The bound calcium of actin

Michael Bárány: a recollection

In this special edition of the Journal of Muscle Research and Cell Motility, we recall the lives and scientific contributions of Michael and Kate Bárány, who died in 2011. Michael and Kate were Holocaust survivors who went on to become leading researchers in muscle contraction. Their research topics included myosin isoforms, phosphorylation as a regulator of muscle contraction and the application of NMR to study muscle metabolism. They were deeply committed to science and to fostering the careers of young investigators.

Tightly-bound divalent cation of actin

Actin is known to undergo reversible monomer-polymer transitions that coincide with various cell activities such as cell shape changes, locomotion, endocytosis and exocytosis. This dynamic state of actin filament self-assembly and disassembly is thought to be regulated by the properties of the monomeric actin molecule and in vivo by the influence of actin-associated proteins. Of major importance to the properties of the monomeric actin molecule are the presence of one tightly-bound ATP and one tightly-bound divalent cation per molecule. In vivo the divalent cation is thought to be Mg2+ (Mg-actin) but in vitro standard purification procedures result in the preparation of Ca-actin. The affinity of actin for a divalent cation at the tight binding site is in the nanomolar range, much higher than earlier thought. The binding kinetics of Mg2+ and Ca2+ at the high affinity site on actin are considered in terms of a simple competitive binding mechanism. This model adequately describes the published observations regarding divalent cation exchange on actin. The effects of the tightly-bound cation, Mg2+ or Ca2+, on nucleotide binding and exchange on actin, actin ATP hydrolysis activity and nucleation and polymerization of actin are discussed. From the characteristics that are reviewed, it is apparent that the nature of the bound divalent cation has a significant effect on the properties of actin.

Thermodynamics of actin polymerization; influence of the tightly bound divalent cation and nucleotide

Previous work by this laboratory has shown that the tightly bound divalent cation of actin affects the enthalpy of the polymerization reaction for ATP-actin (Selden et al. (1986) J. Muscle Res. Cell Motil. 7, 215-224). In the present study, we have measured the temperature dependence of polymerization for actin containing ATP or ADP as the bound nucleotide and Mg2+ or Ca2+ (Mg-actin or Ca-actin) as the tightly bound divalent cation. In contrast to the marked effect of the tightly bound divalent cation on enthalpy and entropy changes for the polymerization of ATP-actin, ADP-actin polymerization is affected very little by the tightly bound divalent cation. The Arrhenius and van't Hoff plots for polymerization of Ca-ATP-, Mg-ADP- and Ca-ADP-actin were found to be non-linear. The free energy data for actin polymerization have been analyzed as a second order function of absolute temperature (Osborne et al. (1976) Biochemistry 15, 317-320). The values of the enthalpy change and activation enthalpy change for Ca-ATP-, Mg-ADP- and Ca-ADP-actin polymerization were found to be temperature-dependent, in contrast to those for Mg-ATP-actin, which were nearly constant over the temperature range studied. These results suggest that (1) polymerization of actin which does not contain both Mg2+ and ATP may be a multi-step reaction including a rate-limiting step and (2) Mg-ATP-actin has a unique conformation which enhances its ability to polymerize.

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.

The tightly bound divalent cation regulates actin polymerization

The polymerization characteristics of Ca++-actin and Mg++-actin were studied by measuring initial rates of polymerization upon addition of phalloidin-stabilized nuclei and neutral salt. Under conditions where the effects of divalent cation exchange were minimized, CaCl2 and MgCl2 were found to be equally effective in polymerizing actin. Mg++-actin was found to nucleate and polymerize more readily than Ca++-actin, having a forward rate constant about twice that of Ca++-actin under a variety of polymerizing conditions. The critical concentration for Ca++-actin is approximately 20 times that for Mg++-actin under equivalent conditions. These data imply that the polymer of Mg++-actin must be more stable than that of Ca++-actin, having a depolymerization rate constant about 10 fold lower. Since Mg++ is probably the tightly-bound cation in vivo, whereas Ca++-actin has been more widely studied in vitro, it would appear that actin in its physiological state is probably more polymerizable and more stable in the polymer form than previously considered.

Cleavage of actin by thrombin

Under certain conditions actin can be split by thrombin. Actin prepared in the presence of excess Ca(++) was found to be resistant to thrombin. However, if actin was purified without added Ca(++), both G- and F- actin underwent thrombic digestion, although a considerable proportion of actin molecules remained intact. Similar results were obtained with actin (in 50% sucrose) devoid of nucleotide and divalent cations but retaining its native characteristic. The removal of tightly bound Ca(++) from actin by EDTA accelerated the thrombic splitting and made the complete fragmentation of G-actin possible. Thrombin first cleaves actin into two pieces and subsequently one of them, fragment K (molecular weight 37,000 on sodium dodecyl sulfate-polyacrylamide gel), splits further, resulting in fragments L (molecular weight 27,000) and M (molecular weight about 10,000).

Actin polymerization and its relationship to adenosine triphosphatase activity

F-actin was prepared from pig heart myofibrils without the use of organic solvents. G-actin, prepared from F-actin by sonication, had adenosine triphosphatase activity. Dephosphorylation of added ATP by G-actin had a broad pH optimum between pH 7.5 and pH 9.0, specifically required Mg 2+ , and was activated by EGTA. G-actin prepared from skeletal muscle of rabbits by similar methods also showed ATPase activity. Cardiac G-actin and G-actin from skeletal muscle had equimolar nucleotide bound to them. Polymerization of cardiac G-actin was greatest at a pH of about 6.5 and declined above pH 6.8. KCl was necessary for polymerization, and EGTA inhibited the conversion to F-actin. By selectively acetylating G-actin, using acetic anhydride, a preparation of G-actin was obtained that did not have adenosine triphosphatase activity but did polymerize under optimal conditions. It is concluded that dephosphorylation of ATP and the polymerization process in cardiac G-actin are two different reactions occurring at different sites on the actin molecule.