Atomic structure of the actin:DNase I complex

Localization of a vitamin-D-binding protein interaction site in the COOH-terminal sequence of actin

The serum vitamin D binding protein is the carrier of vitamin D and its derivatives in the plasma. One of the known roles of this protein is to sequester monomeric actin in the blood, therefore implicating this protein in actin elimination. However, its binding site at the surface of actin is poorly delimited. We report here the results of a study which locates, using several actin fragments together with immunological probes, a vitamin D binding protein site near the COOH-terminal extremity. Thus, the interface is delimited by the sequence 360-372 in subdomain I of actin.

Relaxant effect of phalloidin on Triton-skinned microvascular and other smooth muscle preparations

Guinea pig mesenteric microarteries (diameter 60-100 microns), the main branch of the mesenteric artery and taenia coli were skinned with 1% Triton X-100 for 4 h at 4 degrees C. Microarteries, mounted for circumferential force measurement, developed maximal active force in response to elevation of the free Ca2+ (pCa = 4.52, in EGTA buffer) in the presence of ATP (7.5 mM) and calmodulin (0.1-0.3 microM). In these preparations, addition of phalloidin (1-100 microM) slowly (greater than 1 h) relaxed submaximal contractions (pCa greater than 4.52) in a dose-dependent manner. Relaxation was irreversible as, after phalloidin wash-out, subsequent active force to pCa = 4.52 was also reduced. By contrast, phalloidin preincubation and wash-out under relaxed conditions (pCa greater than 8) only reduced subsequent force to pCa = 4.52 on prolonged stimulation. The extent of phalloidin-induced relaxation was not dependent on free Ca2+ between pCa 6.40 and 4.52. Phalloidin-induced relaxation did not occur during rigor contractions (i.e. absence of ATP and Ca2+). These mechanical effects of phalloidin were accompanied by a decreased leak of actin out of the skinned preparations and by the prevention of guanidine extraction of actin from these preparations. Phalloidin did not inhibit the myosin light chain kinase or phosphatase activity isolated from these preparations. In addition, the relaxant effects were also noted in taenia coli and the main branch of the mesenteric artery but not in skinned porcine ventricular heart muscle. These experiments suggest the possible participation of actin filament dynamics on the maintenance of active force in Triton-skinned smooth muscle.

Structural relationships of actin-binding proteins

The sequences of a large number of actin-binding proteins have been compared. These findings, together with the results of protein-chemical analysis, peptide synthesis and site-directed and deletion mutagenesis, have led to the assignment of actin-binding sites. Within these segments, small actin-binding motifs have been delineated. Most actin-binding proteins interact with actin subdomain-1 but our analyses reveal neither primary nor secondary structure homology among these proteins, suggesting that actin binding does not follow simple structural principles.

The sequence location of the actin metal

We have incorporated Fe2+ into the high-affinity metal-ion-binding site of actin. By supplying the system with oxygen from air and a reductant (dithiothreitol or ascorbate), we have induced free-radical generation, with the intent of causing peptide cleavage at the metal-ion-binding site. By analysis of the resulting fragments from actin in the F-form, we have deduced that cuts occurred at positions 159-160 and 301-302 (at the latter location we could not be sure if more than one cut occurred). We considered that these two cuts occurred in the chain strand coursing from the outer to the inner domain and vice-versa. Our results harmonize very well with the recently reported atomic structure of actin [Kabsch, W., Mannherz, H.G., Suck, D., Pai, E.F. & Holmes, K.C. (1990) Nature 347, 37-44] and remove ambiguities that had remained in the structure. The results partly bear out the homology-based prediction of Strzelecka-Golaszewska et al. [Strzelecka-Golaszewska, H., Boguta, G., Zmorzynshi, S. & Moraczcwska, J. (1989) Eur. J. Biochem. 182, 299-305].

The structure of the F-actin filament and the actin molecule

A consensus view on the three-dimensional structure of the F-actin filament and the relative strength of the intersubunit contacts in the filament has been established from an atomic filament model and recent three-dimensional reconstructions from electron micrographs of F-actin filaments. Functional implications of recent structural and biochemical data indicating a rather dynamic filament structure are discussed.

Stein and Moore Award address. Reconstructing history with amino acid sequences

The main goal of the protein evolutionist is the reconstruction of past events leading to the structures of contemporary proteins. The common strategy is to align amino acid sequences and make inferences about matters of common ancestry. The rate of change of amino acid sequence varies greatly from protein to protein, and this naturally affects how far back a given protein's ancestry can be traced. Happily, the rate of change of many proteins is slow enough that very ancient events can be inferred. Many mainstream metabolic enzymes, for example, are 40-50% identical in prokaryotes and eukaryotes, groups that diverged from a common ancestor more than 1.5 billion years ago. Moreover, some eukaryotic proteins like actin and tubulin change so slowly that they are seldom less than 60% identical, no matter from what source they are drawn. As it happens, prokaryotic counterparts for many eukaryotic cytoskeletal proteins are unknown. A recent exception involves the finding that a heat shock protein cognate is a relative of actin. The gene duplication that gave rise to these two proteins must have been an ancient event. The more recent invention of other proteins whose distribution is restricted to one or the other of the major kingdoms may be easier to trace. Among the factors that can confound the reconstruction of events, however, are occasional horizontal gene transfers and exon shuffling. The latter has led to a number of mosaic proteins, many of which contain various combinations of a relatively small set of modules like the epidermal growth factor domain.

