Non-polymerizable monomeric actin: Difference between revisions
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== The actin structural motif == | == The actin structural motif == | ||
[[Image:2HF4 COMP 1HSC IMAGE.png|400px|right|thumb| Crystal structure of ATP-bound AP-actin, 2HF4, with selected residues shown in red. ATP is shown in green.]] | [[Image:2HF4 COMP 1HSC IMAGE.png|400px|right|thumb| Crystal structure of ATP-bound AP-actin, (PDB entry 2HF4), with selected residues shown in red. ATP is shown in green.]] | ||
[[Image:1HSC COMP 2HF4 IMAGE.png|400px|right|thumb| Crystal structure of the amino-terminal 44K ATPase fragment of the 70K bovine | [[Image:1HSC COMP 2HF4 IMAGE.png|400px|right|thumb| Crystal structure of the amino-terminal 44K ATPase fragment of the 70K bovine Hsc70 protein, (PDB entry [[3HSC]]), bound to ADP. Selected residues are shown in red. ATP is shown in green.]] | ||
[[Image:1JCG COMP 2HF4 IMAGE.png|400px|right|thumb| Crystal structure of MreB from Thermotoga Maritima bound to AMP.PNP, [[1JCG]]. Selected residues are shown in red. AMP.PNP is shown in green.]] | [[Image:1JCG COMP 2HF4 IMAGE.png|400px|right|thumb| Crystal structure of MreB from Thermotoga Maritima bound to AMP.PNP, (PDB entry [[1JCG]]). Selected residues are shown in red. AMP.PNP is shown in green.]] | ||
Through structural comparisons, it’s possible to identify evolutionary relationships among proteins with common folds. The core structure of actin is strikingly similar to the structure of Hsc70 and MreB which both bind ATP with a common motif. Hsc70 is a member of the 70-kDa heat-shock family Hsp70 which binds to unfolded proteins and hydrolyzes ATP. Exchange factors recycle Hsc70 by stimulating ATPase activity thereby mediating dissociation from the protein substrate<ref name="Buchberger">PMID:7656024</ref>. On the right, the crystal structure of a 44-kDa amino-terminal fragment of Hsc70 (PDB entry [[3HSC]]) is shown that retains ATPase activity but is incapable of binding proteins<ref name="Flaherty">PMID: 2143562</ref>. AP-actin, in blue, is shown aligned to this structure with functionally and structurally conserved residues in red. | Through structural comparisons, it’s possible to identify evolutionary relationships among proteins with common folds. The core structure of actin is strikingly similar to the structure of Hsc70 and MreB which both bind ATP with a common motif. Hsc70 is a member of the 70-kDa heat-shock family Hsp70 which binds to unfolded proteins and hydrolyzes ATP. Exchange factors recycle Hsc70 by stimulating ATPase activity thereby mediating dissociation from the protein substrate<ref name="Buchberger">PMID:7656024</ref>. On the right, the crystal structure of a 44-kDa amino-terminal fragment of Hsc70 (PDB entry [[3HSC]]) is shown that retains ATPase activity but is incapable of binding proteins<ref name="Flaherty">PMID: 2143562</ref>. AP-actin, in blue, is shown aligned to this structure with functionally and structurally conserved residues in red. | ||
Subdomains 1 and 3 of actin are composed of a five-stranded β-sheet surrounded by α-helicies. These domains are highly conserved in Hsc70. The interface between subdomains 1, 3, and 4 are required in both structures to bind nucleotide. The adenosine group of ATP fits into a hydrophobic pocket between subdomains 3 and 4, formed by two helices (AP-actin: residues 210-214, 302-306, 336 and Hsc70: residues 268-272, 339-343, 366 as shown in red for both structures). The ribose is bound by Glu214 in actin and Glu268 in Hsc70. These residues form salt bridges with adjacent arginines in both structures. | Subdomains 1 and 3 of actin are composed of a five-stranded β-sheet surrounded by α-helicies. These domains are highly conserved in Hsc70. The interface between subdomains 1, 3, and 4 are required in both structures to bind nucleotide. The adenosine group of ATP fits into a hydrophobic pocket between subdomains 3 and 4, formed by two helices (AP-actin: residues 210-214, 302-306, 336 and Hsc70: residues 268-272, 339-343, 366 as shown in red for both structures). The ribose is bound by Glu214 in actin and Glu268 in Hsc70. These residues form salt bridges with adjacent arginines in both structures. | ||
