Physical, immunochemical, and functional properties of Acanthamoeba profilin

Using ALS to understand profilin 1's diverse roles in cellular physiology

Profilin is an actin monomer-binding protein whose role in actin polymerization has been studied for nearly 50 years. While its principal biochemical features are now well understood, many questions remain about how profilin controls diverse processes within the cell. Dysregulation of profilin has been implicated in a broad range of human diseases, including neurodegeneration, inflammatory disorders, cardiac disease, and cancer. For example, mutations in the profilin 1 gene (PFN1) can cause amyotrophic lateral sclerosis (ALS), although the precise mechanisms that drive neurodegeneration remain unclear. While initial work suggested proteostasis and actin cytoskeleton defects as the main pathological pathways, multiple novel functions for PFN1 have since been discovered that may also contribute to ALS, including the regulation of nucleocytoplasmic transport, stress granules, mitochondria, and microtubules. Here, we will review these newly discovered roles for PFN1, speculate on their contribution to ALS, and discuss how defects in actin can contribute to these processes. By understanding profilin 1's involvement in ALS pathogenesis, we hope to gain insight into this functionally complex protein with significant influence over cellular physiology.

De Novo Transcriptome Profiling of Naegleria fowleri Trophozoites and Cysts via RNA Sequencing

Naegleria fowleri is a pathogenic free-living amoeba, commonly found around the world in warm, fresh water and soil. N. fowleri trophozoites can infect humans by entering the brain through the nose and causing usually fatal primary amebic meningoencephalitis (PAM). Trophozoites can encyst to survive under unfavorable conditions such as cold temperature, starvation, and desiccation. Recent technological advances in genomics and bioinformatics have provided unique opportunities for the identification and pre-validation of pathogen-related and environmental resistance through improved understanding of the biology of pathogenic N. fowleri trophozoites and cysts at a molecular level. However, genomic and transcriptomic data on differential expression genes (DEGs) between trophozoites and cysts of N. fowleri are very limited. Here, we report transcriptome Illumina RNA sequencing (RNA-seq) for N. fowleri trophozoites and cysts and de novo transcriptome assembly. RNA-seq libraries were generated from RNA extracted from N. fowleri sampled from cysts, and a reference transcriptome was generated through the assembly of trophozoite data. In the database, the assembly procedure resulted in 42,220 contigs with a mean length of 11,254 nucleotides and a C+G content of 37.21%. RNA sequencing showed that 146 genes in cysts of N. fowleri indicated 2-fold upregulation in comparison with trophozoites of N. fowleri, and 163 genes were downregulated; these genes were found to participate in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway. The KEGG pathway included metabolic (131 sequences) and genetic information processing (66 sequences), cellular processing (43 sequences), environmental information processing (22 sequences), and organismal system (20 sequences) pathways. On the other hand, an analysis of 11,254 sequences via the Gene Ontology database showed that their annotations contained 1069 biological processes including the cellular process (228 sequences) and metabolic process (214 sequences); 923 cellular components including cells (240 sequences) and cell parts (225 sequences); and 415 molecular functions including catalytic activities (195 sequences) and binding processes (186 sequences). Differential expression levels increased in cysts of N. fowleri compared to trophozoites of N. fowleri, which were mainly categorized as serine/threonine protease, kinase, and lipid metabolism-related proteins. These results may provide new insights into pathogen-related genes or environment-resistant genes in the pathogenesis of N. fowleri.

Interspersed 5'cis-regulatory elements ascertain the spatio-temporal transcription of cytoskeletal profilin gene family in Arabidopsis

Spatio-temporal expression patterns of cytoskeleton-associated profilin (PRF) family proteins in response to varied environmental stimuli are tightly regulated. Functional analyses of PRFs have revealed their crucial roles in varied developmental and stress related traits, but very little is implicit pertaining to cis-acting regulatory elements that regulate such intricate expression patterns. Here, we identified cis-elements with their varying distribution frequencies by scanning 1.5kbp upstream sequences of 5'regulatory regions of PRFs of dicot and monocot plant species. Predicted cis-elements in the regulatory sub-regions of Arabidopsis PRFs (AtPRFs) were predominantly associated with development-responsive motifs (DREs), light responsive elements (LREs), hormonal responsive elements (HREs), core motifs and stress-responsive elements (SREs). Interestingly, DREs, LREs and core promoter motifs, were extensively distributed up to the distal end of 5'regulatory regions on contrary to HREs present closer to the translational start site in Arabidopsis. The evolutionary footprints of predicted orthologous cis-elements were conserved, and preferably located in the proximal regions of 5'regulatory regions of evolutionarily diverged plant species. We also explored comprehensive tissue-specific global gene expression levels of PRFs under diverse hormonal and abiotic stress regimes. In response, the PRFs exhibited large transcriptional biases in a time- and organ-dependent manner. Further, the methodical elucidation of spatial expression analysis of predicted cis-elements binding transcription factors and relevant PRFs showed notable correlation. Results indicate that binding transcription factors' expression data is largely informative for envisaging their precise roles in the spatial regulation of target PRFs. These results highlight the importance of PRFs during plant development; and establish a relationship between their spatial expression patterns and presence of respective regulatory motifs in their promoter sequences. This information could be employed in future studies and field-utilization of cell wall structural genes.

Reconsidering an active role for G-actin in cytoskeletal regulation

Globular (G)-actin, the actin monomer, assembles into polarized filaments that form networks that can provide structural support, generate force and organize the cell. Many of these structures are highly dynamic and to maintain them, the cell relies on a large reserve of monomers. Classically, the G-actin pool has been thought of as homogenous. However, recent work has shown that actin monomers can exist in distinct groups that can be targeted to specific networks, where they drive and modify filament assembly in ways that can have profound effects on cellular behavior. This Review focuses on the potential factors that could create functionally distinct pools of actin monomers in the cell, including differences between the actin isoforms and the regulation of G-actin by monomer binding proteins, such as profilin and thymosin β4. Owing to difficulties in studying and visualizing G-actin, our knowledge over the precise role that specific actin monomer pools play in regulating cellular actin dynamics remains incomplete. Here, we discuss some of these unanswered questions and also provide a summary of the methodologies currently available for the imaging of G-actin.

RhoA/mDia-1/profilin-1 signaling targets microvascular endothelial dysfunction in diabetic retinopathy

BACKGROUND

Diabetic retinopathy (DR) is a major cause of blindness in the working-age populations of developed countries, and effective treatments and prevention measures have long been the foci of study. Patients with DR invariably demonstrate impairments of the retinal microvascular endothelium. Many observational and preclinical studies have shown that angiogenesis and apoptosis play crucial roles in the pathogenesis of DR. Increasing evidence suggests that in DR, the small guanosine-5'-triphosphate-binding protein RhoA activates its downstream targets mammalian Diaphanous homolog 1 (mDia-1) and profilin-1, thus affecting important cellular functions, including cell morphology, motility, secretion, proliferation, and gene expression. However, the specific underlying mechanism of disease remains unclear.

