Mutational analysis of capping protein function in Saccharomyces cerevisiae

Functional requirements for a Samd14-capping protein complex in stress erythropoiesis

Acute anemia induces rapid expansion of erythroid precursors and accelerated differentiation to replenish erythrocytes. Paracrine signals—involving cooperation between stem cell factor (SCF)/Kit signaling and other signaling inputs—are required for the increased erythroid precursor activity in anemia. Our prior work revealed that the sterile alpha motif (SAM) domain 14 (Samd14) gene increases the regenerative capacity of the erythroid system in a mouse genetic model and promotes stress-dependent Kit signaling. However, the mechanism underlying Samd14’s role in stress erythropoiesis is unknown. We identified a protein-protein interaction between Samd14 and the α- and β-heterodimers of the F-actin capping protein (CP) complex. Knockdown of the CP β subunit increased erythroid maturation in murine ex vivo cultures and decreased colony forming potential of stress erythroid precursors. In a genetic complementation assay for Samd14 activity, our results revealed that the Samd14-CP interaction is a determinant of erythroid precursor cell levels and function. Samd14-CP promotes SCF/Kit signaling in CD71^med spleen erythroid precursors. Given the roles of Kit signaling in hematopoiesis and Samd14 in Kit pathway activation, this mechanism may have pathological implications in acute/chronic anemia.

Anemia is a condition in which the body has a shortage of healthy red blood cells to carry enough oxygen to support its organs. A range of factors are known to cause anemia, including traumatic blood loss, toxins or nutritional deficiency. An estimated one-third of all women of reproductive age are anemic, which can cause tiredness, weakness and shortness of breath. Severe anemia drives the release of hormones and growth factors, leading to a rapid regeneration of precursor red blood cells to replenish the supply in the blood.

To understand how red blood cell regeneration is controlled, Ray et al. studied proteins involved in regenerating blood using mice in which anemia had been induced with chemicals. Previous research had shown that the protein Samd14 is produced at higher quantities in individuals with anemia, and is involved with the recovery of lost red blood cells. However, it is not known how the Samd14 protein plays a role in regenerating blood cells, or whether Samd14 interacts with other proteins required for red blood cell production.

To shed light on these questions, mouse cells exposed to anemia conditions were used to see what proteins Samd14 binds to. Purifying Samd14 revealed that it interacts with the actin capping protein. This interaction relies on a specific region of Samd14 that is similar to regions in other proteins that bind capping proteins. Ray et al. found that the interaction between Samd14 and the actin capping protein increased the signals needed for the development and survival of new red blood cells.

These results identify a signaling mechanism that, if disrupted, could cause anemia to develop. They lead to a better understanding of how our bodies recover from anemia, and potential avenues to treat this condition.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Physiological role of actin regulation in male fertility: Insight into actin capping proteins in spermatogenic cells

BACKGROUND

During spermatogenesis, cytoskeletal elements are essential for spermatogenic cells to change morphologically and translocate in the seminiferous tubule. Actin filaments have been revealed to be concentrated in specific regions of spermatogenic cells and are regulated by a large number of actin-binding proteins. Actin capping protein is one of the essential actin regulatory proteins, and a recent study showed that testis-specific actin capping protein may affect male infertility.

METHODS

The roles of actin during spermatogenesis and testis-specific actin capping protein were reviewed by referring to the previous literature.

MAIN FINDINGS RESULTS

Actin filaments are involved in several crucial phases of spermatogenesis including acrosome biogenesis, flagellum formation, and nuclear processes such as the formation of synaptonemal complex. Besides, an implication for capacitation and acrosome reaction was also suggested. Testis-specific actin capping proteins are suggested to be associated with the removal of excess cytoplasm in mice. By the use of high-throughput sperm proteomics, lower protein expression of testis-specific actin capping protein in infertile men was also reported.

CONCLUSION

Actin is involved in the crucial phases of spermatogenesis, and the altered expression of testis-specific actin capping proteins is suggested to be a cause of male infertility in humans.

