Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein

Cell processes require precise regulation of actin polymerization that is mediated by plus-end regulatory proteins. Detailed mechanisms that explain plus-end dynamics involve regulators with opposing roles, including factors that enhance assembly, e.g., the formin mDia1, and others that stop growth (Capping Protein, CPz). We explore IQGAP1's roles regulating actin filament plus-ends and the consequences of perturbing its activity in cells. We confirm that IQGAP1 pauses elongation and interacts with plus ends through two residues (C756 and C781). We directly visualize the dynamic interplay between IQGAP1 and mDia1, revealing that IQGAP1 displaces the formin to influence actin assembly. Using four-color TIRF we show that IQGAP1's displacement activity extends to formin-CPz 'decision complexes', promoting end-binding protein turnover at plus-ends. Loss of IQGAP1 or its plus-end activities disrupts morphology and migration, emphasizing its essential role. These results reveal a new role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how plus-end actin assembly is regulated in cells.

Purification of Human Cytoplasmic Actins From Saccharomyces cerevisiae

Eukaryotic cells rely on actin to support cellular structure, motility, transport, and a wide variety of other cytoplasmic functions and nuclear activities. Humans and other mammals express six closely related isoforms of actin, four of which are found primarily in skeletal, cardiac, and smooth muscle tissues. The final two isoforms, β and γ, are found in non-muscle cells. Due to the ease of purification, many biochemical studies surveying the functions of actin and its regulators have been carried out with protein purified from skeletal muscle. However, it has become increasingly clear that some activities are isoform specific, necessitating more accessible sources of non-muscle actin isoforms. Recent innovations permit the purification of non-muscle actins from human cell culture and heterologous systems, such as insect cell culture and the yeast Pichia pastoris. However, these systems generate mixtures of actin types or require additional steps to remove purification-related tags. We have developed strains of Saccharomyces cerevisiae (budding yeast) that express single untagged isoforms of either human non-muscle actin (β or γ) as their sole actin, allowing the purification of individual homogeneous actin isoforms by conventional purification techniques. Key features • Easy growth of yeast as a source of human cytoplasmic actin isoforms. Uses well-established actin purification methods. • The tag-free system requires no post-purification processing.

Purification of human β- and γ-actin from budding yeast

Biochemical studies of human actin and its binding partners rely heavily on abundant and easily purified α-actin from skeletal muscle. Therefore, muscle actin has been used to evaluate and determine the activities of most actin regulatory proteins but there is an underlying concern that these proteins perform differently from actin present in non-muscle cells. To provide easily accessible and relatively abundant sources of human β- or γ-actin (i.e. cytoplasmic actins), we developed Saccharomyces cerevisiae strains that express each as their sole source of actin. Both β- or γ-actin purified in this system polymerize and interact with various binding partners, including profilin, mDia1 (formin), fascin and thymosin-β4 (Tβ4). Notably, Tβ4 and profilin bind to β- or γ-actin with higher affinity than to α-actin, emphasizing the value of testing actin ligands with specific actin isoforms. These reagents will make specific isoforms of actin more accessible for future studies on actin regulation.

F1-ATPase of Escherichia coli: the ε- inhibited state forms after ATP hydrolysis, is distinct from the ADP-inhibited state, and responds dynamically to catalytic site ligands

F1-ATPase is the catalytic complex of rotary nanomotor ATP synthases. Bacterial ATP synthases can be autoinhibited by the C-terminal domain of subunit ε, which partially inserts into the enzyme's central rotor cavity to block functional subunit rotation. Using a kinetic, optical assay of F1·ε binding and dissociation, we show that formation of the extended, inhibitory conformation of ε (εX) initiates after ATP hydrolysis at the catalytic dwell step. Prehydrolysis conditions prevent formation of the εX state, and post-hydrolysis conditions stabilize it. We also show that ε inhibition and ADP inhibition are distinct, competing processes that can follow the catalytic dwell. We show that the N-terminal domain of ε is responsible for initial binding to F1 and provides most of the binding energy. Without the C-terminal domain, partial inhibition by the ε N-terminal domain is due to enhanced ADP inhibition. The rapid effects of catalytic site ligands on conformational changes of F1-bound ε suggest dynamic conformational and rotational mobility in F1 that is paused near the catalytic dwell position.