CONCLUSION

This review focuses on the RhoA/mDia-1/profilin-1 signaling pathway that specifically triggers endothelial dysfunction in diabetic patients. Recently, RhoA and profilin-1 signaling has attracted a great deal of attention in the context of diabetes-related research. However, the precise molecular mechanism by which the RhoA/mDia-1/profilin-1 pathway is involved in progression of microvascular endothelial dysfunction (MVED) during DR has not been determined. This review briefly describes each feature of the cascade before exploring the most recent findings on how the pathway may trigger endothelial dysfunction in DR. When the underlying mechanisms are understood, novel therapies seeking to restore the endothelial homeostasis comprised in DR will become possible.

Profilin-1 serves as a gatekeeper for actin assembly by Arp2/3-dependent and -independent pathways

Cells contain multiple F-actin assembly pathways, including the Arp2/3 complex, formins, and Ena/VASP, which have largely been analyzed separately. They collectively generate the bulk of F-actin from a common pool of G-actin; however, the interplay and/or competition between these pathways remains poorly understood. Using fibroblast lines derived from an Arpc2 conditional knockout mouse, we established matched-pair cells with and without the Arp2/3 complex. Arpc2(-/-) cells lack lamellipodia and migrate more slowly than WT cells but have F-actin levels indistinguishable from controls. Actin assembly in Arpc2(-/-) cells was resistant to cytochalasin-D and was highly dependent on profilin-1 and Ena/VASP but not formins. Profilin-1 depletion in WT cells increased F-actin and Arp2/3 complex in lamellipodia. Conversely, addition of exogenous profilin-1 inhibited Arp2/3 complex actin nucleation in vitro and in vivo. Antagonism of the Arp2/3 complex by profilin-1 in cells appears to maintain actin homeostasis by balancing Arp2/3 complex-dependent and -independent actin assembly pathways.

Stimulus-dependent phosphorylation of profilin-1 in angiogenesis

Angiogenesis, the formation of new blood vessels, is fundamental to development and post-injury tissue repair. Vascular endothelial growth factor (VEGF)-A guides and enhances endothelial cell migration to initiate angiogenesis. Profilin-1 (Pfn-1) is an actin-binding protein that enhances actin filament formation and cell migration, but stimulus-dependent regulation of Pfn-1 has not been observed. Here, we show that VEGF-A-inducible phosphorylation of Pfn-1 at Tyr 129 is critical for endothelial cell migration and angiogenesis. Chemotactic activation of VEGF receptor kinase-2 (VEGFR2) and Src induces Pfn-1 phosphorylation in the cell leading edge, promoting Pfn-1 binding to actin and actin polymerization. Conditional endothelial knock-in of phosphorylation-deficient Pfn1(Y129F) in mice reveals that Pfn-1 phosphorylation is critical for angiogenesis in response to wounding and ischaemic injury, but not for developmental angiogenesis. Thus, VEGFR2/Src-mediated phosphorylation of Pfn-1 bypasses canonical, multistep intracellular signalling events to initiate endothelial cell migration and angiogenesis, and might serve as a selective therapeutic target for anti-angiogenic therapy.

Structure and functions of profilins

Profilins are small actin-binding proteins found in eukaryotes and certain viruses that are involved in cell development, cytokinesis, membrane trafficking, and cell motility. Originally identified as an actin sequestering/binding protein, profilin has been involved in actin polymerization dynamics. It catalyzes the exchange of ADP/ATP in actin and increases the rate of polymerization. Profilins also interact with polyphosphoinositides (PPI) and proline-rich domains containing proteins. Through its interaction with PPIs, profilin has been linked to signaling pathways between the cell membrane and the cytoskeleton, while its role in membrane trafficking has been associated with its interaction with proline-rich domain-containing proteins. Depending on the organism, profilin is present in a various number of isoforms. Four isoforms of profilin have been reported in higher organisms, while only one or two isoforms are expressed in single-cell organisms. The affinity of these isoforms for their ligands varies between isoforms and should therefore modulate their functions. However, the significance and the functions of the different isoforms are not yet fully understood. The structures of many profilin isoforms have been solved both in the presence and the absence of actin and poly-L-proline. These structural studies will greatly improve our understanding of the differences and similarities between the different profilins. Structural stability studies of different profilins are also shedding some light on our understanding of the profilin/ligand interactions. Profilin is a multifaceted protein for which a dramatic increase in potential functions has been found in recent years; as such, it has been implicated in a variety of physiological and pathological processes.

Profilin oligomerization and its effect on poly (L-proline) binding and phosphorylation

Profilin is a cytoskeletal protein that interacts specifically with actin, phosphoinositides and poly (l-proline). Experimental results and in silico studies revealed that profilin exists as dimer and tetramer. Profilin oligomers possess weak affinity to poly (l-proline) due to unavailability of binding sites in dimers and tetramers. Phosphorylation studies indicate that profilin dimers are not phosphorylated while teramers are preferentially phosphorylated over monomers. In silico studies revealed that PKC phosphorylation site, S137 is buried in dimer while it is accessible in tetramer.

Model of formin-associated actin filament elongation

Formin FH2 domains associate processively with actin-filament barbed ends and modify their rate of growth. We modeled how the elongation rate depends on the concentrations of profilin and actin for four different formins. We assume that (1) FH2 domains are in rapid equilibrium among conformations that block or allow actin addition and that (2) profilin-actin is transferred rapidly to the barbed end from multiple profilin binding sites in formin FH1 domains. In agreement with previous experiments discussed below, we find an optimal profilin concentration with a maximal elongation rate that can exceed the rate of actin alone. High profilin concentrations suppress elongation, largely because free profilin displaces profilin-actin from FH1. The model supports a common polymerization mechanism for the four formin FH1FH2 constructs with differences attributed to varying parameter values. The mechanism does not require ATP hydrolysis by polymerized actin, but we cannot exclude that formins accelerate hydrolysis.

Interactions with PIP2, ADP-actin monomers, and capping protein regulate the activity and localization of yeast twinfilin

Twinfilin is a ubiquitous actin monomer-binding protein that regulates actin filament turnover in yeast and mammalian cells. To elucidate the mechanism by which twinfilin contributes to actin filament dynamics, we carried out an analysis of yeast twinfilin, and we show here that twinfilin is an abundant protein that localizes to cortical actin patches in wild-type yeast cells. Native gel assays demonstrate that twinfilin binds ADP-actin monomers with higher affinity than ATP-actin monomers. A mutant twinfilin that does not interact with actin monomers in vitro no longer localizes to cortical actin patches when expressed in yeast, suggesting that the ability to interact with actin monomers may be essential for the localization of twinfilin. The localization of twinfilin to the cortical actin cytoskeleton is also disrupted in yeast strains where either the CAP1 or CAP2 gene, encoding for the alpha and beta subunits of capping protein, is deleted. Purified twinfilin and capping protein form a complex on native gels. Twinfilin also interacts with phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2), and its actin monomer-sequestering activity is inhibited by PI(4,5)P2. Based on these results, we propose a model for the biological role of twinfilin as a protein that localizes actin monomers to the sites of rapid filament assembly in cells.

