Villin-type headpiece domains show a wide range of F-actin-binding affinities

HP35 Protein in the Mesopore of MIL-101(Cr) MOF: A Model to Study Cotranslocational Unfolding

The immobilization of enzymes in metal-organic framework (MOF) cages is important in biotechnology. In this context, the mechanism of translocation of proteins through the cavities of the MOF and the roles played by confinement and MOF chemistry in giving rise to stable protein intermediates that are otherwise transiently populated in the physiological environment are important questions to be addressed. These unexplored aspects are examined with villin headpiece (HP35) as a model protein confined within a mesopore of MIL-101(Cr) using molecular dynamics simulations. At equilibrium, the protein is located farther from the center of the cavity and closer to the MOF surface. Molecular interactions with the MOF partially unfold helix-1 at its N-terminus. Umbrella sampling simulations inform the range of conformations that HP35 undertakes during translocation from one cavity to another and associated changes in free energy. Relative to its equilibrium state within the cavity, the free energy barrier for the unfolded protein at the cage window is estimated to be 16 kcal/mol. This study of MOF-based protein conformation can also be a general approach to observing intermediates in folding-unfolding pathways.

Dematin Regulates Calcium Mobilization, Thrombosis, and Early Akt Activation in Platelets

The complex intrinsic and extrinsic pathways contributing to platelet activation profoundly impact hemostasis and thrombosis. Detailed cellular mechanisms that regulate calcium mobilization, Akt activation, and integrin signaling in platelets remain incompletely understood. Dematin is a broadly expressed actin binding and bundling cytoskeletal adaptor protein regulated by phosphorylation via cAMP-dependent protein kinase. Here, we report the development of a conditional mouse model specifically lacking dematin in platelets. Using the new mouse model termed PDKO, we provide direct evidence that dematin is a major regulator of calcium mobilization, and its genetic deletion inhibits the early phase of Akt activation in response to collagen and thrombin agonists in platelets. The aberrant platelet shape change, clot retraction, and in vivo thrombosis observed in PDKO mice will enable future characterization of dematin-mediated integrin activation mechanisms in thrombogenic as well as nonvascular pathologies.

Structural basis of membrane skeleton organization in red blood cells

The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

The Role of Liquid-Liquid Phase Separation in Actin Polymerization

To date, it has been shown that the phenomenon of liquid-liquid phase separation (LLPS) underlies many seemingly completely different cellular processes. This provided a new idea of the spatiotemporal organization of the cell. The new paradigm makes it possible to provide answers to many long-standing, but still unresolved questions facing the researcher. In particular, spatiotemporal regulation of the assembly/disassembly of the cytoskeleton, including the formation of actin filaments, becomes clearer. To date, it has been shown that coacervates of actin-binding proteins that arise during the phase separation of the liquid-liquid type can integrate G-actin and thereby increase its concentration to initiate polymerization. It has also been shown that the activity intensification of actin-binding proteins that control actin polymerization, such as N-WASP and Arp2/3, can be caused by their integration into liquid droplet coacervates formed by signaling proteins on the inner side of the cell membrane.

Self-construction of actin networks through phase separation-induced abLIM1 condensates

The abLIM1 is a nonerythroid actin-binding protein critical for stable plasma membrane-cortex interactions under mechanical tension. Its depletion by RNA interference results in sparse, poorly interconnected cortical actin networks and severe blebbing of migrating cells. Its isoforms, abLIM-L, abLIM-M, and abLIM-S, contain, respectively four, three, and no LIM domains, followed by a C terminus entirely homologous to erythroid cortex protein dematin. How abLIM1 functions, however, remains unclear. Here we show that abLIM1 is a liquid-liquid phase separation (LLPS)-dependent self-organizer of actin networks. Phase-separated condensates of abLIM-S-mimicking ΔLIM or the major isoform abLIM-M nucleated, flew along, and cross-linked together actin filaments (F-actin) to produce unique aster-like radial arrays and interconnected webs of F-actin bundles. Interestingly, ΔLIM condensates facilitated actin nucleation and network formation even in the absence of Mg²⁺. Our results suggest that abLIM1 functions as an LLPS-dependent actin nucleator and cross-linker and provide insights into how LLPS-induced condensates could self-construct intracellular architectures of high connectivity and plasticity.

F-actin-bundling sites are conserved in proteins with villin-type headpiece domains

Villin is a major actin-bundling protein that assembles the brush border of intestinal and renal epithelial cells. The villin "headpiece" domain and the actin-binding residues within it regulate its actin-bundling function. Substantial experimental and theoretical information about the three-dimensional structure of the isolated villin headpiece, including a description of the actin-binding residues within the headpiece, is available. Despite that, the actin-bundling site in the full-length (FL) villin protein remains unidentified. We used this existing villin headpiece nuclear magnetic resonance data and performed mutational analysis and functional assays to identify the actin-bundling site in FL human villin protein. By careful evaluation of these conserved actin-binding residues in human advillin protein, we demonstrate their functional significance in the over 30 proteins that contain a villin-type headpiece domain. Our study is the first that combines the available structural data on villin headpiece with functional assays to identify the actin-binding residues in FL villin that regulate its filament-bundling activity. Our findings could have wider implications for other actin-bundling proteins that contain a villin-type headpiece domain.

abLIM1 constructs non-erythroid cortical actin networks to prevent mechanical tension-induced blebbing

The cell cortex is a layer of cytoskeletal networks underneath the plasma membrane, formed by filamentous actin (F-actin) and cortex proteins including spectrin, adducin, and myosin. It provides cells with proper stiffness, elasticity, and surface tension to allow morphogenesis, division, and migration. Although its architecture and formation have been widely studied in red blood cells, they are poorly understood in non-erythrocytes due to structural complexity and versatile functions. In this study, we identify the actin-binding protein abLIM1 as a novel non-erythroid cell-specific cortex organizer. Endogenous abLIM1 colocalized with cortical βII spectrin but upon overexpression redistributed to thick cortical actin bundles. abLIM1 associated with major cortex proteins such as spectrins and adducin in vivo. Depletion of abLIM1 by RNAi induced prominent blebbing during membrane protrusions of spreading or migrating RPE1 cells and impaired migration efficiency. Reducing cortical tensions by culturing the cells to confluency or inhibiting myosin activity repressed the blebbing phenotype. abLIM1-depleted RPE1 or U2OS cells lacked the dense interwoven cortical actin meshwork observed in control cells but were abundant in long cortical actin bundles along the long axis of the cells. In-vitro assays indicated that abLIM1 was able to crosslink and bundle F-actin to induce dense F-actin network formation. Therefore, abLIM1 governs the formation of dense interconnected cortical actin meshwork in non-erythroid cells to prevent mechanical tension-induced blebbing during cellular activities such as spreading and migration.

