The structure of nonvertebrate actin: implications for the ATP hydrolytic mechanism

The structures of Saccharomyces cerevisiae, Dictyostelium, and Caenorhabditis elegans actin bound to gelsolin segment-1 have been solved and refined at resolutions between 1.9 and 1.75 A. These structures reveal several features relevant to the ATP hydrolytic mechanism, including identification of the nucleophilic water and the roles of Gln-137 and His-161 in positioning and activating the catalytic water, respectively. The involvement of these residues in the catalytic mechanism is consistent with yeast genetics studies. This work highlights both structural and mechanistic similarities with the small and trimeric G proteins and restricts the types of mechanisms responsible for the considerable enhancement of ATP hydrolysis associated with actin polymerization. The conservation of functionalities involved in nucleotide binding and catalysis also provide insights into the mechanistic features of members of the family of actin-related proteins.

F-actin-like ATPase activity in a polymerization-defective mutant yeast actin (V266G/L267G)

Polymerization increases a low level G-actin ATPase activity yielding ADP-P(i) F-actin and then ADP F-actin following release of P(i). By monitoring P(i) release, we explored the relationship between the ATPase activity and polymerization characteristics of a mutant yeast actin, GG. In this mutant, two hydrophobic residues at the tip of a proposed hydrophobic plug between actin subdomains 3 and 4, Val(266) and Leu(267), were mutated to Gly. Although GG-actin does not polymerize by itself in vitro, GG cells are viable. We show that GG-actin ATPase activity increases under normal polymerization conditions, although stable filaments do not form. A plot of P(i) release rate versus actin concentration yields an apparent critical concentration, like that seen for actin polymerization, of approximately 8 microm for Mg(2+) GG-actin and 11 microm for Ca(2+) GG-actin. In contrast to WT-actin, P(i) release from GG-actin is cold-sensitive, reflecting the temperature sensitivity associated with mutations that decrease hydrophobicity in this region. Thus, under polymerization conditions, GG-actin exhibits a continuous F-actin-like ATPase activity resulting from the temperature-sensitive formation of unstable cycling F-actin oligomers. Tropomyosin limits the extent and rate of this activity and restores polymerization by capturing and stabilizing these oligomers rather than enhancing filament nucleation.

Tropomyosin-dependent filament formation by a polymerization-defective mutant yeast actin (V266G,L267G)

A major function of tropomyosin (TPM) in nonmuscle cells may be stabilization of F-actin by binding longitudinally along the actin filament axis. However, no clear evidence exists in vitro that TPM can significantly affect the critical concentration of actin. We previously made a polymerization-defective mutant actin, GG (V266G, L267G). This actin will not polymerize alone at 25 degrees C but will in the presence of phalloidin or beryllium fluoride. With beryllium fluoride, but not phalloidin, this polymerization rescue is cold-sensitive. We show here that GG-actin polymerizability was restored by cardiac tropomyosin and yeast TPM1 and TPM2 at 25 degrees C with rescue efficiency inversely proportional to TPM length (TPM2 > TPM1 > cardiac tropomyosin), indicating the importance of the ends in polymerization rescue. In the presence of TPM, the apparent critical concentration of actin is 5.5 microm, 10-15-fold higher than that of wild type actin but well below that of the GG-actin alone (>20 microm). Non N-acetylated TPMs did not rescue GG-actin polymerization. The TPMs did not prevent cold-induced depolymerization of GG F-actin. TPM-dependent GG-actin polymerization did not occur at temperatures below 20 degrees C. Polymerization rescue may depend initially on the capture of unstable GG-F-actin oligomers by the TPM, resulting in the strengthening of actin monomer-monomer contacts along the filament axis.