Molecular mechanisms controlling actin filament dynamics in nonmuscle cells

We review how motile cells regulate actin filament assembly at their leading edge. Activation of cell surface receptors generates signals (including activated Rho family GTPases) that converge on integrating proteins of the WASp family (WASp, N-WASP, and Scar/WAVE). WASP family proteins stimulate Arp2/3 complex to nucleate actin filaments, which grow at a fixed 70 degrees angle from the side of pre-existing actin filaments. These filaments push the membrane forward as they grow at their barbed ends. Arp2/3 complex is incorporated into the network, and new filaments are capped rapidly, so that activated Arp2/3 complex must be supplied continuously to keep the network growing. Hydrolysis of ATP bound to polymerized actin followed by phosphate dissociation marks older filaments for depolymerization by ADF/cofilins. Profilin catalyzes exchange of ADP for ATP, recycling actin back to a pool of unpolymerized monomers bound to profilin and thymosin-beta 4 that is poised for rapid elongation of new barbed ends.

Characterization and localization of profilin in pollen grains and tubes of Lilium longiflorum

Pollen tubes show a rapid and dramatically polarized growth in which the actin cytoskeleton appears to play a central role. In order to understand the regulation of actin we characterized its associated protein, profilin, in pollen tubes of Lilium longiflorum. By using purified polyclonal antibodies prepared against bean root profilin [Vidali et al., 1995: Plant Physiol. 108:115-123] we detected in pollen grains and tubes two profilin polypeptides with molecular masses of 14.4 and 13.4 KDa, and an identical isoelectric point of 5.05. Profilin comprises approximately 0.47% of the total grain protein, with actin being approximately 1.4%. We were unable to detect a statistically significant profilin increase after germination, while the actin increased approximately 68%. We also spatially localized the distribution of profilin using immunocytochemistry of fixed cells at both the light and electron microscope level, and by fluorescent analog cytochemistry on live cells. The results show that profilin is evenly distributed throughout the cytoplasm and does not specifically associate with any cellular structure.

Localization of actobindin, profilin I, profilin II, and phosphatidylinositol-4,5-bisphosphate (PIP2) in Acanthamoeba castellanii

Specific polyclonal antisera were raised against purified Acanthamoeba actobindin and synthetic peptides corresponding to regions of maximum charge differences in Acanthamoeba profilin I and profilin II. Immunofluorescence studies with these antibodies showed profilin I to be distributed throughout the Acanthamoeba cytoplasm, except for lamellipodia, with the highest fluorescence intensity in cortical regions in which monomeric actin also was present, as shown by labeling with fluorescent DNase. In contrast, profilin II appeared to be uniformly associated with the plasma membrane except at sites of pseudopod extension, where the concentration was frequently decreased, in addition to cortical regions. Immunofluorescence studies using a monoclonal antibody specific for phosphatidylinositol-4,5-bisphosphate (PIP2) suggested that its distribution is mostly limited to the plasma membrane. In contrast to the distribution of profilin II, PIP2 immunofluorescence was prominent at the leading edge of cells, including the plasma membrane of lamellipodia. Quantitative immunoelectron microscopy showed that profilin II was approximately 36 times more likely to localize to the plasma membrane than profilin I. Immunofluorescence and confocal microscopy localized actobindin to the base of lamellipodia. The differential localization of the three actin monomer-binding proteins suggests that they have different biologic functions in Acanthamoeba and is consistent with the hypotheses that (1) profilin I functions predominantly as an actin monomer-binding protein; (2) profilin II regulates, or is regulated by, PIP2; and (3) actobindin inhibits nucleation of new filaments and facilitates elongation of existing polarized filaments in actively motile regions.

Differential colocalization of profilin with microfilaments in PtK2 cells

Profilins are thought to be involved in the control of actin dynamics in eukaryotic cells. In accordance with this concept, profilin was found to be colocalized with the cortical microfilament webs in leading lamellae of locomoting and spreading fibroblasts. However, so far, there is little information on the distribution of profilin in other cell types. In this study, we report on the colocalization of profilin with various microfilament suprastructures in the epithelial cell line PtK2. This cell line, which is derived from rat kangaroo, contains a profilin sharing an N-terminal epitope with bovine and human profilin I, as seen by immunoblotting with monoclonal antibodies. By using immunofluorescence in conjunction with conventional fluorescence microscopy and confocal laser-scanning microscopy, we found profilin in ruffling areas of the peripheral lamellae and nascent stress fibers of spreading cells, whereas the peripheral belts of stationary cells growing in epithelioid sheets lacked profilin staining. In these cells, profilin was primarily distributed in a fine reticular or vesicular network that was not related to the microfilament system. Conspicuously low levels of profilins was not related to the contractile ring of mitotic cells. This was found for different fixation protocols and antibodies of the IgG and IgM type, respectively, indicating that lack of staining of the cleavage furrow was not due to antibody penetration problems. Depending on the fixation protocol, the nuclear matrix appeared strongly positive or negative for profilin. Cells microinjected with birch pollen profilin and labeled with a birch profilin-specific monoclonal antibody corroborated the results obtained with the endogeneous protein: The injected profilin was targeted to the cortical web and to nascent stress fibers of spreading cells but not to the cleavage ring of mitotic cells. These results suggest that high concentrations of a profilin I homologue are preferentially located with those microfilament suprastructures in PtK2 cells that are subject to rapid modulation by external signals.

Profilin promotes barbed-end actin filament assembly without lowering the critical concentration

The mechanism of profilin-promoted actin polymerization has been systematically reinvestigated. Rates of barbed-end elongation onto Spectrin.4.1. Actin seeds were measured by right angle light scattering to avoid confounding effects of pyrenyl-actin, and KINSIM was used to analyze elongation progress curves. Without thymosin-beta4, both actin and Profilin. Actin (P.A) are competent in barbed-end polymerization, and kinetic simulations yielded the same bimolecular rate constant ( approximately 10 x 10(6) M(-1) s(-1)) for actin monomer or Profilin. Actin. When measured in the absence of profilin, actin assembly curves over a 0.7-4 microM thymosin-beta4 concentration range fit a simple monomer sequestering model (1 microM K(D) for Thymosin-beta4. Actin). The corresponding constant for thymosin-beta4.pyrenyl-Actin, however, was significantly higher ( approximately 9-10 microM), suggesting that the fluorophore markedly weakens binding to thymosin-beta4. With solutions of actin (2 microM) and thymosin-beta4 (2 or 4 microM), the barbed-end assembly rate rose with increasing profilin concentration (0.7-2 microM). Actin assembly in presence of thymosin-beta4 and profilin fit a simple thermodynamic energy cycle, thereby disproving an earlier claim (D. Pantaloni and M.-F. Carlier (1993) Cell 75, 1007-1014) that profilin promotes nonequilibrium filament assembly by accelerating hydrolysis of filament-bound ATP. Our findings indicate that profilin serves as a polymerization catalyst that captures actin monomers from Thymosin-beta4. Actin and ushers actin as a Profilin. Actin complex onto growing barbed filament ends.

