Characterization of the Ca2+-induced conformational changes in gelsolin and identification of interaction regions between actin and gelsolin

Serum gelsolin, a Ca2+-dependent protein regulating the length of actin filaments, undergoes conformational changes upon binding Ca2+. These were detected and analyzed by several approaches including ultraviolet difference spectroscopy, circular dichroism studies, analytical ultracentrifugation, thiol group titration, and limited proteolytic digestions. The effect of Ca2+ binding on the UV absorption difference spectrum and the near-UV circular dichroism spectrum was consistent with changes in the environments of tyrosine and phenylalanine residues. In the presence of Ca2+, the S0(20),w value decreased from 5.3 to 4.7. This latter result implies a transformation to a more asymmetric molecular shape. Gelsolin contained only two accessible thiol groups per mole of protein, one of which was titratable in the native protein; it was more accessible to 5,5'-dithiobis(2-nitrobenzoic acid) in the absence than in the presence of Ca2+. The limited digestion of gelsolin from serum and bovine aorta smooth muscle by two different proteases, chymotrypsin and trypsin, proceeded much faster in the presence of Ca2+ than in its absence with the production of three main fragments of about 40K, 32K, and 21K. This fragment mixture was found still able to shorten F-actin in a Ca2+-dependent manner; this severing activity was expressed by the isolated 40K peptide. Gelsolin was cross-linked to F- and G-actin by the zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide (EDC), generating a covalent 130K binary complex (actin1-gelsolin1) followed by a covalent 180K ternary complex (actin2-gelsolin1).(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotide trapping at the ATPase site of myosin subfragment 1 by a new interthiol crosslinking

When myosin subfragment 1 derivatives in which the reactive sulfhydryl SH1 has been blocked react with N,N'-p-phenylenedimaleimide or 5,5'-dithiobis(2-nitrobenzoic acid), the reactive sulfhydryl group SH2 of the 20-kDa domain is crosslinked with a thiol of the 50-kDa domain of the heavy chain. The crosslink induces the stable trapping of a significant amount of Mg2+-nucleotide in the ATPase site.

Abolition of ATPase activities of skeletal myosin subfragment 1 by a new selective proteolytic cleavage within the 50-kilodalton heavy chain segment

We have isolated and chemically characterized several 5-thio-2-nitrobenzoate-subfragment 1 derivatives (TNB-S-1) generated by the reaction of 5,5'-dithiobis(2-nitrobenzoic acid) (DNTB, up to 10-fold molar excess) with native S-1, N-acetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine-S-1 (AEDANS-S-1), and N,N'-p-phenylenedimaleimide-S-1 (pPDM-S-1) at 4 degrees C, pH 8.0. The reaction of the reagent with AEDANS-S-1, which has a blocked -SH1 group, induced the formation of an intramolecular cystine disulfide between two vicinal -SH groups in S-1; in contrast, the treatment of pPDM-S-1 with DTNB resulted in the formation of TNB mixed disulfides only. The incorporation of the TNB groups (up to 3 mol/mol of S-1) into the native or premodified S-1 led to a local conformational change in the 50K heavy chain region that was fully reversed upon disulfide reduction. Exploiting this peculiarity of the DTNB-modified S-1's, we have realized a highly selective proteolysis of the S-1 heavy chain by thrombin and chymotrypsin, which do not act at all on the normal S-1. The 95K heavy chain was cut by thrombin into two fragments with apparent masses of 68K and 30K, whereas the "connector segments" and the light chains were unaffected. The two new fragments were issued from a primary peptide-bound cleavage between Lys-560 and Ser-561 within the amino acid sequence of the 50K region (M. Elzinga, personal communication).(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of the ATP hydrolysis and actin binding properties of thrombin-cut myosin subfragment 1

