Effects of single amino acid substitutions in the actin-binding site on the biological activity of bovine profilin I

FgPfn participates in vegetative growth, sexual reproduction, pathogenicity, and fungicides sensitivity via affecting both microtubules and actin in the filamentous fungus Fusarium graminearum

Fusarium head blight (FHB), caused by Fusarium graminearum species complexes (FGSG), is an epidemic disease in wheat and poses a serious threat to wheat production and security worldwide. Profilins are a class of actin-binding proteins that participate in actin depolymerization. However, the roles of profilins in plant fungal pathogens remain largely unexplored. Here, we identified FgPfn, a homolog to profilins in F. graminearum, and the deletion of FgPfn resulted in severe defects in mycelial growth, conidia production, and pathogenicity, accompanied by marked disruptions in toxisomes formation and deoxynivalenol (DON) transport, while sexual development was aborted. Additionally, FgPfn interacted with Fgα1 and Fgβ2, the significant components of microtubules. The organization of microtubules in the ΔFgPfn was strongly inhibited under the treatment of 0.4 μg/mL carbendazim, a well-known group of tubulin interferers, resulting in increased sensitivity to carbendazim. Moreover, FgPfn interacted with both myosin-5 (FgMyo5) and actin (FgAct), the targets of the fungicide phenamacril, and these interactions were reduced after phenamacril treatment. The deletion of FgPfn disrupted the normal organization of FgMyo5 and FgAct cytoskeleton, weakened the interaction between FgMyo5 and FgAct, and resulting in increased sensitivity to phenamacril. The core region of the interaction between FgPfn and FgAct was investigated, revealing that the integrity of both proteins was necessary for their interaction. Furthermore, mutations in R72, R77, R86, G91, I101, A112, G113, and D124 caused the non-interaction between FgPfn and FgAct. The R86K, I101E, and D124E mutants in FgPfn resulted in severe defects in actin organization, development, and pathogenicity. Taken together, this study revealed the role of FgPfn-dependent cytoskeleton in development, DON production and transport, fungicides sensitivity in F. graminearum.

Taro raphide-associated proteins: Allergens and crystal growth

Calcium oxalate raphide crystals are found in bundles in intravacuolar membrane chambers of specialized idioblasts cells of most plant families. Aroid raphides are proposed to cause acridity in crops such as taro (Colocasia esculenta (L.) Schott). Acridity is irritation that causes itchiness and pain when raw/insufficiently cooked tissues are eaten. Since raphides do not always cause acridity and since acridity can be inactivated by cooking and/or protease treatment, it is possible that a toxin or allergen-like compound is associated with the crystals. Using two-dimensional (2D) gel electrophoresis and mass spectrometry (MS) peptide sequencing of selected peptides from purified raphides and taro apex transcriptome sequencing, we showed the presence on the raphides of peptides normally associated with mitochrondria (ATP synthase), chloroplasts (chaperonin ~60 kDa), cytoplasm (actin, profilin), and vacuole (V-type ATPase) that indicates a multistage biocrystallation process ending with possible invagination of the tonoplast and addition of mucilage that may be derived from the Golgi. Actin might play a crucial role in the generation of the needle-like raphides. One of the five raphide profilins genes was highly expressed in the apex and had a 17-amino acid insert that significantly increased that profilin's antigenic epitope peak. A second profilin had a 2-amino acid insert and also had a greater B-cell epitope prediction. Taro profilins showed 83% to 92% similarity to known characterized profilins. Further, commercial allergen test strips for hazelnuts, where profilin is a secondary allergen, have potential for screening in a taro germplasm to reduce acridity and during food processing to avoid overcooking.

Profilin Isoforms in Health and Disease - All the Same but Different

Profilins are small actin binding proteins, which are structurally conserved throughout evolution. They are probably best known to promote and direct actin polymerization. However, they also participate in numerous cell biological processes beyond the roles typically ascribed to the actin cytoskeleton. Moreover, most complex organisms express several profilin isoforms. Their cellular functions are far from being understood, whereas a growing number of publications indicate that profilin isoforms are involved in the pathogenesis of various diseases. In this review, we will provide an overview of the profilin family and "typical" profilin properties including the control of actin dynamics. We will then discuss the profilin isoforms of higher animals in detail. In terms of cellular functions, we will focus on the role of Profilin 1 (PFN1) and Profilin 2a (PFN2a), which are co-expressed in the central nervous system. Finally, we will discuss recent findings that link PFN1 and PFN2a to neurological diseases, such as amyotrophic lateral sclerosis (ALS), Fragile X syndrome (FXS), Huntington's disease and spinal muscular atrophy (SMA).