Interaction of the gonococcal porin P.IB with G- and F-actin

The invasion of epithelial cells by N. gonorrheae is accompanied by formation of a halo of actin filaments around the enveloped bacterium. The transfer of the bacterial major outer membrane protein, porin, to the host cell membrane during invasion makes it a candidate for a facilitator for the formation of this halo. Western analysis shows here that gonococcal porin P.IB associates with the actin cytoskeleton in infected cells. Using the pyrene-labeled Mg forms of yeast and muscle actins, we demonstrate that under low ionic strength conditions, P.IB causes formation of filamentous actin assemblies, although they, unlike F-actin, cannot be internally cross-linked with N,N'-4-phenylenedimaleimide (PDM). In F-buffer, low porin concentrations appear to accelerate actin polymerization. Higher P.IB concentrations lead to the formation of highly decorated fragmented F-actin-like filaments in which the actin can be cross-linked by PDM. Co-assembly of P.IB with a pyrene-labeled mutant actin, S(265)C, prevents formation of a pyrene excimer present with labeled S(265)C F-actin alone. Addition of low concentrations of porin to preformed F-actin results in sparsely decorated F-actin. Higher P.IB concentrations extensively decorate the filaments, thereby altering their morphology to a state like that observed when the components are copolymerized. With preformed labeled S(265)C F-actin, P.IB quenches the pyrene excimer. This decrease is prevented by the F-actin stabilizers phalloidin and to a lesser extent beryllium fluoride. P.IB's association with the actin cytoskeleton and its ability to interact with and remodel actin filaments support a direct role for porin in altering the host cell cytoskeleton during invasion.

Cross-linking constraints on F-actin structure

The DNase I binding loop (residues 38-52), the hydrophobic plug (residues 262-274), and the C terminus region are among the structural elements of monomeric (G-) actin proposed to form the intermonomer interface in F-actin. To test the proximity and interactions of these elements and to provide constraints on models of F-actin structure, cysteine residues were introduced into yeast actin either at residue 41 or 265. These mutations allowed for specific cross-linking of F-actin between C41 and C265, C265 and C374, and C41 and C265 using dibromobimane and disulfide bond formation. The cross-linked products were visualized on SDS-PAGE and by electron microscopy. Model calculations carried out for the cross-linked F-actins revealed that considerable flexibility or displacement of actin residues is required in the disulfide cross-linked segments to fit these filaments into model F-actin structures. The calculated, cross-linked structures showed a better fit to the Holmes rather than the refined Lorenz model of F-actin. It is predicted on the basis of such calculations that image reconstruction of electron micrographs of disulfide cross-linked C41-C374 F-actin should provide a conclusive test of these two similar models of F-actin structure.

Structural transition at actin's N-terminus in the actomyosin cross-bridge cycle

Force and motion generation by actomyosin involves the cyclic formation and transition between weakly and strongly bound complexes of these proteins. Actin's N-terminus is believed to play a greater role in the formation of the weakly bound actomyosin states than in the formation of the strongly bound actomyosin states. It has been the goal of this project to determine whether the interaction of actin's N-terminus with myosin changes upon transition between these two states. To this end, a yeast actin mutant, Cys-1, was constructed by the insertion of a cysteine residue at actin's N-terminus and replacement of the C-terminal cysteine with alanine. The N-terminal cysteine was labeled stoichiometrically with pyrene maleimide, and the properties of the modified mutant actin were examined prior to spectroscopic measurements. Among these properties, actin polymerization, strong S1 binding, and the activation of S1 ATPase by pyrenyl-Cys-1 actin were not significantly different from those of wild-type yeast actin, while small changes were observed in the weak S1 binding and the in vitro motility of actin filaments. Fluorescence changes upon binding of S1 to pyrenyl-Cys-1 actin were measured for the strongly (with or without ADP) and weakly (with ATP and ATPgammaS) bound acto-S1 states. The fluorescence increased in each case, but the increase was greater (by about 75%) in the presence of MgATP and MgATPgammaS than in the rigor state. This demonstrates a transition at the S1 contact with actin's N-terminus between the weakly and strongly bound states, and implies either a closer proximity of the pyrene probe on Cys-1 to structural elements on S1 (most likely the loop of residues 626-647) or greater S1-induced changes at the N-terminus of actin in the weakly bound acto-S1 states.

His(73), often methylated, is an important structural determinant for actin. A mutagenic analysis of HIS(73) of yeast actin

His(73), has been proposed to regulate the release of P(i) from the interior of actin following polymerization-dependent hydrolysis of bound ATP. Although it is a 3-methylhistidine in the vast majority of actins, His(73) is unmethylated in S. cerevisiae actin. We mutated His(73) in yeast actin to Arg, Lys, Ala, Gln, and Glu and detected no altered phenotypes associated with the mutations in vivo. However, they significantly affect actin function in vitro. Substitution of the more basic residues resulted in enhanced thermal stability, decreased rate of nucleotide exchange, and decreased susceptibility to controlled proteolysis relative to wild-type actin. The opposite effects are observed with the neutral and anionic substitutions. All mutations reduced the rate of polymerization. Molecular dynamics simulations predict a new conformation for the His(73) imidazole in the absence of a methyl group. It also predicts that Arg(73) tightens and stabilizes the actin and that Glu(73) causes a rearrangement of the bottom of actin's interdomain cleft leading possibly to our observed destabilization of actin. Considering the exterior location of His(73), this work indicates a surprisingly important role for the residue as a major structural determinant of actin and provides a clue to the impact caused by methylation of His(73).