We have characterized various structural and enzymatic properties of the (68K-30K)-S-1 derivative obtained by thrombic cleavage [Chaussepied, P., Mornet, D., Audemard, E., Derancourt, J., & Kassab, R. (1986) Biochemistry (preceding paper in this issue)]. The far-ultraviolet CD spectra and thiol reactivity measurements indicated an unchanged overall polypeptide conformation of the enzyme whereas the CD spectra in the near-ultraviolet region suggested a local change in the environments of phenylalanine side chains; the latter finding was rationalized by considering the existence of about five of these amino acids in the vicinity of the cleavage sites. When the binding of Mg2+-ATP and Mg2+-ADP to the derivative was assessed by CD spectroscopy, distinct spectra were obtained with the two nucleotides as with native subfragment 1 (S-1), but some spectral features were unique to the nicked S-1. Stern-Volmer fluorescence quenching studies using acrylamide and the analogues 1,N6-ethenoadenosine 5'-triphosphate and 1,N6-ethenoadenosine 5'-diphosphate indicated that the complexes formed with the modified S-1 have a solute quencher accessibility close to that observed for the complexes with the normal S-1. However, in contrast to the parent enzyme, the thrombin-cut S-1 was unable to bind irreversibly Mg2+-ATP, nor did it form a stable Mg2+-ADP-sodium vanadate complex or achieve the entrapping of Mg2+-ADP after cross-linking of SH1 and SH2 with N,N'-p-phenylenedimaleimide. Additionally, the amplitude of the Pi burst was very low, indicating that the inactivation of the proteolyzed S-1 was linked to the suppression of the hydrolysis step in the ATPase cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient kinetics of adenosine 5'-triphosphate hydrolysis by covalently cross-linked actomyosin complex in water and 40% ethylene glycol by the rapid flow quench method

The initial steps of the ATPase of covalently cross-linked actomyosin subfragment 1 (acto-SF-1) were studied by the rapid flow quench method, and the results obtained were compared with those with reversible (i.e., non-cross-linked) acto-SF-1 and SF-1 under identical conditions. Cross-linked acto-SF-1 plus [gamma-32P]ATP reaction mixture milliseconds old was quenched either in a large excess of unlabeled ATP (ATP chase) or in acid (Pi burst). The conditions were pH 8 and 15 degrees C at 5 mM or 0.15 M KCl and with or without 40% ethylene glycol. In 40% ethylene glycol (5 mM KCl), as with SF-1 and reversible acto-SF-1, the ATP chase was used to titrate active sites and to study the kinetics of ATP binding. Unlike those with SF-1 or reversible acto-SF-1, saturation kinetics were not obtained. The second-order rate constant for ATP binding was 3.1 X 10(6) M-1 s-1 for cross-linked acto-SF-1, 1.8 X 10(6) M-1 s-1 for reversible acto-SF-1, and 2 X 10(6) M-1 s-1 for SF-1. In Pi burst experiments, a transient phase could not be discerned. Because of a high kcat, cross-linked acto-SF-1 was difficult to study in aqueous solution, but at 5 mM KCl, the ATP chase and Pi burst curves were similar to those obtained in 40% ethylene glycol. At 0.15 M KCl the ATP chase curve was difficult to interpret (small amplitude), and there was a small Pi burst.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular movements promoted by metal nucleotides in the heavy-chain regions of myosin heads from skeletal muscle