Specific cross-linking of the SH1 thiol of skeletal myosin subfragment 1 to F-actin and G-actin

Recently, we reported that (maleimidobenzoyl)-G-actin (MBS-G-actin), which was resistant to the salt and myosin subfragment 1 (S-1) induced polymerizations, reacts reversibly and covalently in solution with the S-1 heavy chain at or near the strong F-actin binding region [Bettache, N., Bertrand, R., & Kassab, R. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6028-6032]. Here, we have readily converted the MBS-G-actin into MBS-F-actin in the presence of phalloidin and salts. The binding of S-1 to the two actin derivatives carrying on their surface free reactive maleimidobenzoyl groups was investigated comparatively in cross-linking experiments performed under various conditions to probe further the molecular structure of the actin-heavy chain complex before and after the polymerization process. Like MBS-G-actin, the isolated MBS-F-actin, which did not undergo any intersubunit cross-linking, bound stoichiometrically to S-1, generating two kinds of actin-heavy chain covalent complexes migrating on electrophoretic gels at 180 and 140 kDa. The relative extent of their production was essentially dependent on pH for both G-and F-actins. At pH 8.0, the 180-kDa species was predominant, and at pH 7.0, the amount of the 140-kDa adduct increased at the expense of the 180-kDa entity. The cross-linking of MBS-F-actin to S-1 led to the superactivation of the MgATPase substantiating the ability of this derivative to stimulate the S-1 ATPase as the native protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin molecules promote neurite outgrowth of chick telencephalic neurons in vitro

Chick brain extract contains protein factors for neurite outgrowth from embryonic chick telencephalic neurons in dissociated cell cultures. In the course of purification of the factors using a bioassay system, a 41 kDa protein was detected to be one of the factors. Immunoblot analysis showed that this protein is identical to actin. In addition, G-actin purified from rabbit skeletal muscle exhibits neurite outgrowth activity in the same dose-dependent manner as the 41 kDa protein. The same bioassay carried with other purified proteins did not show such a remarkable activity.

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.

New yeast actin-like gene required late in the cell cycle

Actin, a major cytoskeletal component of all eukaryotic cells, is one of the most highly conserved proteins. It is involved in various cellular processes such as motility, cytoplasmic streaming, chromosome segregation and cytokinesis. The actin from the yeast Saccharomyces cerevisiae, encoded by the essential ACT1 gene, is 89% identical to mouse cytoplasmic actin and is involved in the organization and polarized growth of the cell surface. We report here the characterization of ACT2, a previously undescribed yeast split gene encoding a putative protein (391 amino acids, relative molecular mass (Mr) 44,073) that is 47% identical to yeast actin. The requirement of the ACT2 gene for vegetative growth of yeast cells and the existence of related genes in other eukaryotes indicate an important and conserved role for these actin-like proteins. Superimposition of the Act2 polypeptide onto the three-dimensional structure of known actins reveals that most of the divergence occurred in loops involved in actin polymerization, DNase I and myosin binding, leaving the core domain mainly unaffected. To our knowledge, the Act2 protein from S. cerevisiae is the first highly divergent actin molecule described. Structural and physiological data suggest that the Act2 protein might have an important role in cytoskeletal reorganization during the cell cycle.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Cryo-electron microscopy of vitrified muscle samples

A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure. A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitrified samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the accessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (S1) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thereby visualizing intermediate states of the crossbridge cycle.

Actin in the ciliated protozoan Climacostomum virens: purification by DNAse I affinity chromatography, electrophoretic characterization, and immunological analysis

The anti-actin monoclonal antibody (mab) JLA20 (Lin: Proc. Natl. Acad. Sci. U.S.A. 78:2335-2339, 1981) labels a 43 kD protein on Western blots of Climacostomum cell extracts; this protein does not react with an anti-alpha-smooth muscle actin mab (Skalli et al.: J. Cell Biol. 103:2787-2796, 1986) nor with an anti-alpha-sarcomeric actin mab (Skalli et al.: Am. J. Pathol. 130:515-531, 1988). This protein binds to DNAse I and can be purified by DNAse I affinity chromatography. The affinity-purified actin also reacts with mab JLA20. Two-dimensional gel analysis reveals that Climacostomum actin focuses as three spots which are more basic than the mammalian actin isoforms. After addition of KCl, the affinity-purified actin polymerizes into filaments as shown by electron microscopy after negative staining.

