Effects of macrophage profilin on actin in the presence and absence of acumentin and gelsolin

The molecular mechanism of load adaptation by branched actin networks

Branched actin networks are self-assembling molecular motors that move biological membranes and drive many important cellular processes, including phagocytosis, endocytosis, and pseudopod protrusion. When confronted with opposing forces, the growth rate of these networks slows and their density increases, but the stoichiometry of key components does not change. The molecular mechanisms governing this force response are not well understood, so we used single-molecule imaging and AFM cantilever deflection to measure how applied forces affect each step in branched actin network assembly. Although load forces are observed to increase the density of growing filaments, we find that they actually decrease the rate of filament nucleation due to inhibitory interactions between actin filament ends and nucleation promoting factors. The force-induced increase in network density turns out to result from an exponential drop in the rate constant that governs filament capping. The force dependence of filament capping matches that of filament elongation and can be explained by expanding Brownian Ratchet theory to cover both processes. We tested a key prediction of this expanded theory by measuring the force-dependent activity of engineered capping protein variants and found that increasing the size of the capping protein increases its sensitivity to applied forces. In summary, we find that Brownian Ratchets underlie not only the ability of growing actin filaments to generate force but also the ability of branched actin networks to adapt their architecture to changing loads.

Effects of actin-binding proteins on the thermal stability of monomeric actin

Differential scanning calorimetry (DSC) was applied to investigate the thermal unfolding of rabbit skeletal muscle G-actin in its complexes with actin-binding proteins, cofilin, twinfilin, and profilin. The results show that the effects of these proteins on the thermal stability of G-actin depend on the nucleotide, ATP or ADP, bound in the nucleotide-binding cleft between actin subdomains 2 and 4. Interestingly, cofilin binding stabilizes both ATP-G-actin and ADP-G-actin, whereas twinfilin increases the thermal stability of the ADP-G-actin but not that of the ATP-G-actin. By contrast, profilin strongly decreases the thermal stability of the ATP-G-actin but has no appreciable effect on the ADP-G-actin. Comparison of these DSC results with literature data reveals a relationship between the effects of actin-binding proteins on the thermal unfolding of G-actin, stabilization or destabilization, and their effects on the rate of nucleotide exchange in the nucleotide-binding cleft, decrease or increase. These results suggest that the thermal stability of G-actin depends, at least partially, on the conformation of the nucleotide-binding cleft: the actin molecule is more stable when the cleft is closed, while an opening of the cleft leads to significant destabilization of G-actin. Thus, DSC studies of the thermal unfolding of G-actin can provide new valuable information about the conformational changes induced by actin-binding proteins in the actin molecule.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

Actin filament architecture and movements in macrophage cytoplasm

Actin filaments are the predominant structural elements in macrophage cortical cytoplasm. These fibres form a unique orthogonal network that fills all lamellae extended from the cell and which, in the cell body, bifurcates to form layers 0.2-0.5 micron thick on the cell top and bottom. Single short filaments, 0.1 micron in length, intersect in space in either T-shaped or X-shaped overlaps to form this ultrastructure. Network assembly and pseudopod extension occur when actin filaments within the network elongate. This filament growth is driven by a large storage pool of actin bound to the sequestering protein, profilin. Elongation is regulated by acumentin, gelsolin and possibly severin, proteins that bind to the end of the filaments, preventing the addition of actin monomers to the filaments. The cytosolic concentration of calcium controls whether filaments assemble or disassemble. Filaments can assemble when the filament ends are not blocked by gelsolin, a condition predicted to occur when the calcium concentration is less than 0.1 micron. Orthogonality results when actin filaments are cross-linked by molecules of actin-binding protein.