Plant Villin Headpiece Domain Demonstrates a Novel Surface Charge Pattern and High Affinity for F-Actin

Plants utilize multiple isoforms of villin, an F-actin regulating protein with an N-terminal gelsolin-like core and a distinct C-terminal headpiece domain. Unlike their vertebrate homologues, plant villins have a much longer linker polypeptide connecting the core and headpiece. Moreover, the linker-headpiece connection region in plant villins lacks sequence homology to the vertebrate villin sequences. It is unknown to what extent the plant villin headpiece structure and function resemble those of the well-studied vertebrate counterparts. Here we present the first solution NMR structure and backbone dynamics characterization of a headpiece from plants, villin isoform 4 from Arabidopsis thaliana. The villin 4 headpiece is a 63-residue domain (V4HP63) that adopts a typical headpiece fold with an aromatics core and a tryptophan-centered hydrophobic cap within its C-terminal subdomain. However, V4HP63 has a distinct N-terminal subdomain fold as well as a novel, high mobility loop due to the insertion of serine residue in the canonical sequence that follows the variable length loop in headpiece sequences. The domain binds actin filaments with micromolar affinity, like the vertebrate analogues. However, the V4HP63 surface charge pattern is novel and lacks certain features previously thought necessary for high-affinity F-actin binding. Utilizing the updated criteria for strong F-actin binding, we predict that the headpiece domains of all other villin isoforms in A. thaliana have high affinity for F-actin.

The F-actin-binding RapGEF GflB is required for efficient macropinocytosis in Dictyostelium

Macropinocytosis involves the uptake of large volumes of fluid, which is regulated by various small GTPases. The Dictyostelium discoideum protein GflB is a guanine nucleotide exchange factor (GEF) of Rap1, and is involved in chemotaxis. Here, we studied the role of GflB in macropinocytosis, phagocytosis and cytokinesis. In plate culture of vegetative cells, compared with the parental strain AX2, gflB-knockout (KO) cells were flatter and more polarized, whereas GflB-overproducing cells were rounder. The gflB-KO cells exhibited impaired crown formation and retraction, particularly retraction, resulting in more crowns (macropinocytic cups) per cell and longer crown lifetimes. Accordingly, gflB-KO cells showed defects in macropinocytosis and also in phagocytosis and cytokinesis. F-actin levels were elevated in gflB-KO cells. GflB localized to the actin cortex most prominently at crowns and phagocytic cups. The villin headpiece domain (VHP)-like N-terminal domain of GflB directly interacted with F-actin in vitro Furthermore, a domain enriched in basic amino acids interacted with specific membrane cortex structures such as the cleavage furrow. In conclusion, GflB acts as a key local regulator of actin-driven membrane protrusion possibly by modulating Rap1 signaling pathways.

Drosophila Dunc-115 mediates axon projection through actin binding

A central step in organizing the central nervous system development is the growth cone of an axon navigating through guidance cues to reach its specific target. While a great deal of this process has been understood especially in identifying the extracellular guidance cues and their membrane receptors, much less is known about how guidance signals are further relayed to the actin filaments that are central to the mobility of the growth cone. The previous results from our laboratory have shown that Drosophila gene dunc-115 regulates axon projection in the eye and the central nervous system. Furthermore, Dunc-115 has a villin-headpiece (VHD) domain, implying the possibility of binding to actin. To further characterize Dunc-115's functions, we have identified the isoform Dunc-115L as a possible downstream target in relaying guidance cues further down to the cytoskeleton. Specifically, we have shown that Dunc-115 regulates neural connections in both the eye and the central nervous system in Drosophila and that Dunc-115 contains an actin-binding domain potentially capable of binding to actin filaments. In this report, we show that Dunc-115 binds to actin via its VHD domain directly, suggesting a possible mechanism for how Dunc-115 relays guidance signals.

Supervillin binding to myosin II and synergism with anillin are required for cytokinesis

Cytokinesis, the process by which cytoplasm is apportioned between dividing daughter cells, requires coordination of myosin II function, membrane trafficking, and central spindle organization. Most known regulators act during late cytokinesis; a few, including the myosin II-binding proteins anillin and supervillin, act earlier. Anillin's role in scaffolding the membrane cortex with the central spindle is well established, but the mechanism of supervillin action is relatively uncharacterized. We show here that two regions within supervillin affect cell division: residues 831-1281, which bind central spindle proteins, and residues 1-170, which bind the myosin II heavy chain (MHC) and the long form of myosin light-chain kinase. MHC binding is required to rescue supervillin deficiency, and mutagenesis of this site creates a dominant-negative phenotype. Supervillin concentrates activated and total myosin II at the furrow, and simultaneous knockdown of supervillin and anillin additively increases cell division failure. Knockdown of either protein causes mislocalization of the other, and endogenous anillin increases upon supervillin knockdown. Proteomic identification of interaction partners recovered using a high-affinity green fluorescent protein nanobody suggests that supervillin and anillin regulate the myosin II and actin cortical cytoskeletons through separate pathways. We conclude that supervillin and anillin play complementary roles during vertebrate cytokinesis.

The allosteric mechanism induced by protein kinase A (PKA) phosphorylation of dematin (band 4.9)

Dematin (band 4.9) is an F-actin binding and bundling protein best known for its role within red blood cells, where it both stabilizes as well as attaches the spectrin/actin cytoskeleton to the erythrocytic membrane. Here, we investigate the structural consequences of phosphorylating serine 381, a covalent modification that turns off F-actin bundling activity. In contrast to the canonical doctrine, in which phosphorylation of an intrinsically disordered region/protein confers affinity for another domain/protein, we found the converse to be true of dematin: phosphorylation of the well folded C-terminal villin-type headpiece confers affinity for its intrinsically disordered N-terminal core domain. We employed analytical ultracentrifugation to demonstrate that dematin is monomeric, in contrast to the prevailing view that it is trimeric. Next, using a series of truncation mutants, we verified that dematin has two F-actin binding sites, one in the core domain and the other in the headpiece domain. Although the phosphorylation-mimicking mutant, S381E, was incapable of bundling microfilaments, it retains the ability to bind F-actin. We found that a phosphorylation-mimicking mutant, S381E, eliminated the ability to bundle, but not bind F-actin filaments. Lastly, we show that the S381E point mutant caused the headpiece domain to associate with the core domain, leading us to the mechanism for cAMP-dependent kinase control of dematin's F-actin bundling activity: when unphosphorylated, dematin's two F-actin binding domains move independent of one another permitting them to bind different F-actin filaments. Phosphorylation causes these two domains to associate, forming a compact structure, and sterically eliminating one of these F-actin binding sites.