Allosteric Coupling of CARMIL and V-1 Binding to Capping Protein Revealed by Hydrogen-Deuterium Exchange

Actin assembly is important for cell motility. The ability of actin subunits to join or leave filaments via the barbed end is critical to actin dynamics. Capping protein (CP) binds to barbed ends to prevent subunit gain and loss and is regulated by proteins that include V-1 and CARMIL. V-1 inhibits CP by sterically blocking one binding site for actin. CARMILs bind at a distal site and decrease the affinity of CP for actin, suggested to be caused by conformational changes. We used hydrogen-deuterium exchange with mass spectrometry (HDX-MS) to probe changes in structural dynamics induced by V-1 and CARMIL binding to CP. V-1 and CARMIL induce changes in both proteins' binding sites on the surface of CP, along with a set of internal residues. Both also affect the conformation of CP's ββ subunit "tentacle," a second distal actin-binding site. Concerted regulation of actin assembly by CP occurs through allosteric couplings between CP modulator and actin binding sites.

Capping Protein Insulates Arp2/3-Assembled Actin Patches from Formins

How actin structures of distinct identities and functions coexist within the same environment is a critical self-organization question. Fission yeast cells have a simple actin cytoskeleton made of four structures: Arp2/3 assembles actin patches around endocytic pits, and the formins For3, Cdc12, and Fus1 assemble actin cables, the cytokinetic ring during division, and the fusion focus during sexual reproduction, respectively. The focus concentrates the delivery of hydrolases by myosin V to digest the cell wall for cell fusion. We discovered that cells lacking capping protein (CP), a heterodimer that blocks barbed-end dynamics and associates with actin patches, exhibit a delay in fusion. Consistent with CP-formin competition for barbed-end binding, Fus1, F-actin, and the linear filament marker tropomyosin hyper-accumulate at the fusion focus in cells lacking CP. CP deletion also rescues the fusion defect of a mutation in the Fus1 knob region. However, myosin V and exocytic cargoes are reduced at the fusion focus and diverted to ectopic foci, which underlies the fusion defect. Remarkably, the ectopic foci coincide with Arp2/3-assembled actin patches, which now contain low levels of Fus1. We further show that CP localization to actin patches is required to prevent the formation of ectopic foci and promote efficient cell fusion. During mitotic growth, actin patches lacking CP similarly display a dual identity, as they accumulate the formins For3 and Cdc12, normally absent from patches, and are co-decorated by the linear filament-binding protein tropomyosin and the patch marker fimbrin. Thus, CP serves to protect Arp2/3-nucleated structures from formin activity.

Systematic characterization of human testis-specific actin capping protein β3 as a possible biomarker for male infertility

STUDY QUESTION

Is actin capping protein (CP) β3 involved in human spermatogenesis and male infertility?

SUMMARY ANSWER

Human CPβ3 (hCPβ3) is expressed in testis, changes its localization dynamically during spermatogenesis, and has some association with male infertility.

WHAT IS KNOWN ALREADY

The testis-specific α subunit of CP (CPα3) was previously identified in human, and mutations in the cpα3 gene in mouse were shown to induce malformation of the sperm head and male infertility. However, CPβ3, which is considered to be a heterodimeric counterpart of CPα3, has been neither characterized in human nor reported in association with male infertility.

STUDY DESIGN, SIZE, DURATION

To confirm the existence of CPβ3 in human testis, fresh semen samples from proven fertile men were analyzed. To investigate protein expression during spermatogenesis, cryopreserved testis obtained from men with obstructive azoospermia were examined by immunofluorescent analysis. To assess the association of CP with male infertility, we compared protein expression of human CPα3 (hCPα3) and hCPβ3 using immunofluorescent analysis of cryopreserved sperm between men with normozoospermia (volunteers: Normo group, n = 20) and infertile men with oligozoospermia and/or asthenozoospermia (O + A group, n = 21).

