The effects of calcium and magnesium ions on the adenosine triphosphatase and inosine triphosphatase activities of myosin A

Importance of melastatin-like transient receptor potential 7 and magnesium in the stimulation of osteoblast proliferation and migration by platelet-derived growth factor

Bone is a dynamic tissue that is continuously being remodeled throughout life. Specialized cells called osteoclasts transiently break down old bone (resorption process) at multiple sites as other cells known as osteoblasts are replacing it with new tissue (bone formation). Usually, both resorption and formation processes are in balance and thereby maintain skeletal strength and integrity. This equilibrium is assured by the coordination of proliferation, migration, differentiation, and secretory functions of the osteoblasts, which are essential for adequate formation and resorption processes. Disturbances of this equilibrium may lead to decreased bone mass (osteoporosis), increased bone fragility, and susceptibility to fractures. Epidemiological studies have linked insufficient dietary magnesium (Mg2+) intake in humans with low bone mass and osteoporosis. Here, we investigated the roles of Mg2+ and melastatin-like transient receptor potential 7 (TRPM7), known as Mg2+ channels, in human osteoblast cell proliferation and migration induced by platelet-derived growth factor (PDGF), which has been involved in the bone remodeling process. PDGF promoted an influx of Mg2+, enhanced cell migration, and stimulated the gene expression of TRPM7 channels in human osteoblast MG-63 cells. The stimulation of osteoblast proliferation and migration by PDGF was significantly reduced under culture conditions of low extracellular Mg2+ concentrations. Silencing TRPM7 expression in osteoblasts by specific small interfering RNA prevented the induction by PDGF of Mg2+ influx, proliferation, and migration. Our results indicate that extracellular Mg2+ and TRPM7 are important for PDGF-induced proliferation and migration of human osteoblasts. Thus Mg2+ deficiency, a common condition among the general population, may be associated with altered osteoblast functions leading to inadequate bone formation and the development of osteoporosis.

Theoretical estimation of the calcium-binding constants for proteins from the troponin C superfamily based on a secondary structure prediction method. I. Estimation procedure

Proteins belonging to the TNC superfamily are known to be built of two, three, four, or six domains of closely similar amino acid sequences. Each domain binds no more than one calcium ion and shows a characteristic helix-loop-helix structure when in the calcium-bound state. Conformational properties of all the domains known so far have been analysed by us using a secondary structure prediction method (Garnier, J., Osguthorpe, D.J. & Robson, B. (1978). J. molec. Biol. 120, 97). Significant differences in distribution of residues predicted as being in the helical, beta-turn, and coil conformations have been found between the strongly, weakly, and non-binding domains. We could determine the ideal prediction pattern characteristic for the domains with the highest affinity for calcium. On the basis of our analysis and observations made by other authors we worked out a few simple rules which made it possible to compare conformational properties of a given domain with the ideal reference pattern and estimate, in this way, the Ca2+-binding constant of the domain. In native proteins the domains are known to be organized in pairs. The Ca2+-binding constant for a two-domain region could be evaluated from the sum of the estimation points attributed to each of its components. Using our method it is possible to predict the binding constants of typical domains and two-domain regins with a precision of one order of magnitude. Data on amino acid sequences and calcium-binding constants of all known proteins, believed to be the members of the TNC superfamily, have been reviewed. References to virtually all papers published on this subject before the end of 1987 are given.

Binding of calcium and magnesium to myosin in skeletal muscle myofibrils

Binding profiles for divalent cation to myosin have been obtained in myofibrils where myosin is assembled in arrays typical of the in vivo organization. Protection by Ca2+ and Mg2+ ions of the regions of myosin susceptible to chymotryptic attack provided the means to monitor metal ion binding. The effect of various concentrations of divalent cations on the chymotryptic digestion patterns was assessed by densitometry of Coomassie Blue stained gels obtained by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and by the rate of myofibrillar solubilization. The results indicate the presence of two classes of binding sites differing in affinity by 4 orders of magnitude. The fractional saturation of the high-affinity site associated with the 5,5'-dithiobis(2-nitrobenzoic acid)-dissociable light chains of myosin regulated the production of subfragment 1 of myosin. From the digestion profiles as a function of metal ion concentration, binding constants for Mg2+ and Ca2+ were obtained. The value for Mg2+ was 5.7 x 10(6) M-1, which is about 1 order of magnitude higher than the most recently determined values for free myosin in solution; the value for Ca2+ was 6.3 x 10(6) M-1. Binding to the low-affinity site regulated the production of the heavy meromyosin fragment and yielded association constants for Ca2+ and Mg2+ of 0.9 x 10(3) and 0.7 x 10(3) M-1, respectively.

