Sandbox Reserved 471: Difference between revisions

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== Introduction ==

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Power Stroke Mechanism

Myosin II molecules bound to actin filaments generate force in skeletal muschles through a mechanism known as the power stroke. The mechanism is comprised of a cycle of ATP-binding, hydrolysis and phosphate release. At the beginning of the first step, the myosin head lacks an ATP and is attached to an actin filament in a conformation known as the “rigor confirmation.” In step two, the ATP binds to the myosin head which induces a conformational shift in the actin-binding site that reduces its binding to actin and cause myosin to release the actin. In the third step the binding of ATP leads to the myosin head being positioned further along the filament. The ATP is then hydrolysed and the inorganic phosphate and ADP remain bound to myosin. In the fourth step, the myosin head makes contact with the actin filament leading to another conformational change that promotes the release of inorganic phosphate. In step five, the binding between myosin and actin is reinforced and triggers the power stroke. Forces are generated on the actin filament as the myosin head returns to its original position. In the final step, the ADP is released and the myosin head remains bound to the filament at a new position from where it began therefore starting the cycle again.<ref>PMID: 11810692</ref>

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Many of the functions of the cytoskeleton involve cytoskeleton and plasma membrane interactions. Proteins that exist between the two help to control cell shape, define the membrane domains and regulate cell/cell interactions and adhesion. There are at least 15 major protein species involved in the membrane-cytoskeleton of the human red blood cell. A mutation in any of these proteins can lead to cell fragility and cell death. Homologs of the erythrocyte membrane-skeleton can be found in many other cell types, which suggests the significance of these proteins. One of particular importance is the actin associated protein dystrophin. Dystrophin is an important part of the sarcolemmal membrane where it links the membrane to the sides of the actin filament bundles. Mutations in this protein leads to muscular dystrophies such as Duchenne’s disease or Becker muscular dystrophy. A homolog of dystrophin called utropin is thought to play a similar role in non-muscle cells. <ref>Boron, W., & Boulpaep, E. (2008). Medical physiology. (2 ed., pp. 14-20). Saunders Elsevier. Retrieved from http://books.google.com/books/about/Medical_Physiology_E_Book.html?id=HlMJRw08ihgC</ref>

The genome of many mammals, including humans contains six actin genes, ACTA1, ACTA2, ACTB, ACTC, ACTG1 and ACTG2. Four of these are differentially expressed in cardiac (ACTC), smooth (ACTA2) enteric (ACTG2) and skeletal muscles (ACTA1); two are described as cytoplasmic actin genes (ACTB and ACTG1) and are expressed in all cells. <ref>Remedios, C., & Chhabra, D. (2008). Actin-binding proteins and diseases. (8 ed., pp. 16-18). New York: Springer.</ref> Actin mutations were not really known until the last 6-10 years. Mutations that have been found to cause human diseases have been found in the ACTC cardiac muscle gene which can cause dilated or hypertrophic cardiomyopathies including congenital fiber-type disproportion (CFTDP). Recently, mutations in ACTG1 have been associated with autosomal dominant deafness. Majority of actin disease mutations have been found to be dominant mutations. This has been the case for actin mutations is the Drosophila flight muscle-specific actin gene ACT88F. Scientists believe the common dominance of actin mutations is because the major functions of actin occur when it is in its F-form, associated with other proteins. The dominant actin mutations that lead to disease are mostly missense mutations. Actin mutations have been found in a variety of species such as yeast, nematodes and flies whose genomes are more accessible to mutagenesis. It is thought that there are three reasons why actin mutations are likely to be at very low frequencies in human populations: 1. actin is a ubiquitously expressed protein, 2. it is a highly conserved protein with many binding partners, so mutations in a large fraction of actin residues appear to cause a severe phenotype in humans and other organisms and 3. most actin mutations are dominant and given the usually severe effects, are not passed on to offspring. Thus familial actin mutants are likely to be relatively mild dominant alleles or recessive. Severe dominant alleles are likely to be de novo mutations. In the case of cytoplasmic actin, five ACT1G disease-causing mutant alleles are known so far and no disease-causing ACTB alleles have yet been described.

