[Interaction of annexin VI with actin]. / Vzaiemodiia aneksynu VI z aktynom.

Actin polymerization was investigated using fluorescence probe N-(1-pyrenyl)iodoacetamide, which was bound covalently to reactive sulfhydryl group, Cys-373. Labeled actin in the bulk was 0.5 to 1% of total actin concentration. Actin polymerization at concentration 12 mM was started by addition of 20 mM KCl and 2 mM MgCl2. The label fluorescence was excited at 365 nm and registered at 386 nm. Under actin polymerization the label fluorescence increased almost 10 times. Two main phases may be distinguished in the process of actin polymerization: 1) monomer activation and nucleus (trimer) formation, 2) growth of actin filaments on the nuclei. In our experimental conditions, both for pure actin and for that with added annexin VI, the 1st phase continued for about 3 min and after that the 2nd phase was perfectly approximated by exponential dependence. An analysis of the exponential curves showed that actin monomer lifetime increased from 327 s, at annexin absence, to about 373 s at 0.7 microM annexin and more. Calculation of rate constants at two ends of growing actin filament suggests that annexin VI binds with pointed ("slow") end so that at sufficient annexin concentration the filament grows only on barbed ("fast") end. Our results, together with data of other researchers showing that annexin VI binds with the inner membrane surface of smooth muscle cell through Ca2+, may indicate that, at Ca2+ entering the cell, this annexin binds actin filament pointed ends to cell surface making it ready for the act of contraction.

[Effect of preincubation on smooth muscle myosin light chain kinase activity]. / Zalezhnist' aktyvnosti kinazy lehkykh lantsiuhiv miozynu hladen'kykh m'iaziv vid umov peredinkubatsiï.

Phosphorylation of myosin regulatory light chain (RLC) catalysed by myosin light chain kinase (MLCK) is a key reaction in the regulation of actin-myosin interaction in smooth muscle. The activation of MLCK by calmodulin (CaM) and Ca2+ was investigated over a wide range of the enzyme concentrations using myosin or its RLC with Mw = 20 kDa as substrates. Kinase activation by CaM (at saturating Ca2+ concentrations) was characterized by positive cooperativity even though noncooperative activation would be expected from the established 1:1 binding stoichiometry between MLCK and CaM. The activation of the kinase by Ca2+ was also cooperative but only at relatively low CaM levels. This cooperativity was shown to result from time dependent changes in MLCK that take place during its incubation with Ca2+ and CaM before substrate addition in phosphorylation assays. As a result the kinase activity as a function of its concentration at constant CaM level was biphasic: there was the activity optimum at 1:1 ratio of CaM to MLCK and almost complete inhibition at 3 to 7 molar excess of kinase over CaM. Such changes that take place during 10 to 15 min preincubation with Ca2+ and CaM may involve the kinase supramolecular structure formation or/and its conformational rearrangements.

[Conformational modifications of smooth muscle light chain kinase]. / Konformatsiina modyfikatsiia kinazy lehkykh lantsiuhiv miozynu hladenkykh m'iaziv.

Our recent investigations have shown that smooth muscle myosin light chain kinase (MLCK) exists in solution as a mixture of oligomeric, dimeric and monomeric species; besides during preincubation (maintaining of the activated enzyme without substrate) with substoichiometric amounts of calmodulin (CaM) it undergoes definite changes leading to several fold lowering of its activity. Fluorescent data obtained in this work suggest that such kinase inhibition must not be connected with quantitative redistribution of different kinase species but rather it is the result of conformational modifications of this enzyme activated molecules leading to the reduction of their affinity to CaM. Such conformational rearrangements took place also at equimolar kinase to CaM ratio (or CaM excess) but in this case they were characterized by lower depth and insignificant MLCK activity fall. The nature of these conformational changes is discussed.

Smooth muscle myosin light chain kinase, supramolecular organization, modulation of activity, and related conformational changes.

It has recently been suggested that activation of smooth muscle myosin light chain kinase (MLCK) can be modulated by formation of supramolecular structures (Sobieszek, A. 1991. Regulation of smooth muscle myosin light chain kinase. Allosteric effects and co-operative activation by CaM. J. Mol. Biol. 220:947-957). The present light scattering data demonstrate that the inactive (calmodulin-free) MLCK apoenzyme exists in solution as a mixture of oligomeric (2% by weight), dimeric (53%), and monomeric (45%) species at physiological ionic strength (160 mM salt). These long-living assemblies, the lifetime of which was measured by minutes, were in equilibrium with each other. The most likely form of the oligomer was a spiral-like hexamer, the dimensions of which fit very well the helical structure of self-assembled myosin filaments (Sobieszek, A. 1972. Cross-bridges on self-assembled smooth muscle myosin filaments. J. Mol. Biol. 70:741-744). After activation of the kinase by calmodulin (CaM) we could not detect any appreciable changes in the distribution of the kinase species either when the kinase was saturated with CaM or when its molar concentration exceeded that of CaM. Our fluorescent measurements suggest that the earlier observed inhibition of kinase at substoichiometric amounts of CaM (Sobieszek, A., A. Strobl, B. Ortner, and E. Babiychuk. 1993. Ca2+-calmodulin-dependent modification of smooth-muscle myosin light chain kinase leading to its co-operative activation by calmodulin. Biochem. J. 295:405-411) is associated with slow conformational change(s) of the activated (CaM-bound) kinase molecules. Such conformational rearrangements also took place with equimolar kinase to CaM; however, in this case there was no decrease in MLCK activity. The nature of these conformational changes, which are accompanied by reduction of the kinase for CaM affinity, is discussed.