Ser71 Phosphorylation Inhibits Actin-Binding of Profilin-1 and Its Apoptosis-Sensitizing Activity

The essential actin-binding factor profilin-1 (Pfn1) is a non-classical tumor suppressor with the abilities toboth inhibit cellular proliferation and augment chemotherapy-induced apoptosis. Besides actin, Pfn1 interacts with proteins harboring the poly-L-proline (PLP) motifs. Our recent work demonstrated that both nuclear localization and PLP-binding are required for tumor growth inhibition by Pfn1, and this is at least partially due to Pfn1 association with the PLP-containing ENL protein in the Super Elongation Complex (SEC) and the transcriptional inhibition of pro-cancer genes. In this paper, by identifying a phosphorylation event of Pfn1 at Ser71 capable of inhibiting its actin-binding and nuclear export, we provide in vitro and in vivo evidence that chemotherapy-induced apoptotic sensitization by Pfn1 requires its cytoplasmic localization and actin-binding. With regard to tumor growth inhibition byPfn1, our data indicate a requirement for dynamic actin association and dissociation rendered by reversible Ser71phosphorylation and dephosphorylation. Furthermore, genetic and pharmacological experiments showed that Ser71 of Pfn1 can be phosphorylated by protein kinase A (PKA). Taken together, our data provide novel mechanistic insights into the multifaceted anticancer activities of Pfn1 and how they are spatially-defined in the cell and differentially regulated by ligand-binding.

Ubiquitin chains: a new way of screening for regulatory differences in pulmonary hypertension

Protein ubiquitination serves many regulatory functions; in addition to degradation, ubiquitination has roles in intracellular trafficking, cell cycle, innate immunity, and more. Using mass spectrometry, it is possible to assess the ubiquitination state of every protein simultaneously. In this issue, Wade et al. have for the first time done just that in a hypoxic mouse model of pulmonary hypertension (PH). New techniques drive new discoveries; their work is important not just because they have found new ways to intervene in known PH-related pathways but have found regulation of proteins not previously associated with disease.

Hypoxia-induced alterations in the lung ubiquitin proteasome system during pulmonary hypertension pathogenesis

Pulmonary hypertension (PH) is a clinical disorder characterized by sustained increases in pulmonary vascular resistance and pressure that can lead to right ventricular (RV) hypertrophy and ultimately RV failure and death. The molecular pathogenesis of PH remains incompletely defined, and existing treatments are associated with suboptimal outcomes and persistent morbidity and mortality. Reports have suggested a role for the ubiquitin proteasome system (UPS) in PH, but the extent of UPS-mediated non-proteolytic protein alterations during PH pathogenesis has not been previously defined. To further examine UPS alterations, the current study employed C57BL/6J mice exposed to normoxia or hypoxia for 3 weeks. Lung protein ubiquitination was evaluated by mass spectrometry to identify differentially ubiquitinated proteins relative to normoxic controls. Hypoxia stimulated differential ubiquitination of 198 peptides within 131 proteins (p < 0.05). These proteins were screened to identify candidates within pathways involved in PH pathogenesis. Some 51.9% of the differentially ubiquitinated proteins were implicated in at least one known pathway contributing to PH pathogenesis, and 13% were involved in three or more PH pathways. Anxa2, App, Jak1, Lmna, Pdcd6ip, Prkch1, and Ywhah were identified as mediators in PH pathways that undergo differential ubiquitination during PH pathogenesis. To our knowledge, this is the first study to report global changes in protein ubiquitination in the lung during PH pathogenesis. These findings suggest signaling nodes that are dynamically regulated by the UPS during PH pathogenesis. Further exploration of these differentially ubiquitinated proteins and related pathways can provide new insights into the role of the UPS in PH pathogenesis.

Human profilin 1 is a negative regulator of CTL mediated cell-killing and migration

The actin-binding protein profilin1 (PFN1) plays a central role in actin dynamics, which is essential for cytotoxic T lymphocyte (CTL) functions. The functional role of PFN1 in CTLs, however still remains elusive. Here, we identify PFN1 as the only member of the profilin family expressed in primary human CD8⁺ T cells. Using in vitro assays, we find that PFN1 is a negative regulator of CTL-mediated elimination of target cells. Furthermore, PFN1 is involved in activation-induced lytic granule (LG) release, CTL migration and modulation of actin structures at the immunological synapse (IS). During CTL migration, PFN1 modulates the velocity, protrusion formation patterns and protrusion sustainability. In contrast, PFN1 does not significantly affect migration persistence and the rates of protrusion emergence and retraction. Under in vitro conditions mimicking a tumor microenvironment, we show that PFN1 downregulation promotes CTL invasion into a 3D matrix, without affecting the viability of CTLs in a hydrogen peroxide-enriched microenvironment. Highlighting its potential relevance in cancer, we find that in pancreatic cancer patients, PFN1 expression is substantially decreased in peripheral CD8⁺ T cells. Taken together, we conclude that PFN1 is a negative regulator for CTL-mediated cytotoxicity and may have an impact on CTL functionality in a tumor-related context.

