Changes in actin lysine reactivities during polymerization detected using a competitive labeling method

Lysine acetylation of F-actin decreases tropomyosin-based inhibition of actomyosin activity

Recent proteomics studies of vertebrate striated muscle have identified lysine acetylation at several sites on actin. Acetylation is a reversible post-translational modification that neutralizes lysine's positive charge. Positively charged residues on actin, particularly Lys³²⁶ and Lys³²⁸, are predicted to form critical electrostatic interactions with tropomyosin (Tpm) that promote its binding to filamentous (F)-actin and bias Tpm to an azimuthal location where it impedes myosin attachment. The troponin (Tn) complex also influences Tpm's position along F-actin as a function of Ca²⁺ to regulate exposure of myosin-binding sites and, thus, myosin cross-bridge recruitment and force production. Interestingly, Lys³²⁶ and Lys³²⁸ are among the documented acetylated residues. Using an acetic anhydride-based labeling approach, we showed that excessive, nonspecific actin acetylation did not disrupt characteristic F-actin-Tpm binding. However, it significantly reduced Tpm-mediated inhibition of myosin attachment, as reflected by increased F-actin-Tpm motility that persisted in the presence of Tn and submaximal Ca²⁺ Furthermore, decreasing the extent of chemical acetylation, to presumptively target highly reactive Lys³²⁶ and Lys³²⁸, also resulted in less inhibited F-actin-Tpm, implying that modifying only these residues influences Tpm's location and, potentially, thin filament regulation. To unequivocally determine the residue-specific consequences of acetylation on Tn-Tpm-based regulation of actomyosin activity, we assessed the effects of K326Q and K328Q acetyl (Ac)-mimetic actin on Ca²⁺-dependent, in vitro motility parameters of reconstituted thin filaments (RTFs). Incorporation of K328Q actin significantly enhanced Ca²⁺ sensitivity of RTF activation relative to control. Together, our findings suggest that actin acetylation, especially Lys³²⁸, modulates muscle contraction via disrupting inhibitory Tpm positioning.

Neuroprotein Targets of γ-Diketone Metabolites of Aliphatic and Aromatic Solvents That Induce Central-Peripheral Axonopathy

Peripheral neuropathy associated with chronic occupational and deliberate overexposure to neurotoxic organic solvents results from axonal degeneration in the central and peripheral nervous system. Human and experimental studies show that axonopathy is triggered by the action of neuroprotein-reactive γ-diketone metabolites formed from exposure to certain aliphatic solvents (n-hexane, 2-hexanone) and aromatic compounds (1,2-diethylbenzene, 1,2-4-triethylbenzene, 6-acetyl-1,1,4,4-tetramethyl-7-ethyl-1,2,3,4-tetralin). Neuroprotein susceptibility is related primarily to their differential content of lysine, the ∊-amino group of which is targeted by γ-diketones. Specific neuroprotein targets have been identified, and the sequence of molecular mechanisms leading to axonal pathology has been illuminated. While occupational n-hexane neuropathy continues to be reported, lessons learned from its experimental study may have relevance to other causes of peripheral neuropathy, including those associated with aging and diabetes mellitus.

Distinct sites in tropomyosin specify shared and isoform-specific regulation of myosins II and V

Muscle contraction, cytokinesis, cellular movement, and intracellular transport depend on regulated actin-myosin interaction. Most actin filaments bind one or more isoform of tropomyosin, a coiled-coil protein that stabilizes the filaments and regulates interactions with other actin-binding proteins, including myosin. Isoform-specific allosteric regulation of muscle myosin II by actin-tropomyosin is well-established while that of processive myosins, such as myosin V, which transport organelles and macromolecules in the cell periphery, is less certain. Is the regulation by tropomyosin a universal mechanism, the consequence of the conserved periodic structures of tropomyosin, or is it the result of specialized interactions between particular isoforms of myosin and tropomyosin? Here, we show that striated muscle tropomyosin, Tpm1.1, inhibits fast skeletal muscle myosin II but not myosin Va. The non-muscle tropomyosin, Tpm3.1, in contrast, activates both myosins. To decipher the molecular basis of these opposing regulatory effects, we introduced mutations at conserved surface residues within the six periodic repeats (periods) of Tpm3.1, in positions homologous or analogous to those important for regulation of skeletal muscle myosin by Tpm1.1. We identified conserved residues in the internal periods of both tropomyosin isoforms that are important for the function of myosin Va and striated myosin II. Conserved residues in the internal and C-terminal periods that correspond to Tpm3.1-specific exons inhibit myosin Va but not myosin II function. These results suggest that tropomyosins may directly impact myosin function through both general and isoform-specific mechanisms that identify actin tracks for the recruitment and function of particular myosins.

