NMR of a synthetic peptide spanning the triphosphate binding site of adenosine 5'-triphosphate in actin

Myosin II from rabbit skeletal muscle and Dictyostelium discoideum and its interaction with F-actin studied by (1)H NMR spectroscopy

Mg-F-actin occurs in two conformational states, I and M, where the N-terminal amino acids are either immobile or highly mobile. In the rigor or ADP complex of rabbit myosin S1 with Mg-F-actin the N-terminal acetyl group of actin stays in its highly mobile state. The same is true for the complexes with the myosin motor domain from Dictyostelium discoideum. This excludes a direct strong interaction of the N-terminal amino acids with myosin in the rigor state as suggested. An interaction of the N-terminus of F-actin with myosin is also not promoted by occupying its low-affinity binding site(s) with divalent ions. The N-terminal high-mobility region may be part of a structural system which has evolved for releasing inadequate stress applied to the actin filaments.

Mobile segments in rabbit skeletal muscle F-actin detected by 1H nuclear magnetic resonance spectroscopy

Polymerization of actin by increasing the ionic strength leads to a quenching of almost all 1H NMR signals. Surprisingly, distinct signals with relatively small line widths can still be observed in actin filaments (F-actin) indicating the existence of mobile, NMR visible residues in the macromolecular structure. The intensity of the F-actin spectrum is much reduced if one replaces Mg2+ with Ca2+, and a moderate reduction of the signal intensity can also be obtained by increasing the ionic strength. These results can be explained in a two-state model of the actin promoters with a M- (mobile) state and a I- (immobile) state in equilibrium. In the M-state a number of residues in the actin promoter are mobile and give rise to observable NMR signals. This equilibrium is shifted towards the I-state specifically by replacing Mg2+ with Ca(2+)-ions and unspecifically by addition of monovalent ions such as K+. The binding of phalloidin to its high-affinity site in the filaments does not influence the equilibrium between M- and I-state. Phalloidin itself is completely immobilized in F-actin, its exchange with the solvent being slow on the NMR time scale.

Inhibition of cytochalasin D-stimulated G-actin ATPase by ADP-ribosylation with Clostridium perfringens iota toxin

Clostridium perfringens iota toxin belongs to a novel family of actin-ADP-ribosylating toxins. The effects of ADP-ribosylation of skeletal muscle actin by Clostridium perfringens iota toxin on cytochalasin D-stimulated actin ATPase activity was studied. Cytochalasin D stimulated actin-catalysed ATP hydrolysis maximally by about 30-fold. ADP-ribosylation of actin completely inhibited cytochalasin D-stimulated ATP hydrolysis. Inhibition of ATPase activity occurred at actin concentrations below the critical concentration (0.1 microM), at low concentrations of Mg2+ (50 microM) and even in the actin-DNAase I complex, indicating that ADP-ribosylation of actin blocks the ATPase activity of monomeric actin and that the inhibitory effect is not due to inhibition of the polymerization of actin.

Biochemical and theoretical approach to localization of metal-ion-binding sites in the actin primary structure

The number of Ca2+ ions bound at sites other than the single high-affinity site in CaCl2-induced polymers of rabbit skeletal muscle, chicken gizzard, and bovine aorta actin was determined. The polymer of skeletal muscle and aorta actin contained 4 mol Ca2+/mol, whereas gizzard actin only 3 mol weakly bound Ca2+/mol monomer. This difference correlates with the deletion in smooth muscle gamma-actin of one out of four NH2-terminal acidic residues typical of skeletal and smooth muscle alpha-actin isoforms, suggesting that this additional acidic residue in alpha-actins is involved in the weak binding of cations which is essential for polymerization. This experimental result, as well as a theoretical analysis of the actin primary structure, argue against the implication of the NH2-terminal acidic residues in the high-affinity site for divalent cation. The analysis of the actin primary structure aimed at identification of sequences resembling the known Ca2+-binding patterns has revealed the absence of an EF-hand Ca2+-binding site. The best match was obtained between the sequence of the 292-301 segment and that of Ca2+ site in lectins. However, in the light of experimental data discussed, it is more plausible that the actual high-affinity Ca2+ site in actin involves sequentially distant residues from the NH2- and COOH-terminal portions of the polypeptide chain.

ADP-ribosylation of actin causes increase in the rate of ATP exchange and inhibition of ATP hydrolysis

ADP-ribosylation of skeletal muscle actin by Clostridium perfringens iota toxin increased the rate of exchange of actin-bound [gamma-32P]ATP by unlabelled ATP about twofold. Increased exchange rates were observed with ATP and ATP[gamma S], much less with ADP but not with AMP or NAD. ADP-ribosylation of skeletal muscle actin reduced "basal" and Mg2+ (1 mM)-induced ATP hydrolysis by about 80%. Similar inhibition of ATP hydrolysis was observed with liver actin ADP-ribosylated by Clostridium botulinum C2 toxin. The data indicate that ADP-ribosylation of actin at Arg-177 largely affects the ATP-binding and ATPase activity.

Influence of the bound nucleotide on the molecular dynamics of actin

Rotational dynamics of actin spin-labelled with maleimide probes at the reactive thiol Cys-374 were studied. Replacement of the bound nucleotide by Br8ATP in G-actin and Br8ADP in F-actin causes significant increase of the rotational correlation time of the spin probe, indicating reduced motion in both G and F-actin. The orientation dependence of the electron paramagnetic resonance spectra in oriented F-actin filaments revealed an altered molecular order of the probe when the nucleotide was a Br-substituted one. The bound nucleotide affects the myosin S1 ATPase activation by actin; both Vmax and K(actin) decreased significantly when the bound nucleotide of actin was Br8ADP.

Fluorescence resonance energy transfer between sites in G-actin. The spatial relationship between Cys-10, Tyr-69, Cys-374, the high-affinity metal and the nucleotide

Intramonomer fluorescence resonance energy transfer spectroscopy was employed to investigate the spatial relationship between labels attached to the residues Cys-10, Tyr-69, Cys-374, the high-affinity metal binding site and the nucleotide binding site in G-actin. The separation between the fluorescence donor 5-(dimethylamino)naphthalene-1-sulphonyl (Dns) chloride (dansyl chloride) used to label Tyr-69 and the acceptor 4-dimethylaminophenylazophenyl-4'-maleimide (DABM) used to label Cys-374 was found to be 3.6 nm. The distance separating Dns on Tyr-69 from DABM on Cys-10 was found to be 2.7 nm. The distance separating the acceptor DABM bound to Cys-374 from the fluorescence donor formycin A 5'-triphosphate (FTP) occupying the nucleotide binding site was determined to be 3.0 nm. A slightly larger separation was determined between the FTP site and DABM attached to Cys-10. In this case a value of 3.2 nm was obtained. The distance separating Dns on Tyr-69 from Co2+ in the high-affinity metal binding site was determined to be 1.1 nm. Finally, the separation of FTP, now acting as donor, from the Dns molecule attached to Tyr-69 and acting as the acceptor was determined to be 2.1 nm. The likely relationship between these label sites on actin is represented by a model which is used to assist in the determination of the actin structure, with particular reference to the environment of the metal and nucleotide binding sites.