DULIP: A Dual Luminescence-Based Co-Immunoprecipitation Assay for Interactome Mapping in Mammalian Cells

Mapping of protein-protein interactions (PPIs) is critical for understanding protein function and complex biological processes. Here, we present DULIP, a dual luminescence-based co-immunoprecipitation assay, for systematic PPI mapping in mammalian cells. DULIP is a second-generation luminescence-based PPI screening method for the systematic and quantitative analysis of co-immunoprecipitations using two different luciferase tags. Benchmarking studies with positive and negative PPI reference sets revealed that DULIP allows the detection of interactions with high sensitivity and specificity. Furthermore, the analysis of a PPI reference set with known binding affinities demonstrated that both low- and high-affinity interactions can be detected with DULIP assays. Finally, using the well-characterized interaction between Syntaxin-1 and Munc18, we found that DULIP is capable of detecting the effects of point mutations on interaction strength. Taken together, our studies demonstrate that DULIP is a sensitive and reliable method of great utility for systematic interactome research. It can be applied for interaction screening and validation of PPIs in mammalian cells. Moreover, DULIP permits the specific analysis of mutation-dependent binding patterns.

Fission yeast profilin is tailored to facilitate actin assembly by the cytokinesis formin Cdc12

The evolutionarily conserved small actin-monomer binding protein profilin is believed to be a housekeeping factor that maintains a general pool of unassembled actin. However, despite similar primary sequences, structural folds, and affinities for G-actin and poly-L-proline, budding yeast profilin ScPFY fails to complement fission yeast profilin SpPRF temperature-sensitive mutant cdc3-124 cells. To identify profilin's essential properties, we built a combinatorial library of ScPFY variants containing either WT or SpPRF residues at multiple positions and carried out a genetic selection to isolate variants that support life in fission yeast. We subsequently engineered ScPFY(9-Mut), a variant containing nine substitutions in the actin-binding region, which complements cdc3-124 cells. ScPFY(9-Mut), but not WT ScPFY, suppresses severe cytokinesis defects in cdc3-124 cells. Furthermore, the major activity rescued by ScPFY(9-Mut) is the ability to enhance cytokinesis formin Cdc12-mediated actin assembly in vitro, which allows cells to assemble functional contractile rings. Therefore an essential role of profilin is to specifically facilitate formin-mediated actin assembly for cytokinesis in fission yeast.

Analysis of the effects of polymorphism on pollen profilin structural functionality and the generation of conformational, T- and B-cell epitopes

An extensive polymorphism analysis of pollen profilin, a fundamental regulator of the actin cytoskeleton dynamics, has been performed with a major focus in 3D-folding maintenance, changes in the 2-D structural elements, surface residues involved in ligands-profilin interactions and functionality, and the generation of conformational and lineal B- and T-cell epitopes variability.

Our results revealed that while the general fold is conserved among profilins, substantial structural differences were found, particularly affecting the special distribution and length of different 2-D structural elements (i.e. cysteine residues), characteristic loops and coils, and numerous micro-heterogeneities present in fundamental residues directly involved in the interacting motifs, and to some extension these residues nearby to the ligand-interacting areas. Differential changes as result of polymorphism might contribute to generate functional variability among the plethora of profilin isoforms present in the olive pollen from different genetic background (olive cultivars), and between plant species, since biochemical interacting properties and binding affinities to natural ligands may be affected, particularly the interactions with different actin isoforms and phosphoinositides lipids species.

Furthermore, conspicuous variability in lineal and conformational epitopes was found between profilins belonging to the same olive cultivar, and among different cultivars as direct implication of sequences polymorphism. The variability of the residues taking part of IgE-binding epitopes might be the final responsible of the differences in cross-reactivity among olive pollen cultivars, among pollen and plant-derived food allergens, as well as between distantly related pollen species, leading to a variable range of allergy reactions among atopic patients. Identification and analysis of commonly shared and specific epitopes in profilin isoforms is essential to gain knowledge about the interacting surface of these epitopes, and for a better understanding of immune responses, helping design and development of rational and effective immunotherapy strategies for the treatment of allergy diseases.

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.