Activation of DNA synthesis and AP-1 by profilin, an actin-binding protein, via binding to a cell surface receptor in cultured rat mesangial cells

Profilin is known to bind to actin monomers (to regulate actin polymerization) and to phosphatidylinositol-4,5-bisphosphate (to inhibit hydrolysis by unphosphorylated phospholipase C-gammal). It was recently reported that profilin is overexpressed in glomerular mesangial cells (MC) of rats with anti-Thy-1.1-induced glomerulonephritis and is accumulated in the extracellular space around MC. In this study, the biologic activities of extracellular profilin were examined. Scatchard analysis indicated the existence of a single class of cell surface binding sites, with similar equilibrium dissociation constants for purified splenic profilin and recombinant profilin, in cultured rat MC. Profilin increased [(3)H]thymidine incorporation in a dose-dependent manner and produced additive effects on platelet-derived growth factor-induced [(3)H]thymidine incorporation. Profilin increased AP-1 DNA-binding activity in a concentration-dependent (ED(50) = 30 nM) and time-dependent manner after transient c-jun gene expression, as measured using gel-shift assays and competitive reverse transcription-PCR. Pretreatment of profilin with an anti-profilin inhibitory antibody suppressed profilin-induced AP-1 activation and [(3)H]thymidine incorporation. Furthermore, profilin induced rapid transient activation of protein kinase C, and staurosporine and H-7 reduced the profilin-induced activation of AP-1, suggesting protein kinase C-dependent activation of AP-1. These findings indicate that profilin in the extracellular space can bind to cell surface receptors of MC and act as an inducer of signal transduction. These results suggest that extracellular profilin may be involved in the progression of glomerular diseases, by affecting cell growth.

The role of profilin in actin polymerization and nucleotide exchange

Properties of human profilin I mutated in the major actin-binding site were studied and compared with wild-type profilin using beta/gamma-actin as interaction partner. The mutants ranged in affinity, from those that only weakly affected polymerization of actin to one that bound actin more strongly than wild-type profilin. With profilins, whose sequestering activity was low, the concentration of free actin monomers observed at steady-state of polymerization [Afree], was close to that seen with actin alone ([Acc], critical concentration of polymerization). Profilin mutants binding actin with an intermediate affinity like wild-type profilin caused a lowering of [Afree] as compared to [Acc], indicating that actin monomers and profilin:actin complexes participate in polymer formation. With a mutant profilin, which bound actin more strongly than the wild-type protein, an efficient sequestration of actin was observed, and in this case, the [Afree] at steady state was again close to [Acc], suggesting that the mutant profilin:actin had a greatly lowered ability to incorporate actin subunits at the (+)-end. The results from the kinetic and steady-state experiments presented are consonant with the idea that profilin:actin complexes are directly incorporated at the (+)-end of actively polymerizing actin filaments, while they do not support the view that profilin facilitates polymer formation.

Listeria monocytogenes intracellular migration: inhibition by profilin, vitamin D-binding protein and DNase I

Infection of host cells by Listeria monocytogenes results in the recruitment of cytoplasmic actin into a tail-like appendage that projects from one end of the bacterium. Each filamentous actin tail progressively lengthens, providing the force which drives the bacterium in a forward direction through the cytoplasm and later results in Listeria cell-to-cell spread. Host cell actin monomers are incorporated into the filamentous actin tail at a discrete site, the bacterial-actin tail interface. We have studied the consequences of microinjecting three different actin monomer-binding proteins on the actin tail assembly and Listeria intracellular movement. Introduction of high concentrations of profilin (estimated injected intracellular concentration 11-22 microM) into infected PtK2 cells causes a marked slowing of actin tail elongation and bacterial migration. Lower intracellular concentrations of two other injected higher affinity monomer-sequestering proteins, Vitamin D-binding protein (DBP; 1-2 microM) and DNase I (6-7 microM) completely block bacterial-induced actin assembly and bacterial migration. The onset of inhibition by each protein is gradual (10-20 min) indicating that the mechanisms by which these proteins interfere with Listeria-induced actin assembly are likely to be complex. To exclude the possibility that Listeria recruits preformed actin filaments to generate the tails and that these monomer-binding proteins act by depolymerizing such performed actin filaments, living infected cells have been injected with fluorescently labeled phalloidin (3 microM). Although the stress fibers are labeled, no fluorescent phalloidin is found in the tails of the moving bacteria. These results demonstrate that Listeria-induced actin assembly in PtK2 cells is the result of assembly of actin monomers into new filaments and that Listeria's ability to recruit polymerization competent monomeric actin is very sensitive to the introduction of exogenous actin monomer-binding proteins.