The human erythrocyte plasma membrane: a Rosetta Stone for decoding membrane-cytoskeleton structure

The mammalian erythrocyte, or red blood cell (RBC), is a unique experiment of nature: a cell with no intracellular organelles, nucleus or transcellular cytoskeleton, and a plasma membrane with uniform structure across its entire surface. By virtue of these specialized properties, the RBC membrane has provided a template for discovery of the fundamental actin filament network machine of the membrane skeleton, now known to confer mechanical resilience, anchor membrane proteins, and organize membrane domains in all cells. This chapter provides a historical perspective and critical analysis of the biochemistry, structure, and physiological functions of this actin filament network in RBCs. The core units of this network are nodes of ~35-37 nm-long actin filaments, interconnected by long strands of (α1β1)₂-spectrin tetramers, forming a 2D isotropic lattice with quasi-hexagonal symmetry. Actin filament length and stability is critical for network formation, relying upon filament capping at both ends: tropomodulin-1 at pointed ends and αβ-adducin at barbed ends. Tropomodulin-1 capping is essential for precise filament lengths, and is enhanced by tropomyosin, which binds along the short actin filaments. αβ-adducin capping recruits spectrins to sites near barbed ends, promoting network formation. Accessory proteins, 4.1R and dematin, also promote spectrin binding to actin and, with αβ-adducin, link to membrane proteins, targeting actin nodes to the membrane. Dissection of the molecular organization within the RBC membrane skeleton is one of the paramount achievements of cell biological research in the past century. Future studies will reveal the structure and dynamics of actin filament capping, mechanisms of precise length regulation, and spectrin-actin lattice symmetry.

An N-terminal, 830 residues intrinsically disordered region of the cytoskeleton-regulatory protein supervillin contains Myosin II- and F-actin-binding sites

Supervillin, the largest member of the villin/gelsolin family, is a cytoskeleton regulating, peripheral membrane protein. Supervillin increases cell motility and promotes invasive activity in tumors. Major cytoskeletal interactors, including filamentous actin and myosin II, bind within the unique supervillin amino terminus, amino acids 1-830. The structural features of this key region of the supervillin polypeptide are unknown. Here, we utilize circular dichroism and bioinformatics sequence analysis to demonstrate that the N-terminal part of supervillin forms an extended intrinsically disordered region (IDR). Our combined data indicate that the N-terminus of human and bovine supervillin sequences (positions 1-830) represents an IDR, which is the largest IDR known to date in the villin/gelsolin family. Moreover, this result suggests a potentially novel mechanism of regulation of myosin II and F-actin via the intrinsically disordered N-terminal region of hub protein supervillin.

Identification of a novel role for dematin in regulating red cell membrane function by modulating spectrin-actin interaction

The membrane skeleton plays a central role in maintaining the elasticity and stability of the erythrocyte membrane, two biophysical features critical for optimal functioning and survival of red cells. Many constituent proteins of the membrane skeleton are phosphorylated by various kinases, and phosphorylation of β-spectrin by casein kinase and of protein 4.1R by PKC has been documented to modulate erythrocyte membrane mechanical stability. In this study, we show that activation of endogenous PKA by cAMP decreases membrane mechanical stability and that this effect is mediated primarily by phosphorylation of dematin. Co-sedimentation assay showed that dematin facilitated interaction between spectrin and F-actin, and phosphorylation of dematin by PKA markedly diminished this activity. Quartz crystal microbalance measurement revealed that purified dematin specifically bound the tail region of the spectrin dimer in a saturable manner with a submicromolar affinity. Pulldown assay using recombinant spectrin fragments showed that dematin, but not phospho-dematin, bound to the tail region of the spectrin dimer. These findings imply that dematin contributes to the maintenance of erythrocyte membrane mechanical stability by facilitating spectrin-actin interaction and that phosphorylation of dematin by PKA can modulate these effects. In this study, we have uncovered a novel functional role for dematin in regulating erythrocyte membrane function.

Refined views of multi-protein complexes in the erythrocyte membrane

The erythrocyte membrane has been extensively studied, both as a model membrane system and to investigate its role in gas exchange and transport. Much is now known about the protein components of the membrane, how they are organised into large multi-protein complexes and how they interact with each other within these complexes. Many links between the membrane and the cytoskeleton have also been delineated and have been demonstrated to be crucial for maintaining the deformability and integrity of the erythrocyte. In this study we have refined previous, highly speculative molecular models of these complexes by including the available data pertaining to known protein-protein interactions. While the refined models remain highly speculative, they provide an evolving framework for visualisation of these important cellular structures at the atomic level.

On the unyielding hydrophobic core of villin headpiece

Villin headpiece (HP67) is a small, autonomously-folding domain that has become a model system for understanding the fundamental tenets governing protein folding. In this communication, we explore the role that Leu61 plays in the structure and stability of the construct. Deletion of Leu61 results in a completely unfolded protein that cannot be expressed in Escherichia coli. Omission of only the aliphatic leucine side chain (HP67 L61G) perturbed neither the backbone conformation nor the orientation of local hydrophobic side chains. As a result, a large, solvent-exposed hydrophobic pocket, a negative replica of the leucine side-chain, was created on the surface. The loss of the hydrophobic interface between leucine 61 and the hydrophobic pocket destabilized the construct by ~3.3 kcal/mol. Insertion of a single glycine residue immediately before Leu61 (HP67 L61[GL]) was also highly destabilizing and had the effect of altering the backbone conformation (α-helix to π-helix) in order to precisely preserve the wild-type position and conformation of all hydrophobic residues, including Leu61. In addition to demonstrating that the hydrophobic side-chain of Leu61 is critically important for the stability of villin headpiece, our results are consistent with the notion that the precise interactions present within the hydrophobic core, rather than the hydrogen bonds that define the secondary structure, specify a protein's fold.

On unsatisfied hydrogen bonds in the N-terminal subdomain of villin headpiece

Villin headpiece is a small autonomously folding protein that has emerged as a model system for understanding the fundamental tenets governing protein folding. In this communication, we employ NMR and X-ray crystallography to characterize a point mutant, H41F, which retains actin-binding activity, is more thermostable but, interestingly, does not exhibit the partially folded intermediate observed of either wild-type or other similar point mutants.

