Localization of two myosin-subfragment-1 binding contacts in the 96-132 region of actin subdomain-1

Quaternary Structure and Hetero-Oligomerization of Recombinant Human Small Heat Shock Protein HspB7 (cvHsp)

In this study, a reliable and simple method of untagged recombinant human HspB7 preparation was developed. Recombinant HspB7 is presented in two oligomeric forms with an apparent molecular weight of 36 kDa (probably dimers) and oligomers with an apparent molecular weight of more than 600 kDa. By using hydrophobic and size-exclusion chromatography, we succeeded in preparation of HspB7 dimers. Mild oxidation promoted the formation of large oligomers, whereas the modification of Cys 126 by iodoacetamide prevented it. The deletion of the first 13 residues or deletion of the polySer motif (residues 17–29) also prevented the formation of large oligomers of HspB7. Cys-mutants of HspB6 and HspB8 containing a single-Cys residue in the central part of the β7 strand in a position homologous to that of Cys137 in HspB1 can be crosslinked to the wild-type HspB7 through a disulfide bond. Immobilized on monoclonal antibodies, the wild-type HspB6 interacted with the wild-type HspB7. We suppose that formation of heterodimers of HspB7 with HspB6 and HspB8 may be important for the functional activity of these small heat shock proteins.

Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements

Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.

Filamin isogene expression during mouse myogenesis

The developmental pattern of filamin gene expression has been studied in mouse embryos by using in situ hybridization. The probes used were isoform specific, (35)S-labeled antisense complementary ribonucleic acids (cRNAs) to the 3; untranslated region (3; UTR) of muscle-specific and nonmuscle-specific filamin genes. Northern blot and in situ hybridization results showed that nonmuscle-specific filamin transcripts had a size of 9.5 kb and were expressed in all nonmuscle tissues. Labeling was most intense in tissues containing a substantial proportion of epithelial and smooth muscle cells. Muscle-specific filamin transcripts had a size of 10 kb and were expressed primarily in cardiac and skeletal muscle. The expression of muscle-specific filamin messenger ribonucleicacids (mRNAs) was detected in heart at 8.0 days after coitum, whereas that in the myotomes of somites was not detected until 10.5 days after coitum. The expression of muscle-specific filamin mRNAs in heart and in skeletal muscle continued through the subsequent days of myogenesis. The results showed that muscle-specific filamin gene transcripts are detected before the formation of myotubes in vivo. This is the first study of filamin gene expression at the early stages of skeletal muscle development. Dev Dyn 2000;217:99-108.

The identification of a second cofilin binding site on actin suggests a novel, intercalated arrangement of F-actin binding

The cofilins are members of a protein family that binds monomeric and filamentous actin, severs actin filaments, and increases monomer off-rate from the pointed end. Here, we characterize the cofilin-actin interface. We confirm earlier work suggesting the importance of the lower region of subdomain 1 encompassing the N and C termini (site 1) in cofilin binding. In addition, we report the discovery of a new cofilin binding site (site 2) from residues 112-125 that form a helix toward the upper, rear surface of subdomain 1 in the standard actin orientation (Kabsch, W., Mannherz, H. G., Suck, D., Pai, E. F., and Holmes, K. C. (1990) Nature 347, 37-44). We propose that cofilin binds "behind" one monomer and "in front" of the other longitudinally associated monomer, accounting for the fact that cofilin alters the twist in the actin (McGough, A., Pope, B., Chiu, W., and Weeds, A. (1997) J. Cell Biol. 138, 771-781). The characterization of the cofilin-actin interface will facilitate an understanding of how cofilin severs and depolymerizes filaments and may shed light on the mechanism of the gelsolin family because they share a similar fold with the cofilins (Hatanaka, H., Ogura, K., Moriyama, K., Ichikawa, S., Yahara, I., and Inagiki, F. (1996) Cell 85, 1047-1055).