Profilin is predominantly associated with monomeric actin in Acanthamoeba

We used biochemical fractionation, immunoassays and microscopy of live and fixed Acanthamoeba to determine how much profilin is bound to its known ligands: actin, membrane PIP(2), Arp2/3 complex and polyproline sequences. Virtually all profilin is soluble after gentle homogenization of cells. During gel filtration of extracts on Sephadex G75, approximately 60% of profilin chromatographs with monomeric actin, 40% is free and none voids with Arp2/3 complex or other large particles. Selective monoclonal antibodies confirm that most of the profilin is bound to actin: 65% in extract immunoadsorption assays and 74–89% by fluorescent antibody staining. Other than monomeric actin, no major profilin ligands are detected in crude extracts. Profilin-II labeled with rhodamine on cysteine at position 58 retains its affinity for actin, PIP(2) and poly-L-proline. When syringe-loaded into live cells, it distributes throughout the cytoplasm, is excluded from membrane-bounded organelles, and concentrates in lamellapodia and sites of endocytosis but not directly on the plasma membrane. Some profilin fluorescence appears punctate, but since no particulate profilin is detected biochemically, these spots may be soluble profilin between organelles that exclude profilin. The distribution of profilin in fixed human A431 cells is similar to that in amoebas. Our results show that the major pool of polymerizable actin monomers is complexed with profilin and spread throughout the cytoplasm.

Rho-family GTPases require the Arp2/3 complex to stimulate actin polymerization in Acanthamoeba extracts

BACKGROUND

Actin filaments polymerize in vivo primarily from their fast-growing barbed ends. In cells and extracts, GTPgammaS and Rho-family GTPases, including Cdc42, stimulate barbed-end actin polymerization; however, the mechanism responsible for the initiation of polymerization is unknown. There are three formal possibilities for how free barbed ends may be generated in response to cellular signals: uncapping of existing filaments; severing of existing filaments; or de novo nucleation. The Arp2/3 complex localizes to regions of dynamic actin polymerization, including the leading edges of motile cells and motile actin patches in yeast, and in vitro it nucleates the formation of actin filaments with free barbed ends. Here, we investigated actin polymerization in soluble extracts of Acanthamoeba.

RESULTS

Addition of actin filaments with free barbed ends to Acanthamoeba extracts is sufficient to induce polymerization of endogenous actin. Addition of activated Cdc42 or activation of Rho-family GTPases in these extracts by the non-hydrolyzable GTP analog GTPgammaS stimulated barbed-end polymerization, whereas immunodepletion of Arp2 or sequestration of Arp2 using solution-binding antibodies blocked Rho-family GTPase-induced actin polymerization.

CONCLUSIONS

For this system, we conclude that the accessibility of free barbed ends regulates actin polymerization, that Rho-family GTPases stimulate polymerization catalytically by de novo nucleation of free barbed ends and that the primary nucleation factor in this pathway is the Arp2/3 complex.

Profilin and the Abl tyrosine kinase are required for motor axon outgrowth in the Drosophila embryo

The ability of neuronal growth cones to be guided by extracellular cues requires intimate communication between signal transduction systems and the dynamic actin-based cytoskeleton at the leading edge. Profilin, a small, actin-binding protein, has been proposed to be a regulator of the cell motility machinery at leading edge membranes. However, its requirement in the developing nervous system has been unknown. Profilin associates with members of the Enabled family of proteins, suggesting that Profilin might link Abl function to the cytoskeleton. Here, genetic analysis in Drosophila is used to demonstrate that mutations in Profilin (chickadee) and Abl (abl) display an identical growth cone arrest phenotype for axons of intersegmental nerve b (ISNb). Moreover, the phenotype of a double mutant suggests that these components function together to control axonal outgrowth.

Mena is required for neurulation and commissure formation

Mammalian enabled (Mena) is a member of a protein family thought to link signal transduction pathways to localized remodeling of the actin cytoskeleton. Mena binds directly to Profilin, an actin-binding protein that modulates actin polymerization. In primary neurons, Mena is concentrated at the tips of growth cone filopodia. Mena-deficient mice are viable; however, axons projecting from interhemispheric cortico-cortical neurons are misrouted in early neonates, and failed decussation of the corpus callosum as well as defects in the hippocampal commissure and the pontocerebellar pathway are evident in the adult. Mena-deficient mice that are heterozygous for a Profilin I deletion die in utero and display defects in neurulation, demonstrating an important functional role for Mena in regulation of the actin cytoskeleton.

Arp2/3 complex from Acanthamoeba binds profilin and cross-links actin filaments

The Arp2/3 complex was first purified from Acanthamoeba castellanii by profilin affinity chromatography. The mechanism of interaction with profilin was unknown but was hypothesized to be mediated by either Arp2 or Arp3. Here we show that the Arp2 subunit of the complex can be chemically cross-linked to the actin-binding site of profilin. By analytical ultracentrifugation, rhodamine-labeled profilin binds Arp2/3 complex with a Kd of 7 microM, an affinity intermediate between the low affinity of profilin for barbed ends of actin filaments and its high affinity for actin monomers. These data suggest the barbed end of Arp2 is exposed, but Arp2 and Arp3 are not packed together in the complex exactly like two actin monomers in a filament. Arp2/3 complex also cross-links actin filaments into small bundles and isotropic networks, which are mechanically stiffer than solutions of actin filaments alone. Arp2/3 complex is concentrated at the leading edge of motile Acanthamoeba, and its localization is distinct from that of alpha-actinin, another filament cross-linking protein. Based on localization and actin filament nucleation and cross-linking activities, we propose a role for Arp2/3 in determining the structure of the actin filament network at the leading edge of motile cells.