Identification of act2, an essential gene in the fission yeast Schizosaccharomyces pombe that encodes a protein related to actin

Actins are a family of highly conserved proteins that are ubiquitously found among eukaryotic organisms. All actins that have previously been identified, including those of animals, plants, fungi, and protozoa, are 374-376 amino acids long and exhibit at least 70% amino acid sequence identity when compared with one another. We have cloned a gene from the fission yeast Schizosaccharomyces pombe that encodes a distantly related member of the actin protein family, herein referred to as act2. In contrast to all other actins, the derived amino acid sequence reveals that act2 is 427 residues long and exhibits only 35-40% identity to actins, including act1 from Sch. pombe. Comparison to the known x-ray crystallographic structure of rabbit skeletal muscle actin indicates that the ATP and divalent metal ion binding sites are largely conserved in act2, while regions involved in actin-actin and actin-myosin interactions are relatively divergent. Disruption of the act2 gene demonstrated that this gene encodes a function essential for germination of haploid spores. These findings indicate that while act2 and act1 are related proteins, they appear to have distinct functions. In addition, they demonstrate that the actin protein family is more diverse than was previously thought.

Functional and ultrastructural effects of a missense mutation in the indirect flight muscle-specific actin gene of Drosophila melanogaster

A single-site mutation of the flight-muscle-specific actin gene of Drosophila melanogaster causes a substitution of glutamic acid 93 by lysine in all the actin encoded in the indirect flight muscle (IFM). In these Act88FE93K mutants, myofibrillar bundles of thick and thin filaments are present but lack Z-discs and all sarcomeric repeats. Dense filament bundles, which are probably aberrant Z-discs, are seen in myofibrils of pupal flies, but early in adult life these move to the periphery of the fibrils and are not seen in skinned adult fibres. Consistent with this observation, alpha-actinin and other high molecular weight proteins, possibly associated with Z-discs, are not detected on SDS/polyacrylamide gels or Western blots of skinned adult IFM. The mutation lies at the beginning of a loop in the small domain of actin, near the myosin binding region. However, that the mutant actin binds myosin heads is shown by (1) rigor crossbridges in electron micrographs, (2) the appropriate rise in stiffness when ATP is withdrawn in mechanical experiments, and (3) equal protection against tryptic digestion provided by rigor binding between actin and myosin in both wild-type and mutant fibres. Reversal of rigor chevron angle along some thin filaments reflects reversal of thin-filament polarity due to lattice disorder. The absence of Z-discs, alpha-actinin and two high molecular weight proteins, and binding studies by others, suggest that the substitution at residue 93 affects the binding of the mutant actin to a protein, possibly alpha-actinin, which is necessary for Z-disc assembly or maintenance.

Peptide competition of actin activation of myosin-subfragment 1 ATPase by an amino terminal actin fragment

The amino-terminal region of actin participates in the binding of myosin subfragment 1 (S1) during cross-bridge cycling, thereby assisting in the activation of the magnesium-dependent myosin ATPase. Effects of three actin fragments on the magnesium-dependent S1 and acto-S1 ATPase activities in solution were studied. One of the peptides, containing residues actin 1-44, mimicked the S1 ATPase-activating properties of actin and in turn inhibited acto-S1 ATPase both in a concentration-dependent manner. This suggests peptide competition for the actin binding site on myosin. The other fragments, residues actin 1-18 and 82-119, respectively, had no detectable effect on S1- and acto-S1 ATPase activity.

Characterization of actin filament severing by actophorin from Acanthamoeba castellanii

Actophorin is an abundant 15-kD actinbinding protein from Acanthamoeba that is thought to form a nonpolymerizable complex with actin monomers and also to reduce the viscosity of polymerized actin by severing filaments (Cooper et al., 1986. J. Biol. Chem. 261:477-485). Homologous proteins have been identified in sea urchin, chicken, and mammalian tissues. Chemical crosslinking produces a 1:1 covalent complex of actin and actophorin. Actophorin and profilin compete for crosslinking to actin monomers. The influence of actophorin on the steady-state actin polymer concentration gave a Kd of 0.2 microM for the complex of actophorin with actin monomers. Several new lines of evidence, including assays for actin filament ends by elongation rate and depolymerization rate, show that actophorin severs actin filaments both at steady state and during spontaneous polymerization. This is confirmed by direct observation in the light microscope and by showing that the effects of actophorin on the low shear viscosity of polymerized actin cannot be explained by monomer sequestration. The severing activity of actophorin is strongly inhibited by stoichiometric concentrations of phalloidin or millimolar concentrations of inorganic phosphate.