Stable preanaphase spindle positioning requires Bud6p and an apparent interaction between the spindle pole bodies and the neck

Faithful partitioning of genetic material during cell division requires accurate spatial and temporal positioning of nuclei within dividing cells. In Saccharomyces cerevisiae, nuclear positioning is regulated by an elegant interplay between components of the actin and microtubule cytoskeletons. Regulators of this process include Bud6p (also referred to as the actin-interacting protein Aip3p) and Kar9p, which function to promote contacts between cytoplasmic microtubule ends and actin-delimited cortical attachment points. Here, we present the previously undetected association of Bud6p with the cytoplasmic face of yeast spindle pole bodies, the functional equivalent of metazoan centrosomes. Cells lacking Bud6p show exaggerated movements of the nucleus between mother and daughter cells and display reduced amounts of time a given spindle pole body spends in close association with the neck region of budding cells. Furthermore, overexpression of BUD6 greatly enhances interactions between the spindle pole body and mother-bud neck in a spindle alignment-defective dynactin mutant. These results suggest that association of either spindle pole body with neck components, rather than simply entry of a spindle pole body into the daughter cell, provides a positive signal for the progression of mitosis. We propose that Bud6p, through its localization at both spindle pole bodies and at the mother-bud neck, supports this positive signal and provides a regulatory mechanism to prevent excessive oscillations of preanaphase nuclei, thus reducing the likelihood of mitotic delays and nuclear missegregation.

Role of a Cdc42p effector pathway in recruitment of the yeast septins to the presumptive bud site

The septins are GTP-binding, filament-forming proteins that are involved in cytokinesis and other processes. In the yeast Saccharomyces cerevisiae, the septins are recruited to the presumptive bud site at the cell cortex, where they form a ring through which the bud emerges. We report here that in wild-type cells, the septins typically become detectable in the vicinity of the bud site several minutes before ring formation, but the ring itself is the first distinct structure that forms. Septin recruitment depends on activated Cdc42p but not on the normal pathway for bud-site selection. Recruitment occurs in the absence of F-actin, but ring formation is delayed. Mutant phenotypes and suppression data suggest that the Cdc42p effectors Gic1p and Gic2p, previously implicated in polarization of the actin cytoskeleton, also function in septin recruitment. Two-hybrid, in vitro protein binding, and coimmunoprecipitation data indicate that this role involves a direct interaction of the Gic proteins with the septin Cdc12p.

A genetic dissection of Aip1p's interactions leads to a model for Aip1p-cofilin cooperative activities

Actin interacting protein 1 (Aip1p) and cofilin cooperate to disassemble actin filaments in vitro and are thought to promote rapid turnover of actin networks in vivo. The precise method by which Aip1p participates in these activities has not been defined, although severing and barbed-end capping of actin filaments have been proposed. To better describe the mechanisms and biological consequences of Aip1p activities, we undertook an extensive mutagenesis of AIP1 aimed at disrupting and mapping Aip1p interactions. Site-directed mutagenesis suggested that Aip1p has two actin binding sites, the primary actin binding site lies on the edge of its N-terminal beta-propeller and a secondary actin binding site lies in a comparable location on its C-terminal beta-propeller. Random mutagenesis followed by screening for separation of function mutants led to the identification of several mutants specifically defective for interacting with cofilin but still able to interact with actin. These mutants suggested that cofilin binds across the cleft between the two propeller domains, leaving the actin binding sites exposed and flanking the cofilin binding site. Biochemical, genetic, and cell biological analyses confirmed that the actin binding- and cofilin binding-specific mutants are functionally defective, whereas the genetic analyses further suggested a role for Aip1p in an early, internalization step of endocytosis. A complementary, unbiased molecular modeling approach was used to derive putative structures for the Aip1p-cofilin complex, the most stable of which is completely consistent with the mutagenesis data. We theorize that Aip1p-severing activity may involve simultaneous binding to two actin subunits with cofilin wedged between the two actin binding sites of the N- and C-terminal propeller domains.