Plant and animal profilins are functionally equivalent and stabilize microfilaments in living animal cells

We have analyzed the degree of functional similarity between birth and mammalian profilins, two members of the profilin family which show only a moderate sequence homology (22%) in living animal cells. The plant profilin, derived from birch pollen, was stably expressed in BHK-21 cells. Plant and endogenous profilin synthesis and cellular distribution were monitored by specific monoclonal antibodies. Quantitation of profilin and actin on calibrated immunoblots showed that two stable clones contained in total 1.4 and 2.0 times as much profilin as the parental cells. Using double fluorescence and confocal laser scanning microscopy, it was seen that the endogenous and the plant profilin colocalized with dynamic microfilaments, in particular with F-actin-rich foci and cortical microfilament webs of spreading cells, with dynamic microfilament bundles induced by serum deprival, and with cytochalasin D- and latrunculin-induced transient F-actin aggregates. The increase in the overall profilin concentration correlated with a significantly higher resistance of actin filaments to these drugs. Our data indicate that even profilins of highly distant evolutionary origin can functionally substitute for each other and support the hypothesis that in animal cells, profilins are engaged in regulating either the stability or the kinetic properties of actin filaments.

Sequences, structural models, and cellular localization of the actin-related proteins Arp2 and Arp3 from Acanthamoeba

We cloned and sequenced the two actin-related proteins (Arps) present in the profilin-binding complex of Acanthamoeba (Machesky, L. M., S. J. Atkinson, C. Ampe, J. Vandekerckhove, and T. D. Pollard. 1994, J. Cell Biol. 127:107-115). The sequence of Arp2 is more similar to other Arp2s than to actin, while the sequence of Arp3 is more similar to other Arp3s than to actin. Phylogenetic analysis of all known Arps demonstrates that most group into three major families, which are likely to be shared across all eukaryotic phyla. Together with conventional actins, the Arps form a larger family distinct from structurally related ATPases such as Hsp70's and sugar kinases. Atomic models of the Arps based on their sequences and the structure of actin provide some clues about function. Both Arps have atoms appropriately placed to bind ATP and divalent cation. Arp2, but not Arp3, has a conserved profilin-binding site. Neither Arp has the residues required to copolymerize with actin, but an Arp heterodimer present in the profilin-binding complex might serve as a pointed end nucleus for actin polymerization. Both Acanthamoeba Arps are soluble in cell homogenates, and both are concentrated in the cortex of Acanthamoeba. The cellular concentrations are 1.9 microM Arp2 and 5.1 microM Arp3, substoichiometric to actin (200 microM) but comparable to many actin-binding proteins.

Localization of a binding site for phosphatidylinositol 4,5-bisphosphate on human profilin

Profilin is a small 12-15-kDa actin-binding protein, which in eukaryotic organisms is ubiquitous and necessary for normal cell growth and function. Although profilin's interactions with its three known ligands (actin monomers, phosphatidylinositol 4,5-bisphosphate (PIP2), and poly-L-proline (PLP)) have been well characterized in vitro, its precise role in cells remains largely unknown. By binding to clusters of PIP2, profilin is able to inhibit the hydrolysis of PIP2 by phospholipase C gamma 1 (PLC gamma 1). This ability is the result of profilin's affinity for PIP2, but the specific residues of profilin's amino acid sequence involved in the binding of PIP2 are not known. Using site-directed mutagenesis, we sought to localize regions of profilin important for this interaction by generating the following mutants of human profilin (named according to the wild-type amino acid altered, its position, and the amino acid substituted in its place): Y6F, D8A, L10R, K25Q, K53I, R74L, R88L, R88L/K90E, H119D, G121D, and K125Q. With the exception of L10R, all of the mutants were successfully expressed in Escherichia coli and purified by affinity chromatography on PLP-Sepharose. Only Y6F and K25Q demonstrated moderately less stringent binding to PLP, indicating that most of the mutations did not induce marked alterations of profilin's structure. When tested for their relative abilities to inhibit the hydrolysis of PIP2 by PLC gamma 1, most of the mutants were indistinguishable from wild-type profilin. Exceptions included D8A, which demonstrated increased inhibition of PLC gamma 1, and R88L, which demonstrated decreased inhibition of PLC gamma 1. To assess the importance of the region surrounding residue 88 of human profilin, three synthetic decapeptides selected to correspond to non-overlapping stretches of the human profilin sequence were tested for their abilities to inhibit PLC gamma 1. We found that only te decapeptide that matched the peptide stretch centered around residue 88 was able to inhibit PLC gamma 1 activity substantially and was able to do so at nearly wild-type profilin levels. Taken together with the finding that mutating residue 88 resulted in decreased inhibition of PLC gamma 1 activity, these data provide strong evidence that this region of human profilin represents an important binding site for PIP2.

Purification, characterization, and cDNA cloning of profilin from Phaseolus vulgaris

Profilin from common bean (Phaseolus vulgaris L.) was purified to homogeneity by poly-L-Pro affinity chromatography and gel filtration. The hypocotyl and symbiotic root nodule protein was detected as a single isoform with a 14.4-kD molecular mass and an isoelectric point of 5.3. Partial amino acid and DNA sequencing of a full-length cDNA clone confirmed its identity as profilin. An antibody generated against the purified protein binds to a protein with the same molecular mass in leaves and nodules. Immunolocalization of the protein showed a diffuse distribution in the cytoplasm of hypocotyls and nodules but enhanced staining at the vascular bundles. The strong identity of the sequence among the profilins of birch, maize, and bean suggests that it may play an important role in the signal transduction mechanism of plant cells and plant-bacterial symbioses.

Binding of divalent cation and nucleotide to G-actin in the presence of profilin

The effect of profilin, a G-actin binding protein, on the mechanism of exchange of the tightly bound metal ion and nucleotide on G-actin, has been investigated. 1) In low ionic strength buffer, profilin increases the rates of Ca2+ and Mg2+ dissociation from G-actin 250- and 50-fold, respectively. On the profilin-actin complex as well as on G-actin alone, nucleotide exchange is dependent on the concentration of divalent metal ion and is kinetically limited, at low concentration of metal ion, by the dissociation of the metal ion. 2) Under physiological ionic conditions, nucleotide exchange on G-actin is 1 order of magnitude faster than at low ionic strength. The rate of MgATP dissociation is increased by profilin from 0.05 s-1 to 2 s-1, the rate of MgADP dissociation is increased from 0.2 s-1 to 24 s-1. The dependences of the exchange rates on profilin concentration are consistent with a high affinity (5 x 10(6) to 10(7) M-1) of profilin for ATP-G-actin, and a 20-fold lower affinity for ADP-G-actin. Profilin binding to actin lowers the affinity of metal-nucleotide by about 1 order of magnitude. These results restrain the possible roles of profilin in actin assembly in vivo.