Purification and crystallization of yeast hexokinase isoenzymes. Characterization of different forms by chromatofocusing

The yeast hexokinase isoenzymes PI and PII have been purified in large amounts (20 mg) from overproducing yeast strains. The purification procedures of hexokinase PI and PII include anion-exchange chromatography on DEAE-Sephacel and chromatofocusing on PBE 94, hydrophobic interaction chromatography on phenyl-Sepharose (necessary for the isolation of the isoenzyme PI); in the final step either a Mono Q HR 5/5 or a Fractogel EMD TMAE 650(S) column was used. Hexokinase preparations were characterized before crystallization by chromatofocusing on a Mono P HR 5/20 FPLC column, where different forms of hexokinase can be rapidly distinguished by their elution behaviour. From both purified hexokinase PI and PII, large crystals were grown that diffract X-rays to high resolution.

Detection of conformational changes in actin by fluorescence resonance energy transfer between tyrosine-69 and cysteine-374

The distance between 5-(dimethylamino)naphthalene-1-sulfonyl chloride (dansyl chloride or DNS-Cl) attached to Tyr-69 and N-[[4-[4-(dimethylamino)phenyl]azo]phenyl]maleimide (DABMI) or N-[4-(dimethylamino)-3,5-dinitrophenyl]maleimide (DDPM) attached to Cys-374 in an actin monomer was measured to be 2.51 nm or 2.27 +/- 0.04 nm, respectively, by fluorescence resonance energy transfer. This distance does not change significantly when the actin monomer binds DNase I, when the monomer is polymerized, when the polymer interacts with myosin subfragment 1, or when it interacts with tropomyosin-troponin in the presence and absence of Ca2+. Changes in the distance were within 0.1 nm. The results indicate that the structure of the region involving Tyr-69 and Cys-374 is substantially rigid. A large blue shift (about 15 nm) of the fluorescence spectrum and a large increase (about 80%) in the fluorescence intensity of DNS-actin were observed when DNS-actin was denatured upon addition of EDTA. On the other hand, a red shift (about 7 nm) of the fluorescence spectrum and a large decrease (about 50%) in the fluorescence intensity were observed when DNS-actin was completely unfolded in 8 M urea. The results indicate that dansyl chromophore becomes less exposed to the aqueous environment by EDTA denaturation in contradiction to the case of intrinsic tryptophan residues in G-actin. Resonance energy transfer measurements showed that the distance between probes attached to Tyr-69 and Cys-374 on an actin monomer changes by 0.37 nm during EDTA denaturation, but that the distance becomes longer than 4.0 nm in 8 M urea in which no energy transfer is observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin filaments regulate epithelial Na+ channel activity

The functional role of the cytoskeleton in the control of ion channel activity is unknown. In the present study, immunocolocalization of Na+ channels with specific antibodies and fluorescein isothiocyanate-phalloidin to stain the cortical cytoskeleton indicates that actin is always present in close proximity to apical Na+ channels in A6 cells. The patch-clamp technique was used to assess the effect of cortical actin networks on apical Na+ channels in these A6 epithelial cells. The actin filament disrupter, cytochalasin D (5 micrograms/ml), induced Na+ channel activity in cell-attached patches within 5 min of addition. Cytochalasin D also induced and/or increased Na+ channel activity in 90% of excised patches tested within 2 min. Addition of short actin filaments (greater than 5 microM) to excised patches also induced channel activity. This effect was enhanced by addition of ATP and/or cytochalasin D. The effect of actin on Na+ channel activity was reversed by addition of the G actin-binding protein DNase I or completely prevented by treatment of the excised patches with this enzyme. Addition of the actin-binding protein, filamin, reversibly inhibited both spontaneous and actin-induced Na+ channels. Thus actin filament networks, achieved by either depolymerizing endogenous actin filaments by treatment with cytochalasin D, the addition of exogenous short actin filaments plus ATP, or actin plus cytochalasin D, regulate apical Na+ channel activity. This conclusion was supported by the observation that the addition of short actin filaments in the form of actin-gelsolin complexes in molar ratios less than 8:1 was also effective in activating Na+ channels. We have thus demonstrated a functional role for the cortical actin network in the regulation of epithelial Na+ channels that may complement a structural role for membrane protein targetting and assembly.

The structural basis for the intrinsic disorder of the actin filament: the "lateral slipping" model

