Number and location of adenosine triphosphatase sites of myosin

Probes of eukaryotic DNA-dependent RNA polymerase II-I. Binding of 9-beta-D-arabinofuranosyl-6-mercaptopurine to the elongation subsite

9-beta-D-Arabinofuranosyl-6-mercaptopurine (ara-6-MP) was used to affinity-label wheat germ DNA-dependent RNA polymerase II (or B) (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6). This nucleoside analogue was found to be a competitive inhibitor with respect to [3H]UMP incorporation. Natural substrates protected the enzyme from inactivation by ara-6-MP when the enzyme was preincubated with excess concentrations of substrates, suggesting that the inhibitor binds at the elongation subsite. The inhibitor bound the catalytic center of the enzyme with a stoichiometry of 0.6:1. The sulfhydryl reagent, dithiothreitol, reversed the inhibition by ara-6-MP, suggesting that the 6-thiol group of the inhibitor was interacting closely with an essential cysteine residue in the catalytic center of the enzyme. Chromatographic analysis of the pronase-digestion products of the RNA polymerase II-ara-6-MP complex also showed that ara-6-MP had bound a cysteine residue. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the denatured [6-35S]ara-6-MP-labeled RNA polymerase II revealed that over 80% of the radioactivity was associated with the IIb subunit of the enzyme.

Circular dichroic spectra of 6-thioguanosine nucleotides and their complexes with myosin subfragment 1

The circular dichroic spectra of the thione form of 2-amino-6-mercapto-9 beta-ribofuranosylpurine 5'-triphosphate (thioGTP), thioGDP, thioGTP(gamma S), thioGMP-P(NH)P, and 6-mercapto-9 beta-ribofuranosylpurine 5'-diphosphate (thioIDP) and their complexes with myosin subfragment 1 and heavy meromyosin have been measured between 300 and 400 nm. The free nucleotides have a weak negative circular dichroic spectral peak at their absorption maxima, with a longer wavelength shoulder. On binding to the proteolytic fragments of myosin, the negative peak in enhanced approximately 10-fold and a longer wavelength positive peak appears. These effects are attributed to a change in the stereochemical structure of the nucleotide and specific interactions within the nucleotide binding site. All four thioguanosine nucleotides give these spectral changes although minor significant differences do occur. The CD spectra of the subfragment 1 steady-state complexes with thioGTP and thioGTP(gamma S) are similar to each other but different from the complexes of subfragment 1 with thioGDP and thioGMP-P(NH)P. Interactions between nucleotides in the complexes are excluded by a study of their spectra in 2-propanol when base stacking occurs. This is the first investigation where circular dichroism has been used to determine structural differences between different myosin--nucleotide states and provides evidence that the nucleotide in the gamma S bound state is similar to that of the triphosphate bound state.

Interaction between actomyosin and 8-substituted ATP analogs

Various 8-substituted ATP analogs were synthesized, and their reactions with myosin and actomyosin were studied. The nucleoside triphosphates (NTPs) with an amino group at the 6 position and hydrogen at the 8 position, and formycin 5'-triphosphate (FTP) were hydrolyzed by myosin very slowly in the presence of Mg2+ and rapidly in the presence of EDTA and K+. In contrast, NTPs with substitution of the 8 position, other than FTP, were readily hydrolyzed by myosin in the presence of Mg2+ but were hardly hydolyzed in the presence of EDTA and K+. The Michaelis constant (Km) for hydrolysis of 8-substituted NTP by heavy meromyosin was much larger than the dissociation constant (Kfl) for binding of heavy meromyosin with NTP estimated from the change in tryptophan fluorescence. All the NTPs with no substitution at the 8 position, and FTP, caused an initial Pi burst, actin activation of myosin NTPase, superprecipitation of actomyosin, and myofibrillar contraction. On the other hand, all the 8-substituted NTPs in three possible conformations did not cause these phenomena, regardless of the conformation. These results were discussed in relation to the hindrance of rotation about the glycosidic bond accompanying an 8 substitution.

