggesting that the peripheral site may be a physiological store containing a Ca2+ influx factor. 3,3'-Dihexyloxacarbocyanine iodide staining organelles, which correlate with Ca2+ release sites, coalesced in neutrophils after treatment with cytochalasins. We propose that peripheral Ca2+ storage sites are restricted from coalescence by cortical polymerized actin and that Ca2+ store coalescence and Ca2+ release are coupled events.

Actins↗

PubMed1996

Altered actin polymerization dynamics in various malignant cell types: evidence for differential sensitivity to cytochalasin B.

Using the DNase I inhibition assay, fluorimetric measurements, and immunoblot analysis, we studied quantitatively changes in the actin polymerization dynamics in primary cultures of normal and malignant human lymphocytes, normal human endometrial cells, and in various leukemic and endometrial adenocarcinoma cell lines. The G/total-actin ratio of malignant cells was found to be 1.37 to 1.81-fold higher compared to normal cells, indicating that malignant cells express reduced amounts of polymerized actin. The above findings were corroborated by fluorescence measurements of the amounts of rhodamine-phalloidin-labeled F-actin in normal and neoplastic cells, which showed significantly lower F-actin content in malignant cell preparations. Moreover, the total actin content, as quantitated by the DNase I inhibition assay and by immunoblot analysis, was found to be significantly decreased in the primary cultures of malignant human lymphocytes and endometrial cells when compared to the total actin levels in corresponding normal cells. Proliferation and viability measurements of normal and neoplastic cells in culture, treated equally with cytochalasin B (CB), revealed an increased susceptibility of malignant cells to this anticytoskeletal agent. This was not due to increased CB incorporation in neoplastic cells, as indicated by 3H-CB uptake experiments. In addition, fluorescence microscopy, in the presence of graded concentrations of CB, showed destabilization of microfilaments in the poorly differentiated endometrial adenocarcinoma HEC-50 cells, compared to the well-differentiated Ishikawa cells. In conclusion, all investigated malignant cells are characterized by: (a) higher G/total-actin ratio; (b) decreased F- and total-actin content; and (c) lower resistance to CB treatment. These quantitatively determined parameters may represent potential biochemical indicators reflecting malignant transformation. Moreover, it seems worthwhile to explore whether or not the differential sensitivity of malignant cells to anticytoskeletal drugs may provide a valuable approach to the manipulation of malignant cells.

Actins↗

PubMed1984

Effect of cAMP-elevating drugs on Ca2+ efflux and actin polymerization in peritoneal macrophages stimulated with N-formyl chemotactic peptide.

To investigate intracellular cAMP inhibitory mechanisms related to migration of guinea-pig peritoneal macrophages, we examined the effects of cAMP-elevating drugs on the Ca2+ efflux and actin polymerization in macrophages stimulated with fMet-Leu-Phe, a chemotactic peptide. The stimulation with 1 X 10(-8) M fMet-Leu-Phe enhanced the Ca2+ efflux, and induced actin polymerization. Dibutyryl cAMP, theophylline and papaverine, which continuously increased the levels of intracellular cAMP, inhibited the enhancement of Ca2+ efflux and induction of actin polymerization by fMet-Leu-Phe. On the other hand, isoproterenol, which transiently increased the cAMP level, inhibited only the early phase of Ca2+ efflux and not the actin polymerization. As additions of both cAMP and cAMP-dependent protein kinase did not modify the Ca2+ uptake of phagocytic vesicles, the inhibition of Ca2+ efflux by these drugs may be due to the inhibition of the Ca2+ release from the intracellular store site(s). The cAMP-elevating drugs increased the monomeric actin content without change in the total actin content, indicating an induction of the depolymerization of filamentous actin. From these findings, we conclude that the inhibition of macrophage migration induced by cAMP may be due to the inhibition of both the increase of intracellular Ca2+ concentration and actin polymerization. Furthermore, the intracellular levels of cAMP probably play a role in regulating actin states in the macrophages.

Actins↗

PubMed1996

Is actin polymerization relevant to neurosecretion? A study on neuroblastoma cells.

We have investigated the relevance of actin rearrangement to neurotransmitter release, in neuroblastoma cell line SH-SY5Y stimulated with carbachol (1 mM). Carbachol reversibly polymerizes actin and releases norepinephrine in undifferentiated SH-SY5Y cells as well as in tetradecanoylphorbol 13-acetate (16 nM) and retinoic acid (5 microM) differentiated SH-SY5Y cells. Microscopic analysis of F-actin distribution indicates polymerization of cortical actin. Prior treatment with iota toxin E from Clostridium perfringens inhibits both carbachol-induced actin polymerization and norepinephrine release, slightly affecting the basal actin network. These data suggest that actin polymerization is associated with norepinephrine release.

Actins↗

PubMed1988

Cooperative effect of tumor necrosis factor and gamma-interferon on chemotactic peptide receptor expression and stimulus-induced actin polymerization in HL-60 cells.