Three-dimensional (3-D) helical reconstructions computed from electron micrographs of negatively stained dispersed F-actin filaments invariably revealed two uninterrupted columns of mass forming the "backbone" of the double-helical filament. The contact between neighboring subunits along the thus defined two long-pitch helical strands was spatially conserved and of high mass density, while the intersubunit contact between them was of lower mass density and varied among reconstructions. In contrast, phalloidinstabilized F-actin filaments displayed higher and spatially more conserved mass density between the two long-pitch helical strands, suggesting that this bicyclic hepta-peptide toxin strengthens the intersubunit contact between the two strands. Consistent with this distinct intersubunit bonding pattern, the two long-pitch helical strands of unstabilized filaments were sometimes observed separated from each other over a distance of two to six subunits, suggesting that the intrastrand intersubunit contact is also physically stronger than the interstrand contact. The resolution of the filament reconstructions, extending to 2.5 nm axially and radially, enabled us to reproducibly "cut out" the F-actin subunit which measured 5.5 nm axially by 6.0 nm tangentially by 3.2 nm radially. The subunit is distinctly polar with a massive "base" pointing towards the "barbed" end of the filament, and a slender "tip" defining its "pointed" end (i.e., relative to the "arrowhead" pattern revealed after stoichiometric decoration of the filaments with myosin subfragment 1). Concavities running approximately parallel to the filament axis both on the inner and outer face of the subunit define a distinct cleft separating the subunit into two domains of similar size: an inner domain confined to radii less than or equal to 2.5-nm forms the uninterrupted backbone of the two long-pitch helical strands, and an outer domain placed at radii of 2-5-nm protrudes radially and thus predominantly contributes to the outer part of the massive base. Quantitative evaluation of successive crossover spacings along individual F-actin filaments revealed the deviations from the mean repeat to be compensatory, i.e., short crossovers frequently followed long ones and vice versa. The variable crossover spacings and diameter of the F-actin filament together with the local unraveling of the two long-pitch helical strands are explained in terms of varying amounts of compensatory "lateral slipping" of the two strands past each other roughly perpendicular to the filament axis. This intrinsic disorder of the actin filament may enable the actin moiety to play a more active role in actin-myosin-based force generation than merely act as a rigid passive cable as has hitherto been assumed.

Direct photoaffinity labeling of gizzard myosin with vanadate-trapped adenosine diphosphate

The active-site topology of smooth muscle myosin has been investigated by direct photoaffinity-labeling studies with [3H]ADP. Addition of vanadate (Vi) and Co2+ enabled [3H]ADP to be stably trapped at the active site (t1/2 greater than 5 days at 0 degrees C). The extraordinary stability of the myosin.Co2+.[3H]ADP.Vi complex allowed it to be purified free of excess [3H]ADP before irradiation began and ensured that only active-site residues became labeled. Following UV irradiation, approximately 10% of the trapped [3H]ADP became covalently attached at the active site. All of the [3H]ADP incorporated into the 200-kDa heavy chain, confirming earlier results using untrapped [alpha-32P]ATP [Maruta, H., & Korn, E. (1981) J. Biol. Chem. 256, 499-502]. After extensive trypsin digestion of labeled subfragment 1, HPLC separation methods combined with alkaline phosphatase treatment allowed two labeled peptides to be isolated. Sequence analysis of both labeled peptides indicated that Glu-185 was the labeled residue. Since Glu-185 has been previously identified as a residue at the active site of smooth myosin using [3H]UDP as a photolabel [Garabedian, T. E., & Yount, R. G. (1990) J. Biol. Chem. 265, 22547-22553], these results provide further evidence that Glu-185, located immediately adjacent to the glycine-rich loop, is located in the purine binding pocket of the active site of smooth muscle myosin.

Nucleotide-induced changes in the interaction of myosin subfragment 1 with actin: detection by antibodies against the N-terminal segment of actin

The binding of myosin subfragment 1 (S-1) to actin in the presence and absence of nucleotides was determined under conditions of partial saturation of actin, up to 80%, by Fab(1-7), the antibodies against the first seven N-terminal residues on actin. In the absence of nucleotides, the binding constant of S-1 to actin (2 x 10(7) M-1) was decreased by 1 order of magnitude by Fab(1-7). The binding of S-1 to actin caused only limited displacement of Fab, and between 30 and 50% of actin appeared to bind both proteins. In the presence of MgAMP.PNP, MgADP, and MgPPi and at low S-1 concentrations, the same antibodies caused a large decrease in the binding of S-1 to actin. However, the binding of S-1.nucleotide to actin in the presence of Fab(1-7) increased cooperatively with the increase in S-1 concentration. Also, in contrast to rigor conditions, there was no indication for the binding of Fab(1-7) and S-1.nucleotide to the same actin molecules. These results show a nucleotide-induced transition in the actomyosin interface, most likely related to the different roles of the N-terminal segment of actin in the binding of S-1 and S-1.nucleotide. The possible implications of these findings to the regulation of actomyosin interactions are discussed.