N-ethylmaleimide-modified actin filaments do not bundle in the presence of alpha-actinin

We show that the modification of actin subdomain 1 by N-ethylmaleimide (NEM), which binds Cys-374 close to the C-terminus of the molecule, inhibits the alpha-actinin-induced bundling of actin filaments. This effect is not merely related to the block of Cys-374, since N-(1-pyrenyl)iodoacetamide (pyrene-IA) is unable to prevent bundling. Considering that NEM (but not pyrene-IA) influences actin assembly, we suggest that the inhibition of the actin-alpha-actinin interaction is due to the chemical modification of actin Cys-374 which, by inducing a marked spatial reorganization of actin monomers, is able to modify both the intra- and inter-molecular interactions of this protein. Finally, NEM-modified actin filaments form bundles in the presence of polyethylene glycol 6000 since, in this case, the side by side association of actin filaments does not depend on the accessibility of binding sites nor on the formation of chemical bonds.

Interaction of plant profilin with mammalian actin

The mode of interaction of birch and bovine profilins with actin was compared using a number of techniques. Birch profilin was purified from pollen or as a recombinant protein from Escherichia coli, using poly(L-proline) affinity chromatography and a monoclonal antibody for the identification of the isolated product. On two-dimensional gels, the genuine and recombinant proteins were identical in molecular mass and isoelectric point and revealed that birch profilin, in contrast to the basic profilins found in mammals, is an acidic protein, analogous to maize profilins. Bovine profilin was obtained from calf thymus. In viscometric assays, the birch protein was seen to modulate actin filament formation analogous to animal profilin. Birch profilin increased the critical concentration required for muscle and brain actin polymerization in a concentration-dependent manner, supporting the notion of the formation of a heterologous complex between the plant protein and animal actin. The effect was Mg(2+)-sensitive, as had been described for homologous complexes. The dissociation constants obtained for the plant/vertebrate and the vertebrate/vertebrate system were both in the micromolar range. The affinity of birch profilin for muscle actin was slightly lower than that for nonmuscle (brain) actin. A binary complex of birch profilin and skeletal muscle actin could be isolated by gel chromatography. Cross-linking experiments with actin, birch profilin, the G-actin binding peptide thymosin beta 4 and gelsolin segment 1, the N-terminal fragment of an actin capping protein, showed that profilin competed with thymosin beta 4, but had no effect on segment 1 binding to actin. These data indicate that the actin-binding domains in plant and animal profilins are functionally highly conserved, although the overall sequence similarity is less than 25%.

A novel protein accumulated during maturation of the pods of the plant Impatiens balsamina

A high molecular weight protein has been isolated as a major polypeptide comprising 85% of the total extractable proteins in the fruit pericarp of the plant Impatiens balsamina. The protein has been purified to homogeneity following fractionation of the crude cell supernatant with ammonium sulphate, chromatography on Sepharose 4B and Sephacryl-S-300. This protein appeared to be a homo-tetramer consisting of subunits, Mr 75 K. Amino acid analysis showed the presence of more acidic amino acids, with an isoelectric point 5.8. The interaction of this protein with filamentous actin, both from rabbit muscle and Mung bean embryo was monitored by transmission electron microscopy, light scattering analysis, viscometry and sedimentation assay. This protein inhibited appreciably the polymerization of G-actin to F-actin.

How profilin promotes actin filament assembly in the presence of thymosin beta 4

The role of profilin in the regulation of actin assembly has been reexamined. The affinity of profilin for ATP-actin appears 10-fold higher than previously thought. In the presence of ATP, the participation of the profilin-actin complex to filament elongation at the barbed end is linked to a decrease in the steady-state concentration of globular actin. This surprising effect is made possible by the involvement of the irreversible ATP hydrolysis accompanying actin polymerization. As a consequence, in the presence of thymosin beta 4 (T beta 4), low amounts of profilin promote extensive actin assembly off of the pool of actin-T beta 4 complex. When barbed ends are capped, profilin simply sequesters globular actin. A model is proposed for the function of profilin in actin-based motility.