Evolutionarily conserved sites in yeast tropomyosin function in cell polarity, transport and contractile ring formation

Tropomyosin is a coiled-coil protein that binds and regulates actin filaments. The tropomyosin gene in Schizosaccharomyces pombe, cdc8, is required for formation of actin cables, contractile rings, and polar localization of actin patches. The roles of conserved residues were investigated in gene replacement mutants. The work validates an evolution-based approach to identify tropomyosin functions in living cells and sites of potential interactions with other proteins. A cdc8 mutant with near-normal actin affinity affects patch polarization and vacuole fusion, possibly by affecting Myo52p, a class V myosin, function. The presence of labile residual cell attachments suggests a delay in completion of cell division and redistribution of cell patches following cytokinesis. Another mutant with a mild phenotype is synthetic negative with GFP-fimbrin, inferring involvement of the mutated tropomyosin sites in interaction between the two proteins. Proteins that assemble in the contractile ring region before actin do so in a mutant cdc8 strain that cannot assemble condensed actin rings, yet some cells can divide. Of general significance, LifeAct-GFP negatively affects the actin cytoskeleton, indicating caution in its use as a biomarker for actin filaments.

Regulation of nonmuscle myosin II by tropomyosin

The actin cytoskeleton carries out cellular functions, including division, migration, adhesion, and intracellular transport, that require a variety of actin binding proteins, including myosins. Our focus here is on class II nonmuscle myosin isoforms, NMIIA, NMIIB, and NMIIC, and their regulation by the actin binding protein, tropomyosin. NMII myosins are localized to different populations of stress fibers and the contractile ring, structures involved in force generation required for cell migration, adhesion, and cytokinesis. The stress fibers and contractile ring that contain NMII myosins also contain tropomyosin. Four mammalian genes encode more than 40 tropomyosins. Tropomyosins inhibit or activate actomyosin MgATPase and motility depending on the myosin and tropomyosin isoform. In vivo, tropomyosins play a role in cell migration, adhesion, cytokinesis, and NMII isoform localization in an isoform-specific manner. We postulate that the isoform-specific tropomyosin localization and effect on NMII isoform localization reflect modulation of NMII actomyosin kinetics and motile function. In this study, we compare the ability of different tropomyosin isoforms to support actin filament motility with NMIIA, NMIIB, and NMIIC as well as skeletal muscle myosin. Tropomyosins activated, inhibited, or had no effect on motility depending on the myosin, indicating that the myosin isoform is the primary determinant of the isoform-specific effect of tropomyosin on actomyosin regulation. Activation of motility of nonmuscle tropomyosin–actin filaments by NMII myosin correlates with an increased V_max of the myosin MgATPase, implying a direct effect on the myosin MgATPase, in contrast to the skeletal tropomyosin–actin filament that has no effect on the V_max or maximal filament velocity.

A molecular evolution approach to study the roles of tropomyosin in fission yeast

Tropomyosin, a coiled-coil protein that binds along the length of the actin filament, is a universal regulator of the actin cytoskeleton. We have taken a bioinformatics/proteomic approach to studying structure-function relationships in this protein. The presence of a single, essential tropomyosin gene, cdc8, in fission yeast, Schizosaccharomyces pombe, enables a systems-based approach to define the residues that are important for cellular functions. Using molecular evolution methodologies we identified the most conserved residues and related them to the coiled coil structure. Mutants in which one or more of 21 of the most conserved surface residues was mutated to Ala were tested for the ability to rescue growth of a temperature-sensitive cdc8 mutant when overexpressed at the restrictive temperature. Based on altered morphology of the septum and actin cytoskeleton, we selected three sets of mutations for construction of mutant cdc8 strains using marker reconstitution mutagenesis and analysis of recombinant protein in vitro: D16A.K30A, V114S.E117A.H118A and R121A.D131A.E138A. The mutations have sequence-specific effects on cellular morphology including cell length, organization of cytoskeletal structures (actin patches, actin cables and contractile rings), and in vitro actin affinity, lending credence to the proteomic approach introduced here. We propose that bioinformatics is a valid analysis tool for defining structure-function relationships in conserved proteins in this model organism.

A periodic pattern of evolutionarily conserved basic and acidic residues constitutes the binding interface of actin-tropomyosin

Actin filament cytoskeletal and muscle functions are regulated by actin binding proteins using a variety of mechanisms. A universal actin filament regulator is the protein tropomyosin, which binds end-to-end along the length of the filament. The actin-tropomyosin filament structure is unknown, but there are atomic models in different regulatory states based on electron microscopy reconstructions, computational modeling of actin-tropomyosin, and docking of atomic resolution structures of tropomyosin to actin filament models. Here, we have tested models of the actin-tropomyosin interface in the "closed state" where tropomyosin binds to actin in the absence of myosin or troponin. Using mutagenesis coupled with functional analyses, we determined residues of actin and tropomyosin required for complex formation. The sites of mutations in tropomyosin were based on an evolutionary analysis and revealed a pattern of basic and acidic residues in the first halves of the periodic repeats (periods) in tropomyosin. In periods P1, P4, and P6, basic residues are most important for actin affinity, in contrast to periods P2, P3, P5, and P7, where both basic and acidic residues or predominantly acidic residues contribute to actin affinity. Hydrophobic interactions were found to be relatively less important for actin binding. We mutated actin residues in subdomains 1 and 3 (Asp(25)-Glu(334)-Lys(326)-Lys(328)) that are poised to make electrostatic interactions with the residues in the repeating motif on tropomyosin in the models. Tropomyosin failed to bind mutant actin filaments. Our mutagenesis studies provide the first experimental support for the atomic models of the actin-tropomyosin interface.

