Tightly-bound divalent cation of actin

Abstract

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.

Identification of cation-binding sites on actin that drive polymerization and modulate bending stiffness

“…The effects of solution ionic conditions on the assembly and stability of actin filaments have been investigated for several decades (6)(7)(8)(9)(10)(11)(12). The actin C c and (monomer and filament) conformation depend on the nucleotide-associated divalent cation (Ca 2þ or Mg 2þ ) as well as the type and concentration of ions in solution (6,7,(13)(14)(15), a behavior shared among characterized actins and their bacterial homologs (16).…”

“…and/or specific ion binding interactions, potentially at discrete sites. Identification of saturable cation binding sites with different affinities favors specific and discrete binding sites on monomers (8)(9)(10)17), but the location of these sites and their contributions to filament assembly and stiffness are unknown.…”

Non‐hydrolyzable analog of GTP induces activity of Na+ channels via disassembly of cortical actin cytoskeleton

“…Ca-actin polymerizes too slowly to inhibit the channel activity but it can e¡ectively add to the free ends of existing ¢laments, thus preventing their disassembly [25]. Indeed, when 100 WM GTPQS and 300 Wg/ml Ca-actin were applied to the bath solution simultaneously, no channel activation occurred during 9^10 min of the recording (n = 9).…”

“…It is known that actin containing Ca 2þ as a tightly bound cation (Ca-actin) polymerizes much slower than actin containing Mg 2þ (Mg-actin) [25]. In order to elucidate whether Na þ channels activated by GTPQS could be inhibited by polymerizing actin, as it occurs upon cytochalasin-induced channel activity [19], incubation of the membrane fragment with GTPQS was followed by addition of G-actin with or without magnesium ions in the bath (Fig.…”

Characterization of α-actin isoforms in white and red skeletal muscle types of Leporinus macrocephalus (Characiformes, Anostomidae)

“…The alphaskeletal actin plays a key role in biological muscle movement and represents the major protein in muscles along with myosin. This protein has been intensively studied in mammals and its functions include polymerization in neutral salt and binding to Ca 2+ , Mg 2+ , adenine nucleotides, tropomyosin, and myosin (Estes et al 1992, Reisler 1993.…”