The molecular anatomy of caldesmon

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An unusual actin-encoding gene in Physarum polycephalum

Actin is one of the most conserved proteins in eukaryotic organisms. In the present work, we cloned and determined the nucleotide sequence of an unusual actin-encoding gene, ardD, from the slime mold, Physarum polycephalum. The ardD gene encodes an ArdD protein containing 367 amino acids (aa) instead of the 375-376 aa found in a typical actin. The nine missing aa are accounted for by deletions of three aa in the first exon, five in the fifth exon and one in the sixth exon. These deletions in the coding sequence were observed in a polymerase chain reaction (PCR)-generated cDNA fragment, which excludes the possibility of a cloning artifact. In addition, ArdD contains numerous aa substitutions distributed throughout the protein. The ArdD aa sequence was compared with published actin sequences. The most identity is seen with the P. polycephalum ArdA, ArdB and ArdC (84%) and Acanthamoeba (82%) actins, while the least identity is found with Tetrahymena actin (67%). The expression of the ardD gene is developmentally regulated. The highest levels of ardD mRNA were found in spherules, less was seen in plasmodia and no detectable transcripts were observed in amoebae. The PCR amplification of an ardD cDNA from spherules confirmed the presence of mRNA in this developmental stage. The aa deletions and substitutions in the predicted ArdD aa sequence make it one of the most distinctive actins known.

Crystallographic analysis of acrosomal bundle from Limulus sperm

The acrosomal process of Limulus sperm contains a bundle of filaments composed of actin and a 102 kDa protein in a 1:1 molar ratio. The structure of the bundle in true discharge was investigated by electron cryomicroscopy, X-ray scattering and crystallographic image analysis. A bundle can be characterized as a quasi-crystal with continuously varying views along the bundle axis. Each segment of the bundle is found to obey the symmetry of space group P1, with a = b = 147 A, c = 762 A, alpha = 90 degrees, beta = 90.6 degrees, gamma = 120 degrees. A unit cell contains a helical repeat of the filament with a selection rule following that of an actin filament. A 24 A projection map based on the h0l view was reconstructed after averaging 5300 unit cells from six electron images. Filaments in this projection are well separated and clearly display a 21 screw symmetry. This screw symmetry results from the helical parameters of the bundle filament and is found to be a non-crystallographic symmetry element present in the unit cell. Our structural analysis has led to the proposal that the assembly of a stable bundle with a defined maximum diameter can be controlled by the crystallographic packing of the twisted filaments.

Site-directed mutations of Dictyostelium actin: disruption of a negative charge cluster at the N terminus

Aspartic acid residues in the N-terminal negative charge cluster of Dictyostelium actin were replaced with histidine residues by site-directed mutagenesis of the actin gene. The mutant actins were expressed in Dictyostelium cells and were purified to homogeneity by HPLC. Functional properties of the mutant actins were compared with those of the wild-type actin. (i) In vitro assays of the sliding movement of actin filaments driven by myosin showed that the movement was slowed by the mutations. (ii) The mutations diminished the actin-activated ATPase activity of myosin in such a way that the maximum turnover rate at infinite actin concentration (Vmax) dropped sharply without an appreciable change in the apparent affinity of actin and myosin (Kapp). These results indicate that the N-terminal negative charge cluster of actin is essential for the ATP-dependent actin-myosin interaction.

Interference with myosin subfragment-1 binding by site-directed mutagenesis of actin

Three N-terminal double mutants of beta-actin expressed in the yeast Saccharomyces cerevisiae have been characterized with respect to DNase-I interaction, N-terminal post-translational modification, polymerizability and myosin subfragment-1 binding. The results strongly support earlier suggestions that the acidic residues at the N-terminus of actin are part of the myosin-binding site, while they seem to be of no importance for the other aspects of actin biochemistry tested. The suitability of this expression system for production of recombinant actin in general is discussed.

C-terminal structure and mobility of rabbit skeletal muscle light meromyosin as studied by one- and two-dimensional 1H NMR spectroscopy and X-ray small-angle scattering

Intact rabbit myosin and two different C-terminal fragments of rabbit muscle light meromyosin (LMM) expressed in Escherichia coli, LMM-30, and LMM-30C', were studied by 1H NMR spectroscopy. X-ray small-angle scattering shows that at high ionic strength two polypeptide chains of LMM-30 (which consists of the C-terminal 262 amino acids of myosin heavy chain) or LMM-30C' (which corresponds to LMM-30 but lacks the last 17 residues) assemble to form an alpha-helical coiled-coil as it is found also in myosin. The last 12 C-terminal residues of one polypeptide chain of LMM-30 and the last 9 C-terminal residues of the other chain are very mobile. The last 8 residues of the two strands are equivalent from the NMR point of view and unfolded; the valine residues in position 255 in the two strands are not equivalent, suggesting an interaction between the two strands, Ser-252, Arg-253, and Asp-254 are completely immobilized in one of the polypeptide strands and partly mobile in the other. Essentially the same pattern is observed in intact myosin. In spite of the large molecular weights of LMM-30 and LMM-30C', it is possible to resolve almost all aromatic residues and to determine the pK values of all the 4 tyrosine and of 9 (out of 10) histidine residues. The tyrosine residues in the two strands are equivalent in the two polypeptide chains and both have a pK of 10.5. The pK values of the histidine residues vary between 5.7 and 7.0.