NAC1 is an actin-binding protein that is essential for effective cytokinesis in cancer cells

NAC1 is a transcriptional corepressor protein that is essential to sustain cancer cell proliferation and migration. However, the underlying molecular mechanisms of NAC1 function in cancer cells remain unknown. In this study, we show that NAC1 functions as an actin monomer-binding protein. The conserved BTB protein interaction domain in NAC1 is the minimal region for actin binding. Disrupting NAC1 complex function by dominant-negative or siRNA strategies reduced cell retraction and abscission during late-stage cytokinesis, causing multinucleation in cancer cells. In Nac1-deficient murine fibroblasts, restoring NAC1 expression was sufficient to partially avert multinucleation. We found that siRNA-mediated silencing of the actin-binding protein profilin-1 in cancer cells caused a similar multinucleation phenotype and that NAC1 modulated the binding of actin to profillin-1. Taken together, our results indicate that the NAC1/actin/profilin-1 complex is crucial for cancer cell cytokinesis, with a variety of important biologic and clinical implications.

Characterization of profilin polymorphism in pollen with a focus on multifunctionality

Profilin, a multigene family involved in actin dynamics, is a multiple partners-interacting protein, as regard of the presence of at least of three binding domains encompassing actin, phosphoinositide lipids, and poly-L-proline interacting patches. In addition, pollen profilins are important allergens in several species like Olea europaea L. (Ole e 2), Betula pendula (Bet v 2), Phleum pratense (Phl p 12), Zea mays (Zea m 12) and Corylus avellana (Cor a 2). In spite of the biological and clinical importance of these molecules, variability in pollen profilin sequences has been poorly pointed out up until now. In this work, a relatively high number of pollen profilin sequences have been cloned, with the aim of carrying out an extensive characterization of their polymorphism among 24 olive cultivars and the above mentioned plant species. Our results indicate a high level of variability in the sequences analyzed. Quantitative intra-specific/varietal polymorphism was higher in comparison to inter-specific/cultivars comparisons. Multi-optional posttranslational modifications, e.g. phosphorylation sites, physicochemical properties, and partners-interacting functional residues have been shown to be affected by profilin polymorphism. As a result of this variability, profilins yielded a clear taxonomic separation between the five plant species. Profilin family multifunctionality might be inferred by natural variation through profilin isovariants generated among olive germplasm, as a result of polymorphism. The high variability might result in both differential profilin properties and differences in the regulation of the interaction with natural partners, affecting the mechanisms underlying the transmission of signals throughout signaling pathways in response to different stress environments. Moreover, elucidating the effect of profilin polymorphism in adaptive responses like actin dynamics, and cellular behavior, represents an exciting research goal for the future.

Identification of enoxacin as an inhibitor of osteoclast formation and bone resorption by structure-based virtual screening

An interaction between the B2 subunit of vacuolar H(+)-ATPase (V-ATPase) and microfilaments is required for osteoclast bone resorption. An atomic homology model of the actin binding site on B2 was generated and molecular docking simulations were performed. Enoxacin, a fluoroquinolone antibiotic, was identified and in vitro testing demonstrated that enoxacin blocked binding between purified B2 and microfilaments. Enoxacin dose dependently reduced the number of osteoclasts differentiating in mouse marrow cultures stimulated with 1,25-dihydroxyvitamin D(3), as well as markers of osteoclast activity, and the number of resorption lacunae formed on bone slices. Enoxacin inhibited osteoclast formation at concentrations where osteoblast formation was not altered. In summary, enoxacin is a novel small molecule inhibitor of osteoclast bone resorption that acts by an unique mechanism and is therefore an attractive lead molecule for the development of a new class of antiosteoclastic agents.

Structural basis for parasite-specific functions of the divergent profilin of Plasmodium falciparum

Profilins are key regulators of actin dynamics. They sequester actin monomers, forming a pool for rapid polymer formation stimulated by proteins such as formins. Apicomplexan parasites utilize a highly specialized microfilament system for motility and host cell invasion. Their genomes encode only a small number of divergent actin regulators. We present the first crystal structure of an apicomplexan profilin, that of the malaria parasite Plasmodium falciparum, alone and in complex with a polyproline ligand peptide. The most striking feature of Plasmodium profilin is a unique minidomain consisting of a large beta-hairpin extension common to all apicomplexan parasites, and an acidic loop specific for Plasmodium species. Reverse genetics in the rodent malaria model, Plasmodium berghei, suggests that profilin is essential for the invasive blood stages of the parasite. Together, our data establish the structural basis for understanding the functions of profilin in the malaria parasite.

