Sandbox 154: Difference between revisions

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'''G-actin''' is the free monomeric form of actin which transitions to F-actin. The structures of globular and filamentous actin are distinct from one another in numerous ways, despite the fact that G-actin comprises F-actin. When the monomeric actin becomes polymerized into F-actin, the unit becomes flattened. G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/1'>Ca2+</scene> ion ligands in its structure, and also has the ligand RHO as opposed to 4-methyl histidine as found in the F-actin structure.

Formation of F-actin is a dynamic process of assembly and disassembly.

Formation of F-actin is a dynamic process of assembly and disassembly which has been termed “treadmilling”.

The transition between G- and F-actin begins with a stabilized oligomer of actin units ~~forms~~ through a nucleation-condensation type fold pattern<ref>pfaendtner</ref>. Addition of monomeric units to either end subsequently occurs, however, because of a difference in charge polarity in the two ends, there is preferential addition to what is termed the "plus (+) end" or the "barbed-end". On the opposite end, the "minus (-) end" or the "pointed end", there is preferential dissociation of actin units<ref>mitchinson</ref>. Because of the potential for addition or removal of monomeric units to occur at both ends, the assembly of f-actin may be described in terms of equilibrium. However, because the rate of ATP-actin association is ten-fold that of ADP-actin dissociation, the f-actin has the appearance of moving forward, or "treadmilling"<ref>clasier</ref>. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.

The transition between G- and F-actin begins with a stabilized oligomer of ATP-actin units formed through a nucleation-condensation type fold pattern<ref>pfaendtner</ref>. Addition of ATP-monomeric units to either end subsequently occurs, however, because of a difference in charge polarity in the two ends, there is preferential addition to what is termed the "plus (+) end" or the "barbed-end". On the opposite end, the "minus (-) end" or the "pointed end", there is preferential dissociation of actin units<ref>mitchinson</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP+Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref>chen</ref>. Because of the potential for addition or removal of monomeric units to occur at both ends, the assembly of F-actin may be described in terms of equilibrium. However, because the rate of ATP-actin association is ten-fold that of ADP-actin dissociation, the f-actin has the appearance of moving forward, or "treadmilling"<ref>clasier</ref>. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.

According to Oda et al.<ref>Oda</ref>, there is a 20 degree tilt in one of the domains of F-actin as compared to G-actin which gives F-actin the much flatter structure as compared to G-actin. It is not certain whether this flattening occurs before or after ATP hydrolysis.

According to Oda et al.<ref>Oda</ref>, there is a 20 degree tilt in one of the domains of F-actin as compared to G-actin which gives F-actin the much flatter structure as compared to G-actin.

== Structure ==

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=== Polymer F-actin ===

[[image:f-actin.png]]

F-actin has the appearance of two right-handed helices, with a gradual twist around one another. It is actually composed of repeats of 13 actin units for every 6 left-handed turns, spanning a length of 350 Å. <ref> Holmes, K.C., Popp, D., Gebhard, W. and Kabsch, W. 1990. Atomic model of the actin filament. Nature,347(6288):44-49. PMID: [http://www.ncbi.nlm.nih.gov/pubmed/2395461/ 2395461]</ref>. Including the ADP and Ca<sup>2+</sup>, the F-actin molecule as shown here consists of 377 residues (43kDa), two major domains separated by a nucleotide-binding cleft<ref>oda</ref>. Depending on the state of the bound nucleotide, the most stable conformation of F-actin changes. In its ATP and ADP + Pi nucleotide bound states, it has a closed binding cleft. In its ADP only bound state, it has a wider binding cleft<ref>pfaendtner</ref>. A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref>oda</ref>.

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@@ Line 12: / Line 12: @@
 '''G-actin''' is the free monomeric form of actin which transitions to F-actin. The structures of globular and filamentous actin are distinct from one another in numerous ways, despite the fact that G-actin comprises F-actin. When the monomeric actin becomes polymerized into F-actin, the unit becomes flattened. G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/1'>Ca2+</scene> ion ligands in its structure, and also has the ligand RHO as opposed to 4-methyl histidine as found in the F-actin structure.
-Formation of F-actin is a dynamic process of assembly and disassembly.
+Formation of F-actin is a dynamic process of assembly and disassembly which has been termed “treadmilling”.
-The transition between G- and F-actin begins with a stabilized oligomer of actin units forms through a nucleation-condensation type fold pattern<ref>pfaendtner</ref>. Addition of monomeric units to either end subsequently occurs, however, because of a difference in charge polarity in the two ends, there is preferential addition to what is termed the "plus (+) end" or the "barbed-end". On the opposite end, the "minus (-) end" or the "pointed end", there is preferential dissociation of actin units<ref>mitchinson</ref>. Because of the potential for addition or removal of monomeric units to occur at both ends, the assembly of f-actin may be described in terms of equilibrium. However, because the rate of ATP-actin association is ten-fold that of ADP-actin dissociation, the f-actin has the appearance of moving forward, or "treadmilling"<ref>clasier</ref>. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.
+The transition between G- and F-actin begins with a stabilized oligomer of ATP-actin units formed through a nucleation-condensation type fold pattern<ref>pfaendtner</ref>. Addition of ATP-monomeric units to either end subsequently occurs, however, because of a difference in charge polarity in the two ends, there is preferential addition to what is termed the "plus (+) end" or the "barbed-end". On the opposite end, the "minus (-) end" or the "pointed end", there is preferential dissociation of actin units<ref>mitchinson</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP+Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref>chen</ref>. Because of the potential for addition or removal of monomeric units to occur at both ends, the assembly of F-actin may be described in terms of equilibrium. However, because the rate of ATP-actin association is ten-fold that of ADP-actin dissociation, the f-actin has the appearance of moving forward, or "treadmilling"<ref>clasier</ref>. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.
+According to Oda et al.<ref>Oda</ref>, there is a 20 degree tilt in one of the domains of F-actin as compared to G-actin which gives F-actin the much flatter structure as compared to G-actin. It is not certain whether this flattening occurs before or after ATP hydrolysis.
-According to Oda et al.<ref>Oda</ref>, there is a 20 degree tilt in one of the domains of F-actin as compared to G-actin which gives F-actin the much flatter structure as compared to G-actin.
 == Structure ==
@@ Line 27: / Line 26: @@
 === Polymer F-actin ===
 <applet load='2zwh' size='222' color='black' frame='true' align='left' caption='Filamentous Actin (F-actin)' scene='Sandbox_154/2zwh_black_domains/1'/>
+[[image:f-actin.png]]
 F-actin has the appearance of two right-handed helices, with a gradual twist around one another. It is actually composed of repeats of 13 actin units for every 6 left-handed turns, spanning a length of 350 Å. <ref> Holmes, K.C., Popp, D., Gebhard, W. and Kabsch, W. 1990. Atomic model of the actin filament. Nature,347(6288):44-49. PMID: [http://www.ncbi.nlm.nih.gov/pubmed/2395461/ 2395461]</ref>. Including the ADP and Ca<sup>2+</sup>, the F-actin molecule as shown here consists of 377 residues (43kDa), two major domains separated by a nucleotide-binding cleft<ref>oda</ref>. Depending on the state of the bound nucleotide, the most stable conformation of F-actin changes. In its ATP and ADP + Pi nucleotide bound states, it has a closed binding cleft. In its ADP only bound state, it has a wider binding cleft<ref>pfaendtner</ref>. A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref>oda</ref>.