Maternal perinatal undernutrition has long-term consequences on morphology, function and gene expression of the adrenal medulla in the adult male rat

Epidemiological studies suggest that maternal undernutrition sensitises to the development of chronic adult diseases, such as type 2 diabetes, hypertension and obesity. Although the physiological mechanisms involved in this 'perinatal programming' remain largely unknown, alterations of stress neuroendocrine systems such as the hypothalamic-pituitary-adrenal (HPA) and sympathoadrenal axes might play a crucial role. Despite recent reports showing that maternal perinatal undernutrition disturbs chromaffin cells organisation and activity in male rats at weaning, its long-term effects on adrenal medulla in adult animals are unknown. Using a rat model of maternal perinatal 50% food restriction (FR50) from the second week of gestation until weaning, histochemistry approaches revealed alterations in noradrenergic chromaffin cells aggregation and in cholinergic innervation in the adrenal medulla of 8-month-old FR50 rats. Electron microscopy showed that chromaffin cell granules exhibited ultrastructural changes in FR50 rats. These morphological changes were associated with reduced circulating levels and excretion of catecholamines. By contrast, catecholamine plasma levels were significantly increased after a 16 or 72 h of fasting, indicating that the responsiveness of the sympathoadrenal system to food deprivation was accentuated in FR50 adult rats. Among 384 pituitary adenylate cyclase-activating polypeptide-sensitive genes, we identified 129 genes (33.6%) that were under expressed (ratio < 0.7) in FR50 animals. A large number of these genes are involved in cytoskeleton remodelling and vesicle trafficking. Taken together, our results show that maternal perinatal undernutrition programmes adrenomedullary function and gene expression in adult male rats. Because catecholamines contribute to metabolic homeostasis, as well as arterial blood pressure regulation, the alterations observed in the adrenal medulla of adult male FR50 rats may participate in the programming of chronic adult diseases.

Competition between intradomain and interdomain interactions: a buried salt bridge is essential for villin headpiece folding and actin binding

Villin-type headpiece domains are ∼70 residue motifs that reside at the C-terminus of a variety of actin-associated proteins. Villin headpiece (HP67) is a commonly used model system for both experimental and computational studies of protein folding. HP67 is made up of two subdomains that form a tightly packed interface. The isolated C-terminal subdomain of HP67 (HP35) is one of the smallest autonomously folding proteins known. The N-terminal subdomain requires the presence of the C-terminal subdomain to fold. In the structure of HP67, a conserved salt bridge connects N- and C-terminal subdomains. This buried salt bridge between residues E39 and K70 is unusual in a small protein domain. We used mutational analysis, monitored by CD and NMR, and functional assays to determine the role of this buried salt bridge. First, the two residues in the salt bridge were replaced with strictly hydrophobic amino acids, E39M/K70M. Second, the two residues in the salt bridge were swapped, E39K/K70E. Any change from the wild-type salt bridge residues results in unfolding of the N-terminal subdomain, even when the mutations were made in a stabilized variant of HP67. The C-terminal subdomain remains folded in all mutants and is stabilized by some of the mutations. Using actin sedimentation assays, we find that a folded N-terminal domain is essential for specific actin binding. Therefore, the buried salt bridge is required for the specific folding of the N-terminal domain which confers actin-binding activity to villin-type headpiece domains, even though the residues required for this specific interaction destabilize the C-terminal subdomain.

Hydrophobic core formation and dehydration in protein folding studied by generalized-ensemble simulations

Despite its small size, chicken villin headpiece subdomain HP36 folds into the native structure with a stable hydrophobic core within several microseconds. How such a small protein keeps up its conformational stability and fast folding in solution is an important issue for understanding molecular mechanisms of protein folding. In this study, we performed multicanonical replica-exchange simulations of HP36 in explicit water, starting from a fully extended conformation. We observed at least five events of HP36 folding into nativelike conformations. The smallest backbone root mean-square deviation from the crystal structure was 1.1 A. In the nativelike conformations, the stably formed hydrophobic core was fully dehydrated. Statistical analyses of the simulation trajectories show the following sequential events in folding of HP36: 1), Helix 3 is formed at the earliest stage; 2), the backbone and the side chains near the loop between Helices 2 and 3 take nativelike conformations; and 3), the side-chain packing at the hydrophobic core and the dehydration of the core side chains take place simultaneously at the later stage of folding. This sequence suggests that the initial folding nucleus is not necessarily the same as the hydrophobic core, consistent with a recent experimental phi-value analysis.

Novel interactors and a role for supervillin in early cytokinesis

Supervillin, the largest member of the villin/gelsolin/flightless family, is a peripheral membrane protein that regulates each step of cell motility, including cell spreading. Most known interactors bind within its amino (N)-terminus. We show here that the supervillin carboxy (C)-terminus can be modeled as supervillin-specific loops extending from gelsolin-like repeats plus a villin-like headpiece. We have identified 27 new candidate interactors from yeast two-hybrid screens. The interacting sequences from 12 of these proteins (BUB1, EPLIN/LIMA1, FLNA, HAX1, KIF14, KIFC3, MIF4GD/SLIP1, ODF2/Cenexin, RHAMM, STARD9/KIF16A, Tks5/SH3PXD2A, TNFAIP1) co-localize with and mis-localize EGFP-supervillin in mammalian cells, suggesting associations in vivo. Supervillin-interacting sequences within BUB1, FLNA, HAX1, and MIF4GD also mimic supervillin over-expression by inhibiting cell spreading. Most new interactors have known roles in supervillin-associated processes, e.g. cell motility, membrane trafficking, ERK signaling, and matrix invasion; three (KIF14, KIFC3, STARD9/KIF16A) have kinesin motor domains; and five (EPLIN, KIF14, BUB1, ODF2/cenexin, RHAMM) are important for cell division. GST fusions of the supervillin G2-G3 or G4-G6 repeats co-sediment KIF14 and EPLIN, respectively, consistent with a direct association. Supervillin depletion leads to increased numbers of bi- and multi-nucleated cells. Cytokinesis failure occurs predominately during early cytokinesis. Supervillin localizes with endogenous myosin II and EPLIN in the cleavage furrow, and overlaps with the oncogenic kinesin, KIF14, at the midbody. We conclude that supervillin, like its interactors, is important for efficient cytokinesis. Our results also suggest that supervillin and its interaction partners coordinate actin and microtubule motor functions throughout the cell cycle.

Identifying competitive protein antagonists for F-actin with reverse-phase high-performance liquid chromatography

F-actin binding constants are traditionally determined by centrifugal cosedimentation with actin microfilaments, where bound protein is separated from actin with SDS-PAGE and quantitated using densitometry. Here, we demonstrate that UV quantitation of reverse-phase HPLC-separated proteins provides increased accuracy and sensitivity, can be fully automated, and allows one to perform F-actin competition assays on similar sized proteins.