Purification of a cortical complex containing two unconventional actins from Acanthamoeba by affinity chromatography on profilin-agarose

We identified four polypeptides of 47, 44, 40, and 35 kD that bind to profilin-Sepharose and elute with high salt. When purified by conventional chromatography using an antibody to the 47-kD polypeptide, these four polypeptides copurified as a stoichiometric complex together with three additional polypeptides of 19, 18, and 13 kD that varied in their proportions to the other polypeptides. Partial protein sequences showed that the 47-kD polypeptide is a homologue of S. pombe act2 and the 44-kD polypeptide is a homologue of S. cerevisiae ACT2, both unconventional actins. The 40-kD polypeptide contains a sequence similar to the WD40 motif of the G beta subunit of a trimeric G-protein from Dictyostelium discoideum. From partial sequences, the 35-, 19-, and 18-kD polypeptides appear to be novel proteins. On gel filtration the complex of purified polypeptides cochromatograph with a Stokes' radius of 4.8 nm, a value consistent with a globular particle of 220 kD containing one copy of each polypeptide. Cell extracts also contain components of the complex that do not bind the profilin column. Affinity purified antibodies localize 47- and 18/19-kD polypeptides in the cortex and filopodia of Acanthamoeba. Antibodies to the 47-kD unconventional actin cross-react on immunoblots with polypeptides of similar size in Dictyostelium, rabbit muscle, and conventional preparations of rabbit muscle actin but do not react with actin.

Purification of a cortical complex containing two unconventional actins from Acanthamoeba by affinity chromatography on profilin-agarose

We identified four polypeptides of 47, 44, 40, and 35 kD that bind to profilin-Sepharose and elute with high salt. When purified by conventional chromatography using an antibody to the 47-kD polypeptide, these four polypeptides copurified as a stoichiometric complex together with three additional polypeptides of 19, 18, and 13 kD that varied in their proportions to the other polypeptides. Partial protein sequences showed that the 47-kD polypeptide is a homologue of S. pombe act2 and the 44-kD polypeptide is a homologue of S. cerevisiae ACT2, both unconventional actins. The 40-kD polypeptide contains a sequence similar to the WD40 motif of the G beta subunit of a trimeric G-protein from Dictyostelium discoideum. From partial sequences, the 35-, 19-, and 18-kD polypeptides appear to be novel proteins. On gel filtration the complex of purified polypeptides cochromatograph with a Stokes' radius of 4.8 nm, a value consistent with a globular particle of 220 kD containing one copy of each polypeptide. Cell extracts also contain components of the complex that do not bind the profilin column. Affinity purified antibodies localize 47- and 18/19-kD polypeptides in the cortex and filopodia of Acanthamoeba. Antibodies to the 47-kD unconventional actin cross-react on immunoblots with polypeptides of similar size in Dictyostelium, rabbit muscle, and conventional preparations of rabbit muscle actin but do not react with actin.

The sea urchin profilin gene is specifically expressed in mesenchyme cells during gastrulation

Eggs and embryos of the purple sea urchin (Strongylocentrotus purpuratus) contain profilin that is partly supplied from maternal sources and partly produced by the gastrula. The maternal profilin protein content is about 13 microM and it persists in the embryo at least through gastrulation. Transcript quantitation from probe excess titrations show that very few profilin gene transcripts are present in the embryo during cleavage, but that they increase at the onset of gastrulation. By in situ hybridization, the newly synthesized profilin transcripts are localized in mesenchyme cells. Profilin gene expression increases when mesenchyme cells initiate migration and filopodial extension and retraction. We show that there are three isoforms of maternal profilin protein produced from the single copy gene during oogenesis. However, the blastula stage embryo only produces the major isoform, whereas the acidic isoform is produced in the early stages of gastrulation and the basic isoform appears by the end of gastrulation. Based on transcript prevalence and protein production rates, our calculations indicate that the amount of new protein produced in the mesenchyme cells in 12 hr is at maximum < 2% of that supplied from maternal sources. Because of the large amount of maternally supplied profilin present in the egg and embryo, we suggest that it may be used in the cytokinetic processes of cleavage. Alternatively, because of the small amount of embryonically produced profilin, we suggest that it may function in the cytoskeletal shape changes required for filopodial extension and motility in the mesenchyme cells during gastrulation.

Direct visualization by electron microscopy of the weakly bound intermediates in the actomyosin adenosine triphosphatase cycle

We used a novel stopped-flow/rapid-freezing machine to prepare the transient intermediates in the actin-myosin adenosine triphosphatase (ATPase) cycle for direct observation by electron microscopy. We focused on the low affinity complexes of myosin-adenosine triphosphate (ATP) and myosin-adenosine diphosphate (ADP)-Pi with actin filaments since the transition from these states to the high affinity actin-myosin-ADP and actin-myosin states is postulated to generate the molecular motion that drives muscle contraction and other types of cellular movements. After rapid freezing and metal replication of mixtures of myosin subfragment-1, actin filaments, and ATP, the structure of the weakly bound intermediates is indistinguishable from nucleotide-free rigor complexes. In particular, the average angle of attachment of the myosin head to the actin filament is approximately 40 degrees in both cases. At all stages in the ATPase cycle, the configuration of most of the myosin heads bound to actin filaments is similar, and the part of the myosin head preserved in freeze-fracture replicas does not tilt by more than a few degrees during the transition from the low affinity to high affinity states. In contrast, myosin heads chemically cross-linked to actin filaments differ in their attachment angles from ordered at 40 degrees without ATP to nearly random in the presence of ATP when viewed by negative staining (Craig, R., L.E. Greene, and E. Eisenberg. 1985. Proc. Natl. Acad. Sci. USA. 82:3247-3251, and confirmed here), freezing in vitreous ice (Applegate, D., and P. Flicker. 1987. J. Biol. Chem. 262:6856-6863), and in replicas of rapidly frozen samples. This suggests that many of the cross-linked heads in these preparations are dissociated from but tethered to the actin filaments in the presence of ATP. These observations suggest that the molecular motion produced by myosin and actin takes place with the myosin head at a point some distance from the actin binding site or does not involve a large change in the shape of the myosin head.

An interaction between zyxin and alpha-actinin

Zyxin is an 82-kD protein first identified as a component of adhesion plaques and the termini of stress fibers near where they associate with the cytoplasmic face of the adhesive membrane. We report here that zyxin interacts with the actin cross-linking protein alpha-actinin. Zyxin cosediments with filamentous actin in an alpha-actinin-dependent manner and an association between zyxin and alpha-actinin is observed in solution by analytical gel filtration. The specificity of the interaction between zyxin and alpha-actinin was demonstrated by blot overlay experiments in which 125I-zyxin recognizes most prominently alpha-actinin among a complex mixture of proteins extracted from avian smooth muscle. By these blot overlay binding studies, we determined that zyxin interacts with the NH2-terminal 27-kD domain of alpha-actinin, a region that also contains the actin binding site. Solid phase binding assays were performed to evaluate further the specificity of the binding and to determine the affinity of the zyxin-alpha-actinin interaction. By these approaches we have demonstrated a specific, saturable, moderate-affinity interaction between zyxin and alpha-actinin. Furthermore, double-label immunofluorescence reveals that zyxin and alpha-actinin exhibit extensive overlap in their subcellular distributions in both chicken embryo fibroblasts and pigmented retinal epithelial cells. The significant colocalization of the two proteins is consistent with the possibility that the interaction between zyxin and alpha-actinin has a biologically relevant role in coordinating membrane-cytoskeletal interactions.