Dual requirement for flexibility and specificity for binding of the coiled-coil tropomyosin to its target, actin

The coiled coil is a widespread motif involved in oligomerization and protein-protein interactions, but the structural requirements for binding to target proteins are poorly understood. To address this question, we measured binding of tropomyosin, the prototype coiled coil, to actin as a model system. Tropomyosin binds to the actin filament and cooperatively regulates its function. Our results support the hypothesis that coiled-coil domains that bind to other proteins are flexible. We made mutations that alter interface packing and stability as well as mutations in surface residues in a postulated actin binding site. Actin affinity, measured by cosedimentation, was correlated with coiled-coil stability and local instability and side chain flexibility, analyzed with circular dichroism and fluorescence spectroscopy. The flexibility from interruptions in the stable coiled-coil interface is essential for actin binding. The surface residues in a postulated actin binding site participate in actin binding when the coiled coil within it is poorly packed.

Tropomyosin ends determine the stability and functionality of overlap and troponin T complexes

Tropomyosin binds end to end along the actin filament. Tropomyosin ends, and the complex they form, are required for actin binding, cooperative regulation of actin filaments by myosin, and binding to the regulatory protein, troponin T. The aim of the work was to understand the isoform and structural specificity of the end-to-end association of tropomyosin. The ability of N-terminal and C-terminal model peptides with sequences of alternate alpha-tropomyosin isoforms, and a troponin T fragment that binds to the tropomyosin overlap, to form complexes was analyzed using circular dichroism spectroscopy. Analysis of N-terminal extensions (N-acetylation, Gly, AlaSer) showed that to form an overlap complex between the N-terminus and the C-terminus requires that the N-terminus be able to form a coiled coil. Formation of a ternary complex with the troponin T fragment, however, effectively takes place only when the overlap complex sequences are those found in striated muscle tropomyosins. Striated muscle tropomyosins with N-terminal modifications formed ternary complexes with troponin T that varied in affinity in the order: N-acetylated > Gly > AlaSer > unacetylated. The circular dichroism results were corroborated by native gel electrophoresis, and the ability of the troponin T fragment to promote binding of full-length tropomyosins to filamentous actin.

Disease-causing mutations in cardiac troponin T: identification of a critical tropomyosin-binding region

Fifteen percent of the mutations causing familial hypertrophic cardiomyopathy are in the troponin T gene. Most mutations are clustered between residues 79 and 179, a region known to bind to tropomyosin at the C-terminus near the complex between the N- and C-termini. Nine mutations were introduced into a troponin T fragment, Gly-hcTnT(70-170), that is soluble, alpha-helical, binds to tropomyosin, promotes the binding of tropomyosin to actin, and stabilizes an overlap complex of N-terminal and C-terminal tropomyosin peptides. Mutations between residues 92 and 110 (Arg92Leu, Arg92Gln, Arg92Trp, Arg94Leu, Ala104Val, and Phe110Ile) impair tropomyosin-dependent functions of troponin T. Except for Ala104Val, these mutants bound less strongly to a tropomyosin affinity column and were less able to stabilize the TM overlap complex, effects that were correlated with increased stability of the troponin T, measured using circular dichroism. All were less effective in promoting the binding of tropomyosin to actin. Mutations within residues 92-110 may cause disease because of altered interaction with tropomyosin at the overlap region, critical for cooperative actin binding and regulatory function. A model for a five-chained coiled-coil for troponin T in the tropomyosin overlap complex is presented. Mutations outside the region (Ile79Asn, Delta 160Glu, and Glu163Lys) functioned normally and must cause disease by another mechanism.

Alteration of tropomyosin function and folding by a nemaline myopathy-causing mutation

Mutations in the human TPM3 gene encoding gamma-tropomyosin (alpha-tropomyosin-slow) expressed in slow skeletal muscle fibers cause nemaline myopathy. Nemaline myopathy is a rare, clinically heterogeneous congenital skeletal muscle disease with associated muscle weakness, characterized by the presence of nemaline rods in muscle fibers. In one missense mutation the codon corresponding to Met-8, a highly conserved residue, is changed to Arg. Here, a rat fast alpha-tropomyosin cDNA with the Met8Arg mutation was expressed in Escherichia coli to investigate the effect of the mutation on in vitro function. The Met8Arg mutation reduces tropomyosin affinity for regulated actin 30- to 100-fold. Ca(2+)-sensitive regulatory function is retained, although activation of the actomyosin S1 ATPase in the presence of Ca(2+) is reduced. The poor activation may reflect weakened actin affinity or reduced effectiveness in switching the thin filament to the open, force-producing state. The presence of the Met8Arg mutation severely, but locally, destabilizes the tropomyosin coiled coil in a model peptide, and would be expected to impair end-to-end association between TMs on the thin filament. In muscle, the mutation may alter thin filament assembly consequent to lower actin affinity and altered binding of the N-terminus to tropomodulin at the pointed end of the filament. The mutation may also reduce force generation during activation.

