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J E Morel

Publications and source records attributed to J E Morel.

At least 37 records · Page 2Linked to original sources
PubMed1989

Behaviour of the crossbridges in stretched or compressed muscle fibres.

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.

Actin Cytoskeleton↗
PubMed1988

Muscle contraction and movement of cellular organelles: are there two different types of mechanisms for their generation?

It has been shown that myosin molecules attached to Covaspheres can "walk along" polar actin filament in vitro. The driving force for this movement seems to explain only about 1% of the isometric tension developed by a muscle fibre. Therefore, the driving force for the bead movement seems to be incompatible with that found in muscle, and the bead movement cannot be considered as a model for muscle contraction. The origin of the bead movement may be related to a "molecular jet" process, resulting from the rapid ejection of the MgATP splitting products. This "molecular jet" might also explain the movements of many cellular organelles.

Actins↗
PubMed1986

MgATPase activity of myosin subfragment 1. The dimer is more active than the monomer.

The MgATPase activity of the rabbit skeletal myosin subfragment 1 (S1), in the steady state, was measured by means of the intrinsic fluorescence of tryptophan. This technique gave results similar to those obtained by other methods (linked or radioactive assays). The activity was measured under conditions that effect the monomer/dimer ratio. It is shown that there is a close correlation between MgATPase activity and the proportion of dimer. At 20 degrees C, for pH 6.9 to 8.1 and for [KCl] less than or equal to 1 M, the observed activity (kobs) can be linearly related to the proportion of dimer (Ed/Eo) by: kobs(s-1) = 0.016-7 X 10(-3)[KCl] + 0.031(Ed/Eo), where [KCl] is expressed in M. We deduce that, at 20 degrees C and for [KCl] = 0 M, the activity of the monomer is kmobs = 0.016 s-1 (Ed/Eo = 0) and that of the dimer kdobs = 0.047 s-1 (Ed/Eo = 1), i.e. a ratio kdobs/kmobs approximately equal to 3. Beyond pH approximately equal to 8.3, the activities of both the monomer and the dimer increased steeply with increasing pH value. In the standard conditions (pH 8.0, [KCl] = 0 to 100 mM), S1 is mainly in the form of a dimer, and such conditions are not appropriate for study of the S1 monomer. For studying the pure monomer, the conditions required at 20 degrees C and in bis-Tris-propane are: S1 concentration approximately equal to 0.2 mg/ml, pH 6.9 to 7.8, [KCl] approximately equal to 300 mM. For studying the pure dimer, the conditions required are: S1 concentration greater than or equal to 0.2 mg/ml, pH 7.8 to 8.1 and [KCl] approximately equal to 0. In both cases the MgATP concentration is about 50 microM. Finally, if great care is taken concerning the age of the S1 solutions and the evaluation of the proportion of dimer, the values of kobs are extremely precise: the uncertainty regarding the values of kobs, as determined by means of intrinsic fluorescence, does not exceed +/- 0.001 s-1. Beyond this error bar conditions are uncontrolled.

Adenosine Triphosphate↗
PubMed1985

Rabbit skeletal myosin heads in solution, as observed by ultracentrifugation and freeze-fracture electron microscopy: dimerization and maximum chord.

The use of analytical ultracentrifugation and freeze-fracture electron microscopy in solution allowed us to observe the monomeric and dimeric forms of Mg.71. This subfragment of the myosin molecule contains the LC2 light chain and is comparable to a "native" myosin head. Sedimentation-diffusion equilibrium ultracentrifugation shows that it is necessary to use slightly different conditions in order to obtain a pure Mg.S1 dimer, as compared to the case of chymotryptic S1 (LC2-free S1). For example, in a buffer leading to a complete dimerization of chymotryptic S1, Mg.S1 is only in the form of a monomer-dimer mixture, with comparable proportions of monomer and dimer. The freeze-fracture technique, applied to solutions containing Mg.S1 or chymotryptic S1, revealed that the monomeric species both have the same maximum chord (about 120 A) and that both dimeric species also have the same maximum chord (about 250 A). The maximum chord of the monomer is comparable to the surface-to-surface spacing between the myosin and actin filaments, in a fiber at the slack length. In sharp contrast this chord is higher than this spacing in a stretched fiber. The consequences of this fact are discussed, with particular reference to the sarcomere length-tension relationship.

Animals↗
PubMed1985

Discussion on the state of water in the myofilament lattice and other biological systems, based on the fact that the usual concepts of colloid stability can not explain the stability of the myofilament lattice.