PARTICIPANTS/MATERIALS, SETTING, METHODS

The tissue-specific expression of hCPβ3 was investigated by RT-PCR and Western blot analysis. To investigate whether hCPα3 and hCPβ3 form a heterodimer, a tandem expression vector containing hcpα3 tagged with monomeric red fluorescent protein 1 and hcpβ3 tagged with enhanced green fluorescent protein in a single plasmid was constructed and analyzed by co-immunoprecipitation (Co-IP) assay. The protein expression profiles of hCPα3 and hCPβ3 during spermatogenesis were examined by immunohistochemical analysis using human spermatogenic cells. The protein expressions of hCPα3 and hCPβ3 in sperm were compared between the Normo and O + A groups by immunohistochemical analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

RT-PCR showed that mRNA of hcpβ3 was expressed exclusively in testis. Western blot analysis detected hCPβ3 with anti-bovine CPβ3 antibody. Co-IP assay with recombinant protein showed that hCPα3 and hCPβ3 form a protein complex. At each step during spermatogenesis, the cellular localization of hCPβ3 changed dynamically. In spermatogonia, hCPβ3 showed a slight signal in cytoplasm. hCPβ3 expression was conspicuous mainly from spermatocytes, and hCPβ3 localization dynamically migrated from cytoplasm to the acrosomal cap and acrosome. In mature spermatozoa, hCPβ3 accumulated in the postacrosomal region and less so at the midpiece of the tail. Double-staining analysis revealed that hCPα3 localization was identical to hCPβ3 at every step in the spermatogenic cells. Most spermatozoa from the Normo group were stained homogenously by both hCPα3 and hCPβ3. In contrast, significantly more spermatozoa in the O + A versus Normo group showed heterogeneous or lack of staining for either hCPα3 or hCPβ3 (abnormal staining) (P < 0.001). The percentage of abnormal staining was higher in the O + A group (52.4 ± 3.0%) than in the Normo group (31.2 ± 2.5%). Even by confining the observations to morphologically normal spermatozoa selected in accordance with David's criteria, the percentage of abnormal staining was still higher in the O + A group (39.9 ± 2.9%) versus the Normo group (22.5 ± 2.1%) (P < 0.001). hCPβ3 in conjunction with hCPα3 seemed to play an important role in spermatogenesis and may be associated with male infertility.

LARGE SCALE DATA

Not applicable.

LIMITATIONS REASONS FOR CAUTION

Owing to the difficulty of collecting fresh samples of human testis, we used cryopreserved samples from testicular sperm extraction. To examine the interaction of spermatogenic cells or localization in seminiferous tubules, fresh testis sample of healthy males are ideal.

WIDER IMPLICATIONS OF THE FINDINGS

The altered expression of hCPα3 and hCPβ3 may not only be a cause of male infertility but also a prognostic factor for the results of ART. They may be useful biomarkers to determine the fertilization ability of human sperm in ART.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JP16K20133). The authors declare no competing interests.

Internetwork competition for monomers governs actin cytoskeleton organization

Cells precisely control the formation of dynamic actin cytoskeleton networks to coordinate fundamental processes, including motility, division, endocytosis and polarization. To support these functions, actin filament networks must be assembled, maintained and disassembled at the correct time and place, and with proper filament organization and dynamics. Regulation of the extent of filament network assembly and of filament network organization has been largely attributed to the coordinated activation of actin assembly factors through signalling cascades. Here, we discuss an intriguing model in which actin monomer availability is limiting and competition between homeostatic actin cytoskeletal networks for actin monomers is an additional crucial regulatory mechanism that influences the density and size of different actin networks, thereby contributing to the organization of the cellular actin cytoskeleton.

Heterodimeric capping protein from Arabidopsis is a membrane-associated, actin-binding protein