Mutations in the N- and D-helices of the N-domain of troponin C affect the C-domain and regulatory function

Troponin C contains a 14-residue alpha-helix at the amino terminus, the N-helix, that calmodulin lacks. Deletion of the first 11-14 residues of troponin C alters function. In the present investigation a mutant lacking residues 1-7 of the N-helix has normal conformation, Ca2+ binding, and regulatory function. Thus, residues 8-14 of the N-helix are generally sufficient for troponin C function. In the x-ray structures of troponin C there is a salt bridge between Arg 11 in the N-helix and Glu 76 in the D-helix. Destroying the salt bridge by individually mutating the residues to Cys has no effect on function. However, mutation of both residues to Cys reduces troponin C's affinity for the troponin complex on the thin filament, reduces the stability of the N-domain in the absence of divalent cations, increases the Ca2+ affinity and reduces the cooperativity of the Ca2+Mg2+ sites in the C-domain, and alters the conformational change that takes place upon Ca2+ binding (but not Mg2+ binding) to the C-domain. Cross-linking with bis-(maleimidomethylether) partially restores function. The Ca2+-specific sites in the N-domain, those closest to the sites of the mutations, are unaffected in the assays employed. These results show that the N-helix is a critical structural element for interaction with and activation of the thin filament. Moreover, mutations in the N-helix affect the C-terminal domain, consistent with recent structural studies showing that the N-helix and C-terminal domain are physically close.

The sequence of the alternatively spliced sixth exon of alpha-tropomyosin is critical for cooperative actin binding but not for interaction with troponin

Tropomyosins, a family of highly conserved coiled-coil actin binding proteins, can differ as a consequence of alternative expression of several exons (Lees-Miller, J., and Helfman, D. (1991) BioEssays 13, 429-437). Exon 6, which encodes residues 189-213 in long, 284-residue tropomyosins, has two alternative forms, exon 6a or 6b, both highly conserved throughout evolution. In alpha-tropomyosin, exon 6a or 6b is not specific to any one of the nine isoforms. Exon 6b encodes part of a putative Ca2+-sensitive troponin binding site in striated muscle tropomyosins, suggesting that the exon 6-encoded region may be specialized for certain tropomyosin functions. A series of recombinant, unacetylated tropomyosin exon 6 deletion and substitution mutants and chimeras was expressed in Escherichia coli to determine the requirements of exon 6 for tropomyosin function. Functional properties of the tropomyosins were defined by actin affinity measured by cosedimentation, troponin T affinity using a newly developed biosensor assay, and regulation of the actomyosin MgATPase. The region of tropomyosin encoded by exon 6 affects actin affinity but not thin filament assembly, troponin T binding, or regulation with troponin. The tropomyosins with exon 6a or 6b function normally whether a striated muscle exon 9a or smooth/non-muscle exon 9d is present. However, the effect of deleting 21 amino acids encoded by exon 6 or replacing it with a GCN4 leucine zipper sequence depends on the COOH-terminal sequence.

Mapping the functional domains within the carboxyl terminus of alpha-tropomyosin encoded by the alternatively spliced ninth exon

Tropomyosins are highly conserved, coiled-coil actin binding proteins found in most eukaryotic cells. Striated and smooth muscle alpha-tropomyosins differ by the regions encoded by exons 2 and 9. Unacetylated smooth tropomyosin expressed in Escherichia coli binds actin with high affinity, whereas unacetylated striated tropomyosin requires troponin, found only in striated muscle, for strong actin binding. The residues encoded by exon 9 cause these differences (Cho, Y.-J., and Hitchcock-DeGregori, S. E. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 10153-10157). We mapped the functional domains encoded by the alpha-tropomyosin exon 9a (striated muscle-specific) and 9d (constitutively expressed), by measuring actin binding and regulation of the actomyosin MgATPase by tropomyosin exon 9 chimeras and truncation mutants expressed in E. coli. We have shown that: 1) the carboxyl-terminal nine residues define the actin affinity of unacetylated tropomyosin; 2) in the presence of Ca2+, the entire exon 9a is required for troponin to promote fully high affinity actin binding; 3) the first 18 residues encoded by exon 9a are critical for the interaction of troponin with tropomyosin on the thin filament, even in the absence of Ca2+. The results give new insight into the structural requirements of tropomyosin for thin filament assembly and regulatory function.

Integral repeats and a continuous coiled coil are required for binding of striated muscle tropomyosin to the regulated actin filament

Tropomyosin is a coiled-coil protein that binds along the length of filamentous actin and contains sequence repeats that correspond to actin monomers in the filament. Analysis of striated muscle alpha-tropomyosin mutants in which internal sequence has been deleted or replaced with non-tropomyosin sequence showed that the following parameters are important for high affinity, cooperative binding of tropomyosin-troponin to actin. 1) Tropomyosin must be a coiled coil along its entire length. 2) An integral number of repeats corresponding to the actin monomers along its length is more important than the total number. 3) In comparison, the actin affinity is relatively insensitive to changes in the sequence of the internal regions of tropomyosin. The results suggest that the internal sequence repeats function as weakly interacting spacers to allow proper alignment of the ends on the regulated actin filament.