Old yellow enzyme protects the actin cytoskeleton from oxidative stress

Old Yellow Enzyme (OYE) has long served as a paradigm for the study of flavin-containing NADPH oxido-reductases and yet its physiological role has remained a mystery. A two-hybrid interaction between Oye2p and actin led us to investigate a possible function in the actin cytoskeleton. We found that oye deletion strains have an overly elaborate actin cytoskeleton that cannot be attributed to changes in actin concentration but likely reflect stabilization of actin filaments, resulting in excessive actin assembly. Cells expressing the actin mutant act1-123p, which has a weakened interaction with Oye2p, show comparable defects in actin organization to the oye deletion strain that can be suppressed by overexpression of Oye2p. Similarly, mutation of either conserved cysteine of the potential disulfide pair Cys285-Cys374 in actin completely suppresses the actin organization defect of the oyeDelta phenotype. Strains lacking Oye function are also sensitive to oxidative stress as induced by H2O2, menadione, and diamide treatment. Mutation of either Cys285 or Cys374 of actin suppresses the sensitivity of oyeDelta strains to oxidative stress and in fact confers super-resistance to oxidative stress in otherwise wild-type strains. These results suggest that oxidative damage to actin, like that which has been observed in irreversibly sickled red blood cells, may be a general phenomenon and that OYE functions to control the redox state of actin thereby maintaining the proper plasticity of the actin cytoskeleton. In addition to uncovering a long sought biological function for Old Yellow Enzyme, these results establish that cellular sensitivity to oxidative stress can in part be directly attributed to a specific form (C285-C374 disulfide bond formation) of oxidative damage to actin.

A cytoskeletal localizing domain in the cyclase-associated protein, CAP/Srv2p, regulates access to a distant SH3-binding site

In the yeast, Saccharomyces cerevisiae, adenylyl cyclase consists of a 200-kDa catalytic subunit (CYR1) and a 70-kDa subunit (CAP/SRV2). CAP/Srv2p assists the small G protein Ras to activate adenylyl cyclase. CAP also regulates the cytoskeleton through an actin sequestering activity and is directed to cortical actin patches by a proline-rich SH3-binding site (P2). In this report we analyze the role of the actin cytoskeleton in Ras/cAMP signaling. Two alleles of CAP, L16P(Srv2) and R19T (SupC), first isolated in genetic screens for mutants that attenuate cAMP levels, reduced adenylyl cyclase binding, and cortical actin patch localization. A third mutation, L27F, also failed to localize but showed no loss of either cAMP signaling or adenylyl cyclase binding. However, all three N-terminal mutations reduced CAP-CAP multimer formation and SH3 domain binding, although the SH3-binding site is about 350 amino acids away. Finally, disruption of the actin cytoskeleton with latrunculin-A did not affect the cAMP phenotypes of the hyperactive Ras2(Val19) allele. These data identify a novel region of CAP that controls access to the SH3-binding site and demonstrate that cytoskeletal localization of CAP or an intact cytoskeleton per se is not necessary for cAMP signaling.

Polarity and division site specification in yeast

A significant component of polarization in budding yeast involves the regulated restructuring of the actin cytoskeleton in response to defined cellular signals. Recent evidence suggests that such cytoskeletal organization arises through the action of large protein complexes that form in response to signals from small GTP-binding proteins, such as Cdc42, Rho, and Ras. These actin-organizing complexes may be fairly diverse, but generally consist of one or more central scaffold proteins, such as those of the formin class, that bind to signaling molecules and recruit actin-binding proteins to bring about desired polarizing events.

Structure determination and characterization of Saccharomyces cerevisiae profilin

The structure of profilin from the budding yeast Saccharomyces cerevisiae has been determined by X-ray crystallography at 2.3 A resolution. The overall fold of yeast profilin is similar to the fold observed for other profilin structures. The interactions of yeast and human platelet profilins with rabbit skeletal muscle actin were characterized by titration microcalorimetry, fluorescence titrations, and nucleotide exchange kinetics. The affinity of yeast profilin for rabbit actin (2.9 microM) is approximately 30-fold weaker than the affinity of human platelet profilin for rabbit actin (0.1 microM), and the relative contributions of entropic and enthalpic terms to the overall free energy of binding are different for the two profilins. The titration of pyrene-labeled rabbit skeletal actin with human profilin yielded a Kd of 2.8 microM, similar to the Kd of 2.0 microM for the interaction between yeast profilin and pyrene-labeled yeast actin. The binding data are discussed in the context of the known crystal structures of profilin and actin, and the residues present at the actin-profilin interface. The affinity of yeast profilin for poly-L-proline was determined from fluorescence measurements and is similar to the reported affinity of Acanthamoeba profilin for poly-L-proline. Yeast profilin was shown to catalyze adenine nucleotide exchange from yeast actin almost 2 orders of magnitude less efficiently than human profilin and rabbit skeletal muscle actin. The in vivo and in vitro properties of yeast profilin mutants with altered poly-L-proline and actin binding sites are discussed in the context of the crystal structure.