Molecular movements generated in the heavy-chain regions (27-50-20(X 10(3)) Mr) of myosin S1 on interaction with nucleotides ATP, AMPPNP, ADP and PPi were investigated by limited proteolysis of several enzyme-metal nucleotide complexes in the absence and presence of reversibly bound and crosslinked F-actin. The rate and extent of the nucleotide-promoted conversion of the NH2-terminal 27 X 10(3) Mr and 50 X 10(3) Mr segments into products of 22 X 10(3) Mr and 45 X 10(3) Mr, respectively, were estimated to determine the amplitude of the molecular movements. The 22 X 10(3) Mr peptide was identified by amino acid sequence studies as being derived from cleavage of the peptide bond between Arg and Ile (at position 23 to 24). The 45 X 10(3) Mr peptide, previously shown to represent the NH2-terminal part of the 50 X 10(3) Mr region, would be connected to the adjacent C-terminal 20 X 10(3) Mr region by a pre-existing loop segment of about 5 X 10(3) Mr; the proteolytic sensitivity of the latter region is increased particularly by nucleotide binding. The tryptic reaction proved to be a sensitive indicator of the conformational state of the liganded heavy chain as the rate of peptide bond cleavage in the two regions is dependent on the nature of the bound ligand; it decreases in the order: ATP greater than AMPPNP greater than ADP greater than PPi. It depends also on the nature of the metal present, Mg2+ and Ca2+ being much more effective than K+. Binding of F-actin to the S1-MgAMPPNP complex affords significant protection against breakdown of 27 X 10(3) Mr and 50 X 10(3) Mr peptides, but with concomitant hydrolysis of the 50 X 10(3) Mr-20 X 10(3) Mr junction. Additionally, interaction of MgATP with HMM modulates the tryptic fission of the S1-S2 region. The overall data provide a molecular support for the two-state model of the myosin head and emphasize the involvement of the 50 X 10(3) Mr unit in the mechanism of coupling between the actin and nucleotide binding sites.

Interaction of the heavy chain of gizzard myosin heads with skeletal F-actin

To probe the molecular properties of the actin recognition site on the smooth muscle myosin heavy chain, the rigor complexes between skeletal F-actin and chicken gizzard myosin subfragments 1 (S1) were investigated by limited proteolysis and by chemical cross-linking with 1-ethyl-3-[3-(dimethyl-amino)propyl]carbodiimide. Earlier, these approaches were used to analyze the actin site on the skeletal muscle myosin heads [Mornet, D., Bertrand, R., Pantel, P., Audemard, E., & Kassab, R. (1981) Biochemistry 20, 2110-2120; Labbé, J.P., Mornet, D., Roseau, G., & Kassab, R. (1982) Biochemistry 21, 6897-6902]. In contrast to the case of the skeletal S1, the cleavage with trypsin or papain of the sensitive COOH-terminal 50K-26K junction of the head heavy chain had no effect on the actin-stimulated Mg2+-ATPase activity of the smooth S1. Moreover, actin binding had no significant influence on the proteolysis at this site whereas it abolished the scission of the skeletal S1 heavy chain. The COOH-terminal 26K segment of the smooth papain S1 heavy chain was converted by trypsin into a 25K peptide derivative, but it remained intact in the actin-S1 complex. A single actin monomer was cross-linked with the carbodiimide reagent to the intact 97K heavy chain of the smooth papain S1. Experiments performed on the complexes between F-actin and the fragmented S1 indicated that the site of cross-linking resides within the COOH-terminal 25K fragment of the S1 heavy chain. Thus, for both the striated and smooth muscle myosins, this region appears to be in contact with F-actin.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of the structural and functional properties of the cross-linked complex between F-actin and myosin S-1