Interaction of 75-106 actin peptide with myosin subfragment-1 and its trypsin modified derivative

To explore the role of a hydrophobic domain of actin in the interaction with a myosin chain we have synthesized a peptide corresponding to residues 75-106 of native actin monomer and studied by fluorescence and ELISA the interaction (13+/-2.6x10(-6) M) with both S-1 and (27 kDa-50 kDa-20 kDa) S-1 trypsin derivative of myosin. The loop corresponding to 96-103 actin residues binds to the S-1 only in the absence of Mg-ATP and under similar conditions but not to the trypsin derivative S-1. Biotinylated C74-K95 and I85-K95 peptide fragments were purified after actin proteolysis with trypsin. The C74-K95 peptide interacted with both S-1 and the S-1 trypsin derivative with an apparent Kd(app) of 6+/-1.2x10(-6) M in the presence or absence of nucleotides. Although peptide fragment I85-K95 binds to S-1 with a Kd(app) of 12+/-2.4x10(-6) M, this fragment did not bind to the trypsin S-1 derivative. We concluded that the actin 85-95 sequence should be a potential binding site to S-1 depending of the conformational state of the intact 70 kDa segment of S-1.

Inhibitory actions of synthetic actin peptides and 2,3-butanedion monoxime on actomyosin system

Interaction of myosin and actin has been studied in the presence of inhibitors in order to identify and characterize sites essential for the motor function. Acidic amino acid cluster of actin at the amino terminal region is known to interact with basic residue around 50K-20K connecting loop of the myosin heavy chain. In this study, an amino terminal peptide region of actin without the initial four acidic residue cluster is also shown to have important role in the interaction with myosin in the presence of ATP. A set of peptides corresponding to actin sequences from Leu8 to Arg28 or to Gly36 were synthesized. In the absence of actin, neither activation nor inhibition of MgATPase activity of S1 alone has been observed with both types of peptide. In the presence of actin, however, the longer peptide (L8G36) at 50 microM shows about 50% inactivation of the control value of actoS1 Mg2+ATPase activity. The shorter one (L8R28) has been a non competitive and weaker inhibitor. The inhibition could be interpreted as a results of impairing of one of the interactions necessary for normal progression of actomyosin ATPase reaction. Moreover, these results indicate a contribution of amino terminal sequence of actin without initial four acidic residues for the acto myosin interaction. Examination of the inhibitory mechanism of BDM has shown that the effect may not be due to lowering the binding affinity between actin and myosin.

Analysis of long-range structural effects induced by DNase-I interaction with actin monomeric form or complexed to CapZ

Two fundamental properties of monomeric actin were examined in this study, ie its interaction with DNase-I, and the inhibition of endonuclease activity consecutive to the association of the two molecules. In particular, the topological independence between catalytic site of DNase-I and interface with actin, structural changes in actin monomer and the absence of conformational changes in DNase-I were described. We demonstrated a loss of flexibility of antigenic structures in actin subdomain I (ie epitopes 18-28 and 95-105) as well as modification in the exposure of Cys10 and Cys374 after DNase-I binding. Furthermore, the conformational changes induced by DNase-I into the actin molecule weakened the interaction of CapZ to its binding site located in the C-terminal region of actin monomer. These structural changes were time-dependent. When actin was cleaved in the DNase-I binding loop (sequence 38-52) at position 42 by E coli A2 strain protease, a tight DNase-I binding to split actin and the conformational changes were still observed, whereas the DNase-I inhibition activity was completely abolished. Finally, when we substitute Ca2+ by Mg2+ (ATP-Mg2+ monomeric actin) which induces a tighter conformation of actin and partially restores the inhibitory ability of split actin, long-range conformational effects of DNase-I are prevented and the ternary complex DNase-I-actin-CapZ is obtained.

Conformational and functional studies of three gelsolin subdomain-1 synthetic peptides and their implication in actin polymerization

Gelsolin, a calcium and inositol phospholipid-sensitive protein, regulates actin filament length. Its activity is complex (capping, severing, etc.) and is supported by several functional domains. The N-terminal domain alone (S1), in particular, is able to impede actin polymerization. Our investigations were attempted to precise this inhibitory process by using synthetic peptides as models mimicking gelsolin S1 activity. Three peptides issued from S1 and located in gelsolin-actin interfaces were synthesized. The peptides (15-28, 42-55, and 96-114 sequences) were tested for their conformational and actin binding properties. Although the three peptides interact well with actin, only peptide 42-55 affects actin polymerization. A detailed kinetic study shows that the latter peptide essentially inhibits the nucleation step during actin polymerization. In conclusion, the present work shows that the binding of a synthetic peptide to a small sequence located outside the actin-actin interface is essential in the actin polymerization process.