The actin cytoskeleton is a major regulator of cell morphogenesis and responses to biotic and abiotic stimuli. The organization and activities of the cytoskeleton are choreographed by hundreds of accessory proteins. Many actin-binding proteins are thought to be stimulus-response regulators that bind to signaling phospholipids and change their activity upon lipid binding. Whether these proteins associate with and/or are regulated by signaling lipids in plant cells remains poorly understood. Heterodimeric capping protein (CP) is a conserved and ubiquitous regulator of actin dynamics. It binds to the barbed end of filaments with high affinity and modulates filament assembly and disassembly reactions in vitro. Direct interaction of CP with phospholipids, including phosphatidic acid, results in uncapping of filament ends in vitro. Live-cell imaging and reverse-genetic analyses of cp mutants in Arabidopsis (Arabidopsis thaliana) recently provided compelling support for a model in which CP activity is negatively regulated by phosphatidic acid in vivo. Here, we used complementary biochemical, subcellular fractionation, and immunofluorescence microscopy approaches to elucidate CP-membrane association. We found that CP is moderately abundant in Arabidopsis tissues and present in a microsomal membrane fraction. Sucrose density gradient separation and immunoblotting with known compartment markers were used to demonstrate that CP is enriched on membrane-bound organelles such as the endoplasmic reticulum and Golgi. This association could facilitate cross talk between the actin cytoskeleton and a wide spectrum of essential cellular functions such as organelle motility and signal transduction.

Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

Capping protein modulates the dynamic behavior of actin filaments in response to phosphatidic acid in Arabidopsis

Remodeling of actin filament arrays in response to biotic and abiotic stimuli is thought to require precise control over the generation and availability of filament ends. Heterodimeric capping protein (CP) is an abundant filament capper, and its activity is inhibited by membrane signaling phospholipids in vitro. How exactly CP modulates the properties of filament ends in cells and whether its activity is coordinated by phospholipids in vivo is not well understood. By observing directly the dynamic behavior of individual filament ends in the cortical array of living Arabidopsis thaliana epidermal cells, we dissected the contribution of CP to actin organization and dynamics in response to the signaling phospholipid, phosphatidic acid (PA). Here, we examined three cp knockdown mutants and found that reduced CP levels resulted in more dynamic activity at filament ends, and this significantly enhanced filament-filament annealing and filament elongation from free ends. The cp mutants also exhibited more dense actin filament arrays. Treatment of wild-type cells with exogenous PA phenocopied the actin-based defects in cp mutants, with an increase in the density of filament arrays and enhanced annealing frequency. These cytoskeletal responses to exogenous PA were completely abrogated in cp mutants. Our data provide compelling genetic evidence that the end-capping activity of CP is inhibited by membrane signaling lipids in eukaryotic cells. Specifically, CP acts as a PA biosensor and key transducer of fluxes in membrane signaling phospholipids into changes in actin cytoskeleton dynamics.

Requirements of Slm proteins for proper eisosome organization, endocytic trafficking and recycling in the yeast Saccharomyces cerevisiae

Eisosomes are large immobile assemblies at the cortex of a cell under the membrane compartment of Can1 (MCC) in yeast. Slm1 has recently been identified as an MCC component that acts downstream of Mss4 in a pathway that regulates actin cytoskeleton organization in response to stress. In this study, we showed that inactivation of Slm proteins disrupts proper localization of the primary eisosome marker Pil1, providing evidence that Slm proteins play a role in eisosome organization. Furthermore, we found that slm ts mutant cells exhibit actin defects in both the ability to polarize cortical F-actin and the formation of cytoplasmic actin cables even at the permissive temperature (30 degrees C). We further demonstrated that the actin defect accounts for the slow traffic of FM4-64- labelled endosome in the cytoplasm, supporting the notion that intact actin is essential for endosome trafficking. However, our real-time microscopic analysis of Abp1-RFP revealed that the actin defect in slm ts cells was not accompanied by a noticeable defect in actin patch internalization during receptor-mediated endocytosis. In addition, we found that slm ts cells displayed impaired membrane recycling and that recycling occurred in an actin-independent manner. Our data provide evidence for the requirement of Slm proteins in eisosome organization and endosome trafficking and recycling.