Several structural and functional properties of the covalent complex, formed upon cross-linking of the myosin heads (S-1) to F-actin with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, were characterized. The elevated Mg2+-ATPase activity was measured during a 1-month storage of the complex under various conditions. In aqueous medium it showed a rapid time-dependent decrease but it was significantly more stable in the presence of 50% ethylene glycol at -20 degrees C. The ATPase loss most likely reflects a progressive conformational change within the S-1 ATPase site resulting from its greater exposure to the medium, induced by the permanently bound F-actin. The covalent acto-S1 complex was submitted to depolymerization-repolymerization experiments using different depolymerizing agents (0.6 M KI; 4.7 M NH4Cl; low-ionic-strength solution). The depolymerization led to an immediate loss of the enhanced Mg2+-ATPase activity; this activity was almost entirely recovered upon repolymerization of the complex. The protein material formed upon depolymerization of the covalent acto-S1 was analyzed by gel chromatography, gel electrophoresis, analytical ultracentrifugation and electron microscopy. It comprised mainly small-sized actin oligomers associated with the covalently bound S-1 and only a limited amount of free G-actin. The results illustrate the relationships between the filamentous state of actin and its ability to stimulate the Mg2+-ATPase activity of S-1. They also indicate that the binding of S-1 to F-actin is transmitted to several neighbouring actin subunits and strengthens the interactions between actin monomers. Acto-S1 cross-linked complexes were prepared in the presence of tropomyosin and the tropomyosin-troponin system. Under the conditions employed, the regulatory proteins were not cross-linked to actin or S-1 and did not affect the extent or the pattern of S-1 cross-linking to F-actin. Measurements of the elevated Mg2+-ATPase activity of the cross-linked preparations revealed that tropomyosin and the tropomyosin-troponin complex, in the absence of Ca2+, inhibit ATP hydrolysis; the extent of ATPase inhibition (up to 50%) was dependent on the amount of covalently bound S-1, being larger at low level of S-1 cross-linking; the addition of Ca2+ restored the ATPase activity to the control value. The data provide direct evidence that the regulatory proteins can modulate directly the kinetics of ATP hydrolysis by the covalent acto-S1 complex as has earlier been suggested for the reversible complex [Chalovich, J. M. and Eisenberg, E. (1982) J. Biol. Chem. 257, 2432-2437].(ABSTRACT TRUNCATED AT 400 WORDS)

Recurrent systemic embolic events with valve prosthesis

Among 1436 patients who underwent valve replacement, the 400 first cases were studied to assess the features of recurrent systemic embolic event. The mean follow-up was 87 months. Three groups of patients were compared: groups A-289 patients without any thromboembolic event (72.25%); group B-78 patients with only one embolic event (19.5%); group C-33 patients with several embolic events (8.25%). The frequency of recurrence was high: one patient out of three (linearized mean 8.9% in group C considering only one recurrence, vs 3.8% in group B). The recurrence have the same location in 45% of patients. The consequences of these embolisms are serious; each event has a 30 to 40% risk of death or major disability. Four variables seem statistically to promote the occurrence of embolic events: mitral prostheses, pre-operative fibrillation, left atrial enlargement, poor anticoagulant therapy. 54 months after the first embolic event, 60% of the patients with poor anticoagulant therapy experience a recurrent thromboembolism vs 20% with adequate therapy. Twenty-six patients of groups B and C had a pathological study of prostheses. Thrombosis of the prostheses was found in 12 out of 18 patients in group B and in 7 out of 8 patients; in group C. Strict observance of anticoagulant therapy is the better way to prevent thromboembolism and especially recurrences. A reoperation is sometimes necessary. Valve re-replacement was performed in 27 cases out of 1436 patients.

The interaction of skeletal myosin subfragment 1 with the polyanion, heparin

The association between chymotryptic skeletal muscle myosin subfragment 1 (S1) and the polyanion, heparin, was investigated as an experimental approach in probing the functional importance of the cationic sites on S1 and their involvement in ionic interactions within the myosin head during energy transduction. The direct binding of heparin, used at micromolar concentrations, and its influence on the structural and functional properties of S1 were followed by gel chromatography, electron microscopy, chemical cross-linking techniques and limited digestion studies. 1. The limited tryptic digestion of S1 showed that the presence of heparin, as well as of the homopolymer, poly-(L-glutamic acid) causes a specific structural change in the 50-kDa heavy chain region of S1 and accelerates the breakdown of this segment into a 45-kDa species by a proteolytic cleavage restricted to its COOH-terminal portion. Under similar experimental conditions, the binding of MgATP and MgADP to S1 led also to the 50-kDa----45-kDa conversion, suggesting that the S1-nucleotide interactions exhibit some resemblances to the polyanion-S1 binding of polyanionic ligands to S1. This particular area is adjacent to the actin site containing the 45-kDa and 20-kDa segments of the S1 heavy chain. On the other hand, the polyanions as well as nucleotides induced changes in the interface between the heavy chain and the alkali light chains. 2. Moreover, the binding of heparin to S1 resulted in the self-association of the enzyme and the production of stable small S1 oligomers, most likely dimers, which were demonstrated by the alteration of the size of the S1 particles examined by electron microscopy and their freezing by chemical cross-linking agents. These findings are relevant to the recently reported property of skeletal chymotryptic S1 to form dimers under convenient ionic conditions, in particular in the presence of Mg-nucleotides. The interaction of cationic sites on S1 and possibly on the 50-kDa region of the heavy chain with polyanions promotes the dimerization of the S1 molecules. The binding of S1 to F-actin abolished S1 aggregation.