The covalent maleimidobenzoyl-actin-myosin head complex. Cross-linking of the 50 kDa heavy chain region to actin subdomain-2

We have identified the region of actin involved in the covalent coupling of maleimidobenzoyl-G-actin to the central 50 kDa segment of the myosin-S-1 heavy chain by analyzing the structure of the maleimidobenzoyl-G-actin-S-1 conjugate using selective proteolytic digestions, amino acid sequence determinations and novel cross-linking reactions between S-1 and different maleimidobenzoyl-G-actin derivatives. The cross-linking is shown to occur only on the stretch of residues 48-67 in actin subdomain-2 with Lys-50, residing on the outer part of the DNase-I-binding loop, as the most likely site of cross-linking. Because the maleimidobenzoyl-F-actin-S-1 complex undergoes the same coupling process, the data provide experimental evidence in favor of the recent model of the rigor F-actin-S-1 complex suggesting a close contact between structural elements of the lower domain of the 50 kDa fragment and the top of actin subdomain-2.

Gln-41 is intermolecularly cross-linked to Lys-113 in F-actin by N-(4-azidobenzoyl)-putrescine

The bifunctional reagent N-(4-azidobenzoyl)-putrescine was synthesized and covalently bound to rabbit skeletal muscle actin. The incorporation was mediated by guinea pig liver transglutaminase under conditions similar to those described by Takashi (1988, Biochemistry 27, 938-943); up to 0.5 M/M were incorporated into G-actin, whereas F-actin was refractory to incorporation. Peptide fractionation showed that at least 90% of the label was bound to Gln-41. The labeled G-actin was polymerized, and irradiation of the F-actin led to covalent intermolecular cross-linking. A cross-linked peptide complex was isolated from a tryptic digest of the cross-linked actin in which digestion was limited to arginine; sequence analysis as well as mass spectrometry indicated that the linked peptides contained residues 40-62 and residues 96-116, and that the actual cross-link was between Gln-41 and Lys-113. Thus the gamma-carboxyl group of Gln-41 must be within 10.7 A of the side chain (probably the amino group) of Lys-113 in an adjacent actin monomer. In the atomic model for F-actin proposed by Holmes et al. (1990, Nature 347, 44-49), the alpha-carbons of these residues in adjacent monomers along the two-start helices are sufficiently close to permit cross-linking of their side chains, and, pending atomic resolution of the side chains, the results presented here seem to support the proposed model.

Localization of the tightly bound divalent-cation-dependent and nucleotide-dependent conformation changes in G-actin using limited proteolytic digestion

Using proteolytic susceptibility as a probe, we have identified four regions of the actin polypeptide chain where structural rearrangements, dependent on the nature of the tightly bound metal ion and/or nucleotide, take place. Replacement of the tightly bound Ca2+ by Mg2+ in ATP-actin strongly affected the regions around Arg26 and Lys68, as judged from nearly complete inhibition of tryptic cleavages of the polypeptide chain at these residues. It also significantly diminished the rates of splitting by trypsin of the peptide bonds involving carbonyl groups of Arg372 and of Lys373 in the C-terminal segment. Conversion of ATP-actin to ADP-actin (with Mg2+ as the tightly bound cation) abolished the protective effect of Mg2+ on specific tryptic cleavage and, in contrast, largely inhibited proteolysis at specific sites for subtilisin and for a novel protease from Escherichia coli A2 strain within a surface loop of residues 39-51. We also examined the effect of proteolytic cleavage or chemical modification at certain sites on the kinetics of proteolysis at other sites of the molecule. These experiments demonstrated structural relationships between loop 39-51 and regions involving Lys61 and Lys68. It is suggested that the conformational transitions reflected in the observed changes in proteolytic susceptibility may underlie the known influence of the nature of the tightly bound cation and nucleotide on the kinetics of actin polymerization and stability of the polymer.

Incorporation of microinjected mutant and wildtype recombinant tropomyosins into stress fibers in fibroblasts

The structural requirements for assembly of tropomyosin into stress fibers were investigated by microinjecting wildtype and four mutant striated chicken muscle alpha-tropomyosins expressed in E. coli as fusion and nonfusion proteins into cultured rat embryo fibroblasts, followed by localization of tropomyosin using indirect immunofluorescence. The results show that the determinants for stress fiber incorporation in living cells correlate with the in vitro actin affinity of these tropomyosins. Wildtype recombinant protein incorporated into stress fibers both when the amino terminus was unacetylated and when it was blocked with an 80-residue fusion protein [Hitchcock-DeGregori, S.E., and Heald, R.W. (1987): J. Biol. Chem. 262:9730-9735]. The pattern of incorporation was indistinguishable from that of tropomyosin isolated from chicken pectoral muscle. The striated alpha-tropomyosin incorporated into stress fibers, even though this isoform is not found in nonmuscle cells. Three recombinant mutant tropomyosins in which one-half, two-thirds, or one actin binding site was deleted were tested [Hitchcock-DeGregori, S.E., and Varnell, T.A. (1990): J. Mol. Biol. 214:885-896]. Only the fusion protein with a full actin binding site deleted incorporated into stress fibers. However, the unacetylated, nonfusion proteins with one half and one actin binding site deleted incorporated into stress fibers, consistent with the ability of troponin to promote the actin binding in vitro. A fourth mutant, in which the conserved amino-terminal nine residues were deleted, did not incorporate into stress fibers, consistent with the complete loss of function of this mutant [Cho, Y.J., Liu, J., and Hitchcock-DeGregori, S.E. (1990): J. Biol. Chem. 265:538-545].