Profilin-actin complexes directly elongate actin filaments at the barbed end

We demonstrate that the profilin-G-actin complex can elongate actin filaments directly at the barbed end but cannot bind to the pointed end. During elongation, the profilin-actin complex binds to the barbed filament end, whereupon profilin is released, leaving the actin molecule behind. This was first proposed by Tilney [Tilney, L. G., et al. (1983) J. Cell Biol. 97, 112-124] and demonstrated by Pollard and Cooper [(1984) Biochemistry 23, 6631-6641] by electron microscopy. We show that a model without any outside energy supply, in contrast to the mechanism proposed by Pollard and Cooper, can be fitted to our and their [Kaiser et al. (1986) J. Cell Biol. 102, 221-226] findings. Input of outside energy is necessary only if profilin-mediated elongation continues after free G-actin has been lowered to or below the critical concentration observed at the barbed end in the absence of profilin.

Mechanism of the interaction of human platelet profilin with actin

We have reexamined the interaction of purified platelet profilin with actin and present evidence that simple sequestration of actin monomers in a 1:1 complex with profilin cannot explain many of the effects of profilin on actin assembly. Three different methods to assess binding of profilin to actin show that the complex with platelet actin has a dissociation constant in the range of 1 to 5 microM. The value for muscle actin is similar. When bound to actin, profilin increases the rate constant for dissociation of ATP from actin by 1,000-fold and also increases the rate of dissociation of Ca2+ bound to actin. Kinetic simulation showed that the profilin exchanges between actin monomers on a subsecond time scale that allows it to catalyze nucleotide exchange. On the other hand, polymerization assays give disparate results that are inconsistent with the binding assays and each other: profilin has different effects on elongation at the two ends of actin filaments; profilin inhibits the elongation of platelet actin much more strongly than muscle actin; and simple formation of 1:1 complexes of actin with profilin cannot account for the strong inhibition of spontaneous polymerization. We suggest that the in vitro effects on actin polymerization may be explained by a complex mechanism that includes weak capping of filament ends and catalytic poisoning of nucleation. Although platelets contain only 1 profilin for every 5-10 actin molecules, these complex reactions may allow substoichiometric profilin to have an important influence on actin assembly. We also confirm the observation of I. Lassing and U. Lindberg (1985. Nature [Lond.] 318:472-474) that polyphosphoinositides inhibit the effects of profilin on actin polymerization, so lipid metabolism must also be taken into account when considering the functions of profilin in a cell.

The affinities of human platelet and Acanthamoeba profilin isoforms for polyphosphoinositides account for their relative abilities to inhibit phospholipase C

In light of recent work implicating profilin from human platelets as a possible regulator of both cytoskeletal dynamics and inositol phospholipid-mediated signaling, we have further characterized the interaction of platelet profilin and the two isoforms of Acanthamoeba profilin with inositol phospholipids. Profilin from human platelets binds to phosphatidylinositol-4-monophosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2) with relatively high affinity (Kd approximately 1 microM for PIP2 by equilibrium gel filtration), but interacts only weakly (if at all) with phosphatidylinositol (PI) or inositol trisphosphate IP3) in small-zone gel-filtration assays. The two isoforms of Acanthamoeba profilin both have a lower affinity for PIP2 than does human platelet profilin, but the more basic profilin isoform from Acanthamoeba (profilin-II) has a much higher (approximately 10-microM Kd) affinity than the acidic isoform (profilin-I, 100 to 500-microM Kd). None of the profilins bind to phosphatidylserine (PS) or phosphatidylcholine (PC) in small-zone gel-filtration experiments. The differences in affinity for PIP2 parallel the ability of these three profilins to inhibit PIP2 hydrolysis by soluble phospholipase C (PLC). The results show that the interaction of profilins with PIP2 is specific with respect to both the lipid and the proteins. In Acanthamoeba, the two isoforms of profilin may have specialized functions on the basis of their identical (approximately 10 microM) affinities for actin monomers and different affinities for PIP2.

The actin released from profilin--actin complexes is insufficient to account for the increase in F-actin in chemoattractant-stimulated polymorphonuclear leukocytes

Chemoattractant stimulation of polymorphonuclear leukocytes is associated with a nearly two-fold rise in actin filament content. We examined the role of the actin monomer sequestering protein, profilin, in the regulation of PMN actin filament assembly during chemoattractant stimulation using a Triton extraction method. Poly-L-proline-conjugated Sepharose beads were used to assess the relative concentration of actin bound to profilin with high enough affinity to withstand dilution (profilin-actin complex) and DNase I-conjugated beads to measure the relative concentration of actin in the Triton-soluble fraction not bound to profilin. Actin associated with the Triton-insoluble fraction (F-actin) was also measured. In unstimulated PMN, the relative concentration of actin bound to profilin was maximum. After FMLP stimulation, profilin released actin monomers within 10 s, with the profilin-actin complex concentration reaching a nadir by 40 s and remaining low as long as the cells were exposed to chemoattractant (up to 30 min). If FMLP was dissociated from PMN membrane receptors using t-BOC, actin reassociated with profilin within 20 s. Quantitative analysis of these reactions, however, revealed that profilin release of and rebinding to actin could account for only a small percentage of the total change in F-actin content. Determination of the total profilin and actin concentrations in PMN revealed that the molar ratio of profilin to actin was 1 to 5.2. When purified actin was polymerized in PMN Triton extract containing EGTA, removal of profilin from the extract minimally affected (12% reduction) the high apparent critical concentration at which actin began to assemble. Although profilin released actin at the appropriate time to stimulate actin assembly during exposure to chemoattractants, the concentration of profilin in PMN was insufficient to explain the high unpolymerized actin content in unstimulated PMN and the quantity of actin released from profilin too small to account for the large shifts from unpolymerized to polymerized actin associated with maximal chemoattractant stimulation.

