ADP Actin

Pollard et al., 1992] Pollard, T. D., Goldberg, I., and Schwarz, W. H. Nucleotide exchange, structure, and mechanical properties of filaments assembled from ATP-actin and ADP-actin. J. Biol. Chem. 267 (1992) 20339-20345... 

The critical concentration, that is, the monomer <=> polymer equilibrium dissociation constant for polymerization of ADP-actin, is 25-fold that for polymerization of ATP-actin. However, in both cases the filament is made of F-ADP subunits, and the rate constant for association of ADP-actin to filament ends is only 2.5-fold lower than the rate constant for association of ATP-actin. In the absence of free ATP, the... 

ATP-actin complex can polymerize, but the polymer once formed spontaneously depolymerizes. Depolymerization stops when the concentration of ADP-monomer in the medium reaches the value of the critical concentration for polymerization of ADP-actin (for review see Kom et al., 1987 Carrier, 1991). 

In reversible polymerization, the critical concentration is equal to the equilibrium dissociation constant for polymer formation. This parameter is therefore independent of the number of polymers in solution. Confirmation comes from smdying reversible polymerization of ADP-actin when sonic vibration is applied to a solution of F-ADP-actin filaments at equilibrium with G-ADP monomers, no change is observed in the proportion of G- and F-actin (Carlier et al., 1985). Therefore, the only effect of sonic vibration is to increase the number of filaments without affecting the rates of monomer association to and dissociation from filament ends. 

Because ATP hydrolysis on F-actin takes place with a delay following the incorporation of ATP-subunits, and because in the transient F-ATP state filaments are more stable than in the final F-ADP state, polymerization under conditions of sonication can be complete, within a time short enough for practically all subunits of the filaments to be F-ATP. At a later stage, as Pj is liberated, the F-ADP filament becomes less stable and loses ADP-subunits steadily. The G-ADP-actin liberated in solution is not immediately converted into easily polymerizable G-ATP-actin, because nucleotide exchange on G-actin is relatively slow, and is not able to polymerize by itself unless a high concentration (the critical concentration of ADP-actin) is reached. Therefore, G-ADP-actin accumulates in solution. A steady-state concentration of G-ADP-actin is established when the rate of depolymerization of ADP-actin (k [F]) is equal to the sum of the rates of disappearance of G-ADP-actin via nucleotide exchange and association to filament ends. [G-ADP]ss in this scheme is described by the following equation (Pantaloni et al., 1984) ... 

The fact that the concentration of G-actin at steady-state in the presence of ATP varies with the number of filaments may have some biological significance indeed, in cells, large pools of G-ADP-actin may accumulate in regions where a large number of short filaments exist. This behavior is the direct consequence of two combined features of actin polymerization namely, the hydrolysis of ATP, and the relatively slow rate of ATP exchange for ADP on G-actin. 

This effect will be even greater if there is any ADP-actin in the total pool of actin monomers, especially if ADP-actin inhibits nucleation. 

Because of these differences in Cc, and the energy provided by the hydrolysis of ATP, the filaments at steady state can treadmill. There is a net loss of monomers (mostly ADP-actin) from the pointed (or minus) ends since the value of Cc is higher than the concentration of G-actin. There is a net gain of ATP-actin at the barbed (or plus) ends since the value of Cc is lower than the concentration of G-actin. 

ADP-actin released from the filaments can exchange ADP for ATP in solution and rebind to the filaments. Due to a difference in conformation, monomers in filaments cannot exchange their... 

Above its critical concentration, ATP-actin will polymerize. The ATP will hydrolyze through time to form ADP-actin, which has a higher critical concentration. Thus, if the initial subunit concentration is between the critical concentrations of ATP-actin and ADP-actin, filaments will form initially and then disappear on ATP hydrolysis. 

Figure 1. Models of coronin function. A) Speculative model of coronin function (1995, E. L. de Hostos, unpublished) suggesting the involvement of coronin in actin dynamics in partnership with other actin-binding proteins. B) Current model of core coronin functions. " 1) Coronin recruits Arp2/3 complex to existing actin filaments and promotes the formation of branches. 2) Coronin stimulates the activity of cofilin to depolymerize actin filaments at their pointed (ADP-actin containing ends) directly, or by recruiting the SSH1L phosphatase. 3) In the absence of F-actin, coronin inhibits the nucleation activity of Arp2/3.

H. sapiens Coronin 1 B Binding (Kd = 170 nM ATP/ADP-Pi actin, Kd = 8 pM ADP-actin), Bundling Arp2/3 inhibition Regulation of cofilin-mediated actin disassembly 7,26... 

Matdla PK, Quintcro-Monzon O, Kuglcr J et al. A high-affinity interaction with ADP-actin monomers imderlies the mechanism and in vivo function of Srv2/cyclase-associatcd protein. Mol Biol Cell 2004 15(11) 5158-5171. 

ATP-binding cleft, it can recharge ADP-actin monomers released from a filament, thereby replenishing the pool of ATP-actin (Figure 19-10). 

Pollard, T. D. (1986) Rate constants for the reactions of ATP- and ADP-actin with the ends of actin filaments. J. Cell Biol. 103, 2747-2754. 

Kovar DR, Harris ES, Mahaffy R, Higgs HN, Pollard TD (2006) Control of the assembly of ATP- and ADP-actin by formins and profilin. Cell 124 423-435... 

Most eukaryotic cells contain the major cytoskeletal protein, actin, which can exist as a monomer (G-actin) and reversibly polymerize into long filamentous structures (F-actin). The F-actin filament is polarized such that the rates of polymerization and depolymerization are different at the two ends. At the plus end (also called barbed end) monomer addition occurs readily, relative to the minus end (also called pointed end) where polymerization is less favored. In addition, the actin subunit binds adenine nucleotides and has an intrinsic ATPase activity. ATP-actin binds preferably at the plus end and, after incorporation into the filament, is converted to ADP-actin. These properties have a significant impact on the dynamics and stability of the filament [54, 293]. In the cell, polymerization and depolymerization are highly regulated reactions, this regulation being mediated by multiple actin-binding proteins. 

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