Profilin is required for the normal timing of actin polymerization in response to thermal stress

We have used a fluorometric assay to determine the relative amounts of polymerized actin (F-actin) in wild-type and profilin mutant yeast cells. Our results indicate that profilin plays a role in maintaining normal F-actin levels in response to shifts to high temperature. Cells lacking profilin display a greater drop in F-actin levels upon such temperature shifts, and are slower to recover to initial F-actin levels than are wild-type cells. Interestingly, shifts to cold temperatures result in rapid increases of F-actin levels in wild-type and profilin null cells. We have further determined that shifting to high-osmolarity growth conditions causes a relatively slow decrease in F-actin levels in wild-type cells, and a small but rapid increase in the F-actin levels in profilin null cells. Profilin null cells contain normal concentrations of F-actin while growing exponentially at room temperature, indicating that profilin is not essential for maintaining F-actin concentrations during steady-state growth. Our data suggest that actin is inherently unstable in vivo at high temperatures, and that profilin helps to maintain actin in its filamentous state at these temperatures, perhaps by stimulating actin polymerization in a proper temporal and spatial fashion.

SEC3 mutations are synthetically lethal with profilin mutations and cause defects in diploid-specific bud-site selection

Replacement of the wild-type yeast profilin gene (PFY1) with a mutated form (pfy1-111) that has codon 72 changed to encode glutamate rather than arginine results in defects similar to, but less severe than, those that result from complete deletion of the profilin gene. We have used a colony color-sectoring assay to identify mutations that cause pfy1-111, but not wild-type, cells to be inviable. These profilin synthetic lethal (psl) mutations result in various degrees of abnormal growth, morphology, and temperature sensitivity in PFY1 cells. We have examined psl1 strains in the most detail. Interestingly, these strains display a diploid-specific defect in bud-site selection; haploid strains bud normally, while homozygous diploid strains show a dramatic increase in random budding. We discovered that PSL1 is the late secretory gene, SEC3, and have found that mutations in several other late secretory genes are also synthetically lethal with pfy1-111. Our results are likely to reflect an interdependence between the actin cytoskeleton and secretory processes in directing cell polarity and growth. Moreover, they indicate that the secretory pathway is especially crucial for maintaining budding polarity in diploids.

Identification of MYO4, a second class V myosin gene in yeast

We have isolated a fourth myosin gene (MYO4) in yeast (Saccharomyces cerevisiae). MYO4 encodes a approximately 170 kDa (1471 amino acid) class V myosin, using the classification devised by Cheney et al. (1993a; Cell Motil. Cytoskel. 24, 215–223); the motor domain is followed by a neck region containing six putative calmodulin-binding sites and a tail with a short potential ‘coiled-coil’ domain. A comparison with other myosins in GenBank reveals that Myo4 protein is most closely related to the yeast Myo2 protein, another class V myosin. Deletion of MYO4 produces no detectable phenotype, either alone or in conjunction with mutations in myo2 or other myosin genes, the actin gene, or secretory genes. However, overexpression of MYO4 or MYO2 results in several morphological abnormalities, including the formation of short strings of unseparated cells in diploid strains, or clusters of cells in haploid strains. Alterations of MYO4 or MYO2 indicate that neither the motor domains nor tails of these myosins are required to confer the overexpression phenotype, whereas the neck region may be required. Although this phenotype is similar to that seen upon MYO1 deletion, we provide evidence that the overexpression of Myo4p or Myo2p is not simply interfering with Myo1p function.

Mutational analysis of yeast profilin

We have mutated two regions within the yeast profilin gene in an effort to functionally dissect the roles of actin and phosphatidylinositol 4,5-bisphosphate (PIP2) binding in profilin function. A series of truncations was carried out at the C terminus of profilin, a region that has been implicated in actin binding. Removal of the last three amino acids nearly eliminated the ability of profilin to bind polyproline in vitro but had no dramatic in vivo effects. Thus, the extreme C terminus is implicated in polyproline binding, but the physiological relevance of this interaction is called into question. More extensive truncation, of up to eight amino acids, had in vivo effects of increasing severity and resulted in changes in conformation and expression level of the mutant profilins. However, the ability of these mutants to bind actin in vitro was not eliminated, suggesting that this region cannot be solely responsible for actin binding. We also mutagenized a region of profilin that we hypothesized might be involved in PIP2 binding. Alteration of basic amino acids in this region produced mutant profilins that functioned well in vivo. Many of these mutants, however, were unable to suppress the loss of adenylate cyclase-associated protein (Cap/Srv2p [A. Vojtek, B. Haarer, J. Field, J. Gerst, T. D. Pollard, S. S. Brown, and M. Wigler, Cell 66:497-505, 1991]), indicating that a defect could be demonstrated in vivo. In vitro assays demonstrated that the inability to suppress loss of Cap/Srv2p correlated with a defect in the interaction with actin, independently of whether PIP2 binding was reduced. Since our earlier studies of Acanthamoeba profilins suggested the importance of PIP2 binding for suppression, we conclude that both activities are implicated and that an interplay between PIP2 binding and actin binding may be important for profilin function.

