Sandbox 154: Difference between revisions

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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 ATP-actin units formed through a nucleation-condensation type fold pattern<ref name="Pfaendtner>PMID:19620726</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 name="Mitchinson">PMID:1493331</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP and Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref name="Chen">PMID:10637608</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 name="Carlier">PMID:3801442</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 name="Pfaendtner">PMID:19620726</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 name="Mitchinson">PMID:1493331</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP and Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref name="Chen">PMID:10637608</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 name="Carlier">PMID:3801442</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.

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[[image:2zwh_domain_colours2.png|thumb|Domains of F-actin monomer|frame|left]]

==== Polymer ====

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 name="Holmes2"/>. 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 name="oda" />. 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 name="Pfaendtner"/>. A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref name="oda" />.

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 name="Holmes2"/>. 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 name="oda" />. 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 name="Pfaendtner"/>A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref name="oda" />.

=== Nucleotide-State-Dependent Conformational Changes ===

Sandbox 154: Difference between revisions

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@@ Line 14: / Line 14: @@
 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 ATP-actin units formed through a nucleation-condensation type fold pattern<ref name="Pfaendtner>PMID:19620726</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 name="Mitchinson">PMID:1493331</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP and Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref name="Chen">PMID:10637608</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 name="Carlier">PMID:3801442</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 name="Pfaendtner">PMID:19620726</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 name="Mitchinson">PMID:1493331</ref>. After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP and Pi bound state. Subsequent loss of a Pi leaves the ADP-actin state<ref name="Chen">PMID:10637608</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 name="Carlier">PMID:3801442</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.
@@ Line 25: / Line 25: @@
 [[image:2zwh_domain_colours2.png|thumb|Domains of F-actin monomer|frame|left]]
 ==== Polymer ====
-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 name="Holmes2"/>. 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 name="oda" />. 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 name="Pfaendtner"/>. A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref name="oda" />.
+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 name="Holmes2"/>. 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 name="oda" />. 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 name="Pfaendtner"/>A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref name="oda" />.
 === Nucleotide-State-Dependent Conformational Changes ===