Cloning and expression of a novel human profilin variant, profilin II

We have isolated a 1.7 kbp cDNA encoding a 140 amino acid protein (15.1 kDa, pI 5.91) with a high sequence similarity (62%) to human profilin (profilin I). We have termed this variant profilin II. Northern blot analysis showed that profilin II is highly expressed in brain, skeletal muscle and kidney and less strongly in heart, placenta, lung and liver. In addition, three different transcript lengths were detected. Only one transcript of profilin I was found. The expression level of this was low in brain and skeletal muscle, medium in heart and high in placenta, lung, liver and kidney.

High levels of profilin suppress the lethality caused by overproduction of actin in yeast cells

Overproduction of actin is lethal to yeast cells. In contrast, overexpression of the profilin gene, PFY1, encoding an actin-binding protein, leads to no very obvious phenotype. Interestingly, profilin overproduction can compensate for the deleterious effects of too much actin in a profilin concentration-dependent manner. Our results, thus, document that actin and profilin interact in vivo. Immunofluorescence studies suggest that suppression works by reducing actin assembly. We observed, however, that even massive overproduction of profilin fails to fully restore the wild-type phenotype (e.g. the wild-type appearance of the actin microfilament system). This may indicate that actin monomer sequestration is not the only mechanism by which the balance of actin polymerization is controlled.

Isolation of the phosphatidylinositol 4-monophosphate dissociable high-affinity profilin-actin complex

Profilin was originally discovered in a tight complex with monomeric actin from bovine spleen, leading to its description as an actin monomer sequestering protein that maintains a pool of unpolymerized actin in cells. Subsequent purifications of profilin using different methods from diverse cells have consistently yielded preparations that affect the kinetics of actin assembly but do not efficiently maintain actin monomeric at steady state in solutions containing mM magnesium. Recent evidence that profilin inhibits phospholipase C and enhances nucleotide exchange of actin has led some to question whether profilin is ever truly an actin monomer sequestering agent. Here we report that the extraction of bovine spleen with fluoride- and pyrophosphate-containing solutions facilitates isolation of monomeric actin that is bound to profilin and does not polymerize in mM magnesium ion. The integrity of this complex depends on the presence of ATP. Phosphatidylinositol 4-monophosphate (PIP), previously shown to dissociate the low-affinity profilin-actin complex (Kd = 0.4 microM in mM Mg2+), also dissociates the high-affinity profilin-actin complex (Kd less than 0.02 microM in mM Mg2+) yielding actin that is polymerization competent and profilin that functions like profilins purified by conventional methods. Although the chemical basis of these results is not known, they indicate that profilin can tightly sequester actin monomers and support the earlier suggestion that the affinity of profilin for actin may be under metabolic control.

GAP-43 as a plasticity protein in neuronal form and repair

Neurons exhibit a remarkable plasticity of form, both during neural development and during the subsequent remodelling of synaptic connectivity. Here we review work on GAP-43 and G0, and focus upon the thesis that their interaction may endow neurons with such plasticity. We also present new data on the role of G proteins in neurite growth, and on the interaction of GAP-43 and actin. GAP-43 is a protein induced during periods of axonal extension and highly enriched on the inner surface of the growth cone membrane. Its membrane localization is primarily due to a short amino terminal sequence which is subject to palmitoylation. Binding to actin filaments may also assist in restricting the protein to specific cellular domains. Consistent with its role as a "plasticity protein," there is evidence that GAP-43 can directly alter cell shape and neurite extension, and several theses have been advanced for how it might do so. Two other prominent components of the growth cone membrane are the alpha and beta subunits of G0. GAP-43 regulates their guanine nucleotide exchange, which is an unusual role for an intracellular protein. We speculate that GAP-43 may adjust the "set point" of responsiveness for G0 stimulation by receptors, thereby altering the neuronal propensity to growth, without actually causing growth. To begin to address how G protein activity affects axon growth, we have developed a means to introduce guanine nucleotide analogs into sympathetic neurons. Stimulation of G proteins with GTP-gamma-S retards axon growth, whereas GDP-beta-S enhances it. This is compatible with G protein registration of inhibitory signals.