We studied the effect of tumor necrosis factor (TNF) and gamma interferon (IFN-gamma), alone and in combination, on the expression of chemotactic peptide receptors, stimulus-induced actin polymerization, hydrogen peroxide production (H2O2), and expression of nonspecific esterase (NSE) positivity in human promyelocytic leukemic cell line HL-60. These parameters were analyzed following a five-day culture with the cytokines. Chemotactic peptide receptor expression was studied using the fluoresceinated hexapeptide, formyl-norleucyl-leucyl-phenylalanyl-norleucyl-tyrosyl-lysine and flow cytometry. Actin polymerization, an important event required for chemotaxis and phagocytosis, was studied using NBD-phallacidin labeling, following stimulation with the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP) or phorbol myristate acetate (PMA). TNF increased the expression of chemotactic peptide receptors in a dose-dependent fashion, and there was good correlation between the receptor expression, stimulus-induced actin polymerization, H2O2 production, and NSE positivity. IFN-gamma was less potent in inducing all the parameters studied but exerted a positive cooperative effect when combined with TNF. IFN-gamma at high concentrations induced chemotactic peptide receptors comparable in magnitude to that seen with TNF but failed to prime these cells to undergo actin polymerization in response to FMLP or PMA. Undifferentiated HL-60 cells showed a decrease in F-actin content on stimulation with PMA. This suggests that protein kinase C might have a negative regulatory role in stimulus-induced actin polymerization. The observations reported here indicate that appropriate combinations of different inducing agents with different modes of action might be necessary to duplicate the functional abilities of mature phagocytic cells.

Actins↗

PubMed1993

Methylisocyanate and actin polymerization: the in vitro effects of carbamylation.

Uremia has been implicated in cataractogenesis due to protein carbamylation by cyanate derived from urea. The present study was designed to directly identify the effects of carbamylation on actin polymerization and the possible contribution to cataract formation. The susceptibility of actin to carbamylation is expected because of the 19 lysines distributed along its length. The lysines of actin were selectively carbamylated by methylisocyanate (MIC) at pH 8.0 and 4 degrees C and actin polymerization assayed by high-shear viscometry, fluorescence and transmission electron microscopy. Our results provide evidence that non-enzymatic carbamylation of the lysine residues prevents the polymerization of actin. In addition, this carbamylated actin inhibited the polymerization of nascent, unmodified actin. High-shear viscosity measurements demonstrated decreased initial apparent rates and decreased steady-states (final specific viscosities) of polymerization. Fluorescence measurements showed decreased relative intensities of fluorescence versus control and confirmed the inhibitory effects of carbamylation by MIC on the steady state of F-actin. Transmission electron microscopy (TEM) showed the presence of disorganized filaments when carbamylated actin was added to polymerizing unmodified actin. Our results suggest that carbamylation of actin can cause a loss of ordered filament structure and shape of the lens fiber cell, thus predisposing it to cataract development.

Actins↗

PubMed2006

Hem-1 complexes are essential for Rac activation, actin polymerization, and myosin regulation during neutrophil chemotaxis.

Migrating cells need to make different actin assemblies at the cell's leading and trailing edges and to maintain physical separation of signals for these assemblies. This asymmetric control of activities represents one important form of cell polarity. There are significant gaps in our understanding of the components involved in generating and maintaining polarity during chemotaxis. Here we characterize a family of complexes (which we term leading edge complexes), scaffolded by hematopoietic protein 1 (Hem-1), that organize the neutrophil's leading edge. The Wiskott-Aldrich syndrome protein family Verprolin-homologous protein (WAVE)2 complex, which mediates activation of actin polymerization by Rac, is only one member of this family. A subset of these leading edge complexes are biochemically separable from the WAVE2 complex and contain a diverse set of potential polarity-regulating proteins. RNA interference-mediated knockdown of Hem-1-containing complexes in neutrophil-like cells: (a) dramatically impairs attractant-induced actin polymerization, polarity, and chemotaxis; (b) substantially weakens Rac activation and phosphatidylinositol-(3,4,5)-tris-phosphate production, disrupting the (phosphatidylinositol-(3,4,5)-tris-phosphate)/Rac/F-actin-mediated feedback circuit that organizes the leading edge; and (c) prevents exclusion of activated myosin from the leading edge, perhaps by misregulating leading edge complexes that contain inhibitors of the Rho-actomyosin pathway. Taken together, these observations show that versatile Hem-1-containing complexes coordinate diverse regulatory signals at the leading edge of polarized neutrophils, including but not confined to those involving WAVE2-dependent actin polymerization.

Actins↗

PubMed1984

Characterization of the action of porcine brain profilin on actin polymerization.

When porcine brain actin is polymerized in either KCl/MgCl2 or KCl alone, porcine brain profilin prolongs the lag phase and inhibits the rate and extent of polymerization in a concentration-dependent manner. Profilin also decreases the elongation rate in a concentration-dependent manner. Moreover, addition of profilin to steady-state actin filaments causes slow depolymerization. All these actions of profilin are explainable by a monomer sequestering mechanism. The inhibition by profilin of both the extent of polymerization and the elongation rate is stronger in KCl alone than in KCl/MgCl2. Moreover, it was found that brain profilin inhibits the polymerization of brain actin more strongly than that of muscle actin. Brain 88K protein/actin complex (88K/A), which has been shown to cap the barbed end of actin filaments, potentiates the inhibitory action of profilin; i.e. the extent of polymerization is much more reduced by profilin in the presence of 88K/A than in its absence.

Actins↗