A comparison of these two structures shows that not only is the nucleotide binding cleft conserved but also helix packing against β-sheets in subdomains 1 and 3. This same mode of structural conservation is described for hexokinase and glycerol kinase<ref name="Kabsch">PMID: 7781919</ref>. | A comparison of these two structures shows that not only is the nucleotide binding cleft conserved but also helix packing against β-sheets in subdomains 1 and 3. This same mode of structural conservation is described for hexokinase and glycerol kinase<ref name="Kabsch">PMID: 7781919</ref>. | ||
While Hsp70 and sugar kinases show ~50% sequence identity, the bacterial cytoskeletal protein MreB shares less than 15% sequence identity. However, a comparison of the crystal structures of actin and MreB reveals high structural homology. Like actin, MreB contains two domains composed of a five-stranded β-sheet. These domains are connected by a helix, similar to actin. Subdomains 2 and 4 in actin are more diverse in the actin superfamily and share little structural homology with Hsp70 and sugar kinases. Remarkably, MreB shares the same topology as actin in these domains<ref name="Ent">PMID:11544518</ref>. MreB can be superimposed on actin (PDB entry [[1ATN]]) with an r.m.s. deviation of 3.7 Å over 310 C atoms<ref name="Holm">PMID: 8377180</ref>. Also, MreB can be superimposed on Hsp70 (PDB entry [[1HPM]]) with an r.m.s. deviation of 3.4 Å over 299 C atoms. However, Hsp70 has a large insert in subdomain 4 not present in actin or MreB. This result suggests that MreB is closely related to Hsp70. Shown right is a crystal structure of MreB (PDB entry [[1JCG]]) bound to | While Hsp70 and sugar kinases show ~50% sequence identity, the bacterial cytoskeletal protein MreB shares less than 15% sequence identity. However, a comparison of the crystal structures of actin and MreB reveals high structural homology. Like actin, MreB contains two domains composed of a five-stranded β-sheet. These domains are connected by a helix, similar to actin. Subdomains 2 and 4 in actin are more diverse in the actin superfamily and share little structural homology with Hsp70 and sugar kinases. Remarkably, MreB shares the same topology as actin in these domains<ref name="Ent">PMID:11544518</ref>. MreB can be superimposed on actin (PDB entry [[1ATN]]) with an r.m.s. deviation of 3.7 Å over 310 C atoms<ref name="Holm">PMID: 8377180</ref>. Also, MreB can be superimposed on Hsp70 (PDB entry [[1HPM]]) with an r.m.s. deviation of 3.4 Å over 299 C atoms. However, Hsp70 has a large insert in subdomain 4 not present in actin or MreB. This result suggests that MreB is closely related to Hsp70. Shown right is a crystal structure of MreB (PDB entry [[1JCG]]) bound to AMP.PNP and aligned with AP-actin. Structurally conserved residues corresponding to those in actin are indicated in red. They include residues at the interface between subdomains 1 and 3, and a functionally conserved glutamic acid at position 204. This residue in MreB forms a salt bridge with an adjacent lysine residue similar to glutamic acid 214 in actin and glutamic acid 268 in Hsc70. | ||
The presence of a structurally conserved ATP-binding motif in the actin superfamily and MreB suggests that these proteins are likely the result of divergent evolution from a common ancestor. | The presence of a structurally conserved ATP-binding motif in the actin superfamily and MreB suggests that these proteins are likely the result of divergent evolution from a common ancestor. | ||