The interaction of chromophoric nucleotides with subfragment 1 of myosin

The interaction of a series of chromophoric nucleotides derived from 6-mercapto-9-beta-ribofuranosylpurine (thioinosine, thiol) and 2-amino-6-mercapto-9-beta-ribofuranosyl-purine (thioguanosine, thioG) with myosin subfragment 1 isolated from rabbit skeletal muscle was investigated kinetically and spectroscopically. The Mg2+-dependent hydrolyses of thioITP and thioGTP are catalysed by subfragment 1 and probably proceed by a similar mechanism as for ATP hydrolysis, although with different rate constants. For example, the binary thioGDP-protein complex only comprises 8% of the steady-state intermediate of the thioGTPase at 5 degrees C and pH 6.5. Long-lived analogues of intermediates of the thioGTPase were generated by using thioGTP(gammaS) [thioguanosine 5'-(3-thio)-triphosphate], thioGMP-P(NH)P (5'-thioguanylylimidodiphosphate) and thioGDP. The near-u.v. spectra of the thioguanosine nucleotides bound to subfragment 1 were measured and showed that in all cases the purine ring is bound to the protein in a hydrophobic environment, although the pK of the purine thiol group only increases by 0.2-0.3. ThioGTP caused glycerinated rabbit psoas muscle to contract, but in contrast with thioITP was not able to relax muscle. The applications of these chromophoric nucleotides for investigating the mechanism of muscle contraction and other biological systems, particularly those involving guanosine nucleotide regulation, are discussed.

Fluorescence properties of 2' (or 3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate and its use in the study of binding to heavy meromyosin ATPase

2' (or 3')-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (N3ph-ATP), which contains a Meisenheimer complex moiety, is one of the class of compounds which do not fluoresce in water but fluoresce both in low polarity solvents and when bound to the protein molecule. Fluorescence intensity of N3ph-ATP in the range of 540 nm, when excited at 410 nm, decreased with increasing the solvent polarity accompanying the increment of the wavelength of maximum emission. When bound to heavy meromyosin ATPase, the fluorescence properties of N3ph-ADP were almost the same as those of N3ph-ATP in a low polarity solvent, suggesting that N3ph-ADP was bound to hydrophobic area on heavy meromyosin ATPase.

Kinetic analysis of ATPase mechanisms

At even the simplest level we can expect an ATPase mechanism to comprise the following four steps: the binding of ATP, the reaction of ATP with water on the enzyme, and the release of the products ADP and P1. So at the outset techniques are needed to investigate these four processes. The range of techniques needed is soon extended once questions are asked about the role of protons and metal ions, the possibility of a multistep hydrolytic process, multistep substrate and product binding processes, and protein–lipid or protein–protein interactions. Since ATPases and ATP synthases are almost universally involved in some form of energy transduction there is a particular need in an ATPase or ATP synthase reaction to evaluate the equilibrium constants of the steps in the mechanism and to investigate the possibility of alternate reaction pathways. The nature of the coupling process by the protein of the chemical reactions of ATP to the other energetic process, be it muscle contraction, active transport, respiration or photosynthesis, is likewise of profound interest. Finally we would like to know as much as possible about the ATPase or ATP synthase mechanism during the period when the various forms of energy transduction are occurring.

Affinity of myosin S-1 for F-actin, measured by time-resolved fluorescence anisotropy

The association constant for myosin subfragment-1 (S-1) and actin was measured, using a new application of fluorescence depolarization which capitalizes on the fact that S-1 has high rotational mobility while F-actin does not. Uncoupling of the time dependences of the anisotropy decay and the association/dissociation phenomena allowed the experimentally determined anisotropy decay curve to be fitted by a sum of two terms weighted by the mole fractions of the free and bound S-1. At 4 degrees C, ionic strength 0.16 M, and pH 7.0, the association constant Ka is (1.73 +/- 0.35) X 10(6) M-1 at infinite dilution. This makes the -deltaG degrees of binding of F-actin to S-1 similar to the -deltaG degrees of binding of ATP to S-1, and the possible physiological relevance of the similarity to muscle contraction is discussed.