Diaphanous-related formin 2 and profilin I are required for gastrulation cell movements

Intensive cellular movements occur during gastrulation. These cellular movements rely heavily on dynamic actin assembly. Rho with its associated proteins, including the Rho-activated formin, Diaphanous, are key regulators of actin assembly in cellular protrusion and migration. However, the function of Diaphanous in gastrulation cell movements remains unclear. To study the role of Diaphanous in gastrulation, we isolated a partial zebrafish diaphanous-related formin 2 (zdia2) clone with its N-terminal regulatory domains. The GTPase binding domain of zDia2 is highly conserved compared to its mammalian homologues. Using a yeast two-hybrid assay, we showed that zDia2 interacts with constitutively-active RhoA and Cdc42. The zdia2 mRNAs were ubiquitously expressed during early embryonic development in zebrafish as determined by RT-PCR and whole-mount in situ hybridization analyses. Knockdown of zdia2 by antisense morpholino oligonucleotides (MOs) blocked epiboly formation and convergent extension in a dose-dependent manner, whereas ectopic expression of a human mdia gene partially rescued these defects. Time-lapse recording further showed that bleb-like cellular processes of blastoderm marginal deep marginal cells and pseudopod-/filopod-like processes of prechordal plate cells and lateral cells were abolished in the zdia2 morphants. Furthermore, zDia2 acts cell-autonomously since transplanted zdia2-knockdown cells exhibited low protrusive activity with aberrant migration in wild type host embryos. Lastly, co-injection of antisense MOs of zdia2 and zebrafish profilin I (zpfn 1), but not zebrafish profilin II, resulted in a synergistic inhibition of gastrulation cell movements. These results suggest that zDia2 in conjunction with zPfn 1 are required for gastrulation cell movements in zebrafish.

Phosphorylation of profilin by ROCK1 regulates polyglutamine aggregation

Y-27632, an inhibitor of the Rho-associated kinase ROCK, is a therapeutic lead for Huntington disease (HD). The downstream targets that mediate its inhibitory effects on huntingtin (Htt) aggregation and toxicity are unknown. We have identified profilin, a small actin-binding factor that also interacts with Htt, as being a direct target of the ROCK1 isoform. The overexpression of profilin reduces the aggregation of polyglutamine-expanded Htt and androgen receptor (AR) peptides. This requires profilin's G-actin binding activity and its direct interaction with Htt, which are both inhibited by the ROCK1-mediated phosphorylation of profilin at Ser-137. Y-27632 blocks the phosphorylation of profilin in HEK293 cells and primary neurons, which maintains profilin in an active state. The knockdown of profilin blocks the inhibitory effect of Y-27632 on both AR and Htt aggregation. A signaling pathway from ROCK1 to profilin thus controls polyglutamine protein aggregation and is targeted by a promising therapeutic lead for HD.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

Profilin and Rop GTPases are localized at infection sites of plant cells

We have found 5 profilin cDNAs in cultured parsley cells, representing a small gene family of about 5 members in parsley. Specific antibodies were produced using heterologously expressed parsley profilin as antigen. Western blot analysis revealed the occurrence of similar amounts of profilin in roots and green parts of parsley plants. Immunocytochemical staining of parsley cells infected with the oomycetous plant pathogen Phytophthora infestans clearly revealed that profilin accumulates at the site on the plasma membrane subtending the oomycetous appressorium, where the actin cables focus. We also observed the accumulation of Rop GTPases around this site, which might point to a potential function in signaling to the cytoskeleton.

Reversible, activity-dependent targeting of profilin to neuronal nuclei

The actin cytoskeleton in pyramidal neurons plays a major role in activity-dependent processes underlying neuronal plasticity. The small actin-binding protein profilin shows NMDA receptor-dependent accumulation in dendritic spines, which is correlated with suppression of actin dynamics and long-term stabilization of synaptic morphology. Here we show that following NMDA receptor activation profilin also accumulates in the nucleus of hippocampal neurons via a process involving rearrangement of the actin cytoskeleton. This simultaneous targeting to dendritic spines and the cell nucleus suggests a novel mechanism of neuronal plasticity in which profilin both tags activated synapses and influences nuclear events.

Interactions Between Vacuolar H+-ATPases and Microfilaments in Osteoclasts

Vacuolar H(+)-ATPases (V-ATPases) are transported from cytosolic compartments to the ruffled plasma membrane of osteoclasts as they activate to resorb bone. Transport of V-ATPases is essential for bone resorption, and is associated with binding interactions between V-ATPases and microfilaments that are mediated by an actin-binding site in subunit B. This site is contained within 44 amino acids in the amino terminal domain, and requires a sequence motif that resembles an actin-binding motif found in mammalian profilin 1. Small alterations in the profilin-like sequence disrupt the actin-binding activity of subunit B. The interaction between V-ATPases and microfilaments in osteoclasts is regulated in response to changes in phosphatidylinositol-3 kinase activity. During internalization of V-ATPases from the plasma membrane of osteoclasts after a cycle of resorption, V-ATPases bind microfilaments that are in podosomes, dynamic actin-based structures, also present in metastatic cancer cells. Studies are ongoing to establish the physiological role of the microfilament-binding activity of subunit B in osteoclasts and in other cells.

