Sandbox 154: Difference between revisions

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== Introduction ==

Actin is found in nearly all eukaryotic cells and is known primarily for its function as a structural and translocation protein. It also has an ATPase function, as it hydrolyzes ATP to ADP and P<sub>i</sub> and undergoes conformational changes with each hydrolysis. Actin belongs to the actin superfamily, which includes other proteins such as Hsp70(DnaK), Hsc70, and hexokinase, because of its nucleotide-dependent conformational change<ref name="Graceffa">PMID:12813032</ref>. Because of the similarity observed in ''Escherichia Coli'''s, Hsc70 and ATPase domain of actin, it is believed that the two proteins have a common ~~ancestory~~<ref name="Holmes1">PMID:19158779</ref>. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of possible common ancestory<ref name="Holmes2">PMID:2395461</ref>.

Actin is found in nearly all eukaryotic cells and is known primarily for its function as a structural and translocation protein. It also has an ATPase function, as it hydrolyzes ATP to ADP and P<sub>i</sub> and undergoes conformational changes with each hydrolysis. Actin belongs to the actin superfamily, which includes other proteins such as Hsp70(DnaK), Hsc70, and hexokinase, because of its nucleotide-dependent conformational change<ref name="Graceffa">PMID:12813032</ref>. Because of the similarity observed in ''Escherichia Coli'''s, Hsc70 and ATPase domain of actin, it is believed that the two proteins have a common ancestry<ref name="Holmes1">PMID:19158779</ref>. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of possible common ancestory<ref name="Holmes2">PMID:2395461</ref>.

Actin occurs in two forms: globular actin (G-actin), the free monomeric units of actin, and filamentous actin (F-actin) which is the polymer form. These two forms exist in a dynamic equilibrium with one another as ATP-associated polymerization and depolymerization occur continuously within the cell. The monomer units in the F-actin possess a form that is distinct from the free monomeric form and it is a result of that change that the more specific ATPase activity may be observed.

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== Assembly ==

'''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 an ATPase function which is minimal in G-actin.The domains and active site are the same in terms of constituent components and will be discussed later in terms of the F-actin monomer.

'''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 an ATPase function which is minimal in G-actin. The domains and active site are the same in terms of constituent components and will be discussed later in terms of the F-actin monomer.

G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/3'>calcium ion</scene> ligands in its structure, external to the active site. Only 3 of the 5 are believed to actually exist in solution and are believed to contribute to the polymerization of G-actin to F-actin<ref name="otterbein">PMID:11474115</ref>. This representation of G-actin also possesses an <scene name='Sandbox_154/1j6z_black_true_dloop/1'>alpha-helical fold in the D-loop</scene> which is observed in some actin crystalline structures but not necessarily<ref name="otterbein"/>. The observed molecule on Cys374, <scene name='Sandbox_154/1j6z_black_true_rho/1'>tetramethylrhodamine-5-maleimide</scene> was used to block polymerization activity so the crystal of G-actin could be observed<ref name="otterbein"/>

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==== [http://en.wikipedia.org/wiki/Cytoskeleton#Actin_filaments_.2F_Microfilaments Cytoskeleton] ====

F-actin is the most abundant component of the cytoskeleton of eukaryotes. It provides large amounts of tensile strength, considering its thin size. In cases where the flexibility is not desirable as a ~~strucutral~~ component, crosslinkages can be formed between F-actin polymers to give greater stiffness and support<ref name="Mitchinson"/>.

F-actin is the most abundant component of the cytoskeleton of eukaryotes. It provides large amounts of tensile strength, considering its thin size. In cases where the flexibility is not desirable as a structural component, crosslinkages can be formed between F-actin polymers to give greater stiffness and support<ref name="Mitchinson"/>.

Elongation of F-actin branches leads to the phenomenon of pushing of the plasma membrane forward in lamellopodial and filopodial extension<ref name="Chen"/>. This process relies on the dynamic equilibrium state in which G- and F-actin exist, as it is the continual polymerization of actin units on the leading edge that propels the membrane extension. Without the enzymatic ATPase function of F-actin, this process would not be possible.