Interaction in vivo and in vitro between the yeast fimbrin, SAC6P, and a polymerization-defective yeast actin (V266G and L267G)

A mutant yeast actin (GG) has decreased hydrophobicity in a subdomain 3/4 hydrophobic plug believed to be involved in a hydrophobic cross-strand "plug-pocket" interaction necessary for actin filament stability. This actin will not polymerize in vitro but is compatible with cell viability. We have assessed the ability of Sac6p, the yeast homologue of the actin filament stabilizing and bundling protein fimbrin, to restore polymerization in vitro and to facilitate GG-actin function in vivo. Sac6p rescues GG-actin polymerization at 25 degrees C but not at 4 degrees C. The actin polymerizes into bundles at room temperature with a fimbrin:actin molar ratio of 1:4. At this ratio, every actin monomer contacts a Sac6p actin binding domain. Following cold-induced depolymerization, actin/Sac6p mixtures repolymerize beginning at 15 degrees C instead of the 25 degrees C required for de novo assembly, because of the presence of residual actin-Sac6p nuclei. Generation of haploid Deltasac6/GG-actin cells from either diploid or haploid cells was unsuccessful. The facile isolation of cells with either mutation alone indicates a synthetic lethal relationship between this actin allele and the SAC6 gene. Sac6p may allow GG-actin function in vivo by stabilizing the actin in bundles thereby helping maintain sufficient levels of an otherwise destabilized actin monomer within the cell.

A highly conserved 3-methylhistidine modification is absent in yeast actin

To identify a protein histidine methyltransferase from Saccharomyces cerevisiae, we examined purified actin for the presence of the highly conserved 3-methylhistidine residue at position 73 by amino acid analysis of the whole protein and by amino acid analysis and mass spectrometry of the corresponding tryptic fragment. Surprisingly, we found that His-73 is not modified. A similar lack of modification was also found in actin from the yeast Candida albicans, while rabbit muscle actin revealed the expected 3-methylhistidine residue. Phylogenetic analysis of actin sequences suggests that this modification was introduced in evolution after the divergence of yeast from higher eukaryotic organisms, including unicellular eukaryotes such as Acanthamoeba, Dictyostelium, and Physarum, whose actins contain 3-methylhistidine. Our methodology for the analytical determination of 3-methylhistidine in actin offers an improved approach for investigating histidine methylation in proteins.

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.

Modulation of yeast F-actin structure by a mutation in the nucleotide-binding cleft

Although the actin sequence is very highly conserved across evolution, tissue-specific expression of different isoforms in high eukaryotes suggests that different isoforms carry out different functions. However, little information exists about either the differences in filaments made from different actins or the effects on filament structure caused by the various mutations in actin that have been introduced to gain insight into actin function. Using electron microscopy and three-dimensional reconstruction, we have studied the differences in the filaments made by yeast and rabbit skeletal muscle actin, two proteins with 88% homologous sequences, and we have assessed the changes in filament structure caused by the introduction of the S14A mutation into yeast actin. Elimination of the S14 hydroxyl group, assumed to bind to the gamma-phosphate of actin-bound ATP, results in a 40 to 60-fold decrease in actin's affinity for ATP. We show that yeast actin displays less extensive contacts between the two long-pitch helical strands than does muscle actin, and displays the large cooperativity within filaments previously observed for muscle actin. Finally, we demonstrate that the S14A mutation narrows the cleft between the two lobes of the actin subunit and strengthens the inter-strand connections in F-actin.