Actin polymerization in the equatorial and postacrosomal regions of guinea pig spermatozoa during the acrosome reaction is regulated by G proteins

The acrosome reaction (AR) is an exocytotic process of spermatozoa, and an absolute requirement for fertilization. During AR, actin polymerization is necessary in the equatorial and postacrosomal regions of guinea pig sperm for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane (PM) fusion nor the early steps of egg activation. To identify the mechanisms involved in this sperm actin polymerization, we searched for the protein members, known to be involved in a highly conserved model, that may apply to any cellular process in which de novo actin polymerization occurs from G protein activation. WASP, Arp 2/3, profilins I and II, and Cdc42, RhoA and RhoB GTPases were localized by indirect immunofluorescence (IIF) in guinea pig spermatozoa and their presence corroborated by Western blotting. WASP and profilin II were translocated to the postacrosomal region (Arp2/3 already were there) in long-term capacitated and acrosome-reacted spermatozoa, at the same time as actin polymerization occurred. These events were inhibited by GDP-beta-S and promoted by lysophosphatidic acid (LPA) and GTP-gamma-S, a small GTPase inhibitor and two activators, respectively. By immunoprecipitation, Cdc42-WASp association was identified in capacitated but not in noncapacitated gametes. Polymerized actin in the postacrosomal region is apparently anchored both to the postacrosomal perinuclear theca region and the overlying PM. Results suggest that GTPases are involved in sperm actin polymerization, in the postacrosomal region and the mechanism for polymerization might fit a previously proposed model (Mullins, 2000: Curr Opin Cell Biol 12:91-96).

The role of profilin complexes in cell motility and other cellular processes

Profilins are small actin-binding proteins that are essential in all organisms that have been examined to date. In vitro, profilins regulate the dynamics of actin polymerization, which is their key role in vivo during cell motility. However, there is growing evidence that, apart from actin binding, profilins function as hubs that control a complex network of molecular interactions. Profilins interact with a plethora of proteins and the importance of this aspect of their function is just beginning to be understood. In this article, I will summarize recent findings in mammalian cells and mice, and discuss the evidence of a role for profilins in cellular processes such as membrane trafficking, small-GTPase signaling and nuclear activities, in addition to neurological diseases and tumor formation.

Tumor suppressor activity of profilin requires a functional actin binding site

Profilin 1 (PFN1) is a regulator of the microfilament system and is involved in various signaling pathways. It interacts with many cytoplasmic and nuclear ligands. The importance of PFN1 for human tissue differentiation has been demonstrated by the findings that human cancer cells, expressing conspicuously low PFN1 levels, adopt a nontumorigenic phenotype upon raising their PFN1 level. In the present study, we characterize the ligand binding site crucial for profilin's tumor suppressor activity. Starting with CAL51, a human breast cancer cell line highly tumorigenic in nude mice, we established stable clones that express PFN1 mutants differentially defective in ligand binding. Clones expressing PFN1 mutants with reduced binding to either poly-proline-stretch ligands or phosphatidyl-inositol-4,5-bisphosphate, but with a functional actin binding site, were normal in growth, adhesion, and anchorage dependence, with only a weak tendency to elicit tumors in nude mice, similar to controls expressing wild-type PFN1. In contrast, clones expressing a mutant with severely reduced capacity to bind actin still behaved like the parental CAL51 and were highly tumorigenic. We conclude that the actin binding site on profilin is instrumental for normal differentiation of human epithelia and the tumor suppressor function of PFN1.