Muscle gelsolin: isolation from heart tissue and characterization as an integral myofibrillar protein

A 92-kDa polypeptide present in rabbit and dog cardiac muscle was purified to homogeneity and some of its properties were investigated using biochemical and cytochemical approaches. The protein was found to be similar, if not identical to macrophage gelsolin; it cross-reacts immunologically with anti-rabbit macrophage gelsolin antibody, has a Ca2+-sensitive shortening effect on the actin filaments as judged by the high shear viscometry and sedimentation experiments, and has a similar amino acid composition. In addition, immunoblot and SDS polyacrylamide gel analysis of cardiac muscle extracts obtained at high and low ionic strength showed that this protein is tightly bound to myofibrils, both in the absence and presence of Ca2+, in ventricular as well as in atrial muscle cells. Indirect immunofluorescence microscopy revealed a striated gelsolin staining pattern analogous to that previously observed for the skeletal muscle gelsolin, suggesting that in the muscle cell this protein is sharing the same localisation as actin. Because of its severing and nucleating properties the gelsolin may play a major role in the organization, assembly and turnover of the thin filaments within the muscle cells.

[Role of surgery in the treatment of chronic aortic insufficiency]

The long-term prognostic factors which determine the indications for surgery are studied on the basis of a series of 198 cases operated for chronic aortic incompetence, in terms of their influence on the actuarial long-term mortality rate due to heart disease and irreversible myocardial dysfunction. The mortality in the first month after surgery is 5 per cent. The factors which significantly modify the long-term outcome of operated patients are clinical (cardiomegaly, degree of preoperative heart failure, ventricular extrasystole, functional grade) and haemodynamic (mean pulmonary artery pressure, end-systolic and-end diastolic left ventricular pressures, left ventricular ejection fraction). Multifactorial studies have shown that ventricular dilatation is the most important prognostic factor. Left ventricular function is severely altered, even in patients with few or no symptoms, but, in the absence of marked functional disturbance, the prognosis is usually good, whatever the repercussions on the myocardium. Surgery should be performed routinely in patients with severe functional disturbance, in the absence of reliable individual criteria capable of predicting when the indications have been exceeded. Surgery is also legitimate in patients with few or no symptoms when investigations reveal severe effects on the left ventricular function.

Selective cleavage of the connector segments within the myosin-S1 heavy chain by staphylococcal protease

The existence of the two connector segments linking the tryptic 50 kDa fragment of skeletal S1 heavy chain to the adjacent 27 kDa and 20 kDa peptides was ascertained by digestion of S1 with staphylococcal protease which was found to act specifically at these particular regions. Three new peptides of Mr 28000, 48000 and 22000 were produced and the novel S1 derivative formed had an intact actin-activated ATPase activity. Amino acid sequence analyses indicated that the 48 kDa and 22 kDa peptides overlap the two connector elements.

