The divalent cation bound to actin and thin filament

Biochemical and theoretical approach to localization of metal-ion-binding sites in the actin primary structure

The number of Ca2+ ions bound at sites other than the single high-affinity site in CaCl2-induced polymers of rabbit skeletal muscle, chicken gizzard, and bovine aorta actin was determined. The polymer of skeletal muscle and aorta actin contained 4 mol Ca2+/mol, whereas gizzard actin only 3 mol weakly bound Ca2+/mol monomer. This difference correlates with the deletion in smooth muscle gamma-actin of one out of four NH2-terminal acidic residues typical of skeletal and smooth muscle alpha-actin isoforms, suggesting that this additional acidic residue in alpha-actins is involved in the weak binding of cations which is essential for polymerization. This experimental result, as well as a theoretical analysis of the actin primary structure, argue against the implication of the NH2-terminal acidic residues in the high-affinity site for divalent cation. The analysis of the actin primary structure aimed at identification of sequences resembling the known Ca2+-binding patterns has revealed the absence of an EF-hand Ca2+-binding site. The best match was obtained between the sequence of the 292-301 segment and that of Ca2+ site in lectins. However, in the light of experimental data discussed, it is more plausible that the actual high-affinity Ca2+ site in actin involves sequentially distant residues from the NH2- and COOH-terminal portions of the polypeptide chain.

Calcium and magnesium binding to thin and thick filaments in skinned muscle fibres: electron probe analysis

Electron probe analysis of ultrathin cryosections with high spatial resolution was used to determine in situ the concentrations of Ca2+ and Mg2+ bound in the absence of ATP to myofilaments in the I and A-bands of skinned frog skeletal muscle. At 2.2 x 10(-11) M Ca2+ and 2.7 x 10(-9) M Mg2+, the inexchangeably bound Mg2+ in the I-band was equivalent to the amount of divalent cations known to be inexchangeably bound to F-actin, while the Ca2+ bound to the I-band was not significantly above zero. The bound Mg2+ in the I-band was not exchangeable with Ca2+ even when the skinned fibres were exposed to 10 mM Ca2+ solution. These results clearly indicate that Mg2+, rather than Ca2+, is the divalent cation bound to F-actin in the thin filaments in situ. In the presence of 1 mM Mg2+, the exchangeable Ca2+ bound to the I-band was increased as a function of the free Ca2+, while that in the A-band was not significantly changed with [Ca2+] up to 2 x 10(-5) M, and increased to approximately 0.8 mol Ca2+ per mol myosin at 10(-4) M Ca2+. At a saturating free Ca2+ in Tris-Cl solution, the bound Ca2+ content (2-3 mol Ca2+ per mol troponin) of the nonoverlapping I-band was unexpectedly low; the replacement of Tris with Na+ enhanced Ca2+ binding to the level equivalent to 3-4 mol Ca2+ per mol troponin. The depressant effect of Tris on Ca2+ binding was greater in the absence of Mg2+. High concentrations of Tris also reduced the maximum tension induced by 10(-4) M Ca2+ buffered with 10 mM EGTA. At 1.3 x 10(-7) M Ca2+, thought to be close to the cytoplasmic free Ca2+ in resting muscle, the I-band bound a significant amount of Ca2+: equivalent to about 1 mol Ca2+ per mol troponin. In rabbit myofibrils there was a significant amount (approximately 1.5 mol/mol myosin) of Ca2+ bound by the A-band at a free Ca2+ of 10(-4) M.

Chicken-gizzard actin: polymerization and stability

Preparations of chicken gizzard actin obtained from acetone-dried muscle powders prepared with various methods developed for skeletal muscle contain variable amounts of a beta-actinin-like protein. This contamination is minimized if the procedure of muscle powder preparation includes washing with EDTA solution, and can be completely removed by gel filtration of G-actin on Sephadex G-100. The presence of beta-actinin activity manifests itself in an increased rate of actin polymerization, low filament lengths resulting in low reduced viscosity and enhanced ATP-splitting activity of actin polymer, and instability of the polymer in the absence of free ATP. Gizzard actin purified on a Sephadex G-100 column does not differ from rabbit skeletal muscle actin in its polymerization properties. The distinct property of gizzard actin is the instability of its G form in the absence of added Ca2+, indicating that the affinity of this cation for the single high-affinity site in gizzard actin is lower than in skeletal muscle actin.