Phosphorylation and the binding of calcium and magnesium to skeletal myosin

It has previously been shown that the binding of calcium and magnesium ions to the isolated metal-binding light chains, i.e. those dissociable by 5,5'-dithiobis(2-nitrobenzoate), of rabbit skeletal muscle myosin is moderated by phosphorylation and is accompanied by a sizeable conformational change. As judged by circular dichroism in the region of the aromatic Cotton effects, this conformational change occurs when calcium ions bind to the light chain in situ on the myosin head. Moreover the affinity for calcium is again changed by phosphorylation. The change in chymotryptic digestion patterns, in particular the protection of the head-rod junction in insoluble myosin, by divalent cations, has been used to obtain binding profiles. The results are consistent with the presence of a single class of independent sites, showing no cooperativity. The affinity of the site for both calcium and magnesium ions is enhanced by 1-2 orders of magnitude when the light chain in incorporated in the myosin heads. The effect of phosphorylation on the affinity persists in these circumstances, being marked for calcium and small for magnesium. On phosphorylation the calcium binding constant falls from 8 x 10(6) M-1 to 4 x 10(6) M-1 at physiological ionic strength, compared with 2.5 x 10(5) M-1 and 5 x 10(4) M-1 for the isolated light chains. The sensitivity of the proteolytic cleavage sites is affected by phosphorylation. Thus in the absence of calcium ions the yield of subfragment 1 at a low chymotrypsin concentration is substantially greater in dephosphorylated than phosphorylated myosin, whereas at saturating concentrations of calcium ions attack at the light meromyosin/heavy meromyosin junction is favoured by phosphorylation. These observations may signify a structural effect of phosphorylation on the prevailing interactions within the myosin filament in physiological solvent conditions.

Calcium-induced structural changes in chemically skinned muscle fibers. Detection by optical diffractometry

The intensity of the first order diffraction line produced by chemically skinned muscle fibers was detected by a self scanning photodiode array and minicomputer system. Line intensity was observed to decrease in fibers stretched to zero filament overlap when subjected to calcium-EGTA buffers in the physiological pCa range. Calcium dependent intensity decreases were not observed for myosin extracted fibers indicating that the thick filament proteins may be the source of the calcium effect seen in non-extracted fibers. These results can be interpreted in terms of calcium dependent effects on thick filament disordering which are not dependent upon cross bridge formation.

The effects of divalent cations on the rotational mobility of myosin, heavy meromyosin and myosin subfragment-1 and on the binding of heavy meromyosin to actin

The effects of the divalent cations Mg2+, Mn2+ and Ca2+ on the Brownian rotational motion of fluorescently labeled myosin, heavy meromyosin and myosin subfragment-1 were measured by the method of time-resolved fluorescence depolarization. When Mg2+ was added to solutions of myosin or heavy meromyosin and EDTA, their rotational mobility increased. Ca2+ had no effect. Mn2+ increased the mobility of heavy meromyosin but decreased that of myosin. None of these divalent cations effected the mobility of subfragment-1. The binding of heavy meromyosin to actin was affected very little by Mg2+ or EDTA over a wide range of conditions. Divalent cations appear to change the swivel about which the heads of myosin rotate, presumably by binding to light chain 2 (also called DTNB light chain). However, the heads are still able to bind actin in nearly the same way whether Mg2+ is present or not. The concentration of free Mg2+ for the mid-point of the change in heavy meromyosin mobility is in good agreement with that for EDTA activation of ATPase activity. This suggests that EDTA activation is due to removal of Mg2+ bound to myosin itself.