Partially folded equilibrium intermediate of the villin headpiece HP67 defined by 13C relaxation dispersion

Identification and characterization of ensembles of intermediate states remains an important objective in describing protein folding in atomic detail. The 67-residue villin headpiece, HP67, consists of an N-terminal subdomain (residues 10-42) that transiently unfolds at equilibrium under native-like conditions and a highly stable C-terminal subdomain (residues 43-76). The transition between folded and unfolded states of the N-terminal domain has been characterized previously by (15)N NMR relaxation dispersion measurements (Grey et al. in J Mol Biol 355:1078, 2006). In the present work, (13)C spin relaxation was used to further characterize backbone and hydrophobic core contributions to the unfolding process. Relaxation of (13)C(alpha) spins was measured using the Hahn echo technique at five static magnetic fields (11.7, 14.1, 16.4, 18.8, and 21.1 T) and the Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion method at a static magnetic field of 14.1 T. Relaxation of methyl (13)C spins was measured using CPMG relaxation dispersion experiments at static magnetic fields of 14.1 and 18.8 T. Results for (13)C and (15)N spins yielded a consistent model in which the partially unfolded intermediate state of the N-terminal subdomain maintains residual structure for residues near the unprotonated His41 imidazole ring and in the interface between the N- and C-terminal subdomains. In addition, a second faster process was detected that appears to represent local dynamics within the folded state of the molecule and is largely confined to the hydrophobic interface between the N- and C-terminal subdomains.

Dematin exhibits a natively unfolded core domain and an independently folded headpiece domain

Dematin is an actin-binding protein originally identified in the junctional complex of the erythrocyte plasma membrane, and is present in many nonerythroid cells. Dematin headpiece knockout mice display a spherical red cell phenotype and develop a compensated anemia. Dematin has two domains: a 315-residue, proline-rich "core" domain and a 68-residue carboxyl-terminal villin-type "headpiece" domain. Expression of full-length dematin in E. coli as a GST recombinant protein results in truncation within a proline, glutamic acid, serine, threonine rich region (PEST). Therefore, we designed a mutant construct that replaces the PEST sequence. The modified dematin has high actin binding activity as determined by actin sedimentation assays. Negative stain electron microscopy demonstrates that the modified dematin also exhibits actin bundling activity like that of native dematin. Circular dichroism (CD) and NMR spectral analysis, however, show little secondary structure in the modified dematin. The lack of secondary structure is also observed in native dematin purified from human red blood cells. (15)N-HSQC NMR spectra of modified dematin indicate that the headpiece domain is fully folded whereas the core region is primarily unfolded. Our finding suggests that the core is natively unfolded and may serve as a scaffold to organize the components of the junctional complex.

How to arm a supervillin: designing F-actin binding activity into supervillin headpiece

Villin-type headpiece domains are compact motifs that have been used extensively as model systems for protein folding. Although the majority of headpiece domains bind actin, there are some that lack this activity. Here, we present the first NMR solution structure and (15)N-relaxation analysis of a villin-type headpiece domain natively devoid of F-actin binding activity, that of supervillin headpiece (SVHP). The structure was found to be similar to that of other headpiece domains that bind F-actin. Our NMR analysis demonstrates that SVHP lacks a conformationally flexible region (V-loop) present in all other villin-type headpiece domains and which is essential to the phosphoryl regulation of dematin headpiece. In comparing the electrostatic surface potential map of SVHP to that of other villin-type headpiece domains with significant affinity for F-actin, we identified a positive surface potential conserved among headpiece domains that bind F-actin but absent from SVHP. A single point mutation (L38K) in SVHP, which creates a similar positive surface potential, endowed SVHP with specific affinity for F-actin that is within an order of magnitude of the tightest binding headpiece domains. We propose that this effect is likely conferred by a specific buried salt bridge between headpiece and actin. As no high-resolution structural information exists for the villin-type headpiece F-actin complex, our results demonstrate that through positive mutagenesis, it is possible to design binding activity into homologous proteins without structural information of the counterpart's binding surface.

Phosphorylation-induced changes in backbone dynamics of the dematin headpiece C-terminal domain

Dematin is an actin-binding protein abundant in red blood cells and other tissues. It contains a villin-type 'headpiece' F-actin-binding domain at its extreme C-terminus. The isolated dematin headpiece domain (DHP) undergoes a significant conformational change upon phosphorylation. The mutation of Ser74 to Glu closely mimics the phosphorylation of DHP. We investigated motions in the backbone of DHP and its mutant DHPS74E using several complementary NMR relaxation techniques: laboratory frame (15)N NMR relaxation, which is sensitive primarily to the ps-ns time scale, cross-correlated chemical shift modulation NMR relaxation detecting correlated mus-ms time scale motions of neighboring (13)C' and (15)N nuclei, and cross-correlated relaxation of two (15)N-(1)H dipole-dipole interactions detecting slow motions of backbone NH vectors in successive amino acid residues. The results indicate a reduction in mobility upon the mutation in several regions of the protein. The additional salt bridge formed in DHPS74E that links the N- and C-terminal subdomains is likely to be responsible for these changes.

Crystal structure of a pH-stabilized mutant of villin headpiece

Villin-type headpiece domains are compact F-actin-binding motifs that have been used extensively as a model system to investigate protein folding by both experimental and computational methods. Villin headpiece (HP67) harbors a highly helical, thermostable, and autonomously folding subdomain in the C terminus (HP35), and because of this feature, HP67 is usually considered to be composed of a N- and C-terminal subdomain. Unlike the C-terminal subdomain, the N-terminal subdomain consists mainly of loops and turns, and the folding is dependent upon the presence of the C-terminal subdomain. The pH sensitivity of this subdomain is thought to arise from, at least partially, protonation of H41 buried in the hydrophobic core. Substitution of this histidine with tyrosine, another permissive residue at this position for naturally occurring sequences, increases not only the pH stability of HP67 but also the thermal stability and the cooperativity of thermal unfolding over a wide pH range (0.9-7.5). The crystal structures of wild-type HP67 and the H41Y mutant, determined under the same conditions, indicate that the H41Y substitution causes only localized rearrangement around the mutated residue. The F-actin-binding motif remains essentially the same after the mutation, accounting for the negligible effect of the mutation on F-actin affinity. The hydrogen bond formed between the imidazole ring of H41 and the backbone carbonyl of E14 of HP67 is eliminated by the H41Y mutation, which renders the extreme N terminus of H41Y more mobile; the hydrogen bond formed between the imidazole ring of H41 and the backbone nitrogen of D34 is replaced with that between the hydroxyl group of Y41 and the backbone nitrogen of D34 after the H41Y substitution. The increased hydrophobicity of tyrosine compensates for the loss of hydrogen bonds in the extreme N terminus and accounts for the increased stability and cooperativity of the H41Y mutant.