The role of myosin I and II in cell motility

It has been recognized since the turn of the century that cell motility by non-muscle cells requires virtually continuous restructuring of the cytoskeleton (see refs [1-4]). It is also clear that cell motility requires a mechanism for converting chemical energy into mechanical work. The proteins actin and myosin, two important constituents of the cytoskeleton, have been postulated to act as the chemicomechanical transducer in motile cells. Central to their role as a force generating mechanism in motile cells is the ability of myosin (a) to hydrolyze ATP when it interacts with actin and (b) to form filaments. Recent studies on mammalian cells and on the cellular slime mold Dictyostelium discoideum have shed light and at the same time raised questions regarding the involvement of myosin in cell motility. Moreover, they have demonstrated the presence of two types of myosins, called myosin II and myosin I, that have unique biochemical and regulatory properties and that may play different roles in mediating cell motility. In this chapter we will discuss the properties of these two myosins and then describe what is known about their involvement in Dictyostelium and mammalian cell motility.

Distribution of profilin in fibroblasts correlates with the presence of highly dynamic actin filaments

We have used polyclonal and monoclonal antibodies raised against calf thymus profilin to localize the corresponding protein in translocating, spreading, and stationary rat fibroblasts. Immunofluorescence of whole cells and immunogold labeling on ventral membranes of lysis-squirted cells showed that profilin was markedly enriched in the highly dynamic lamellipodia or pseudopodial lobes. Within these regions, a significant fraction was colocalized with dynamic actin filaments organized in actin ribs, cortical filaments, or stress fiber-like bundles, and little profilin was found in membrane areas appearing free of actin. In contrast, stress fibers of stationary cells as well as actin arcs and ring-like bundles of spreading and migrating cells showed very little label. These results are discussed in context with the proposed role of profilin in regional membrane dynamics typical for fibroblasts and are compared to previous data (Hartwig et al.: J. Cell Biol. 109:1571-1579, 1989) on profilin distribution in platelets and granulocytes.

Production, isolation and characterization of human profilin from Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used to express human profilin cDNA. The recombinant protein, isolated by affinity chromatography on poly(L-proline)-Sepharose followed by ion exchange chromatography, associates with non-muscle actin and phosphatidylinositol-(4,5)-bisphosphate as authentic profilin.

Primary structure of profilins from two species of Echinoidea and Physarum polycephalum

Profilin is a small G-actin-binding protein, the amino acid sequence of which was previously reported for calf, human, Acanthamoeba and yeast. Here the amino acid sequences of three profilins obtained from eggs of two species of Echinoidea, Clypeaster japonicus (order, Clypeasteroida) and Anthocidaris crassispina (order, Echinoida), and plasmodium of Physarum polycephalum were determined. Two echinoid profilins were composed of 139 amino acid residues, N-termini were acylated and the molecular mass was calculated to be 14.6 kDa, slightly larger than that of 13 kDa estimated by SDS/PAGE [Mabuchi, I. & Hosoya, H. (1982) Biomed. Res. 3, 465-476]. On the other hand, Physarum profilin was composed of 124 amino acid residues, the N-terminus was acylated, and the calculated molecular mass was 13132 Da. The sequences of C. japonicus and A. crassispina profilins were homologous (84% identical). However, the similarity of these profilins with those form other organisms was low. The sequence of Physarum profilin was homologous with Acanthamoeba profilin isoforms (51% identical) and with yeast profilin (42% identical), but not with other profilins. The relatively conservative sequence of profilins from yeast, Physarum, Acanthamoeba, echinoid eggs and mammalian cells was found in the N-terminal region, which was suggested to be a common actin-binding region. The C-terminal region was also conserved, although to a lesser extent than the N-terminal region.

