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Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.It is found in essentially all eukaryotic cells, where it may be present at a concentration of over 100 μM; its mass is roughly 42 kDa, with a diameter of 4 to 7 nm.
Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin , but are modified by and interact with numerous other proteins in the cell.
These filament forming proteins have been classified into 4 classes. Tubulin-like, actin-like, Walker A cytoskeletal ATPases (WACA-proteins), and intermediate filaments. [8] [28] Tubulin-like proteins are tubulin in eukaryotes and FtsZ, TubZ, RepX in prokaryotes. Actin-like proteins are actin in eukaryotes and MreB, FtsA in prokaryotes.
The actin filaments depolymerize and disassemble on the proximal end to allow free monomers to migrate to the leading edge (distal end) of the actin filament where it can polymerize and thus reattach. Actin filaments are also constantly being transported away from the leading edge by a myosin-motor driven process known as retrograde F-actin flow.
Actin filaments are shown in red, microtubules are in green, and the nuclei are in blue. The cytoskeleton provides the cell with an inner framework and enables it to move and change shape. The cytoskeleton provides the cell with an inner framework and enables it to move and change shape.
These filaments are made of two strands of actin monomers (or protofilaments) wrapping around each other, to create a single actin filament. Because actin monomers are not symmetrical molecules, their filaments have polarity based upon the structure of the actin monomer, which will allow one end of the actin filament to polymerize faster than ...
Cytochalasin is a toxin that will bind to the actin polymer, so it can no longer bind to the incoming actin monomers. Actin originally attached in the polymer is still leaving the microfilament causing depolymerization. Phalloidin is a toxin that will bind to actin locking the filament in place. Monomers are neither adding or leaving this ...
Thick filaments consist primarily of the protein myosin, that is responsible for force generation. It is composed of a globular head with both ATP and actin binding sites, and a long tail involved in its polymerization into myosin filaments. Elastic filaments are made up of a giant protein called titin and hold the thick filaments in place.