=References=

Sandbox Reserved 471: Difference between revisions

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+<Structure load='1atn' size='500' frame='true' align='right' caption='Actin' scene='Insert optional scene name here' />
 == Introduction ==
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 Power Stroke Mechanism
 Myosin II molecules bound to actin filaments generate force in skeletal muschles through a mechanism known as the power stroke. The mechanism is comprised of a cycle of ATP-binding, hydrolysis and phosphate release. At the beginning of the first step, the myosin head lacks an ATP and is attached to an actin filament in a conformation known as the “rigor confirmation.” In step two, the ATP binds to the myosin head which induces a conformational shift in the actin-binding site that reduces its binding to actin and cause myosin to release the actin. In the third step the binding of ATP leads to the myosin head being positioned further along the filament. The ATP is then hydrolysed and the inorganic phosphate and ADP remain bound to myosin. In the fourth step, the myosin head makes contact with the actin filament leading to another conformational change that promotes the release of inorganic phosphate. In step five, the binding between myosin and actin is reinforced and triggers the power stroke. Forces are generated on the actin filament as the myosin head returns to its original position. In the final step, the ADP is released and the myosin head remains bound to the filament at a new position from where it began therefore starting the cycle again.<ref>PMID: 11810692</ref>
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 Many of the functions of the cytoskeleton involve cytoskeleton and plasma membrane interactions. Proteins that exist between the two help to control cell shape, define the membrane domains and regulate cell/cell interactions and adhesion. There are at least 15 major protein species involved in the membrane-cytoskeleton of the human red blood cell. A mutation in any of these proteins can lead to cell fragility and cell death. Homologs of the erythrocyte membrane-skeleton can be found in many other cell types, which suggests the significance of these proteins. One of particular importance is the actin associated protein dystrophin. Dystrophin is an important part of the sarcolemmal membrane where it links the membrane to the sides of the actin filament bundles. Mutations in this protein leads to muscular dystrophies such as Duchenne’s disease or Becker muscular dystrophy. A homolog of dystrophin called utropin is thought to play a similar role in non-muscle cells. <ref>Boron, W., & Boulpaep, E. (2008). Medical physiology. (2 ed., pp. 14-20). Saunders Elsevier. Retrieved from http://books.google.com/books/about/Medical_Physiology_E_Book.html?id=HlMJRw08ihgC</ref>
-  The genome of many mammals, including humans contains six actin genes, ACTA1, ACTA2, ACTB, ACTC, ACTG1 and ACTG2. Four of these are differentially expressed in cardiac (ACTC), smooth (ACTA2) enteric (ACTG2) and skeletal muscles (ACTA1); two are described as cytoplasmic actin genes (ACTB and ACTG1) and are expressed in all cells. <ref>Remedios, C., & Chhabra, D. (2008). Actin-binding proteins and diseases. (8 ed., pp. 16-18). New York: Springer.</ref> Actin mutations were not really known until the last 6-10 years. Mutations that have been found to cause human diseases have been found in the ACTC cardiac muscle gene which can cause dilated or hypertrophic cardiomyopathies including congenital fiber-type disproportion (CFTDP). Recently, mutations in ACTG1 have been associated with autosomal dominant deafness. Majority of actin disease mutations have been found to be dominant mutations. This has been the case for actin mutations is the Drosophila flight muscle-specific actin gene ACT88F. Scientists believe the common dominance of actin mutations is because the major functions of actin occur when it is in its F-form, associated with other proteins. The dominant actin mutations that lead to disease are mostly missense mutations. Actin mutations have been found in a variety of species such as yeast, nematodes and flies whose genomes are more accessible to mutagenesis. It is thought that there are three reasons why actin mutations are likely to be at very low frequencies in human populations: 1. actin is a ubiquitously expressed protein, 2. it is a highly conserved protein with many binding partners, so mutations in a large fraction of actin residues appear to cause a severe phenotype in humans and other organisms and 3. most actin mutations are dominant and given the usually severe effects, are not passed on to offspring. Thus familial actin mutants are likely to be relatively mild dominant alleles or recessive. Severe dominant alleles are likely to be de novo mutations. In the case of cytoplasmic actin, five ACT1G disease-causing mutant alleles are known so far and no disease-causing ACTB alleles have yet been described.
+The genome of many mammals, including humans contains six actin genes, ACTA1, ACTA2, ACTB, ACTC, ACTG1 and ACTG2. Four of these are differentially expressed in cardiac (ACTC), smooth (ACTA2) enteric (ACTG2) and skeletal muscles (ACTA1); two are described as cytoplasmic actin genes (ACTB and ACTG1) and are expressed in all cells. <ref>Remedios, C., & Chhabra, D. (2008). Actin-binding proteins and diseases. (8 ed., pp. 16-18). New York: Springer.</ref> Actin mutations were not really known until the last 6-10 years. Mutations that have been found to cause human diseases have been found in the ACTC cardiac muscle gene which can cause dilated or hypertrophic cardiomyopathies including congenital fiber-type disproportion (CFTDP). Recently, mutations in ACTG1 have been associated with autosomal dominant deafness. Majority of actin disease mutations have been found to be dominant mutations. This has been the case for actin mutations is the Drosophila flight muscle-specific actin gene ACT88F. Scientists believe the common dominance of actin mutations is because the major functions of actin occur when it is in its F-form, associated with other proteins. The dominant actin mutations that lead to disease are mostly missense mutations. Actin mutations have been found in a variety of species such as yeast, nematodes and flies whose genomes are more accessible to mutagenesis. It is thought that there are three reasons why actin mutations are likely to be at very low frequencies in human populations: 1. actin is a ubiquitously expressed protein, 2. it is a highly conserved protein with many binding partners, so mutations in a large fraction of actin residues appear to cause a severe phenotype in humans and other organisms and 3. most actin mutations are dominant and given the usually severe effects, are not passed on to offspring. Thus familial actin mutants are likely to be relatively mild dominant alleles or recessive. Severe dominant alleles are likely to be de novo mutations. In the case of cytoplasmic actin, five ACT1G disease-causing mutant alleles are known so far and no disease-causing ACTB alleles have yet been described.
 =References=