Invertebrate coronins

Coronins are highly conserved among species, but their function is far from being understood in detail. Here we will introduce members of the family of coronin like proteins from Drosophila melanogaster, Caenorhabditis elegans and the social amoeba Dictyostelium discoideum. Genetic data from D. discoideum and D. melanogaster revealed that coronins in general are important regulators of many actin-dependent processes.

Rational design, structural and thermodynamic characterization of a hyperstable variant of the villin headpiece helical subdomain

A hyperstable variant of the small independently folded helical subdomain (HP36) derived from the F-actin binding villin headpiece was designed by targeting surface electrostatic interactions and helical propensity. A double mutant N68A, K70M was significantly more stable than wild type. The Tm of wild type in aqueous buffer is 73.0 degrees C, whereas the double mutant did not display a complete unfolding transition. The double mutant could not be completely unfolded even by 10 M urea. In 3 M urea, the Tm of wild type is 54.8 degrees C while that of the N68AK70M double mutant is 73.9 degrees C. Amide H/2H exchange studies show that the pattern of exchange is very similar for wild type and the double mutant. The structures of a K70M single mutant and the double mutant were determined by X-ray crystallography and are identical to that of the wild type. Analytical ultracentrifugation demonstrates that the proteins are monomeric. The hyperstable mutant described here is expected to be useful for folding studies of HP36 because studies of the wild type domain have sometimes been limited by its marginal stability. The results provide direct evidence that naturally occurring miniature protein domains have not been evolutionarily optimized for global stability. The stabilizing effect of this double mutant could not be predicted by sequence analysis because K70 is conserved in the larger intact headpiece for functional reasons.

Role of Drosophila gene dunc-115 in nervous system

Axonal guidance signals are transduced through growth cone surface receptors to the interior leading to changes of actin dynamics and actin binding proteins, which are critical in determining the outcome of actin cytoskeleton reorganization. We report here the characterization of the Drosophila actin binding protein abLIM/Unc-115 homolog Dunc-115 and its role in the nervous system. Three Dunc-115 isoforms are identified as Dunc-115L, M and S, respectively. While Dunc-115L is a canonical homolog of Unc-115 with four LIM domains and one villin headpiece domain, Dunc-115M and S are novel isoforms without counterparts in other species. Our molecular modeling shows Dunc-115L is likely to bind to actin. Mutant analysis reveals that Dunc-115 is involved in axonal projection in both the visual and central nervous system.

Folding free-energy landscape of villin headpiece subdomain from molecular dynamics simulations

High-accuracy ab initio folding has remained an elusive objective despite decades of effort. To explore the folding landscape of villin headpiece subdomain HP35, we conducted two sets of replica exchange molecular dynamics for 200 ns each and three sets of conventional microsecond-long molecular dynamics simulations, using AMBER FF03 force field and a generalized-Born solvation model. The protein folded consistently to the native state; the lowest C(alpha)-rmsd from the x-ray structure was 0.46 A, and the C(alpha)- rmsd of the center of the most populated cluster was 1.78 A at 300 K. ab initio simulations have previously not reached this level. The folding landscape of HP35 can be partitioned into the native, denatured, and two intermediate-state regions. The native state is separated from the major folding intermediate state by a small barrier, whereas a large barrier exists between the major folding intermediate and the denatured states. The melting temperature T(m) = 339 K extracted from the heat-capacity profile was in close agreement with the experimentally derived T(m) = 342 K. A comprehensive picture of the kinetics and thermodynamics of HP35 folding emerges when the results from replica exchange and conventional molecular dynamics simulations are combined.

Two novel members of the ABLIM protein family, ABLIM-2 and -3, associate with STARS and directly bind F-actin

In addition to regulating cell motility, contractility, and cytokinesis, the actin cytoskeleton plays a critical role in the regulation of transcription and gene expression. We have previously identified a novel muscle-specific actin-binding protein, STARS (striated muscle activator of Rho signaling), which directly binds actin and stimulates serum-response factor (SRF)-dependent transcription. To further dissect the STARS/SRF pathway, we performed a yeast two-hybrid screen of a skeletal muscle cDNA library using STARS as bait, and we identified two novel members of the ABLIM protein family, ABLIM-2 and -3, as STARS-interacting proteins. ABLIM-1, which is expressed in retina, brain, and muscle tissue, has been postulated to function as a tumor suppressor. ABLIM-2 and -3 display distinct tissue-specific expression patterns with the highest expression levels in muscle and neuronal tissue. Moreover, these novel ABLIM proteins strongly bind F-actin, are localized to actin stress fibers, and synergistically enhance STARS-dependent activation of SRF. Conversely, knockdown of endogenous ABLIM expression utilizing small interfering RNA significantly blunted SRF-dependent transcription in C2C12 skeletal muscle cells. These findings suggest that the members of the novel ABLIM protein family may serve as a scaffold for signaling modules of the actin cytoskeleton and thereby modulate transcription.

Identification of the PXW sequence as a structural gatekeeper of the headpiece C-terminal subdomain fold

The HeadPiece (HP) domain, present in several F-actin-binding multi-domain proteins, features a well-conserved, solvent-exposed PXWK motif in its C-terminal subdomain. The latter is an autonomously folding subunit comprised of three alpha-helices organised around a hydrophobic core, with the sequence motif preceding the last helix. We report the contributions of each conserved residue in the PXWK motif to human villin HP function and structure, as well as the structural implications of the naturally occurring Pro to Ala mutation in dematin HP. NMR shift perturbation mapping reveals that substitution of each residue by Ala induces only minor, local perturbations in the full villin HP structure. CD spectroscopic thermal analysis, however, shows that the Pro and Trp residues in the PXWK motif afford stabilising interactions. This indicates that, in addition to the residues in the hydrophobic core, the Trp-Pro stacking within the motif contributes to HP stability. This is reinforced by our data on isolated C-terminal HP subdomains where the Pro is also essential for structure formation, since the villin, but not the dematin, C-terminal subdomain is structured. Proper folding can be induced in the dematin C-terminal subdomain by exchanging the Ala for Pro. Conversely, the reverse substitution in the villin C-terminal subdomain leads to loss of structure. Thus, we demonstrate a crucial role for this proline residue in structural stability and folding potential of HP (sub)domains consistent with Pro-Trp stacking as a more general determinant of protein stability.