Actin filament structure probed with monoclonal antibodies

The interaction of two monoclonal antibodies (mAbs) with actin has been characterized to map the epitopes defined by these mAbs and to determine the accessibility of these sites in the actin filament (F-actin). Both mAbs react specifically with actin in radioimmunoassays and Western blot assays, and by immunoprecipitation. The location of the epitopes within the primary structure of actin has been determined using limited proteolysis of actin and Western blots, or using immunoprecipitation of truncated actin fragments synthesized in a cell free translation assay. Both mAbs bind to the C-terminal fragment of actin (residues 68-375) produced by chymotrypsin cleavage. One epitope is further localized to a 9.9 kD peptide corresponding to residues 5-93. Therefore, the epitope defined by this mAb (2G11.4) lies between residues Lys68 and Glu93 of actin. The location of the other epitope was determined by immunoprecipitation of actin fragments synthesized in vitro. Removal of residues 356-365 from the C-terminus of actin completely abolished the binding of mAb 4E3.adl. Therefore, this mAb defines an epitope that involves residues between Trp356 and Ala365. The accessibility of these epitopes in native F-actin was determined with solution binding assays and characterized by immunoelectron microscopy. Monoclonal antibody 4E3.adl binds strongly to filaments, resulting in bundling or decoration of F-actin depending on the valency of the mAb, and indicating that the epitope is readily accessible in F-actin. In contrast, mAb 2G11.4 disrupts F-actin structure, resulting in the formation of an amorphous immunoprecipitate. These results place constraints on models of actin filament structure.

Covalent modification of G-actin by pyridoxal 5'-phosphate: polymerization properties and interaction with DNase I and myosin subfragment 1

Pyridoxal 5'-phosphate (PLP), a lysine-specific reagent, has been used to modify G-actin. At pH 7.5, PLP reacted with 1.7-2 lysines on G-actin. Limited proteolytic digestion experiments indicated that, in agreement with previous works, essentially lysine-61 was modified in a 1:1 fashion by PLP, other lysines being much less reactive. A PLP-derivatized affinity label of ATP binding sites, AMPPLP, reacted with two additional lysines that do not appear to be located in the ATP site on G-actin. PLP-G-actin did not polymerize spontaneously up to 30 microM; however, it retained other essential native properties of G-actin. PLP-actin bound to the barbed ends of actin filaments with an equilibrium dissociation constant of 4 microM and prevented dilution-induced depolymerization like a capping protein. PLP-actin copolymerized with unmodified actin. The stability of F-actin copolymers decreased with the fraction of PLP-actin incorporated, consistent with a model within which the actin-PLP-actin interactions in the copolymer are 50-fold weaker, and PLP-actin-PLP-actin interactions are 200-fold weaker than regular actin-actin interactions. PLP-actin bound DNase I with an equilibrium association constant of 2 nM-1, i.e., 10-fold lower than that of unmodified actin. PLP modification did not affect the binding of G-actin to myosin subfragment 1. However, polymerization of PLP-actin by myosin subfragment 1 was not observed in low ionic strength buffers, whereas PLP-F-actin-S1 filaments, in which the stoichiometry PLP-actin:S1 is 1:1, were formed with an apparent critical concentration of 4.5 microM in the presence of 0.1 M KCl.

Atomic model of the actin filament

The F-actin filament has been constructed from the atomic structure of the actin monomer to fit the observed X-ray fibre diagram from oriented gels of F-actin. A unique orientation of the monomer with respect to the actin helix has been found. The main interactions are along the two-start helix with a contribution from a loop extending across the filament axis provided by the molecule in the adjacent strand. There are also contacts along the left-handed genetic helix.

Tropomyosin has discrete actin-binding sites with sevenfold and fourteenfold periodicities