Three-dimensional structure of myosin subfragment-1 from electron microscopy of sectioned crystals

Image analysis of electron micrographs of thin-sectioned myosin subfragment-1 (S1) crystals has been used to determine the structure of the myosin head at approximately 25-A resolution. Previous work established that the unit cell of type I crystals of myosin S1 contains eight molecules arranged with orthorhombic space group symmetry P212121 and provided preliminary information on the size and shape of the myosin head (Winkelmann, D. A., H. Mekeel, and I. Rayment. 1985. J. Mol. Biol. 181:487-501). We have applied a systematic method of data collection by electron microscopy to reconstruct the three-dimensional (3D) structure of the S1 crystal lattice. Electron micrographs of thin sections were recorded at angles of up to 50 degrees by tilting the sections about the two orthogonal unit cell axes in sections cut perpendicular to the three major crystallographic axes. The data from six separate tilt series were merged to form a complete data set for 3D reconstruction. This approach has yielded an electron density map of the unit cell of the S1 crystals of sufficient detail. to delineate the molecular envelope of the myosin head. Myosin S1 has a tadpole-shaped molecular envelope that is very similar in appearance to the pear-shaped myosin heads observed by electron microscopy of rotary-shadowed and negatively stained myosin. The molecule is divided into essentially three morphological domains: a large domain on one end of the molecule corresponding to approximately 60% of the total molecular volume, a smaller central domain of approximately 30% of the volume that is separated from the larger domain by a cleft on one side of the molecule, and the smallest domain corresponding to a thin tail-like region containing approximately 10% of the volume. This molecular organization supports models of force generation by myosin which invoke conformational mobility at interdomain junctions within the head.

Crystal structure of the catalytic subunit of cyclic adenosine monophosphate-dependent protein kinase

The crystal structure of the catalytic subunit of cyclic adenosine monophosphate-dependent protein kinase complexed with a 20-amino acid substrate analog inhibitor has been solved and partially refined at 2.7 A resolution to an R factor of 0.212. The magnesium adenosine triphosphate (MgATP) binding site was located by difference Fourier synthesis. The enzyme structure is bilobal with a deep cleft between the lobes. The cleft is filled by MgATP and a portion of the inhibitor peptide. The smaller lobe, consisting mostly of amino-terminal sequence, is associated with nucleotide binding, and its largely antiparallel beta sheet architecture constitutes an unusual nucleotide binding motif. The larger lobe is dominated by helical structure with a single beta sheet at the domain interface. This lobe is primarily involved in peptide binding and catalysis. Residues 40 through 280 constitute a conserved catalytic core that is shared by more than 100 protein kinases. Most of the invariant amino acids in this conserved catalytic core are clustered at the sites of nucleotide binding and catalysis.

A method to identify protein sequences that fold into a known three-dimensional structure

The inverse protein folding problem, the problem of finding which amino acid sequences fold into a known three-dimensional (3D) structure, can be effectively attacked by finding sequences that are most compatible with the environments of the residues in the 3D structure. The environments are described by: (i) the area of the residue buried in the protein and inaccessible to solvent; (ii) the fraction of side-chain area that is covered by polar atoms (O and N); and (iii) the local secondary structure. Examples of this 3D profile method are presented for four families of proteins: the globins, cyclic AMP (adenosine 3',5'-monophosphate) receptor-like proteins, the periplasmic binding proteins, and the actins. This method is able to detect the structural similarity of the actins and 70- kilodalton heat shock proteins, even though these protein families share no detectable sequence similarity.

Mechanism of the interaction of human platelet profilin with actin

We have reexamined the interaction of purified platelet profilin with actin and present evidence that simple sequestration of actin monomers in a 1:1 complex with profilin cannot explain many of the effects of profilin on actin assembly. Three different methods to assess binding of profilin to actin show that the complex with platelet actin has a dissociation constant in the range of 1 to 5 microM. The value for muscle actin is similar. When bound to actin, profilin increases the rate constant for dissociation of ATP from actin by 1,000-fold and also increases the rate of dissociation of Ca2+ bound to actin. Kinetic simulation showed that the profilin exchanges between actin monomers on a subsecond time scale that allows it to catalyze nucleotide exchange. On the other hand, polymerization assays give disparate results that are inconsistent with the binding assays and each other: profilin has different effects on elongation at the two ends of actin filaments; profilin inhibits the elongation of platelet actin much more strongly than muscle actin; and simple formation of 1:1 complexes of actin with profilin cannot account for the strong inhibition of spontaneous polymerization. We suggest that the in vitro effects on actin polymerization may be explained by a complex mechanism that includes weak capping of filament ends and catalytic poisoning of nucleation. Although platelets contain only 1 profilin for every 5-10 actin molecules, these complex reactions may allow substoichiometric profilin to have an important influence on actin assembly. We also confirm the observation of I. Lassing and U. Lindberg (1985. Nature [Lond.] 318:472-474) that polyphosphoinositides inhibit the effects of profilin on actin polymerization, so lipid metabolism must also be taken into account when considering the functions of profilin in a cell.