Molecular basis for barbed end uncapping by CARMIL homology domain 3 of mouse CARMIL-1

Capping protein (CP) is a ubiquitously expressed, 62-kDa heterodimer that binds the barbed end of the actin filament with approximately 0.1 nm affinity to prevent further monomer addition. CARMIL is a multidomain protein, present from protozoa to mammals, that binds CP and is important for normal actin dynamics in vivo. The CARMIL CP binding site resides in its CAH3 domain (CARMIL homology domain 3) located at or near the protein's C terminus. CAH3 binds CP with approximately 1 nm affinity, resulting in a complex with weak capping activity (30-200 nm). Solution assays and single-molecule imaging show that CAH3 binds CP already present on the barbed end, causing a 300-fold increase in the dissociation rate of CP from the end (i.e. uncapping). Here we used nuclear magnetic resonance (NMR) to define the molecular interaction between the minimal CAH3 domain (CAH3a/b) of mouse CARMIL-1 and CP. Specifically, we show that the highly basic CAH3a subdomain is required for the high affinity interaction of CAH3 with a complementary "acidic groove" on CP opposite its actin-binding surface. This CAH3a-CP interaction orients the CAH3b subdomain, which we show is also required for potent anti-CP activity, directly adjacent to the basic patch of CP, shown previously to be required for CP association to and high affinity interaction with the barbed end. The importance of specific residue interactions between CP and CAH3a/b was confirmed by site-directed mutagenesis of both proteins. Together, these results offer a mechanistic explanation for the barbed end uncapping activity of CARMIL, and they identify the basic patch on CP as a crucial regulatory site.

Structural basis for capping protein sequestration by myotrophin (V-1)

Capping protein (CP) is a ubiquitously expressed, heterodimeric 62-kDa protein that binds the barbed end of the actin filament with high affinity to block further filament elongation. Myotrophin (V-1) is a 13-kDa ankyrin repeat-containing protein that binds CP tightly, sequestering it in a totally inactive complex in vitro. Here, we elucidate the molecular interaction between CP and V-1 by NMR. Specifically, chemical shift mapping and intermolecular paramagnetic relaxation enhancement experiments reveal that the ankyrin loops of V-1, which are essential for V-1/CP interaction, bind the basic patch near the joint of the alpha tentacle of CP shown previously to drive most of the association of CP with and affinity for the barbed end. Consistently, site-directed mutagenesis of CP shows that V-1 and the strong electrostatic binding site for CP on the barbed end compete for this basic patch on CP. These results can explain how V-1 inactivates barbed end capping by CP and why V-1 is incapable of uncapping CP-capped actin filaments, the two signature biochemical activities of V-1.

Direct observation of the uncapping of capping protein-capped actin filaments by CARMIL homology domain 3

Bulk solution assays have shown that the isolated CARMIL homology 3 (CAH3) domain from mouse and Acanthamoeba CARMIL rapidly and potently restores actin polymerization when added to actin filaments previously capped with capping protein (CP). To demonstrate this putative uncapping activity directly, we used total internal reflection microscopy to observe single, CP-capped actin filaments before and after the addition of the CAH3 domain from mouse CARMIL-1 (mCAH3). The addition of mCAH3 rapidly restored the polymerization of individual capped filaments, consistent with uncapping. To verify uncapping, filaments were capped with recombinant mouse CP tagged with monomeric green fluorescent protein (mGFP-CP). Restoration of polymerization upon the addition of mCAH3 was immediately preceded by the complete dissociation of mGFP-CP from the filament end, confirming the CAH3-driven uncapping mechanism. Quantitative analyses showed that the percentage of capped filaments that uncapped increased as the concentration of mCAH3 was increased, reaching a maximum of approximately 90% at approximately 250 nm mCAH3. Moreover, the time interval between mCAH3 addition and uncapping decreased as the concentration of mCAH3 increased, with the half-time of CP at the barbed end decreasing from approximately 30 min without mCAH3 to approximately 10 s with a saturating amount of mCAH3. Finally, using mCAH3 tagged with mGFP, we obtained direct evidence that the complex of CP and mCAH3 has a small but measurable affinity for the barbed end, as inferred from previous studies and kinetic modeling. We conclude that the isolated CAH3 domain of CARMIL (and presumably the intact molecule as well) possesses the ability to uncap CP-capped actin filaments.