Structural and actin-binding properties of the trypsin-produced HMM and S1 from gizzard smooth muscle myosin

The reaction of trypsin on the heavy chain of gizzard myosin and chymotryptic HMM was investigated under restricted fragmentation conditions. The three fragments of the head part with 29 kDa, 50 kDa and 26 kDa were isolated and identified. The 66 K heavy chain segment containing the S1-S2 junction was slowly but extensively degraded liberating a S1-like entity which lacked an intact COOH-terminal 26 kDa region; this isolated species displayed full intrinsic ATPase activities but little actin-binding ability. Tryptic HMM was also formed bearing a fragmented heavy chain and lacking the 20 kDa light chain. Its actin-activated ATPase was derepressed upon cleavage of the 66 kDa segment by papain. We propose that the integral 66 kDa heavy chain component is directly involved in the regulation of the gizzard actomyosin ATPase.

Structure of the actin-myosin interface

The topography of the rigor complex between F-actin and myosin heads (S1) has been investigated by carbodiimide zero-length cross-linking. The results demonstrate for the first time that the 95,000-molecular weight (95K) heavy chain of the myosin head enters into van der Waals contact with two neighbouring actin monomers; one is bound to the 50K domain and the other to the 20K domain of the myosin chain. The covalent F-actin-S1 complex can be isolated; it shows a vastly elevated Mg2+-ATPase. Each pair of actin subunits in the thin filament seems to act as a functional unit for specific binding of a myosin head and stimulation of its Mg2+-ATPase activity.

Structural aspects of actomyosin interaction

Actin binding to myosin-S1 modulates the limited tryptic cleavage of the COOH-terminal region of the 95K heavy chain at the joint connecting the 75K and 20K peptide units; concomitantly actin affords total protection against the resulting loss of acto-S1 Mg2+-ATPase activity. The specificity of the actin effect is illustrated by the fact that it exerts itself not only on free S1 but also on the intact myosin molecule. Mg2+-ATP and Mg2+-ADP impair the protective action of actin to an extent closely related to their respective affinity for the acto-S1 complex. Tryptic fragmentation of S1 heavy chain under highly controlled conditions, using trypsin to S1 weight ratios in the range 1:1000 - 1:1500 led us to establish that peptide bond cleavage at the 75K-20K junction is a sequential process giving rise first to a 22K peptide intermediate which is subsequently converted to the stable 20K fragment. Most importantly, it is also demonstrated that the loss of S1 activation by actin is not due to the initial scission of the 75K-22K linkage but is intimately associated with the breakdown of the 22K precursor into its 20K moiety. Three trypsin-modified S1 derivatives, the heavy chain of which is a complex of two or three fragments, were purified. A detailed analysis of the C-termini of these fragments, as compared to the C-terminal structure of the intact heavy chain, indicated that the 20K fragment is formed mainly through the degradation of a NH2-terminal 2K segment in the 22K precursor and that this proteolytic event is the only one accounting for the acto-S1 ATPase loss. Cross-linking experiments exploiting the reaction of a carbodiimide reagent with rigor complexes containing either fluorescent actin or fluorescent fragmented S1 revealed unequivocally the attachment of the actin monomer to recognition sites on the 20K and 50K units of S1 heavy chain. Specific interactions between the C-terminal 20K domain and light chain LC2 are proposed as being part of the molecular mechanism of the myosin-linked regulation of actomyosin interaction.

Primary structure of lobster-muscle arginine kinase. Amino and carboxyl-terminal structure of the enzyme and complete alignment of the cyanogen-bromide peptides

The acetylation state of the blocked NH2-terminal end of arginine kinase was characterized by the occurrence of 1 mol acetyl group/mol protein; a 5-residue segment corresponding to the amino-terminal portion was isolated from a pronase digest of the enzyme and its amino acid sequence determined as N-acetyl Ala-Asx-Ala-Thr. Arginine kinase is terminated at the carboxyl end by the sequence Lys-GluMetOH; this particular 3-residue sequence is repeated three times in the overall structure of the protein and is present in three CNBr fragments. One of these, a peptide of 14 amino acid residues, was identified in the course of this study and its amino acid sequence determined. Its location at the COHO-terminal end of the enzyme was recognized on the basis of investigations carried out with des-MetOH-Glu-arginine kinase, a specific proteolytic derivative. The alignment of the eight CNBr-fragments which constitute the arginine kinase molecule was established according to the sequential and compositional properties of seven unique tryptic methionyl peptides isolated from the whole protein. The alignment was confirmed by using BNPS-skatole fragments of the enzyme as another protein source.