Fluorescence probing of yeast actin subdomain 3/4 hydrophobic loop 262-274. Actin-actin and actin-myosin interactions in actin filaments

Residues 262-274 form a loop between subdomains 3 and 4 of actin. This loop may play an important role in actin filament formation and stabilization. To assess directly the behavior of this loop, we mutated Ser265 of yeast actin to cysteine (S265C) and created another mutant (S265C/C374A) by changing Cys374 of S265C actin to alanine. These changes allowed us to attach a pyrene maleimide stoichiometrically to either Cys374 or Cys265. These mutations had no detectable effects on the protease susceptibility, intrinsic ATPase activity, and thermal stability of labeled or unlabeled G-actin. The presence of the loop cysteine, either labeled or unlabeled, did not affect the actin-activated S1 ATPase activity or the in vitro motility of the actin. Both mutant actins, either labeled or unlabeled, nucleated filament formation considerably faster than wild-type (WT) actin, although the critical concentration was not affected. Whereas the fluorescence of the C-terminal (WT) probe increased during polymerization, that of the loop (S265C/C374A) probe decreased, and the fluorescence of the doubly labeled actin (S265C) was approximately 50% less than the sum of the fluorescence of the individual fluorophores. Quenching was also observed in copolymers of labeled WT and S265C/C374A actins. An excimer peak was present in the emission spectrum of labeled S265C F-actin and in the labeled S265C/C374A-WT actin copolymers. These results show that in the filaments, the C-terminal pyrene of a substantial fraction of monomers directly interacts with the loop pyrene of neighboring monomers, bringing the two cysteine sulfurs to within 18 A of one another. Finally, when bound to labeled S265C/C374A F-actin, myosin S1, but not tropomyosin, caused an increase in fluorescence of the loop probe. Both proteins had no effect on excimer fluorescence. These results help establish the orientation of monomers in F-actin and show that the binding of S1 to actin subdomains 1 and 2 affects the environment of the loop between subdomains 3 and 4.

The effects of severely decreased hydrophobicity in a subdomain 3/4 loop on the dynamics and stability of yeast G-actin

The hydrophobicity of the subdomain 3/4 hydrophobic loop (262-274) has been implicated to be essential for actin's function. We previously showed (Kuang, B., and Rubenstein, P. A. (1997) J. Biol. Chem. 272, 1237-1247) that a mutant yeast actin (V266G/L267G) with markedly decreased hydrophobicity in this loop conferred severe cold sensitivity to its polymerization. Here we further tested the mutational effect on the conformation and function of G-actin. This GG mutation caused no significant changes in overall secondary structure or in the microenvironment around actin's tryptophan residues, nor did it alter the dissociation constant of G-actin for ATP. However, it lowers the intrinsic ATPase activity and the melting temperature for Mg-GG actin from 51 to 33 degrees C and transforms the conformation of subdomain 2 and the central cleft of G-actin into an F-monomer-like structure. The results suggest that the hydrophobic plug may not only play a role in actin filament stabilization but also may be important for controlling the stability of G-actin and for promoting the conformational change of the monomer needed for addition to a growing actin filament.

Beryllium fluoride and phalloidin restore polymerizability of a mutant yeast actin (V266G,L267G) with severely decreased hydrophobicity in a subdomain 3/4 loop

Holmes proposed that in F-actin, hydrophobic residues in a subdomain 3/4 loop interact with a hydrophobic pocket on the opposing strand resulting in helix stabilization. We have determined how a decreased hydrophobicity of this plug affects yeast actin function. Cells harboring only the V266G, V266D, V266F, L267G, L269D, or L269K actins appear normal, although V266G cells display an altered budding pattern. However, V266G,L267G (GG) double mutant cells are cold-sensitive with randomly oriented thick actin assemblies seen in rhodamine phalloidin-stained GG cells. V266D actin polymerizes slower than wild-type actin at room temperature. At 4 degrees C, not only is polymerization slowed, but there is also an effect on critical concentration. However, the polymerization defects are milder than those associated with substitution of Asp for the neighboring Leu267. Purified GG-actin does not polymerize in vitro alone or in the presence of wild-type F-actin seeds. GG-actin polymerization can be restored by larger amounts of wild-type actin, beryllium fluoride, or phalloidin at room temperature, although at 4 degrees C only phalloidin is effective. These results suggest that the diminished hydrophobicity of the plug in GG-actin leads to filament destabilization. However, the V266D actin results require a modification of the original Holmes filament model.