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@@ Line 4: / Line 4: @@
 == Introduction ==
-Actin is found in nearly all eukaryotic cells and is known primarily for its function as a structural and translocation protein. It also has an ATPase function, as it hydrolyzes ATP to ADP and P<sub>i</sub> and undergoes conformational changes with each hydrolysis. Actin belongs to the actin superfamily, which includes other proteins such as Hsp70(DnaK), Hsc70, and hexokinase, because of its nucleotide-dependent conformational change<ref name="Graceffa">PMID:12813032</ref>. Because of the similarity observed in ''Escherichia Coli'''s, Hsc70 and ATPase domain of actin, it is believed that the two proteins have a common ancestory<ref name="Holmes1">PMID:19158779</ref>. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of possible common ancestory<ref name="Holmes2">PMID:2395461</ref>.
+Actin is found in nearly all eukaryotic cells and is known primarily for its function as a structural and translocation protein. It also has an ATPase function, as it hydrolyzes ATP to ADP and P<sub>i</sub> and undergoes conformational changes with each hydrolysis. Actin belongs to the actin superfamily, which includes other proteins such as Hsp70(DnaK), Hsc70, and hexokinase, because of its nucleotide-dependent conformational change<ref name="Graceffa">PMID:12813032</ref>. Because of the similarity observed in ''Escherichia Coli'''s, Hsc70 and ATPase domain of actin, it is believed that the two proteins have a common ancestry<ref name="Holmes1">PMID:19158779</ref>. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of possible common ancestory<ref name="Holmes2">PMID:2395461</ref>.
 Actin occurs in two forms: globular actin (G-actin), the free monomeric units of actin, and filamentous actin (F-actin) which is the polymer form. These two forms exist in a dynamic equilibrium with one another as ATP-associated polymerization and depolymerization occur continuously within the cell. The monomer units in the F-actin possess a form that is distinct from the free monomeric form and it is a result of that change that the more specific ATPase activity may be observed.
@@ Line 10: / Line 10: @@
 == Assembly ==
 <applet load='1j6z' size='250' 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_true/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 an ATPase function which is minimal in G-actin.The domains and active site are the same in terms of constituent components and will be discussed later in terms of the F-actin monomer.
+'''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 an ATPase function which is minimal in G-actin. The domains and active site are the same in terms of constituent components and will be discussed later in terms of the F-actin monomer.
 G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/3'>calcium ion</scene> ligands in its structure, external to the active site. Only 3 of the 5 are believed to actually exist in solution and are believed to contribute to the polymerization of G-actin to F-actin<ref name="otterbein">PMID:11474115</ref>. This representation of G-actin also possesses an <scene name='Sandbox_154/1j6z_black_true_dloop/1'>alpha-helical fold in the D-loop</scene> which is observed in some actin crystalline structures but not necessarily<ref name="otterbein"/>. The observed molecule on Cys374, <scene name='Sandbox_154/1j6z_black_true_rho/1'>tetramethylrhodamine-5-maleimide</scene> was used to block polymerization activity so the crystal of G-actin could be observed<ref name="otterbein"/>
@@ Line 63: / Line 63: @@
 ==== [http://en.wikipedia.org/wiki/Cytoskeleton#Actin_filaments_.2F_Microfilaments Cytoskeleton] ====
-F-actin is the most abundant component of the cytoskeleton of eukaryotes. It provides large amounts of tensile strength, considering its thin size. In cases where the flexibility is not desirable as a strucutral component, crosslinkages can be formed between F-actin polymers to give greater stiffness and support<ref name="Mitchinson"/>.
+F-actin is the most abundant component of the cytoskeleton of eukaryotes. It provides large amounts of tensile strength, considering its thin size. In cases where the flexibility is not desirable as a structural component, crosslinkages can be formed between F-actin polymers to give greater stiffness and support<ref name="Mitchinson"/>.
 Elongation of F-actin branches leads to the phenomenon of pushing of the plasma membrane forward in lamellopodial and filopodial extension<ref name="Chen"/>. This process relies on the dynamic equilibrium state in which G- and F-actin exist, as it is the continual polymerization of actin units on the leading edge that propels the membrane extension. Without the enzymatic ATPase function of F-actin, this process would not be possible.