Cellular morphogenesis in the Saccharomyces cerevisiae cell cycle: localization of the CDC3 gene product and the timing of events at the budding site

Budding cells of the yeast Saccharomyces cerevisiae possess a ring of 10-nm-diameter filaments, of unknown biochemical nature, that lies just inside the plasma membrane in the neck connecting the mother cell to its bud. Electron microscopic observations suggest that these filaments assemble at the budding site coincident with bud emergence and disassemble shortly before cytokinesis (Byers, B. and L. Goetsch. 1976. J. Cell Biol. 69:717-721). Mutants defective in any of four genes (CDC3, CDC10, CDC11, or CDC12) lack these filaments and display a pleiotropic phenotype that involves abnormal bud growth and an inability to complete cytokinesis. We showed previously by immunofluorescence that the CDC12 gene product is probably a constituent of the ring of 10-nm filaments (Haarer, B. and J. Pringle. 1987. Mol. Cell. Biol. 7:3678-3687). We now report the use of fusion proteins to generate polyclonal antibodies specific for the CDC3 gene product. In immunofluorescence experiments, these antibodies decorated the neck regions of wild-type and mutant cells in patterns suggesting that the CDC3 gene product is also a constituent of the ring of 10-nm filaments. We also used the CDC3-specific and CDC12-specific antibodies to investigate the timing of localization of these proteins to the budding site. The results suggest that the CDC3 protein is organized into a ring at the budding site well before bud emergence and remains so organized for some time after cytokinesis. The CDC12 product appears to behave similarly, but may arrive at the budding site closer to the time of bud emergence, and disappear from that site more quickly after cytokinesis, than does the CDC3 product. Examination of mating cells and cells responding to purified mating pheromone revealed novel arrangements of the CDC3 and CDC12 products in the regions of cell wall reorganization. Both proteins were present in normal-looking ring structures at the bases of the first zygotic buds.

Purification of profilin from Saccharomyces cerevisiae and analysis of profilin-deficient cells

We have isolated profilin from yeast (Saccharomyces cerevisiae) and have microsequenced a portion of the protein to confirm its identity; the region microsequenced agrees with the predicted amino acid sequence from a profilin gene recently isolated from S. cerevisiae (Magdolen, V., U. Oechsner, G. Müller, and W. Bandlow. 1988. Mol. Cell. Biol. 8:5108-5115). Yeast profilin resembles profilins from other organisms in molecular mass and in the ability to bind to polyproline, retard the rate of actin polymerization, and inhibit hydrolysis of ATP by monomeric actin. Using strains that carry disruptions or deletions of the profilin gene, we have found that, under appropriate conditions, cells can survive without detectable profilin. Such cells grow slowly, are temperature sensitive, lose the normal ellipsoidal shape of yeast cells, often become multinucleate, and generally grow much larger than wild-type cells. In addition, these cells exhibit delocalized deposition of cell wall chitin and have dramatically altered actin distributions.

Immunofluorescence localization of the Saccharomyces cerevisiae CDC12 gene product to the vicinity of the 10-nm filaments in the mother-bud neck

Budding cells of the yeast Saccharomyces cerevisiae possess a ring of 10-nm-diameter filaments, of unknown biochemical nature, that lies just inside the plasma membrane in the neck connecting the mother cell to its bud (B. Byers and L. Goetsch, J. Cell Biol. 69:717-721, 1976). Mutants defective in any of four genes (CDC3, CDC10, CDC11, and CDC12) lack these filaments and display a pleiotropic phenotype that involves abnormal bud growth and cell-wall deposition and an inability to complete cytokinesis. We fused the cloned CDC12 gene to the Escherichia coli lacZ and trpE genes and used the resulting fusion proteins to raise polyclonal antibodies specific for the CDC12 gene product. In immunofluorescence experiments with affinity-purified antibodies, the neck region of wild-type and mutant cells stained in patterns consistent with the hypothesis that the CDC12 gene product is a constituent of the ring of 10-nm filaments. Without careful affinity purification of the CDC12-specific antibodies, these staining patterns were completely obscured by the staining of residual cell wall components in the neck by antibodies present even in the "preimmune" sera of all rabbits tested.