Vacuolar H+-ATPase binding to microfilaments: regulation in response to phosphatidylinositol 3-kinase activity and detailed characterization of the actin-binding site in subunit B

Vacuolar H(+)-ATPase (V-ATPase) binds microfilaments, and that interaction may be mediated by an actin binding domain in subunit B of the enzyme. To test for possible physiologic functions of the actin binding activity of V-ATPase, early responses of resorbing osteoclasts to inhibition of phosphatidylinositol 3-kinase activity by wortmannin and LY294002 were examined. Rapid co-localization between V-ATPase and F-actin was demonstrated by immunocytochemistry, and corresponding association between V-ATPase and F-actin in immunoprecipitations and pelleting assays was detected. This response was reversed as osteoclasts recovered resorptive activity after inhibitors were removed. By expressing and characterizing fusion proteins containing segments of the actin-binding amino-terminal regions of the B subunits of V-ATPase, we mapped the actin-binding site to a 44-amino acid domain. An 11-amino acid segment with a sequence similar to the actin-binding site of human profilin I was detected within this region. 13-Mers containing these profilin-like segments bound actin in fluorescent anisotropy studies and competed with profilin for binding to actin. Using site-directed mutagenesis, the 11-amino acid profilin-like actin-binding motifs (amino acids 49-59 of B1 and 55-65 of B2) were replaced with an 11-amino acid spacer with a sequence based on the homologous sequence from subunit B of Pyrococcus horikoshii, an organism that lacks an actin cytoskeleton. These substitutions eliminated the actin-binding activity of the B subunit fusion proteins. In summary, binding between V-ATPase and F-actin in osteoclasts occurs in response to blocking phosphatidylinositol 3-kinase activity. This response was fully reversible. The actin binding activities of the B subunits of V-ATPase required 11-amino acid actin-binding motifs that are similar in sequence to the actin-binding site of mammalian profilin I.

The vasodilator-stimulated phosphoprotein promotes actin polymerisation through direct binding to monomeric actin

The vasodilator-stimulated phosphoprotein (VASP) functions as a cellular regulator of actin dynamics. VASP may initialise actin polymerisation, suggesting a direct interaction with monomeric actin. The present study demonstrates that VASP directly binds to actin monomers and that complex formation depends on a conserved four amino acid motif in the EVH2 domain. Point mutations within this motif drastically weaken VASP/G-actin interactions, thereby abolishing any actin-nucleating activity of VASP. Additionally, actin nucleation was found to depend on VASP oligomerisation since VASP monomers fail to induce the formation of actin filaments. Phosphorylation negatively affects VASP/G-actin interactions preventing VASP-induced actin filament formation.

Evidence for two interaction regions for phosphatidylinositol(4,5)-bisphosphate on mammalian profilin I

The binding of phosphatidylinositol(4,5)-bisphosphate (PI(4,5)P(2)) to profilin at a region distinct from the actin interaction surface is demonstrated by experiments with covalently cross-linked profilin:beta-actin. The result is in agreement with observations made with several mutant profilins and provides strong evidence for two regions on mammalian profilin mediating electrostatic interaction with phosphatidylinositol lipids; one close to the binding site for poly(L-proline), and one partially overlapping with the actin-binding surface. Congruent with this, two plant profilins, which have a reduced number of positive amino acids in one of these regions, displayed a dramatically lower binding to PI(4,5)P(2) compared to human profilin I.

Phosphorylation of the vasodilator-stimulated phosphoprotein regulates its interaction with actin

The vasodilator-stimulated phosphoprotein (VASP) is a major substrate for cyclic nucleotide-dependent kinases in platelets and other cardiovascular cells. It promotes actin nucleation and binds to actin filaments in vitro and associates with stress fibers in cells. The VASP-actin interaction is salt-sensitive, arguing for electrostatic interactions. Hence, phosphorylation may significantly alter the actin binding properties of VASP. This hypothesis was investigated by analyzing complex formation of recombinant murine VASP with actin after phosphorylation with cAMP-dependent kinase in different assays. cAMP-dependent kinase phosphorylation had a negative effect on both actin nucleation and VASP interaction with actin filaments, with the actin nucleating capacity being more affected than actin filament binding and bundling. Replacing VASP residues known to be phosphorylated in vivo by acidic residues to mimic phosphorylation had similar although less dramatic effects on VASP-actin interactions. In contrast, phosphorylation had no significant effect on VASP oligomerization or its interaction with its known ligands profilin, vinculin, and zyxin. When overexpressing VASP mutants in eukaryotic cells, they all showed targeting to focal contacts and stress fibers. Our results imply that VASP phosphorylation may act as an immediate negative regulator of actin dynamics.