On the location of the divalent metal binding sites and the light chain subunits of vertebrate myosin

The divalent metal ion binding sites of skeletal myosin were investigated by electron paramagnetic resonance (EPR) spectroscopy using the paramagnetic (Mn(II) ion as a probe. Myosin possesses two high affinity sites (K less than 1 muM) for Mn(II), which are located on the 5,5'-dithiobis(2-nitrobenzoate) (DTNB) light chains. Mn(II) bound to the isolated DTNB light chain gives rise to an EPR spectrum similar to that of Mn(II) bound to myosin and this indicates that the metal binding site comprises ligands from the DTNB light chain alone. Myosin preparations in which the DTNB light chain content is reduced by treatment with 5,5'-dithiobis(2-nitrobenzoate) show a corresponding reduction in the stoichiometry of Mn(II) binding, but the stoichiometry is recovered on reassociation of the DTNB light chain. Chymotryptic digestion of myosin filaments in the presence of ethylenediaminetetraacetic acid yields subfragment 1, but digestion in the presence of divalent metal ions produces heavy meromyosin. Myosin with a depleted DTNB light chain content gives rise to subfragment 1 on proteolysis, even in the presence of divalent metal ions. It is proposed that saturation of the DTNB light chain site with divalent ions protects this subunit against proteolysis, which, in turn, inhibits the cleavage of the subfragment 1-subfragment 2 link. Either the DTNB light chain is located near the region of the link and sterically blocks chymotryptic attack, or it is bound to the subfragment 1 moiety and affects the conformation of the link region. When the product heavy meromyosin was examined by sodium dodecyl sulfate gel electrophoresis, an apparent anomaly arose in that there was no trace of the 19 000-dalton band corresponding to the DTNB light chain. This was resolved by following the time course of chymotryptic digestion of the myosin heavy chain, the DTNB light chain, and the divalent metal binding site. The 19 000-dalton DTNB light chain is rapidly degraded to a 17 000-dalton fragment which comigrates with the alkali 2 light chain. The divalent metal site remains intact, despite this degradation, and the 17 000 fragment continues to protect the subfragment 1-subfragment 2 link. In the absence of divalent metal ions, the 17 000-dalton fragment is further degraded and attack of the subfragment 1 link ensues. Mn(II) bound to cardiac myosin gives an EPR spectrum basically similar to that of skeletal myosin, suggesting that their 19 000-dalton light chains are analogous with respect to their divalent metal binding sites, despite their chemical differences. The potential of EPR spectroscopy for characterizing the metal binding sites of myosin from different sources and of intact muscle fibers is discussed.

Calcium binding to rabbit skeletal myosin under physiological conditions

At a free Mg2+ concentration of 1.0 mM, myosin binds one Ca2+ per molecule when the Ca2+ concentration is 20 muM, a value in the concentration range expected during contraction of skeletal muscle. Mg2+ alters Ca2+ binding in a complex manner, not by simple competition. In the range from 20 to 100 muM Mg2+ it produces positive cooperativity between the high-affinity Ca2+ binding sites, in addition to shifting binding to higher Ca2+ concentrations. High-affinity Ca2+ binding is not significantly affected by the addition of ATP, increase in ionic strength to 0.1 and changes in temperature. Ca2+ binding did not increase actin-activated ATPase activity in the absence of regulatory proteins, but rather inhibited it.

Myosin ATP hydrolysis: a mechanism involving a magnesium chelate complex

It is suggested that under physiological conditions (> 1 mM Mg(2+)) MgATP binds to myosin to form a chelate involving the two reactive sulfhydryl sites (SH(1) and SH(2)). The stability of the chelate structure results in marked inhibition of the myosin ATPase in the presence of millimolar magnesium ion. The inhibitory effect of magnesium ion can be eliminated chemically by blocking either the SH(1) or SH(2) site since this precludes formation of the chelate. In muscle, actin apparently behaves in a similar fashion in that its interaction with myosin causes a disruption of the chelate structure.