Divalent Cations, Nucleotides, and Actin Structure

Regulation of Phospholipase D Activity by Actin

“…In the presence of MgCl 2 , KCl potentiated the level of PLD activation. In contrast, the addition of either MgCl 2 or KCl alone to AMB is both necessary and sufficient for actin polymerization, and the combination is synergistic (46). In the absence of Mg 2ϩ , KCl promotes actin polymerization but not GTP␥S-dependent stimulation of PLD activity, probably because Mg 2ϩ is required for GTP␥S exchange on Rho and ARF GTPases (which is necessary for activation of PLD1).…”

“…AMB, as the name signifies, does not support actin polymerization (46). Addition of MgCl 2 or KCl to AMB results in actin polymerization, and the combination of MgCl 2 and KCl is synergistic.…”

“…First, certain components of the buffer system that regulate the state of actin polymerization (e.g. concentrations of Ca 2ϩ and Mg 2ϩ , ionic strength, pH) also modulate PLD activity (31,45,46). Second, several actin-binding proteins that regulate both the critical concentration of free actin monomer and the dynamics of filament assembly/disassembly (including fodrin, gelsolin, and ␣-actinin) have recently been shown to potently modulate PLD activity (47)(48)(49)(50).…”

Tropomyosin as a Regulator of Actin Dynamics

“…Before approaching steady state, actin polymerization passes through the initial nucleation step followed by the elongation. Two additional steps of polymerization, a conformational change of G-actin transforming the monomer to a state that is conformationally similar to the F-actin subunit (monomer activation) (Rich and Estes, 1976;Strzelecka-Golaszewska, 2001), and annealing of the fragments formed during elongation (Murphy et al, 1988;Andrianantoandro et al, 2001) are also discussed. As tropomyosin does not bind G-actin or short oligomers (Eaton et al, 1975;Perry, 2001), the direct influence of tropomyosin on F-actin polymerization is rather limited, and the available data are often ambiguous.…”

“…These results imply that the newly formed ADP-Pi-actin filaments are more stable than the old F-ADP-actin (Isambert et al, 1995). Since the conformation of actin monomer depends on a type of bound nucleotide and cation (Strzelecka-Golaszewska, 2001), these factors also affect the filament structure although being not so large as those between the corresponding G-actins Egelman, 1992, 1993;Muhlrad et al, 1994;Orlova et al, 1995;Isambert et al, 1995;Strzelecka-Golaszewska et al, 1996). No difference in flexibility and persistence filament lengths of fluorescently labeled F-Ca-actin and F-Mg-actin was detected by means of fluorescence optical video microscopy (Isambert et al, 1995).…”