New insights into mechanism and regulation of actin capping protein

The heterodimeric actin capping protein, referred to here as "CP," is an essential element of the actin cytoskeleton, binding to the barbed ends of actin filaments and regulating their polymerization. In vitro, CP has a critical role in the dendritic nucleation process of actin assembly mediated by Arp2/3 complex, and in vivo, CP is important for actin assembly and actin-based process of morphogenesis and differentiation. Recent studies have provided new insight into the mechanism of CP binding the barbed end, which raises new possibilities for the dynamics of CP and actin in cells. In addition, a number of molecules that bind and regulate CP have been discovered, suggesting new ideas for how CP may integrate into diverse processes of cell physiology.

Structural basis of actin filament capping at the barbed-end: a cryo-electron microscopy study

The intracellular distribution and migration of many protein complexes and organelles is regulated by the dynamics of the actin filament. Many actin filament end-binding proteins play crucial roles in actin dynamics, since polymerization and depolymerization of actin protomers occur only at the filament ends. We present here an EM structure of the complex of the actin filament and hetero-dimeric capping protein (CP) bound to the barbed-end at 23 A resolution, by applying a newly developed methods of image analysis to cryo-electron micrographs. This structure was fitted by the crystal structure of CP and the proposed actin filament structure, allowing us to construct a model that depicts two major binding regions between CP and the barbed-end. This binding scheme accounted for the results of newly performed and previously published mutation experiments, and led us to propose a two-step binding model. This is the first determination of an actin filament end structure.

Heterodimeric capping protein from Arabidopsis is regulated by phosphatidic acid

The cytoskeleton is a key regulator of morphogenesis, sexual reproduction, and cellular responses to extracellular stimuli. Changes in the cellular architecture are often assumed to require actin-binding proteins as stimulus-response modulators, because many of these proteins are regulated directly by binding to intracellular second messengers or signaling phospholipids. Phosphatidic acid (PA) is gaining widespread acceptance as a major, abundant phospholipid in plants that is required for pollen tube tip growth and mediates responses to osmotic stress, wounding, and phytohormones; however, the number of identified effectors of PA is rather limited. Here we demonstrate that exogenous PA application leads to significant increases in filamentous actin levels in Arabidopsis suspension cells and poppy pollen grains. To investigate further these lipid-induced changes in polymer levels, we analyzed the properties of a key regulator of actin filament polymerization, the heterodimeric capping protein from Arabidopsis thaliana (AtCP). AtCP binds to PA with a K(d) value of 17 muM and stoichiometry of approximately 1:2. It also binds well to PtdIns(4,5)P(2), but not to several other phosphoinositide or acidic phospholipids. The interaction with PA inhibited the actin-binding activity of CP. In the presence of PA, CP is unable to block the barbed or rapidly growing and shrinking end of actin filaments. Precapped filament barbed ends can also be uncapped by addition of PA, allowing rapid filament assembly from an actin monomer pool that is buffered with profilin. The findings support a model in which the inhibition of CP activity in cells by elevated PA results in the stimulation of actin polymerization from a large pool of profilin-actin. Such regulation may be important for the response of plant cells to extracellular stimuli as well as for the normal process of pollen tube tip growth.

Lamellipodial versus filopodial mode of the actin nanomachinery: pivotal role of the filament barbed end

Understanding how a particular cell type expresses the lamellipodial or filopodial form of the actin machinery is essential to understanding a cell's functional interactions. To determine how a cell "chooses" among these alternative modes of "molecular hardware," we tested the role of key proteins that affect actin filament barbed ends. Depletion of capping protein (CP) by short hairpin RNA (shRNA) caused loss of lamellipodia and explosive formation of filopodia. The knockdown phenotype was rescued by a CP mutant refractory to shRNA, but not by another barbed-end capper, gelsolin, demonstrating that the phenotype was specific for CP. In Ena/VASP deficient cells, CP depletion resulted in ruffling instead of filopodia. We propose a model for selection of lamellipodial versus filopodial organization in which CP is a negative regulator of filopodia formation and Ena/VASP has recruiting/activating functions downstream of actin filament elongation in addition to its previously suggested anticapping and antibranching activities.

