4.9 - Actin microfilaments (F-actin) 8 nm diam. Composed of...

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Actin microfilaments (F-actin) 8 nm diam. Composed of globular actin monomers (G-actin) 2 strands of G-actin monomers are wound together into a stiff filament Basis of contractile cytoskeleton in muscle cells and non-muscle cells Also, roles in supporting non-motile cellular structures like microvilli and in forming the membrane skeleton beneath the plasma membrane A major protein in all eukaryotic cells Highly conserved
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Figure 9.46b
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Actin microfilaments (F-actin) F-actin is polarized the microfilament has a (+) and a (-) end. Polarization can be visualized by adding myosin S1 head fragments, which bind to and decorate the actin filaments Since the S1 myosin heads bind at an angle to F- actin, an “arrowhead” appearance is created, as viewed under an e.m. The pointed end is the (-) end.
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F-actin assembly A pool of G-actin monomers exists in the cytoplasm. G-actin binds ATP G-actin w/ATP bound can add onto both ends of filament, but much faster addition occurs at the (+) end. Since G-actin can also dissociate from the filament at both ends, net growth tends to occur at the (+) end and net loss is more likely, sometimes, at the (-) end. A dynamic equilibrium at both ends therefore determines filament length After addition, ATP is hydrolysed to ADP after some time. G-actins with ADP bound are more prone to dissociate from the filament. In fast-growing microfilaments a stabilizing “ATP - cap” is therefore created at the (+) end.
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Drugs that affect microfilaments Cytochalasins promote microfilament dissasembly Phalloidin stabilizes microfilaments and, if fluorescently tagged, can be
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4.9 - Actin microfilaments (F-actin) 8 nm diam. Composed of...

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