Mutational analysis of the role of the N terminus of actin in actomyosin interactions. Comparison with other mutant actins and implications for the cross-bridge cycle

Yeast actin mutants with acidic residues at the N terminus either neutralized (DNEQ) or deleted (delta-DSE) were used to assess the role of N-terminal acidic residues in the interactions of actin with myosin in the contractile cycle. Cosedimentation experiments revealed an approximately 3-fold decrease in the binding constant for DNEQ and delta-DSE actins to myosin subfragment-1 (S1) relative to that of wild type actin both in the presence of MgATP and in the absence of nucleotides (strong binding). DNEQ and delta-DSE actins protected S1 from tryptic digestion as well as the wild type and rabbit actins. The activation of S1 ATPase by DNEQ and delta-DSE actins (up to 50 microM) was very low but increased greatly after cross-linking these mutant actins to S1 by dimethyl suberimidate. Thus, the increased dissociation of mutant actins from S1 in the presence of ATP is the main cause for the low acto-S1 ATPase activities. At low-ionic strength conditions and in the presence of methylcellulose, the DNEQ and delta-DSE actins moved in the in vitro motility assays at a mean velocity similar to that of wild type actin (3.0 microns/s). Yet, the sliding velocity of the N-terminal and D24A/D25A and E99A/E100A mutant actins decreased relative to that of the wild type at all levels of external load introduced into the assay and at low densities of heavy meromyosin (HMM) on the cover slip. This indicates a lower relative force generation with the mutant actins. In contrast, the force generated under the same conditions with the 4Ac mutant actin (with four acidic charges at the N terminus) was higher than with wild type actin. At higher-ionic strength conditions (I = 150 mM), the sliding of the DNEQ and delta-DSE as well as that of the D24A/D25A and E99A/E100A actins ceased even in the presence of methylcellulose, while I341A actin (deficient in strong binding to myosin) still moved. These results indicate the importance of electrostatic actomyosin interactions under physiological salt conditions and show functionally distinct roles for the different myosin binding sites on actin.

The effect of the S14A mutation on the conformation and thermostability of Saccharomyces cerevisiae G-actin and its interaction with adenine nucleotides

The actin Ser14 hydroxyl is one of a number of ligands that binds to the gamma-phosphate of ATP thereby stabilizing the actin.ATP complex. In yeast actin, conversion of Ser14 to Ala (S14A), causes a temperature-sensitive phenotype in vivo and temperature-sensitive polymerization defects in vitro (Chen, X., and Rubenstein, P. A. (1995) J. Biol. Chem. 270, 11406-11414). Here, using a new luciferase-based procedure, we show that the mutation results in a 40-60-fold decrease in actin's affinity for ATP. The mutation causes a decrease in the intrinsic ATPase activity of both Ca- and Mg-G-actin at 30 degrees C and alters the protease susceptibility of sites on subdomain 2. Ca-S14A-actin but not Mg-S14A-actin binds etheno-ATP at 37 degrees C. Intrinsic tryptophan fluorescence measurements show that at 37 degrees C, Mg-S14A-actin but not the calcium form unfolds. CD measurements show the mutation causes a decrease in the apparent denaturation temperature for Ca-actin from 57 to 45 degrees C and for the magnesium form a decrease from 52 to 40 degrees C. Based on a re-examination of actin's crystal structure coordinates, we propose that the Ser14 hydroxyl forms a polar bridge between the ATP gamma-phosphate and the amide nitrogen of Gly74, thus conferring additional stability on the actin small domain.

A mutation in an ATP-binding loop of Saccharomyces cerevisiae actin (S14A) causes a temperature-sensitive phenotype in vivo and in vitro

The Ser14 hydroxyl group of actin is one of six groups that potentially form hydrogen bonds with the gamma-phosphate of the ATP bound in the cleft separating the two domains of the protein. To understand the importance of this group in actin function, we mutated Ser14 of Saccharomyces cerevisiae actin and studied the effects of these mutations in vivo and in vitro. Substitution of Cys of Gly resulted in cell death. Substitution of Thr for Ser resulted in an actin with wild-type properties in vivo and in vitro. Cells carrying the Ser14-->Ala (S14A) mutation were viable but displayed a temperature sensitive lethality at 37 degrees C preceded by delocalization of actin patches, the appearance of bar-like structures, and finally the disappearance of identifiable actin structures. The mutation caused no effect on the critical concentration of polymerization but resulted in an actin with an increased rate of polymerization, an altered protease susceptibility, and a decreased filament ATPase activity. At 37 degrees C, Mg-, but not Ca-S14A-actin irreversibly lost the ability to polymerize. These results demonstrate the importance of the ATP-Ser14 hydroxyl hydrogen bond in regulating actin function in vivo and in vitro and the magnification of the effects of the mutation when Mg2+ is substituted for Ca2+ in the protein.