The covalent modification of myosin's proteolytic fragments by a purine disulfide analog of adenosine triphosphate. Reaction at a binding site other than the active site

A purine disulfide analog of ATP, 6,6'-dithiobis(inosinyl imidodiphosphate), forms mixed disulfide bonds between the 6 thiol group on the purine ring and certain key cysteines on myosin, heavy meromyosin, and subfragment one. The EDTA ATPase activities of myosin and heavy meromyosin were completely inactivated when 4 mol of thiopurine nucleotide was bound. When similarly inactivated, subfragment one, depending on its method of preparation, incorporated either 1 or 2 mol of thiopurine nucleotide. Modification of a single cysteine on subfragment one resulted in an inhibition of both the Ca2+ and the EDTA ATPase activities, but the latter always to a greater extent. Modification of two cysteines per head of heavy meromyosin had the same effect suggesting that the active sites were not blocked by the thiopurine nucleotides. Direct evidence for this suggestion was provided by equilibrium dialysis experiments. Heavy meromyosin and subfragment one bound 1.9 and 0.8 mol of [8-3H]adenylyl imidodiphosphate per mol of enzyme, respectively, with an average dissociation constant of 5 X 10(-7) M. Heavy meromyosin with four thiopurine nucleotides bound or subfragment one with two thiopurine nucleotides bound retained 65-80% of these tight adenylyl imidodiphosphate binding sites confirming the above suggestion. Thus previous work assuming reaction of thiopurine nucleotide analogs at the active site of myosin must be reevaluated. Ultracentrifugation studies showed that heavy meromyosin which had incorporated four thiopurine nucleotides did not bind to F-actin while subfragment one with one thiopurine nucleotide bound interacted only very weakly with F-actin. Thus reaction of 6,6'-dithiobis(inosinyl imidodiphosphate) at nucleotide binding sites other than the active sites reduces the rate of ATP hydrolysis and inhibits actin binding. It is suggested that these second sites may function as regulatory sites on myosin.

The effect of substrate analogs on the circular dichroic spectra of thymidylate synthetase from Lactobacillus casei

Circular dichroism studies from 290 to 400 nm with the thymidylate synthetase from Lactobacillus casei revealed characteristic Cotton effects in the presence of various folate analogs plus 5-fluoro-2'-deoxyuridylate. Omission of either substrate analog prevented the appearance of the Cotton effects. When 5-fluoro-2'-deoxyuridylate and (+/-)-5,10-methylenetetrahydrofolate are mixed with the synthetase, a ternary complex results which yields distinctive minor negative ellipicity bands at 285 and 332 nm and a major negative ellipticity bands at 285 and 332 nm and a major positive band at 305 nm. Similar results were obtained with the ternary complex containing (+)-5,10-methylenetetrahydrofolate, but the enzymically inactive (-) diastereoisomer induced only the positive band at 305 nm. More intense Cotton effects were elicited by (+/-)-5,11-methylenetetrahydrohomofolate with a major positive ellipticity band at 308 nm and a minor negative band at 335 nm. A ternary complex was also formed with dihydrofolate, which provided a major circular dichroic band at 305 nm and a broad minor negative band in the region of 335 nm. Deoxyuridylate and thymidylate also formed ternary complexes with dihydrofolate, but their ellipicity bands were much less intense. Other folate analogs that formed ternary complexes with 5-fluoro-2'-deoxyuridylate to provide characteristic circular dichroic spectra were tetrahydrofolate, tetrahydrohomofolate, 10-methyltetrahydrofolate, and a 2-amino-4-hydroxyquinazoline derivative. By measuring the increment in ellipticity at 305 nm on addition of specific ligands to enzyme solutions, it was determined that the L. casei thymidylate synthetase contains two binding sites for 5-fluoro-2'-deoxyuridylate and for each of the diastereoisomers of 5,10-methylenetetrahydrofolate. An improved procedure is presented for the large-scale purification and crystallization of L. casei thymidylate synthetase.

