Sandbox 154: Difference between revisions
From Proteopedia
Jump to navigationJump to search
Melissa Chow (talk | contribs) No edit summary |
Melissa Chow (talk | contribs) No edit summary |
||
| Line 11: | Line 11: | ||
<applet load='1j6z' size='200' color='black' frame='true' align='left' caption='Globular Actin (G-actin): PDB identifier [http://www.rcsb.org/pdb/explore/explore.do?structureId=1J6Z/ 1J6Z].' scene='Sandbox_154/1j6z_black/2'/> | <applet load='1j6z' size='200' color='black' frame='true' align='left' caption='Globular Actin (G-actin): PDB identifier [http://www.rcsb.org/pdb/explore/explore.do?structureId=1J6Z/ 1J6Z].' scene='Sandbox_154/1j6z_black/2'/> | ||
'''G-actin''' is the free monomeric form of actin which polymerizes 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. Also, F-actin possesses ATPase function where it is minimal in G-actin. G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/1'>calcium ion</scene> ligands in its structure, and also has the ligand RHO as opposed to 4-methyl histidine as found in the F-actin structure. | '''G-actin''' is the free monomeric form of actin which polymerizes 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. Also, F-actin possesses ATPase function where it is minimal in G-actin. G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/1'>calcium ion</scene> ligands in its structure, and also has the ligand RHO as opposed to 4-methyl histidine as found in the F-actin structure. | ||
| Line 17: | Line 16: | ||
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 and 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. | 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 and 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. | ||
== Structure == | == Structure == | ||
| Line 36: | Line 32: | ||
=== Active Site === | === Active Site === | ||
Upon actin binding on the plus end of the actin filament, the ATPase function is activated. The conformational change from G- to F- actin promotes the catalytic activity because of the 20 | Upon actin binding on the plus end of the actin filament, the ATPase function is activated. The conformational change from G- to F- actin promotes the catalytic activity because of the 20° shift leading to a more closed binding site; this conformational change is stabilized also by the diagonal subdomain interaction between Leu110 and Thr194. | ||
Upon binding changes, the Gln137 residue of actin is moved closer to the ATP-Ca2+ ligand. Gln137 holds a water molecule, and placing it in close proximity to ATP allows for the gamma-phosphate to become cleaved. Release of the inorganic phosphate occurs via the conformational change of the flexible "D-loop" into an ordered alpha-helix<ref>graceffa</ref>. | Upon binding changes, the Gln137 residue of actin is moved closer to the ATP-Ca2+ ligand. Gln137 holds a water molecule, and placing it in close proximity to ATP allows for the gamma-phosphate to become cleaved. Release of the inorganic phosphate occurs via the conformational change of the flexible "D-loop" into an ordered alpha-helix<ref>graceffa</ref>. | ||