Functional characterization of green fluorescent protein-profilin fusion proteins

To clarify the role of profilins in cells, fusion proteins constructed with green fluorescent protein (GFP) should be extremely helpful. As profilins are considerably smaller than the GFP fusion partner (14-17 kDa compared with 27 kDa, respectively), we characterized the fusion proteins in vitro, to ascertain their biological function. We fused mouse profilin I and II to either the C-terminus or N-terminus of GFP. These fusion proteins were expressed in Escherichia coli and affinity-purified on polyproline-Sepharose. Interaction with vasodilator-stimulated phosphoprotein, a proline-rich ligand of profilin, was investigated by ELISA, as was binding to PtdIns(4,5)P2. The affinity for actin was quantitatively determined in polymerization assays. Our results show that fusion of GFP to the C-terminus of profilin I abolishes polyproline binding. In contrast, the other fusion proteins bound to polyproline-Sepharose and VASP. Binding to PtdIns(4,5)P2 was not significantly altered. Furthermore, fusion of either isoform with GFP did not decrease the affinity for actin. In localization studies with mammalian cells, all fusion proteins showed the localization expected for profilin in areas of high actin dynamics, such as leading lamellae and ruffles induced by epidermal growth factor. However, with regard to our in vitro data, we suspect that only a minor fraction of profilin I carrying the GFP at the C-terminus can target these sites. Therefore, other constructs should be preferred for further in vivo studies.

Dissection of functional domains by expression of point-mutated profilins in Dictyostelium mutants

Profilin is a ubiquitous cytoskeletal protein whose function is fundamental to the maintenance of normal cell physiology. By site-directed mutagenesis of profilin II from Dictyostelium discoideum the point mutations K114E and W3N were generated by PCR thus changing actin and poly-(L)-proline-binding activity respectively. W3N profilin is no longer able to bind to poly-(L)-proline concomitant with a slight reduction in actin binding. The K114E profilin exhibited a profound decrease in its ability to interact with actin, whereas binding to poly-(L)-proline was essentially unchanged. Binding to phospholipids was indistinguishable from the wild-type profilin. The in vivo properties of the point-mutated profilins were studied by expressing either W3N or K114E in profilin-minus D. discoideum mutants which have defects in the F-actin content, cytokinesis and development (Haugwitz et al., Cell 79, 303-314, 1994). Expression of K114E or W3N displayed a reduction in the F-actin content, normal cell morphology, and the transformants were capable of undergoing complete development. Interestingly, only cells that drastically overexpressed W3N could restore the aberrant phenotype, whereas the mutant protein K114E with its fully functional poly-(L)-proline binding and its strongly reduced actin-binding activities rescued the phenotype at low concentrations. Wild-type and both mutated profilins are enriched in phagocytic cups during uptake of yeast particles. These data suggest a) that a functional poly-(L)-proline-binding activity is more important for suppression of the mutant phenotype than the G-actin binding activity of profilin, and b) that the enrichment of profilin in highly active phagocytic cups might be independent of either poly-(L)-proline or actin-binding activities.

X-ray structure determination of human profilin II: A comparative structural analysis of human profilins

Human profilins are multifunctional, single-domain proteins which directly link the actin microfilament system to a variety of signalling pathways via two spatially distinct binding sites. Profilin binds to monomeric actin in a 1:1 complex, catalyzes the exchange of the actin-bound nucleotide and regulates actin filament barbed end assembly. Like SH3 domains, profilin has a surface-exposed aromatic patch which binds to proline-rich peptides. Various multidomain proteins including members of the Ena/VASP and formin families localize profilin:actin complexes through profilin:poly-L-proline interactions to particular cytoskeletal locations (e.g. focal adhesions, cleavage furrows). Humans express a basic (I) and an acidic (II) isoform of profilin which exhibit different affinities for peptides and proteins rich in proline residues. Here, we report the crystallization and X-ray structure determination of human profilin II to 2.2 A. This structure reveals an aromatic extension of the previously defined poly-L-proline binding site for profilin I. In contrast to serine 29 of profilin I, tyrosine 29 in profilin II is capable of forming an additional stacking interaction and a hydrogen bond with poly-L-proline which may account for the increased affinity of the second isoform for proline-rich peptides. Differential isoform specificity for proline-rich proteins may be attributed to the differences in charged and hydrophobic residues in and proximal to the poly-L-proline binding site. The actin-binding face remains nearly identical with the exception of five amino acid differences. These observations are important for the understanding of the functional and structural differences between these two classes of profilin isoforms.

Characterization of the actin binding properties of the vasodilator-stimulated phosphoprotein VASP

The vasodilator-stimulated phosphoprotein (VASP) colocalizes with the ends of stress fibers in cell-matrix and cell-cell contacts. We report here that bacterially expressed murine VASP directly interacts with skeletal muscle actin in several test systems including cosedimentation, viscometry and polymerization assays. It nucleates actin polymerization and tightly bundles actin filaments. The interaction with actin is salt-sensitive, indicating that the complex formation is primarily based on electrostatic interactions. Actin binding is confined to the C-terminal domain of VASP (EVH2). This domain, when expressed as a fusion protein with EGFP, associates with stress fibers in transiently transfected cells.