Analysis of the periodic distribution of amino acids in tropomyosin has revealed the presence of seven or 14 quasi-equivalent actin-binding sites. We tested the hypothesis of periodic actin-binding sites by making deletions of chicken striated alpha-tropomyosin cDNA using oligonucleotide-directed mutagenesis. The deletions corresponded to one-half (amino acid residues 47 to 67), two-thirds (residues 47 to 74) and one actin-binding site (residues 47 to 88), on the basis of there being seven sites. The mutant cDNAs were expressed as fusion and non-fusion proteins in Escherichia coli and analyzed for actin binding and regulatory function. Fusion tropomyosin binds to actin with an affinity similar to that of muscle tropomyosin. Of the mutant fusion tropomyosins, only that with a full site deleted retained actin affinity and the ability to inhibit the actomyosin S1 ATPase, though it was less effective than wild-type. We conclude that an integral number of half-turns of the tropomyosin coiled-coil, and the consequential sevenfold periodicity, as well as the correct orientation of the ends with respect to each other, are important for actin binding. On the other hand, non-fusion tropomyosin binds well to actin only in the presence of troponin, and the binding is calcium-sensitive. Assay of non-fusion mutant tropomyosins showed that mutants with deletion of one-half and one actin binding site both had high affinity for actin, equal to or slightly less than wild-type. The ability of these two mutants to regulate the actomyosin or acto-S1 ATPase with troponin in the absence of calcium was indistinguishable from that of the wild-type. The normal regulatory function of the mutant with a 1/14 deletion (removal of a quarter turn or half a site) indicates that a 14-fold periodicity is adequate for regulation, consistent with the presence of two sets of seven alpha and seven beta quasi-equivalent actin-binding sites. An alternative explanation is that the alpha-sites are of primary importance and that proper alignment of the alpha-sites in every second tropomyosin, as when half a site is deleted, is sufficient for normal regulatory function. Deletion of a non-integral period (2/3 of a site) severely compromised actin-binding and regulatory function, presumably due to the inability of the mutant to align properly on the actin filament.

Tropomyosin inhibits the rate of actin polymerization by stabilizing actin filaments

Tropomyosin inhibition of the rate of spontaneous polymerization of actin is associated with binding of tropomyosin to actin filaments. Rate constants determined by using a direct electron microscopic assay of elongation showed that alpha alpha- and alpha beta-tropomyosin have a small or no effect on the rate of elongation at either end of the filaments. The most likely explanation for the inhibition of the rate of polymerization of actin in bulk samples is that tropomyosin reduces the number of filament ends by mechanical stabilization of the filaments.

Lysine reactivities of tropomyosin complexed with troponin

The relative reactivities of lysine residues of tropomyosin complexed with troponin have been measured in order to locate the binding site of troponin on tropomyosin in a complex between the two native proteins. The lysines were labeled with acetic anhydride using a competitive labeling procedure and the relative reactivities of tropomyosin lysine containing peptides were compared to those from tropomyosin labeled in the absence of troponin (S. E. Hitchcock-DeGregori, S. F. Lewis, and T. M.-T. Chou, (1985) Biochemistry 24, 3305-3314). Analysis of about two-thirds of the lysines indicates that troponin affects the reactivities of lysines along the length of the tropomyosin, indicating long-range effects. The inferred binding site is more extensive than previously reported, about 25 nm, extending from res. 136 to the carboxy-terminus and to res. 30 beyond the end-to-end overlap in the amino-terminal region of the next tropomyosin molecule.

The molecular dynamics of actin measured by a spin probe attached to lysine

Rabbit skeletal muscle G-actin was labeled with a spin probe, 3-(5-fluoro-2,4-dinitroanilino)proxyl. Tryptic digestion of the labeled actin followed by ultrafiltration and ion-exchange column chromatography indicated that the label was attached to residue Lys-61. This residue is found within a 9-kDa N-terminal segment that is easily degraded by proteolytic enzymes. The rate of reduction of the nitroxide bond by ascorbate was measured to determine the accessibility of the probe to small molecules in the solvent. These experiments showed that label bound to G-actin was relatively inaccessible to ascorbate, suggesting that it is buried within the protein structure. Polymerization further decreased the accessibility of the probe. Replacing bound Ca2+ with Mn2+ decreased the observed intensity of the electron paramagnetic resonance signal, indicating the spin label is about 2 nm distant from the metal binding site on the actin molecule. Labels attached to G-actin displayed an absorption spectrum characteristic of rotational motion with a correlation time (tau c) of 7 X 10(-9) s, which is faster than that for the whole molecule. Labels attached to F-actin had a value of tau c, measured using saturation transfer electron paramagnetic resonance, of 2 X 10(-5) s, which shows that the probe has a greater degree of mobility than the filament. The binding of heavy meromyosin or troponin-tropomyosin to labeled actin resulted in a further increase in the rotational correlation times, with the greatest decrease in mobility (tau c = 1 X 10(-4) s) observed when both were bound. Together the above results suggest that the 9-kDa segment of actin is mobile relative to the rest of the molecule and that this mobility can be influenced by the binding of heavymeromyosin or troponin-tropomyosin.

Changes of lysine reactivities of actin in complex with DNAase I

The reactivities of lysines of actin in the actin-DNAase I complex were measured by the method of reductive methylation. The reactivities of lysines in the amino-terminal part, lysines 18, 50, 61 and 68, decreased 50%, while those of lysines 237, 283 and 290 increased about 30%, in comparison with those in G-actin, when actin was bound to DNAase I. These results are consistent with the view that the amino-terminal region of actin is the binding site for DNAase I. In conjunction with our earlier work on the reactivities of lysines in F-actin (Lu, R.C. and Szilagyi, L. (1981) Biochemistry 20, 5914-5919), these results are also consistent with the view that DNAase I binds to actin at one of the regions that is involved in polymerization.