Photoaffinity labelling of arginine kinase and creatine kinase with a gamma-P-substituted arylazido analogue of ATP

1. An ATP analogue with a photoactivated azide group attached to the gamma-phosphate via an amide bond, ATP gamma-p-azidoanilide, appeared to have potential use as a photoaffinity label for the nucleotide-binding regions of ATP: guanidine phosphotransferases. Upon photolysis in the presence of lobster muscle arginine kinase and rabbit muscle creatine kinase, the analogue is converted to a potent inhibito of these two kinases. This photo-dependent inhibition is specific as it cannot be induced by azidoaniline, a mixture of azidoaniline and ATP or by ATP gamma-p-aminoanilide. Preirradiated under suitable conditions, the photoanalogue still shows a transitory inhibitory effect which, however, slowly vanishes with time (t0.5 = 3 h). 2. The photoinhibition is significantly decreased by the presence of ATP or ADP but is completely prevented by the addition of a mixture of nucleotide and guanidine substrates. Differential spectroscopy and affinity chromatography on Sepharose-ATP demonstrated the inability of photoinactivated arginine kinase and creatine kinase to recognize their nucleotide substrates. 3. Experiments with [14C]ATP gamma-p-azidoanilide indicated that photolysis is associated with an irreversible and stoichiometric binding of the ATP analogue to the enzymes. Autoradiographs made with the peptide maps corresponding to the tryptic digests of each 14C-labelled photomodified enzyme showed an unexpected highly specific labelling of the proteins. 4. Thiiol titrations of the kinases which have been subjected to various photolysis conditions led to the conclusion that the arylnitrene moiety of the photoanalogue is covalently attached to the single reactive cysteinyl side chain present in the active-site region of the two homologous kinases. This amino acid residue appears, therefore, to be located near the phosphate chain binding subsite occupied by the ATP analogue and probably also by the natural nucleotide substrates.

Involvement of an arginyl residue in the catalytic activity of myosin heads

1. Phenylglyoxal reacts rapidly with isolated myosin heads (subfragment 1) and induces two successive and distinguishable effects on their enzymic properties: first, a twofold activation of the Ca2+ and Mg2+-dependent ATPases with no effect onthe K+-ATPase followed by inhibition of the K+, Ca2+ and actin-activated Mg2+-ATPases. A specific protein-reagent reagent complex is formed during the second phase of the modification reaction (Ki approximately 5 x 10(-3) M). 2. ADP and ATP with or without cations provide efficient protection only against the loss of ATPase activities, suggesting that the second inhibitory process is occurring at or close to the active site. 3. On the basis of [14C]phenylglyoxal-labelling experiments and the composition of modified subfragment-1 derivatives, it is demonstrated that the sequential modification of two reactive arginyl residues is responsible for the observed activation-inhibition phenomena. Blocking of the first reactive residue produces a shift in the pH/activity curves related to the Ca2+ and Mg2+-dependent ATPases with an apparent activation effect. Modification of the second guanidino group does not destroy the affinity of the protein for the nucleotide substrates but does alter the nucleotide binding site as reflected in the inability of Mg2+. ATP to dissociate the modified subfragment-1--actin complex. It is concluded that electrostatic interactions between this positively charged group and the negatively charged ATP and ADP molecules may be critical for the hydrolytic efficiency of myosin heads. 4. After dissociation and separation of the polypeptide constituents of the protein in acetic acid medium, both labelled sites are found to reside in the heavy chain.