Localization of a calcium sensitive binding site for gelsolin on actin subdomain I: implication for severing process

The binding of the N-terminal domain (S1) of gelsolin to monomeric actin has been extensively documented. In contrast, the location of the C-terminal calcium dependent domains (S4-6) interacting with the actin filament during the severing process remains uncertain. In this study, we have identified a new interface that supports calcium dependent gelsolin binding to actin. This site is located in a critical position towards actin-actin contact in the filament and in the vicinity of the phalloidin site. Using specific antibody and synthetic peptides derived from actin sequence within 105-132 residues, this interface was finally ascribed to the segment 112-120 on the actin subdomain-1.

Biochemical evidence for the presence of an unconventional actin protein in a prokaryotic organism

The ubiquity of actin, like the functional diversity of many associated proteins, raises a question concerning diversification of motility mechanisms and thus the emergence of an elementary functional system. Our aim was to investigate, in particular, mobiles prokaryotics cells as Synechocystis lacking cilia and flagella, search for actin essential properties and then locate the molecular behaviours. Here we report the presence and purification of a 56-kDa (apparent molecular weight) prokaryotic protein that polymerizes to form filaments, activates myosin Mg(++)-ATPase activity, inhibits DNase-1 activity and affords close antigenic homology to skeletal actin. This protein was found to be associated with thylakoid membranes and extracted in the presence of Triton X-100.

Two identical hydrophobic clusters are present on the same actin monomer: interaction between one myosin subfragment-1 and two actin monomers

Two-dimensional hydrophobic clusters analysis (HCA) was used to compare the distribution of hydrophobic clusters along various actin sequence. HCA-deduced patterns were not altered by amino-acid variations throughout the evolution of actin and we observed similar hydrophobic motifs comprising myosin subfragment-1 ATP-independent binding sites. HCA suggested the presence of two groups of identical hydrophobic motifs (A1 and A2) which bound on each side of the S1 (63 kDa-31 kDa) connecting segment in relation with two actin monomers. This connection is important in communications between actin- and nucleotide-binding sites. We postulate that some relation and message between the two motifs A1 and A2 take place through myosin subfragment-1 (63 kDa-31 kDa) connecting segment.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Interaction of skeletal-muscle myosin subfragment-1 with the actin-(338-348) peptide

The data presented here confirm and provide further experimental evidence that rabbit skeletal-muscle myosin subfragment-1 (S-1) binds to the postulated actin-(338-348) hydrophobic segment [Kabsch, Mannherz, Suck, Pai and Holmes (1990) Nature (London) 347, 37-44] with high affinity in the absence and presence of MgATP. The apparent dissociation constant of the S-1 interaction (5.5 x 10(-7) M) with the actin-(338-348) peptide was of the same order of magnitude as that of the actin-(18-28) binding site (2 x 10(-6) M). In similar conditions, fragmented (27 kDa-50 kDa-20 kDa) S-1 also bound to the peptide. Antibodies directed to the vicinal sequence 348-358 were rapidly eliminated from actin by S-1 interaction and weakened S-1 binding to monomeric or filamentous actin. The antigenic site (348-358) is located very close to the C-terminal S-1-binding site (360-369) and encompasses some residues (Leu-349 and Phe-352) included in the hydrophobic S-1-binding region [Schröder, Manstein, Jahn, Holden, Rayment, Holmes and Spudich (1993) Nature (London) 364, 171-174]. It was observed that anti-[actin-(348-358)] antibodies were also unable to decrease actomyosin ATPase activity, in contrast with previous results obtained with anti-[actin-(18-28)] antibodies [Adams and Reisler (1993) Biochemistry 32, 5051-5056]. The hydrophobic actin-(338-348) peptide used in considerable excess was unable to perturb acto-S-1 and S-1 activities in contrast with results obtained with the N-terminal actin peptide [Kôgler, Moir, Trayer and Ruegg (1991) FEBS Lett. 294, 31-34].