A Phosphorylation-Induced Conformation Change in Dematin Headpiece

Dematin is an actin binding protein from the junctional complex of the erythrocyte cytoskeleton. The protein has two actin binding sites and bundles actin filaments in vitro. This actin bundling activity is reversibly regulated by phosphorylation in the carboxyl terminal "headpiece" domain (DHP). DHP is a typical villin-type headpiece actin binding motif and contains a flexible N-terminal loop and an alpha-helical C-terminal subdomain that is phosphorylated at Ser74. The NMR structure of a Ser74-to-Glu mutant (DHPs74e) closely mimics the conformation of phosphorylated DHP. The negative charge at Ser74 does not alter the conformation of the C-terminal subdomain, but attracts the N-terminal loop toward the C terminus, changing the orientation of the N-terminal subdomain. NMR relaxation studies also indicate reduced mobility in the N-terminal loop in DHPs74e. Thus, phosphorylation in DHP serves as a switch controlling the conformational state of DHP and the actin bundling activity of dematin.

Characterizing a Partially Folded Intermediate of the Villin Headpiece Domain Under Non-denaturing Conditions: Contribution of His41 to the pH-dependent Stability of the N-terminal Subdomain

The contribution of interactions involving the imidazole ring of His41 to the pH-dependent stability of the villin headpiece (HP67) N-terminal subdomain has been investigated by nuclear magnetic resonance (NMR) spin relaxation. NMR-derived backbone N-H order parameters (S2) for wild-type (WT) HP67 and H41Y HP67 indicate that reduced conformational flexibility of the N-terminal subdomain in WT HP67 is due to intramolecular interactions with the His41 imidazole ring. These interactions, together with desolvation effects, contribute to significantly depress the pKa of the buried imidazole ring in the native state. 15N R1rho relaxation dispersion data indicate that WT HP67 populates a partially folded intermediate state that is 10.9 kJ mol(-1) higher in free energy than the native state under non-denaturing conditions at neutral pH. The partially folded intermediate is characterized as having an unfolded N-terminal subdomain while the C-terminal subdomain retains a native-like fold. Although the majority of the residues in the N-terminal subdomain sample a random-coil distribution of conformations, deviations of backbone amide 1H and 15N chemical shifts from canonical random-coil values for residues within 5A of the His41 imidazole ring indicate that a significant degree of residual structure is maintained in the partially folded ensemble. The pH-dependence of exchange broadening is consistent with a linear three-state exchange model whereby unfolding of the N-terminal subdomain is coupled to titration of His41 in the partially folded intermediate with a pKa,I=5.69+/-0.07. Although maintenance of residual interactions with the imidazole ring in the unfolded N-terminal subdomain appears to reduce pKa,I compared to model histidine compounds, protonation of His41 disrupts these interactions and reduces the difference in free energy between the native state and partially folded intermediate under acidic conditions. In addition, chemical shift changes for residues Lys70-Phe76 in the C-terminal subdomain suggest that the HP67 actin binding site is disrupted upon unfolding of the N-terminal subdomain, providing a potential mechanism for regulating the villin-dependent bundling of actin filaments.

High-Resolution Crystal Structures of Villin Headpiece and Mutants with Reduced F-Actin Binding Activity,

Villin-type headpiece domains are approximately 70 amino acid modular motifs found at the C terminus of a variety of actin cytoskeleton-associated proteins. The headpiece domain of villin, a protein found in the actin bundles of the brush border epithelium, is of interest both as a compact F-actin binding domain and as a model folded protein. We have determined the high-resolution crystal structures of chicken villin headpiece (HP67) at 1.4 A resolution as well as two mutants, R37A and W64Y, at 1.45 and 1.5 A resolution, respectively. Replacement of R37 causes a 5-fold reduction in F-actin binding affinity in sedimentation assays. Replacement of W64 results in a much more drastic reduction in F-actin binding affinity without significant changes in headpiece structure or stability. The detailed comparison of these crystal structures with each other and to our previously determined NMR structures of HP67 and the 35-residue autonomously folding subdomain in villin headpiece, HP35, provides the details of the headpiece fold and further defines the F-actin binding site of villin-type headpiece domains.

Identification of a villin-related tobacco protein as a novel cross-reactive plant allergen

In a paradigmatic approach we identified cross-reactive plant allergens for allergy diagnosis and treatment by screening of a tobacco leaf complementary DNA (cDNA) library with serum IgE from a polysensitized allergic patient. Two IgE-reactive cDNA clones were isolated which code for proteins with significant sequence similarity to the actin-binding protein, villin. Northern- and Western-blotting demonstrate expression of the villin-related allergens in pollen and somatic plant tissues. In addition, villin-related proteins were detected in several plant allergen sources (tree-, grass-, weed pollen, fruits, vegetables, nuts). A recombinant C-terminal fragment of the villin-related protein was expressed in Escherichia coli, purified and shown to react specifically with allergic patients IgE. After profilin, villin-related proteins represent another family of cytoskeletal proteins, which has been identified as cross-reactive plant allergens. They may be used for the diagnosis and treatment of patients suffering from multivalent plant allergies.

The LIM domain: from the cytoskeleton to the nucleus

First described 15 years ago as a cysteine-rich sequence that was common to a small group of homeodomain transcription factors, the LIM domain is now recognized as a tandem zinc-finger structure that functions as a modular protein-binding interface. LIM domains are present in many proteins that have diverse cellular roles as regulators of gene expression, cytoarchitecture, cell adhesion, cell motility and signal transduction. An emerging theme is that LIM proteins might function as biosensors that mediate communication between the cytosolic and the nuclear compartments.

Calnexin, calreticulin and cytoskeleton-associated proteins modulate uptake and growth of Legionella pneumophila in Dictyostelium discoideum

The haploid amoebaDictyostelium discoideumis a versatile host system for studying cellular aspects ofLegionellapathogenicity. Previous studies have shown that the internalization ofL. pneumophilaleads to an endoplasmic reticulum (ER)-derived organelle that supports intracellular replication of the bacteria. In this study a roadmap of host-cell factors involved in this process was developed. Phagocytosis assays with specific cellular inhibitors and the effects of well defined host-cell mutants revealed that cytoplasmic calcium levels, cytoskeleton-associated proteins and the calcium-binding proteins of the ER, calreticulin and calnexin, specifically influence the uptake and intracellular growth ofL. pneumophila. Confocal microscopic time series with green fluorescent protein (GFP)-tagged calnexin and calreticulin demonstrated the accumulation of both proteins in the phagocytic cup ofL. pneumophila-infected host cells. In contrast to the control experiment withEscherichia coli-containing phagosomes, both proteins decorated the replicative vacuole ofL. pneumophiladuring the entire growth phase of the bacteria. The cumulative effects of cytosolic calcium levels, the spatial distribution of calnexin and calreticulin, and the defective invasion and replication ofL. pneumophilain calnexin- and calreticulin-minus cells suggest that these factors are part of a regulatory system that leads to the specific vacuole ofL. pneumophila.