Tissue-specific expression of profilin

Expression of profilin and profilin:actin ratios in vertebrates were determined with polyclonal antibodies against profilin and actin. Profilin was detected in a wide variety of bovine tissues and was enriched in smooth muscle of bovine, porcine and avian origin. The protein was purified from pig stomach muscle tissue. Smooth muscle profilin was found to be more effective in inhibiting the polymerization of skeletal muscle actin than thymus profilin purified by the same method.

Synthesis of mammalian profilin in Escherichia coli and its characterization

Profilin is a G-actin binding protein that may have a role in controlling the ratio of G/F actin within the cell. To devise a way for obtaining large amounts of mammalian profilin in an active state, we transfected Escherichia coli with a plasmid containing a full-length rat spleen profilin cDNA adjacent to a promoter inducible by isopropyl thiogalactoside (IPTG). Upon induction, they synthesized a new protein of 15,000 MW constituting approximately 5% of the total cell protein. This protein bound to poly-L-proline Sepharose and could be eluted with 7 M urea, behavior similar to that exhibited by authentic profilin. The protein could be released from the bacteria in soluble form following sonication, and the profilin could then be purified to homogeneity following chromatography on Sephadex G-75 and DEAE A-50 Sephadex. The protein began with an unblocked Ala, indicating that the initiating formyl and methionine residues had been removed. The dissociation of the recombinant profilin from chicken skeletal muscle actin was characterized by a Kd of approximately 2 microM based on gel filtration analysis and actin polymerization assays. These results show that purified active mammalian profilin can be made conveniently in large quantities. This study also demonstrates the feasibility of using bacterially synthesized profilin in structure-function studies involving mutant profilins altered by site-directed mutagenesis.

The effect of divalent cations on the interaction between calf spleen profilin and different actins

The interaction between calf spleen profilin and actin depends critically on the status of the C-terminus of the actin, and in the case of profilin, the C-terminus is of great importance for the physiochemical behaviour of the protein. Both proteins easily lose their C-terminal amino acids during the preparation, and special care has to be taken to ensure the isolation of the proteins in the intact form. Another factor that may seriously influence the study of the interaction of profilin with actin is the presence of varying amounts of an activity that causes an apparent stabilization of the complex even at later stages of its purification. We have found conditions for the isolation of intact profilin and actin, and studied the interaction between the two proteins, including the determination of the Kdiss for the complex formed under various ionic conditions. The complex formed between profilin and actin from calf spleen was found to be significantly stronger (Kdiss less than or equal to 10(-8) M in 50 mM KCl, and Kdiss = 4.10(-7) M in 50 mM KCl, 1 mM MgCl2) than that formed between profilin and muscle alpha-actin (Kdiss = 10(-6) M in 50 mM KCl, +/- 1 mM MgCl2). The profilactin complex formed in the mammalian system was stronger than the complex formed between Acanthamoeba actin and the profilin-like protein isolated from this organism. Analysis of the formation of the calf spleen complex in the presence of varying concentrations of divalent cations gave evidence for the presence of a high-affinity divalent-cation-binding site on the spleen actin (beta, gamma) which appears to regulate the interaction with profilin.

The primary structure of human platelet profilin: reinvestigation of the calf spleen profilin sequence

The primary structure of human platelet profilin was determined by aligning the sequences of its tryptic peptides to the previously determined calf spleen profilin sequence [(1979) FEBS Lett. 101, 161-165]. Comparison of the peptide fingerprints of the two proteins suggested a higher homology than that found by direct sequence comparison. We therefore reinvestigated the sequences of the peptides from calf spleen profilin. We identified four incorrect charge assignments and a deletion of three residues. The similarity between the two vertebrate profilins amounts to 95%.