Tropomyosin lysine reactivities and relationship to coiled-coil structure

We have carried out a detailed analysis of tropomyosin structure using lysines as specific probes for the protein surface in regions of the molecule that have not been investigated by other methods. We have measured the relative reactivities of lysines in rabbit skeletal muscle alpha, alpha-tropomyosin with acetic anhydride using a competitive labeling procedure. We have identified 37 of 39 lysines and find that they range 20-fold in reactivity. The observed reactivities are related to the coiled-coil model of the tropomyosin molecule [Crick, F.H.C. (1953) Acta Crystallogr. 6, 689-697; McLachlan, A.D., Stewart, M., & Smillie, L.B. (1975) J. Mol. Biol. 98, 281-291] and other available chemical and physical information about the structure. In most cases, the observed lysine reactivities can be explained by allowable interactions with neighboring amino acid side chains on the same or facing alpha-helix. However, we found no correlation between reactivity and helical position of a given lysine. For example, lysines in the outer helical positions included lysines of low as well as high reactivity, indicating that they vary widely in their accessibility to solvent and that the coiled coil is heterogeneous along its length. Furthermore, the middle of the molecule (residues 126-182) that is susceptible to proteolysis and known to be the least stable region of the protein also contains some of the least and most reactive lysines. We have discussed the implications of our results on our understanding the structures of tropomyosin and other coiled-coil proteins as well as globular proteins containing helical regions.

Bovine aorta actin. Development of an improved purification procedure and comparison of polymerization properties with actins from other types of muscle

Crude actin extracts from acetone-dried powder of the muscle layer of bovine aorta contain an actin-modulating protein which promotes nucleation of actin monomers and decreases the average length of actin filaments in a Ca2+-dependent manner. This observation has allowed the development of an improved purification procedure for aorta actin which increases the yield 2- to 3-times. The actin obtained with this procedure consists of 77% alpha- and 23% gamma-isoelectric species. Pure aorta actin is indistinguishable from actins from skeletal, cardiac and chicken-gizzard smooth muscle in its polymerization rate, critical concentration, and reduced viscosity when polymerized with KCl at 25 degrees C. It differs from sarcomeric actins, but not from chicken-gizzard smooth muscle actin, in the temperature dependence of polymerization equilibria in KCl. This difference correlates with the amino acid replacements Val-17----Cys-17 and Thr-89----Ser-89, supporting a conclusion drawn from other studies that the N-terminal portion of actin polypeptide chain contains sites important for polymerization.

Modification of actin with fluorescein isothiocyanate

Reaction of rabbit skeletal muscle G-actin at pH 8.5 with fluorescein isothiocyanate (FITC) resulted in incorporation of up to 1.20 mol FITC/mol actin. At pH 8.8, the level of incorporation was raised to 1.98 mol FITC/mol actin. When excited with ultraviolet light, the FITC-actin samples fluoresced strongly with an emission maximum near 517 nm. Tryptic digests of FITC-actin containing about 1.0 mol FITC/mol actin could be separated into a nonfluorescent 33.5 kDa trypsin-resistant core protein and a fluorescent pool of small peptides. Chromatography on DEAE-Bio-Gel or two-dimensional separation on cellulose TLC plates of the peptide pool revealed that FITC was highly selective in the site of its reaction with actin, resulting in a single highly fluorescent peptide after tryptic digestion. NH2-terminal and amino acid analyses demonstrated this peptide to be derived from residues 51 to 62, with Lys-61 proposed as the major FITC-sensitive site on actin. FITC-actin is similar to G-actin in gross conformation; circular dichroism spectra of actin before and after labelling are identical. FITC-actin is also able to interact strongly with deoxyribonuclease I. However, FITC-actin solution viscosities and fluorescence properties are not altered by the addition of KCl or MgCl2. Therefore, either a localized conformational change near Lys-61 or steric hindrance due to the FITC attached to Lys-61 blocks the polymerization of actin.

Fluorescence energy transfer between probes on actin and probes on myosin

The structural relationship between F-actin filaments and the biologically active fragments of myosin (either as myosin subfragment-1 or heavy meromyosin) has been investigated using the technique of fluorescence energy transfer. Donor and acceptor probes were used to obtain the following inter- and intramolecular distances. Energy transfer was measured: (1) from the SH1 groups of the myosin 'heads' to the nucleotide sites of F-actin (in the absence of free nucleotide); (2) from the SH1 groups of myosin to multiple probes on the surface of the actin filament; (3) from the nucleotide-binding sites of F-actin to the ATPase sites of myosin; (4) from the ATPase sites of myosin to the nucleotide-binding sites of F-actin; (5) from the SH1 sites of myosin to the nucleotide-binding sites of F-actin; and (6) from the Cys-373 residues of F-actin to the nucleotide binding sites of F-actin. We observed very little energy transfer between the probes on actin and the probes on myosin (10% or less) and we observed a large transfer between the actin Cys-373 and the actin nucleotide. These data strongly suggest that both the SH1 moiety and the ATPase site of myosin are located more than 6 nm from the actin sites. When these distances are combined with similar measurements by other authors and inserted into the most recent three-dimensional reconstruction of electron micrographs of the acto-subfragment-1 complex, it is apparent that the SH1 and the ATPase sites on myosin are not located adjacent to actin and are most probably located in the half of the myosin head that is distal from actin in the actomyosin complex.