Solution structures of the C-terminal headpiece subdomains of human villin and advillin, evaluation of headpiece F-actin-binding requirements

Headpiece (HP) is a 76-residue F-actin-binding module at the C terminus of many cytoskeletal proteins. Its 35-residue C-terminal subdomain is one of the smallest known motifs capable of autonomously adopting a stable, folded structure in the absence of any disulfide bridges, metal ligands, or unnatural amino acids. We report the three-dimensional solution structures of the C-terminal headpiece subdomains of human villin (HVcHP) and human advillin (HAcHP), determined by two-dimensional 1H-NMR. They represent the second and third structures of such C-terminal headpiece subdomains to be elucidated so far. A comparison with the structure of the chicken villin C-terminal subdomain reveals a high structural conservation. Both C-terminal subdomains bind specifically to F-actin. Mutagenesis is used to demonstrate the involvement of Trp 64 in the F-actin-binding surface. The latter residue is part of a conserved structural feature, in which the surface-exposed indole ring is stacked on the proline and lysine side chain embedded in a PXWK sequence motif. On the basis of the structural and mutational data concerning Trp 64 reported here, the results of a cysteine-scanning mutagenesis study of full headpiece, and a phage display mutational study of the 69-74 fragment, we propose a modification of the model, elaborated by Vardar and coworkers, for the binding of headpiece to F-actin.

The NMR Structure of Dematin Headpiece Reveals a Dynamic Loop That Is Conformationally Altered upon Phosphorylation at a Distal Site

Dematin (band 4.9) is found in the junctional complex of the spectrin cytoskeleton that supports the erythrocyte cell membrane. Dematin is a member of the larger class of cytoskeleton-associated proteins that contain a modular "headpiece" domain at their extreme C termini. The dematin headpiece domain provides the second F-actin-binding site required for in vitro F-actin bundling. The dematin headpiece is found in two forms in the cell, one of 68 residues (DHP) and one containing a 22-amino acid insert near its N terminus (DHP+22). In addition, dematin contains the only headpiece domain that is phosphorylated, in vivo. The 22-amino acid insert in DHP+22 appeared unstructured in NMR spectra; therefore, we have determined the three-dimensional structure of DHP by multidimensional NMR methods. Although the overall three-dimensional structure of DHP is similar to that of the villin headpiece, there are two novel characteristics revealed by this structure. First, unlike villin headpiece that contains a single buried salt bridge, DHP contains a buried charged cluster comprising residues Glu(39), Arg(66), Lys(70), and the C-terminal carboxylate of Phe(76). Second, (15)N relaxation experiments indicate that the longer "variable loop" region near the N terminus of DHP (residues 20-29) is dynamic, undergoing significantly greater motions that the rest of the structure. Furthermore, NMR chemical shift changes indicate that the conformation of the dynamic variable loop is altered by phosphorylation of serine 74, which is far in the sequence from the variable loop region. Our results suggest that phosphorylation of the dematin headpiece acts as a conformational switch within this headpiece domain.

Villidin, a novel WD-repeat and villin-related protein from Dictyostelium, is associated with membranes and the cytoskeleton

Villidin is a novel multidomain protein (190 kDa) from Dictyostelium amoebae containing WD repeats at its N-terminus, three PH domains in the middle of the molecule, and five gelsolin-like segments at the C-terminus, followed by a villin-like headpiece. Villidin mRNA and protein are present in low amounts during growth and early aggregation, but increase during development and reach their highest levels at the tipped mound stage. The protein is present in the cytosol as well as in the cytoskeletal and membrane fractions. GFP-tagged full-length villidin exhibits a similar distribution as native villidin, including a distinct colocalization with Golgi structures. Interestingly, GFP fusions with the gelsolin/villin-like region are uniformly dispersed in the cytoplasm, whereas GFP fusions of the N-terminal WD repeats codistribute with F-actin and are associated with the Triton-insoluble cytoskeleton. Strains lacking villidin because of targeted deletion of its gene grow normally and can develop into fruiting bodies. However, cell motility is reduced during aggregation and phototaxis is impaired in the mutant strains. We conclude that villidin harbors a major F-actin binding site in the N-terminal domain and not in the villin-like region as expected; association of villidin with vesicular membranes suggests that the protein functions as a linker between membranes and the actin cytoskeleton.

A phage display-based method for determination of relative affinities of mutants. Application of the actin-binding motifs in thymosin beta 4 and the villin headpiece

We propose phage display combined with enzyme-linked immunosorbent assay as a tool for the systematic analysis of protein-protein interactions by investigating the binding behavior of variants to a partner protein. Via enzyme-linked immunosorbent assay we determine both the amount of fusion protein presented at the phage surface and the amount of complex formed, the ratio of which is proportional to the affinity. Hence this method enables us to calculate the relative affinities of a large number of mutants. As model systems, we investigated actin-binding motifs conserved in a number of proteins binding monomeric or filamentous actin. The hexapeptide motifs LKKTET, present in thymosin beta4, and LKKEKG, present in the villin headpiece, were mutated, and the variants were analyzed. Study of the positional tolerance allows postulating that the motifs, although similar in primary structures adopt different conformations when bound to actin. In addition, our data show that the second and the fourth amino acid of the thymosin beta4 motif and the first three residues of the villin headpiece motif are most important for actin binding. The latter result challenges the charged crown hypothesis for the villin headpiece filamentous actin interaction.

The actin-binding protein UNC-115 is an effector of Rac signaling during axon pathfinding in C. elegans

Rac GTPases control cell shape by regulating downstream effectors that influence the actin cytoskeleton. UNC-115, a putative actin-binding protein similar to human abLIM/limatin, has previously been implicated in axon pathfinding. We have discovered the role of UNC-115 as a downstream cytoskeletal effector of Rac signaling in axon pathfinding. We show that unc-115 double mutants with ced-10 Rac, mig-2 Rac or unc-73 GEF but not with rac-2/3 Rac displayed synthetic axon pathfinding defects, and that loss of unc-115 function suppressed the formation of ectopic plasma membrane extensions induced by constitutively-active rac-2 in neurons. Furthermore, we show that UNC-115 can bind to actin filaments. Thus, UNC-115 is an actin-binding protein that acts downstream of Rac signaling in axon pathfinding.