Application of the usual concepts of colloid stability shows that the in vivo spacings between the myofilaments, making up the contractile part of the muscle myofibrils, correspond to energies of 10(-4) to 10(-1) kT. Refinements in the calculations of the electrostatic and Van der Waals-London energies do not significantly modify these values. Therefore, theory does not predict the observed stability of the myofilament lattice. It is shown that the interfilament water very likely plays an active role in the myofilament lattice. More generally, the structure of water in living cells is probably different from that of bulk water.

Actins↗
PubMed1983

Reinvestigation of the shape and state of hydration of the skeletal myosin subfragment 1 monomer in solution.

Hydrodynamic calculations lead to the conclusion that chymotryptic (or ethylenediaminetetraacetic acid) myosin S1 in solution (hydrated), at 1-5 degrees C, can be modeled as a prolate ellipsoid, with an axial ratio lying between p = 1.0 and 2.5 (major axis between 100.5 A, for p = 1.0, and 162.5 A, for p = 2.5). The degree of hydration is considerable (1.24 g/g for p = 2.5 and 2.02 g/g for p = 1.0). The dehydrated myosin head is pear-shaped under the electron microscope, and its narrowest part is located near the junction with the tail [Elliott, A., & Offer, G. (1978) J. Mol. Biol. 123, 505-519]. Mendelson & Kretzschmar [Mendelson, R. A., & Kretzschmar, K.M. (1980) Biochemistry 19, 4103-4108] have shown that the pear-shaped molecule does not predict the experimental X-ray scattering curve. Nor is this model able to predict the hydrodynamic values. The three-dimensional model for S1 used by Mendelson and Kretzschmar gives a rather good fit to the experimental X-ray scattering curve, but it does not predict the hydrodynamic values. In order to try to reconcile the three models and to fit the X-ray scattering curve and the hydrodynamic data, we suggest that, in solution, the S1 monomer has the shape of a prolate ellipsoid and that an inclusion of bound water exists at one extremity of the protein. The rest of bound water surrounds the protein. As first approximation, the dry protein and the hole are assumed to have the same shape as the hydrated molecule (prolate ellipsoid; p).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1982

The possible roles of the myosin heads.

It is suggested that, in the conditions which exist in vivo, one head of a myosin molecule interacts with another head of the opposite molecule, inside the backbone of the thick filament. The other head lies outside and can interact with actin. This model is based on the fact that a dimer of the myosin heads exists and that there is a close correlation between the properties of the dimer and those of the thick filament diameter. In natural filaments, there are myosin molecules in excess and it is suggested that these molecules have their two heads outside the backbone.

Actins↗
PubMed1982

Dimerization of the myosin heads in solution.

It is shown, by means of analytical ultracentrifugation, that skeletal myosin S-1 exists in the form of a monomer-dimer mixture, in rapid reversible equilibrium, sensitive to the hydrostatic pressure, the temperature, and the composition of the buffer (at least, pH, ionic strength, presence or absence of a Mg-(phosphate compound), and presence or absence of Mg2+). The dimer is predominant at high pH, at low ionic strength, in the presence of a Mg-(phosphate compound), at high pressure, and at low temperature. The monomer is predominant in the reverse conditions. At atmospheric pressure and at room temperature, in a buffer having a composition close to that of the physiological medium, but containing no Mg-(phosphate compound), the monomer is largely predominant (more than 90% at 1 mg/mL S-1). At atmospheric pressure and at room temperature, in a buffer containing a Mg-(phosphate compound) and having a composition close to that of the physiological medium, S-1 exists in the form of a monomer-dimer mixture, with a noticeable proportion of dimer (more than 25% at 1 mg/mL S-1 in the presence of 2 mM MgADP and 3 mM Mg2+). In such buffers, the monomer:dimer ratio is extremely sensitive to both the pH and the ionic strength. The sedimentation coefficients of the monomer and the dimer are respectively 5.05 +/- 0.05 S and 6.05 +/- 0.05 S. The two protomers making up the dimer are stuck together in an end-to-end arrangement. Both the monomer and the dimer are highly hydrated (about 0.9 g of water/g of protein for the monomer and probably more for the dimer).

Animals↗
PubMed1975

Ultrastructure of the contractile system of striated skeletal muscle and the processes of muscular contraction. II. Releasing system and mechanisms of muscular contraction.

The tropomyosin-troponin system of the thin filaments acts as a regulator of the interaction of myosin with actin by a process of inhibition. This inhibition is released by Ca2+ ions. Muscle contraction occurs when Ca+2 ions are released from the sarcoplasmic reticulum into the myofibril, where they interact with the troponin of the thin filaments. Muscular contraction is due to the sliding of the two kinds of filaments past each other in the hexagonal network of the myofibril. Many hypotheses have been formulated concerning the intimate molecular mechanisms responsible for the sliding of the filaments past each other, but none of these can be considered to be entirely satisfactory.

Actins↗