Caldesmon, N-terminal yeast actin mutants, and the regulation of actomyosin interactions

N-Terminal yeast actin mutants were used to assess the role of N-terminal acidic residues in the interactions of caldesmon with actin. The yeast actins differed only in their N-terminal charge: wild type, two negative charges; 4Ac, four negative charges; DNEQ, neutral charge; delta DSE, one positive charge. Caldesmon inhibition of actomyosin subfragment 1 ATPase was affected by alterations in the N-terminus of actin. This inhibition was similar for skeletal muscle alpha-actin and the yeast 4Ac and wild-type actins (80%), but much smaller for the neutral and deletion mutants (15%). However, cosedimentation experiments revealed similar binding of caldesmon to polymerized rabbit skeletal muscle alpha-actin and each yeast actin. This result shows that the N-terminal acidic residues of actin are not required for the binding of caldesmon to F-actin. Caldesmon-actin interactions were also examined by monitoring the polymerization of G-actin induced by caldesmon. Although the final extent of polymerization was similar for all actins tested, the rates of polymerization differed. Skeletal muscle and 4Ac actins had similar rates of polymerization, and the wild-type actin polymerized at a slower rate. The neutral and deletion mutants had even slower rates of polymerization by caldesmon. The slow polymerization of DNEQ G-actin was traced to a greatly reduced binding of caldesmon to this mutant G-actin when compared to wild-type and alpha-actin. MgCl2-induced actin polymerization proceeded at identical rates for all actins.(ABSTRACT TRUNCATED AT 250 WORDS)

Yeast actin with a mutation in the "hydrophobic plug" between subdomains 3 and 4 (L266D) displays a cold-sensitive polymerization defect

Holmes et al. (Holmes, K. C., D. Popp, W. Gebhard, and W. Kabsch. 1990. Nature [Lond.] 347: 44-49) hypothesized that between subdomains 3 and 4 of actin is a loop of 10 amino acids including a four residue hydrophobic plug that inserts into a hydrophobic pocket formed by two adjacent monomers on the opposing strand thereby stabilizing the F-actin helix. To test this hypothesis we created a mutant yeast actin (L266D) by substituting Asp for Leu266 in the plug to disrupt this postulated hydrophobic interaction. Haploid cells expressing only this mutant actin were viable with no obvious altered phenotype at temperatures above 20 degrees C but were moderately cold-sensitive for growth compared with wild-type cells. The critical concentration for polymerization increased 10-fold at 4 degrees C compared with wild-type actin. The length of the nucleation phase of polymerization increased as the temperature decreased. At 4 degrees C nucleation was barely detectable. Addition of phalloidin-stabilized F-actin nuclei and phalloidin restored L266D actin's ability to polymerize at 4 degrees C. This mutation also affects the overall rate of elongation during polymerization. Small effects of the mutation were observed on the exchange rate of ATP from G-actin, the G-actin intrinsic ATPase activity, and the activation of myosin S1 ATPase activity. Circular dichroism measurements showed a 15 degrees C decrease in melting temperature for the mutant actin from 57 degrees C to 42 degrees C. Our results are consistent with the model of Holmes et al. (Holmes, K. C., D. Popp, W. Gebhard, and W. Kabsch. 1990. Nature [Lond.]. 347:44-49) involving the role of the hydrophobic plug in actin filament stabilization.

Enhanced stimulation of myosin subfragment 1 ATPase activity by addition of negatively charged residues to the yeast actin NH2 terminus

We examined the effects of yeast actin NH2-terminal mutations on actomyosin interactions and the function of actin in vivo through measurements of actin-activated ATPase activity, cosedimentation with rabbit muscle myosin subfragment 1 (S-1), in vitro motility, and invertase secretion assays. As reported earlier (Cook, R. K., Blake, W., and Rubenstein, P. A. (1992) J. Biol. Chem. 267, 9430-9436), elimination of NH2-terminal acidic residues from yeast actin results in an increased actin bundling, decreased actin-activated S-1 ATPase, and complete inhibition of actin filament sliding over myosin. Here we show that the addition of 2 new acidic residues to the NH2 terminus of yeast actin increased the Vmax value and the catalytic efficiency of the actin-activated ATPase activity of S-1. However, the binding of actin to S-1 in the presence of ATP and the velocities of actin sliding over myosin in the in vitro motility assays were not affected by this mutation. Thus, the number of actin NH2-terminal negative charges is important for actin activation of myosin S-1 ATPase activity, while only a minimum number of acidic residues is required for actin sliding over myosin in vitro. The number of actin NH2-terminal negative charges therefore appears to determine the efficiency with which the energy from ATP hydrolysis is converted to filament sliding.