Capping protein binding to actin in yeast: biochemical mechanism and physiological relevance

The mechanism by which capping protein (CP) binds barbed ends of actin filaments is not understood, and the physiological significance of CP binding to actin is not defined. The CP crystal structure suggests that the COOH-terminal regions of the CP α and β subunits bind to the barbed end. Using purified recombinant mutant yeast CP, we tested this model. CP lacking both COOH-terminal regions did not bind actin. The α COOH-terminal region was more important than that of β. The significance of CP's actin-binding activity in vivo was tested by determining how well CP actin-binding mutants rescued null mutant phenotypes. Rescue correlated well with capping activity, as did localization of CP to actin patches, indicating that capping is a physiological function for CP. Actin filaments of patches appear to be nucleated first, then capped with CP. The binding constants of yeast CP for actin suggest that actin capping in yeast is more dynamic than in vertebrates.

Arabidopsis capping protein (AtCP) is a heterodimer that regulates assembly at the barbed ends of actin filaments

The precise regulation of actin filament polymerization and depolymerization is essential for many cellular processes and is choreographed by a multitude of actin-binding proteins (ABPs). In higher plants the number of well characterized ABPs is quite limited, and some evidence points to significant differences in the biochemical properties of apparently conserved proteins. Here we provide the first evidence for the existence and biochemical properties of a heterodimeric capping protein from Arabidopsis thaliana (AtCP). The purified recombinant protein binds to actin filament barbed ends with Kd values of 12-24 nM, as assayed both kinetically and at steady state. AtCP prevents the addition of profilin actin to barbed ends during a seeded elongation reaction and suppresses dilution-mediated depolymerization. It does not, however, sever actin filaments and does not have a preference for the source of actin. During assembly from Mg-ATP-actin monomers, AtCP eliminates the initial lag period for actin polymerization and increases the maximum rate of polymerization. Indeed, the efficiency of actin nucleation of 0.042 pointed ends created per AtCP polypeptide compares favorably with mouse CapZ, which has a maximal nucleation of 0.17 pointed ends per CapZ polypeptide. AtCP activity is not affected by calcium but is sensitive to phosphatidylinositol 4,5-bisphosphate. We propose that AtCP is a major regulator of actin dynamics in plant cells that, together with abundant profilin, is responsible for maintaining a large pool of actin subunits and a surprisingly small population of F-actin.

Crystal structure of CapZ: structural basis for actin filament barbed end capping

Capping protein, a heterodimeric protein composed of alpha and beta subunits, is a key cellular component regulating actin filament assembly and organization. It binds to the barbed ends of the filaments and works as a 'cap' by preventing the addition and loss of actin monomers at the end. Here we describe the crystal structure of the chicken sarcomeric capping protein CapZ at 2.1 A resolution. The structure shows a striking resemblance between the alpha and beta subunits, so that the entire molecule has a pseudo 2-fold rotational symmetry. CapZ has a pair of mobile extensions for actin binding, one of which also provides concomitant binding to another protein for the actin filament targeting. The mobile extensions probably form flexible links to the end of the actin filament with a pseudo 2(1) helical symmetry, enabling the docking of the two in a symmetry mismatch.

Interaction of actin with the capping protein, CapZ from sea bass (Dicentrarchus labrax) white skeletal muscle

We have compared the functional properties of CapZ from fish white skeletal muscle with those of CapZ from chicken muscle. CapZ is a heterodimer, which enhances actin nucleation and inhibits the depolymerization process by binding to the barbed ends of microfilaments. Here, we report the interaction of CapZ not only with F-actin, but also with monomeric actin. The affinity of sea bass CapZ for G-actin estimated by enzyme-linked immunosorbent assay (ELISA) was in the microM range. This association was PIP2 dependent. Binding contacts with the barbed end of actin were delimited by both ELISA and fluorescence approaches. One site (actin sequence 338-348) was located in a helical region of the subdomain 1, region already implicated in the interaction with other actin binding proteins such as gelsolin. Another site implicates the C-terminal region (sequence 360-372) of actin. Finally, the partial competition of antibodies directed against CapZ alpha or beta-subunits towards CapZ interaction with actin filaments suggests both subunits participate in the complex with actin.