Segmental flexibility in the myosin molecule: evidence from binding studies of myosin fragments with actin

From comparative studies of the association with polymeric actin of the bifunctional species heavy meromyosin and its monofunctional constituents, information about the relative freedom of these paired elements can be derived. An isotherm for the former binding process is presented which involves, as an experimentally determinable parameter, the local concentration of second segment after the first of a pair is attached to the lattice. From combined data for these two association reactions a value of 10(-4) M is obtained for this quantity. The large degree of segmental flexibility reported for the free heavy meromyosin is still manifested in the association with actin.

Individual states in the cycle of muscle contraction

By using appropriate analogs of ATP, isometrically-held glycerol-extracted psoas fibers from rabbits are forced successively into states corresponding to molecular species in the contractile cycle. In each state measurements are made of P[unk], a fluorescence polarization parameter thought to relate to attitude of S-1 moieties of the myosin molecules. Also, the value of P[unk] is measured during active tension development. It is suggested that this value is a time-average of the P[unk] as S-1 moieties move through the various states of the cycle. Proposals are made concerning the sequence of states in the cycle.

Elementary processes of the magnesium ion-dependent adenosine triphosphatase activity of heavy meromyosin. A transient kinetic approach to the study of kinases and adenosine triphosphatases and a colorimetric inorganic phosphate assay in situ

Transient kinetic studies of Mg(2+)-dependent heavy-meromyosin ATPase (adenosine triphosphatase) were done by monitoring the release of both ADP and P(i) into the reaction medium by using linked assay systems. The release of P(i) was monitored by its quantitative transfer to ADP, with concomitant reduction of NAD(+) in the presence of d-glyceraldehyde 3-phosphate, d-glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase. The dissociation rates of the products, ADP and P(i), from heavy meromyosin were shown to be faster than the rate-controlling process, which occurs after the initial bond cleavage of ATP. The chromophoric ATP analogue, 6-mercapto-9-beta-d-ribofuranosylpurine 5'-triphosphate (thioATP) was used as a substrate and spectral changes associated with a single turnover of heavy meromyosin could be assigned to elementary processes of the mechanism. It was shown that the dissociation rate of thioADP was not the rate-controlling process of the thioATPase, whose catalytic-centre activity was 7.6 times that of the ATPase at pH8. The dissociation rate of ADP from heavy meromyosin was measured by using thioATP as displacing agent and was found to be 2.3s(-1), which is about 50 times the catalytic-centre activity of the ATPase at pH8. Transient kinetic studies with chromophoric adenosine phosphate analogues have general application for kinases and ATPases both in characterizing the chemical states of the intermediates and in delineating the elementary processes of the enzyme mechanism.

Light chains of myosins from white, red, and cardiac muscles

Purified preparations of rabbit skeletal white, red, and cardiac muscle myosin (WM, RM, and CM) were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Significant differences in both the molecular weights and number of light chains in these myosins were found. WM has three distinct light-chain components (LC(1W), LC(2W), LC(3W)) having molecular weights of 25,500, 17,400, and 15,100, respectively. No component with a molecular weight around 15,000 is present in RM or CM. RM and CM contain components of identical molecular weights close to 25,000 and 17,000 (LC(1CR) and LC(2CR)) which, however, clearly differ in molecular weight from the corresponding subunits in WM. RM has an additional component (LC(1R)) having a slightly higher molecular weight than LC(1W) and LC(1CR). Thus differences and similarities in many biochemical properties between WM, RM, and CM, which have been described earlier, are also reflected in the light-chain components. The present results support the hypothesis that different sets of genes are active in producing components of myosin that make up different isozymic forms characteristic of each muscle type.