The primary structure of the basic isoform of Acanthamoeba profilin

Acanthamoeba profilin-II [Kaiser, D.A., Sato, M., Ebert, R. F. and Pollard, T.D. (1986) J. Cell. Biol. 102, 221-226] was digested with trypsin or cleaved by 2-(2-nitrophenylsulphenyl)-3-methyl-3-bromoindolenine. The tryptic peptides were purified by reversed-phase-high-performance liquid chromatography and completely sequenced using automated gas-phase sequence analysis. The complete profilin-II sequence was deduced by ordering the tryptic peptides using the sequence information of the tryptophan-cleavage products. Acanthamoeba profilin-II was found to be homologous to the previously determined profilin-I sequence [Ampe, C., Vandekerckhove, J., Brenner, L., Tobacman, L. and Korn, E.D. (1985) J. Biol. Chem. 260, 834-840]. Like profilin-I, profilin-II consists of 125 amino acids, has a blocked NH2 terminus and a trimethyllysine residue at position 103. Profilin-II differs in at least 21 positions from one of the profilin-I isoforms. The amino acid exchanges are mainly concentrated in the middle part of the sequence. Profilin-II contains two more basic residues than profilin-I, which explains its higher isoelectric point.

Evaluation of the binding of Acanthamoeba profilin to pyrene-labeled actin by fluorescence enhancement

We have used a fluorescence assay to measure the binding of Acanthamoeba profilin to monomeric Acanthamoeba and rabbit skeletal muscle actin labeled on cysteine-374 with pyrene iodoacetamide. The wavelengths of the pyrene excitation and emission maxima are constant at 346 and 386 nm, but the fluorescence is enhanced up to 50% by profilin. The higher fluorescence is largely due to higher absorbance in the presence of profilin. The fluorescence enhancement has a hyperbolic dependence on the concentration of profilin, suggesting a single class of binding sites. Linear Scatchard plots yield an estimate of the dissociation constant, Kd, of the complex of profilin with pyrenyl-actin. In low-ionic-strength buffers with 2 to 6 mM imidazole (pH 7.0) and 0.1 mM CaCl2 the Kd is 9 microM for both muscle and Acanthamoeba actin. In 50 mM KCl the Kd for the complex with Acanthamoeba actin is 16 microM, while the Kd for the complex with muscle actin is greater than 50 microM.

The organization and regulation of the macrophage actin skeleton

To move, leukocytes extend portions of their cortical cytoplasm as pseudopods. These pseudopods are filled with a three-dimensional actin filament skeleton, the reversible assembly of which in response to receptor stimulation is thought to play a major role in providing the mechanical force for these protrusive movements. The organization of this actin skeleton occurs at different levels within the cell, and a number of macrophage proteins have been isolated and shown to affect the architecture, assembly, stability, and length of actin filaments in vitro. The architecture of cytoplasmic actin is regulated by proteins that cross-link filaments in higher-order structures. Actin-binding protein plays a major role in defining network structure by cross-linking actin filaments into orthogonal networks. Gelsolin may have a central role in regulating network structure. It binds to the sides of actin filaments and severs them, and binds the "barbed" filament end, thereby blocking monomer addition at this end. Gelsolin is activated to bind actin filaments by microM calcium. Dissociation of gelsolin bound on filament ends occurs in the presence of the polyphosphoinositides, PIP and PIP2. Calcium and PIP2 have been shown to be intracellular messengers of cell stimulation.

Effects of serum vitamin-D-binding protein on actin in the presence of plasma gelsolin

The interaction of serum vitamin-D-binding protein (DBP) and plasma gelsolin with actin was studied using fluorescent 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole-actin or N-pyrenylcarboxyamidomethyl-actin. DBP and gelsolin formed very tight complexes with one or two monomeric actin subunits respectively. Kd values of about 10 nm have been found for both complexes. When DBP and gelsolin were added simultaneously to G-actin no ternary complex was observed and the DBP-actin complex was formed preferentially. In the presence of CaCl2 no transfer of actin occurred from the 1:2 gelsolin:actin molar complex to free DBP while in the presence of EGTA one actin monomer could be transferred. DBP did not affect the severing activity of gelsolin. The effects of a mixture of gelsolin and DBP on F-actin suggest that only individual interactions of these two plasma proteins with actin occurred in solution.