[Effect of regulatory light chains of myosin on aggregation of its subfragment-1]. / Vplyv rehuliatyvnykh lehkykh lantsiuhiv miozynu na ahrehatsiiu ioho subfrahmentu-1.

Chymotryptic (Ch-Cl) and Mg(2+)-papain subfragments 1 of (Mg-Cl) skeletal myosin has been studied. Mg-Cl is known to differ from Ch-Cl by the presence of the regulatory light chain (RLC) and elongated heavy chain including C-end hinge segment. Experimental data prove the decisive part of coordination bondings with bivalent cations in stabilization of RLV on myosin head hinge segment. Hydrophobic interactions are also significant that is evidenced by the intensive aggregation between subfragment Mg-Cl molecules, after RLC dissociation.

[Functionally different states of the "hydrophobic pocket" of the myosin ATPase center]. / Funktsional'no razlichnye sostoianiia "gidrofobnogo karmana" ATP-aznogo tsentra miozina.

The influence of an increased temperature (39 degrees C) on a denaturation of 50 kDa-fragment of myosin subfragment 1 was studied in the presence of different nucleoside triphosphates (NTP) and nucleoside diphosphates (NDP). The degree of the denaturation was appreciated evaluated from its trypsinolysis depth. According to their protective influence NTP and NDP were shown to arrange in lines ATP greater than or equal to CTP greater than UTP greater than GTP and ADP greater than GDP greater than CDP greater than UDP, correspondingly. The results received and the literature data allow to suggest that there are at least two states of ATPase site hydrophobic pocket, one of which in responsible for sharp ATPase reaction slowing-down on the stage of macroergic bonding splitting.

[Structural characteristics and aggregation of myosin heads in skeletal muscles]. / Strukturnye osobennosti i agregatsiia golovok miozina skeletnykh myshts.

Native conformational modifications of rabbit skeletal muscle myosin and its subfragment-1 (S-1) within the temperature range of 0-40 degrees C and irreversible unfolding of these proteins structure at temperatures 40-70 degrees C have been studied by the fluorescence and light scattering methods. The results obtained permit stating that myosin and its active subfragments form associates at the concentrations above 0.3 microM. Hydrophobic interactions between definite sites of S-1 are likely to be primarily responsible for the association. The complex profile of S-1 melting curve at high ionic strength indicates the existence of three structural domains in the heavy chain of the myosin head.

[Spectrofluorimetric detection of two states during melting of ribonuclease T1]. / Spektrofluorimetricheskoe obnaruzhenie dvukh sostoianii pri plavlenii ribonukleazy T1.

The melting temperature of ribonuclease T1 was studied by the fluorescent method. It was shown that in the melting region the tryptophanyl fluorescence spectrum of the protein containing a single tryptophanyl is the sum of two simple spectra typical for tryptophanyl located in the hydrophobic environment and for tryptophanyl completely accessible to aqueous solvent, correspondingly. This implies the evidence of two forms of the protein, i.e. native (folded) and denatured (unfolded), in the transition region. No intermediate states were found in measured quantities. Therefore, ribonuclease T1 melting process corresponds to the two states model. The free energy of native structure stabilization of the protein at room temperature is delta G approximately equal to 37 kJ/mol.

[Study of interaction between alcohols (mono- and polhydric) and proteins in aqueous solutions]. / Doslidzhennia vzaemodii mizh spirtami (odno- ta bahatoatomnimi) i bilkami u vodnikh rozchinakh

Phosphorescence properties of protein aqueous solutions (life-time, bend-point on the plots of life-time v. s. temperature, maximum positions in spectra, intensity) were shown to depend upon the concentration in the solution of solute--additions (mono- and polyhydric alcohols, glucose, sucrose, certain amino acids). New parameter tau--average life-time of protein phosphorescence in the persence in the solution of the iodine ion constant concentration, was proposed. By the character of dependence of this parameter upon the solute-addition concentration all solutes, which were used as additions, may be devided into two classes. A rise in the solution of the first class additions (monohydric alcohols) was followed by an increase of tau, that evidences for a decrease in availability of protein tryptophanyls, which may be a result of the adsorption of solute-addition by protein. With the addition concentration about 0.5% tau passes through maximum and then decreases. An increase in the solution of the second class addition (glycol, glycerol, polyethylenglycol, glucose, surcrose) was followed by a decrease of tau, that indicates to an increase of trypophanyl availability, Such an interaction of additions with protein is typical of papain and fibrinogen. In the case of myosin the addition of the second class solutes causes a sharp increase of tau with their sufficient concentration. The obtained results make it possible to state that the solute-additions affect proteins directly besides their indirect influence by a change in the water structure.