Characterization of renatured profilin purified by urea elution from poly-L-proline agarose columns

We present evidence that native profilin can be purified from cellular extracts of Acanthamoeba, Dictyostelium, and human platelets by affinity chromatography on poly-L-proline agarose. After applying cell extracts and washing the column with 3 M urea, homogeneous profilin is eluted by increasing the urea concentration to 6-8 M. Acanthamoeba profilin-I and profilin-II can subsequently be separated by cation exchange chromatography. The yield of Acanthamoeba profilin is twice that obtained by conventional methods. Several lines of evidence show that the profilins fully renature after removal of the urea by dialysis: 1) dialyzed Acanthamoeba and human profilins rebind quantitatively to poly-L-proline and bind to actin in the same way as native, conventionally purified profilin without urea treatment; 2) dialyzed profilins form 3-D crystals under the same conditions as native profilins; 3) dialyzed Acanthamoeba profilin-I has an NMR spectrum identical with that of native profilin-I; and 4) dialyzed human and Acanthamoeba profilins inhibit actin polymerization. We report the discovery of profilin in Dictyostelium cell extracts using the same method. Based on these observations we conclude that urea elution from poly-L-proline agarose followed by renaturation will be generally useful for preparing profilins from a wide variety of cells. Perhaps also of general use is the finding that either myosin-II or alpha-actinin in crude cell extracts can be bound selectively to the poly-L-proline agarose column depending on the ionic conditions used to equilibrate the column. We have purified myosin-II from both Acanthamoeba and Dictyostelium cell extracts and alpha-actinin from Acanthamoeba cell extracts in the appropriate buffers. These proteins are retained as complexes with actin by the agarose and not by a specific interaction with poly-L-proline. They can be eluted by dissociating the complexes with ATP and separated from actin by gel filtration if necessary.

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.

Acanthamoeba profilin binding to fluorescein-labeled actins

The binding constants of Acanthamoeba profilin to fluorescein-labeled actin from Acanthamoeba and from rabbit skeletal muscle have been determined by measuring the reduction in the actin tracer diffusion coefficients, determined by fluorescence photobleaching recovery, as a function of added profilin concentration. Data were analyzed using a two-parameter nonlinear regression analysis to determine the profilin-actin dissociation constant Kd and the profilactin diffusion coefficient, DPA. For fluorescein-labeled Acanthamoeba actin, the least-squares estimates for Kd and DPA, along with approximate single standard deviation confidence intervals, are Kd = 48 (36, 63) microM and DPA = 6.72 (6.62, 6.81) X 10(-7) cm2s-1. For fluorescein-labeled skeletal muscle actin, the corresponding values are Kd = 147 (94, 225) microM and DPA = 6.7 (6.3, 7.0) X 10(-7) cm2s-1. These dissociation constants are the first to be determined from direct physical measurement; they are in agreement with values inferred from earlier studies on the effect of profilin on the assembly of actin that had been fluorescently labeled or otherwise modified at Cys 374. These results place important restrictions on the interpretation of experiments in which fluorescently labeled actin is used as a probe of living cytoplasm or cytoplasmic extracts that include profilin.

Assembly of Acanthamoeba actin in the presence of Acanthamoeba profilin measured by fluorescence photobleaching recovery

Assembly of Acanthamoeba actin, of which trace quantities had been labeled with 5-(iodoacetamido)-fluorescein, was quantified using the modulation detection method of fluorescence photobleaching recovery (FPR). This technique permits explicit determination of the fraction of labeled actin incorporated into filaments and the translational diffusion coefficients of the filaments, from which filament length can be calculated. Addition of Acanthamoeba profilin in molar ratios to actin of about 1.1:1 and 2.3:1 retarded the initial kinetics of assembly (induced by addition of 2mM Mg+2) and reduced the fraction of actin incorporated into filaments. The diffusion coefficients of filaments formed were greatly changed by the presence of profilin at short times, but the differences became increasingly smaller at longer times. After 26 hr. the filaments formed in 1.1:1 profilin were about 12% shorter and in 2.3:1 profilin were about 20% shorter than filaments formed by actin alone under the same conditions.

Assembly and dynamics of the actin filament system in nonmuscle cells

Kinetic analysis has provided a detailed quantitative description of the mechanism of actin polymerization as well as the methods to analyze the mechanisms of action of actin-binding proteins. In Acanthamoeba, five different proteins regulate the pool of monomers available for polymerization, cap the end of filaments, sever filaments, and cross-link filaments. Remarkably, many of these interactions involve very-low-affinity bonds between the protein molecules.

Characterization of alpha-actinin from Acanthamoeba

Characterization of a protein from Acanthamoeba that was originally called gelation protein [T.D. Pollard, J. Biol. Chem. 256:7666-7670, 1981] has shown that it resembles the actin filament cross-linking protein, alpha-actinin, found in other cells. It comprises about 1.5% of the total amoeba protein and can be purified by chromatography with a yield of 13%. The native protein has a molecular weight of 180,000 and consists of two polypeptides of 90,000 Da. The Stokes' radius is 8.5 nm, the intrinsic viscosity is 0.35 dl/dm, and the extinction coefficient at 280 mm is 1.8 X 10(5)M-1 X cm-1. Electron micrographs of shadowed specimens show that the molecule is a rod 48 nm long and 7 nm wide with globular domains at both ends and in the middle of the shaft. On gel electrophoresis in sodium dodecylsulfate the pure protein can run as bands with apparent molecular weights of 60,000, 90,000, 95,000, or 134,000 depending on the method of sample preparation. Rabbit antibodies to electrophoretically purified Acanthamoeba alpha-actinin polypeptides react with all of these electrophoretic variants in samples of purified protein and cell extracts. By indirect fluorescent antibody staining of fixed amoebas, alpha-actinin is distributed throughout the cytoplasmic matrix and concentrated in the hyaline cytoplasm of the cortex. The protein cross-links actin filaments in the presence and absence of Ca++. It inhibits slightly the time course of the spontaneous polymerization of actin monomers but has no effect on the critical concentration for actin polymerization even though it increases the apparent rate of elongation to a small extent. Like some other cross-linking proteins, amoeba alpha-actinin inhibits the actin-activated ATPase of muscle myosin subfragment-1. Although Acanthamoeba alpha-actinin resembles the alpha-actinin from other cells in shape and ability to cross-link actin filaments, antibodies to amoeba and smooth muscle alpha-actinins do not cross react and there are substantial differences in the amino acid compositions and molecular dimensions.

Purification and characterization of two isoforms of Acanthamoeba profilin

Acanthamoeba profilin purified according to E. Reichstein and E.D. Korn (1979, J. Biol. Chem. 254:6174-6179) consists of two isoforms (profilin-I and-II) with approximately the same molecular weight and reactivity to a monoclonal antibody but different isoelectric points and different mobilities on carboxymethyl-agarose chromatography and reversed-phase high-performance liquid chromatography. The isoelectric points of profilin-I is approximately 5.5 and that of profilin-II is greater than or equal to 9.0. Tryptic peptides from the two proteins are substantially different, which suggests that there are major differences in their sequences. At similar concentrations, both profilins prolong the lag phase at the outset of spontaneous polymerization and inhibit the extent of polymerization. Both forms also inhibit elongation weakly at the barbed end and strongly at the pointed end of actin filaments.