Control of profilin and actin expression in muscle and nonmuscle cells

Profilin is a small G-actin binding protein implicated in sequestering actin monomers in vivo. We have quantitated profilin and actin expression in human hepatoma HepG-2 cells and in two mouse myogenic cell lines, BC3H1 and C2C12, to determine whether the expression of profilin and the expression of nonmuscle isoactin or total actin are co-regulated. During differentiation of both muscle cell types, profilin and nonmuscle actin expression decrease in a coordinate manner as shown by measurements of steady state mRNA and newly synthesized protein. In human hepatoma HepG-2 cells, the twofold increase in actin synthesis observed after 24 hours of exposure to cytochalasin D did not result in an increase in profilin synthesis. Thus, profilin and actin expression are not co-regulated in all cells. To determine if there is sufficient profilin to sequester a large portion of cellular G-actin, we measured total profilin and G-actin levels in the three cell types. In each case, profilin accounted for less than 10% of the total G-actin on a molar basis. Thus, profilin is not responsible for total G-actin sequestration in these cells. Finally, using poly-L-proline affinity chromatography, we showed that, in the cell types tested, less than 20% of the poly-L-proline purified profilin existed as a complex with G-actin. The profilin in these cells may be interacting with cellular components other than actin.

Isolation and characterization of the rat liver actin N-acetylaminopeptidase

Actins from most eukaryotes undergo a unique post-translational modification of the amino terminus called "processing." Processing consists of the removal of an amino-terminal Ac-Met or Ac-Cys to leave an acidic amino-terminal residue. We have previously demonstrated that this reaction is not catalyzed by the ribosomally associated methionine aminopeptidase or by other previously described acetylaminopeptidases. Here we present the isolation and characterization of the actin N-acetylaminopeptidase (ANAP) from rat liver. A five-step purification protocol achieves a 4100-fold purification of the enzyme with an overall 8% recovery of activity. ANAP is a 77-kDa monomer with a pI of 4.6. Using unprocessed yeast actin as a substrate, the Km of ANAP is 3.5 microM. Purified ANAP was used to generate a polyclonal antibody. The antibody has been used along with activity assays to demonstrate the presence of ANAP in a variety of rat tissues. Finally, evidence is presented that in mammals, ANAP may function with a second, as yet unpurified, component to process actin amino termini.

Splicing of two alternative exon pairs in beta-tropomyosin pre-mRNA is independently controlled during myogenesis

Two known tissue-specific tropomyosin (TM) isoforms are produced from the rodent beta-TM gene. Skeletal muscle beta-TM uses the alternative exons 6b and 9a and the exon 9a-associated poly(A) site. Fibroblast and smooth muscle TM-1 use exons 6a and 9b and the exon-9b associated poly(A) site. We have identified a new skeletal muscle beta-TM isoform, beta-TM2. beta-TM2 contains exon 6b (muscle) and exon 9b (nonmuscle). Full-length beta-TM2 cDNA clones were isolated from a cDNA library of mouse muscle BC3H1 cells. Its mRNA was also found in mouse skeletal muscle tissue but not in other tissues. beta-TM2 mRNA level and protein synthesis are differentiation-dependent, with a transient high level in the early stages of myogenesis both in BC3H1 cells and in mouse embryo limbs. Trace amounts of beta-TM3 mRNA, the other hybrid form (exons 6a + 9a), were found in less differentiated BC3H1 cells, mouse uterus, heart, and 3T3 fibroblasts but not skeletal muscle tissue. Thus, the selection of the two alternative exons appears to be controlled independently. Furthermore, during myogenesis, there is a sequential switch in the internal alternative exon, the terminal exon, and the poly(A) site from the nonmuscle to the muscle type.