Sulfhydryl groups of the "relaxing protein" from rabbit skeletal muscle

SH-1 modification of rabbit myosin interferes with calcium regulation

The reactive thiol of the myosin head, SH-1, can be selectively labelled in glycerinated rabbit muscle fibres. This residue has been used as an attachment site for either fluorescent or spectroscopic probes which report on head movements and orientations in various functional states of muscle. We have specifically modified SH-1 in vitro, using purified rabbit myosin and conditions similar to those employed in the labelling of muscle fibres (low ionic strength [40 mM NaCl] at 4 degrees C), with stoichiometric amounts of either [14C]-iodoacetamide, 5-(2[iodoacetyl)amino)ethyl) aminonaphthalene-1- sulphonic acid (IAEDANS), or 4-(2-iodoacetamido-2,2,6,6-tetramethyl piperidinooxyl (IASL). The specificity of modification was determined by measuring the well-defined alterations in the high salt ATPase activities of myosin and by localizing both IAAm and IAEDANS to the 20-kDa C-terminal subfragment 1 (S1) which contains SH-1. The low ionic strength actin-activated Mg2+-ATPase of SH-1-modified rabbit myosin was measured in the presence of the thin filament regulatory, complex, troponin-tropomyosin. A significant increase in this activity in the absence of calcium, concomitant with a decrease in activity in the presence of calcium, was observed as the extent of SH-1 modification was incrementally increased from zero to one mole of label bound per mole of SH-1. The elevated myosin Mg2+-ATPase, which results from SH-1 modification, does not account for the increased actin-activated Mg2+-ATPase in resting conditions (i.e. in the absence of calcium).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of chemical cross-linking agents on calcium-induced structural changes in skinned muscle fibers. Origin within thick filaments detected by optical diffraction methods

We have reported earlier (Sabbadini, R.A., Rieser, G.D. and Paolini, P.J. (1979) Biochim. Biophys. Acta 578, 526-533) that physiological levels of calcium (pCa 6.95-5.49) can produce structural changes in thick filaments which are detectable as an intensity loss of the first-order optical diffraction lines from chemically skinned skeletal muscle fibers stretched beyond myofilament overlap. We now show that the calcium-induced intensity decrease results from structural changes within, rather than between, thick filaments. Glycerinated, detergent-treated fibers from frog semitendinosus muscle were incubated in 1-10 mM concentrations of dimethylsuberimidate (DMS), dithiobis(succinimidylpropionate) (DTSP) or dimethyl-3,3'-dithiobispropionimidate (DTBP) for 4 h. These substances are homobifunctional lysine-modifying cross-linking reagents known to restrict movement of S-1 heads and limit changes in the association of myosin rods within the core of the thick filament without affecting interfilament lattice spacing. Diffraction patterns from cross-linked cells in relaxing solution were identical to those in control cells, but Ca2+ (pCa 5.49) totally failed to produce the typical 50-70% attenuation of first-order line intensity. Cleavage of the disulfide bond in DTBP-treated cells with dithiothreitol fully restored the Ca2+ sensitivity. Lysine group modification with methylacetimidate, a monofunctional lysine modification reagent equivalent to DMS, did not block the Ca2+ sensitivity. We observed that intensity reductions can also be produced by numerous other agents and mechanisms, such as nonionic polymeric solutions of polyvinylpyrrolidone, which reduces the lattice spacing, and alkaline pH, which probably displaces the S-1 heads from a resting position close to the thick filament surface. However, the prevention of the Ca2+ effect by cross-linkers indicates that intrafilament rather than interfilament changes in structure are responsible for the light diffraction intensity decrease accompanying activation.

The regulatory proteins of the myofibril. Characterization and biological activity of the calcium-sensitizing factor (troponin A)

1. Electrophoretically homogeneous calcium-sensitizing factor was prepared from the troponin complex by chromatography successively on sulphoethyl-Sephadex and on diethyl-(2-hydroxypropyl)aminoethyl-Sephadex in 6m-urea. It is a protein containing 53% of polar amino acids, of which a net excess consists of acidic residues. 2. On gel filtration the calcium-sensitizing factor was shown to be the only myofibrillar protein that bound (45)Ca(2+) tightly in the presence of 2-6m-urea. 3. Calcium-sensitizing factor effectively neutralized the effect of the inhibitory factor on the ATPase activities of actomyosin systems. Tropomyosin was essential for the regulation, by changes in the Ca(2+) concentration, of the neutralizing effect of calcium-sensitizing factor on the inhibitory factor. 4. Prolonged exposure to chelators of Ca(2+) produced an irreversibly modified form of calcium-sensitizing factor of higher electrophoretic mobility at pH8.6. The modified form neutralized the inhibitory factor action but this property could no longer be controlled by the Ca(2+) concentration in the presence of tropomysin. 5. The calcium-sensitizing factor and tropomyosin could be replaced by their carboxymethylated derivatives in the relaxing-protein system.