Rotational dynamics of spin-labeled F-actin during activation of myosin S1 ATPase using caged ATP

The most probable source of force generation in muscle fibers in the rotation of the myosin head when bound to actin. This laboratory has demonstrated that ATP induces microsecond rotational motions of spin-labeled myosin heads bound to actin (Berger, C. L. E. C. Svensson, and D. D. Thomas. 1989. Proc. Natl. Acad. Sci. USA. 86:8753-8757). Our goal is to determine whether the observed ATP-induced rotational motions of actin-bound heads are accompanied by changes in actin rotational motions. We have used saturation transfer electron paramagnetic resonance (ST-EPR) and laser-induced photolysis of caged ATP to monitor changes in the microsecond rotational dynamics of spin-labeled F-actin in the presence of myosin subfragment-1 (S1). A maleimide spin label was attached selectively to cys-374 on actin. In the absence of ATP (with or without caged ATP), the ST-EPR spectrum (corresponding to an effective rotational time of approximately 150 microseconds) was essentially the same as observed for the same spin label bound to cys-707 (SH1) on S1, indicating that S1 is rigidly bound to actin in rigor. At normal ionic strength (micro = 186 mM), a decrease in ST-EPR intensity (increase in microsecond F-actin mobility) was clearly indicated upon photolysis of 1 mM caged ATP with a 50-ms, 351-nm laser pulse. This increase in mobility is due to the complete dissociation of Si from the actin filament. At low ionic strength (micro, = 36 mM), when about half the Si heads remain bound during ATP hydrolysis, no change in the actin mobility was detected, despite much faster motions of labeled S1 bound to actin. Therefore, we conclude that the active interaction of Si, actin,and ATP induces rotation of myosin heads relative to actin, but does not affect the microsecond rotational motion of actin itself, as detected at cys-374 of actin.

Similarity of the three-dimensional structures of actin and the ATPase fragment of a 70-kDa heat shock cognate protein

Although there is very little sequence identity between the two proteins, the structures of rabbit skeletal muscle actin (375-amino acid residues) and the 44-kDa ATPase fragment of the bovine 70-kDa heat shock cognate protein (HSC70; 386 residues) are very similar. The alpha-carbon positions of 241 pairs of amino acid residues that are structurally equivalent within the two proteins can be superimposed with a root-mean-square difference in distance of 2.3 A; of these, 39 residues are identical, and 56 are conservative substitutions. In addition, the conformations of ADP are very similar in both proteins. A local sequence "fingerprint," which may be diagnostic of the adenine nucleotide beta-phosphate-binding pocket, has been derived. The fingerprint identifies members of the glycerol kinase family as candidates likely to have a similar structure in their nucleotide-binding domains. The structural differences between the two molecules mainly occur in loop regions of actin known to be involved in interactions with other monomers in the actin filament or in the binding of myosin; the corresponding regions in heat shock proteins may have functions that are as yet undetermined. Placing the Ca2+ ATP of actin on the ATPase fragment structure suggests Asp-206 (corresponding to His-161 of actin) as a candidate proton acceptor for the ATPase reaction.

Troponin I and caldesmon restrict alterations in actin structure occurring on binding of myosin subfragment 1

The effect of troponin I and caldesmon on phalloidin-rhodamine- and 1,5-IAEDANS-labelled actin in skeletal muscle ghost fibers was investigated by polarized fluorescence. Both these proteins inhibited the structural alterations in the actin monomer and the increase of flexibility of actin filaments occurring on binding of myosin heads, and their effects were potentiated by tropomyosin. This immobilization of the actin filament through troponin I and caldesmon seems to originate from restriction of the relative motions of the two domains within the monomer.

Characterisation of missense mutations in the Act88F gene of Drosophila melanogaster

We have created missense mutations in the indirect flight muscle (IFM)-specific Act88F actin gene of Drosophila melanogaster by random in vitro mutagenesis. Following P element-mediated transformation into wild-type flies and subsequent transfer of the inserts into Act88F null strains, the effects of the actin mutants on the structure and function of the IFMs were examined. All of the mutants were antimorphic for flight ability. E316K and G368E formed muscle with only relatively small defects in structure whilst the others produced IFMs with large amounts of disruption. E334K formed filaments but lacked Z discs. V339I formed no muscle structure in null flies and did not accumulate actin. E364K and G366D both had relatively stable actin but did not form myofibrils. Using an in vitro polymerisation assay we found no significant effects on the ability of the mutant actins to polymerise. E364K and G366D also caused a strong induction of heat shock protein (hsp) synthesis at normal temperatures and accumulated large amounts of hsp22 which, together with the mutant actin, was resistant to detergent extraction. Both E316K and E334K caused a weak induction of hsp synthesis. We discuss how the stability, structure and function of the different mutant actins affects myofibril assembly and function, and the induction of hsps.