Genomic analysis of male germ cell-specific actin capping protein alpha

The Gsg3 gene which expresses specifically in haploid germ cells is a mouse testicular homolog of somatic cell type actin capping protein alpha (ACP alpha). We have obtained a mouse Gsg3 genomic clone using cDNA as a probe. Sequencing data showed that the Gsg3 gene was not interrupted by introns. The transcription initiation site of the gene was preceded not by a TATA box or GC rich promoter motifs, but by two consensus cAMP-response element (CRE) motifs at the putative position. Southern blotting analysis showed that Gsg3 is a single copy gene in the mouse, and conserved in mammals. Phylogenetic analysis showed that Gsg3 is a novel ACP alpha specific for haploid germ cells.

Hydrophobic residues in the C-terminal region of S100A1 are essential for target protein binding but not for dimerization

S100 proteins are a family of small dimeric proteins characterized by two EF hand type Ca2+ binding motifs which are flanked by unique N- and C-terminal regions. Although shown unequivocally in only a few cases S100 proteins are thought to function by binding to, and thereby regulating, cellular target proteins in a Ca2+ dependent manner. To describe for one member of the family, S100A1, structural requirements underlying target protein binding, we generated specifically mutated S100A1 derivatives and characterized their interaction with the alpha subunit of the actin capping protein CapZ shown here to represent a direct binding partner for S100A1. Chemical cross-linking, ligand blotting and fluorescence emission spectroscopy reveal that removal of, or mutations within, the sequence encompassing residues 88-90 in the unique C-terminal region of S100A1 interfere with binding to CapZ alpha and to TRTK-12, a synthetic CapZ alpha peptide. The S100A1 sequence identified contains a cluster of three hydrophobic residues (Phe-88, Phe-89 and Trp-90) at least one of which--as revealed by qualitative phenyl Sepharose binding and hydrophobic fluorescent probe spectroscopy--is exposed on the protein surface of Ca2+ bound S100A1. As homologous hydrophobic residues in the closely related S100B protein were shown by NMR spectroscopy of Ca(2+)-free S100B dimers to provide intersubunit contacts [Kilby P.M., van Eldik L.J., Roberts G.C.K. The solution structure of the bovine S100B dimer in the calcium-free state. Structure 1996; 4: 1041-1052; Drohat A.C., Amburgey J.C., Abildgaard F., Starich M.R., Baldisseri D., Weber D.J. Solution structure of rat apo-S100B (beta beta) as determined by NMR spectroscopy. Biochemistry 1996; 35: 11,577-11,588], we characterized the physical state of the various S100A1 derivatives. Analytical gel filtration and chemical cross-linking show that dimer formation is not compromised in S100A1 mutants lacking residues 88-90 or containing specific amino acid substitutions in this sequence. Thus a cluster of hydrophobic residues in the C-terminal region of S100A1 is essential for target protein binding but dispensable for dimerization, a situation possibly met in other S100 proteins as well.

Regulation of actin filament length in erythrocytes and striated muscle

Actin filaments polymerize in vitro to lengths which display an exponential distribution, yet in many highly differentiated cells they can be precisely maintained at uniform lengths in elaborate supramolecular structures. Recent results obtained using two classic model systems, the erythrocyte membrane cytoskeleton and the striated muscle sarcomere, reveal surprising similarities and instructive differences in the molecules and mechanisms responsible for determining and maintaining actin filament lengths in these two systems. Tropomodulin caps the slow-growing, pointed filament ends in muscle and in erythrocytes. CapZ caps the fast-growing, barbed filament ends in striated muscle, whereas a newly discovered barbed end capping protein, adducin, may cap the barbed filament ends in erythrocytes. The mechanisms responsible for specifying the characteristic filament lengths in these systems are more elusive and may include strict control of the relative amounts of actin filament capping proteins and side-binding proteins, molecular templates (e.g. tropomyosin and nebulin) and/or verniers (e.g. tropomyosin).