Actin Microfilaments, Myosin and Muscle
Cytoskeletal Control of the Cortex and Cytoplasmic Membrane
Oct 5, 2009
Fingers of the cytoplasmic membrane can be rapidly projected by the cytoskeleton and just as rapidly contracted. The membrane is pushed out by the addition of protein subunits to rigid actin microfilaments. Motor proteins, such as myosin, can attach to different actin microfilaments and force them to slide past each other using ATP. This movement causes the furrowing of membrane in cell division and is highly magnified in muscle contraction.
Dynamic Instability and Actin Polymerization
ATP binds to actin proteins and facilitates polymerization at the plus end of actin microfilaments. As part of the filament, the ATP spontaneously hydrolyzes to ADP. Thus, as long as the actin microfilaments continue to grow, there are freshly added, ATP-containing actin proteins at the growing end. If growth slows, then the terminal actin proteins end up with ADP and they spontaneously depolymerize. This is the same type of dynamic instability observed in the much larger microtubules of the cytoskeleton.
Projection of Membrane Fingers and Sheets by Actin Microfilaments
Some accessory proteins, e.g. tropomyosin, can bind to the growing, plus ends of actin microfilaments to align the microfilaments into bundles. These elongating bundles can form membrane extensions, such as the microvilli of intestinal epithelial cells. The bases of these bundles of actin microfilaments are embedded in proteins that line the under surface of the cytoplasmic membrane and are reinforced with the strong meshwork of intermediate filaments of the cytoskeleton.
Actin-Binding Proteins
In addition to the actin-binding proteins (ABPs) that connect actin microfilaments, e.g. tropomysin, fimbrin or filamin, other proteins connect microfilaments to membranes, spectrin. These ABPs determine the structure and function of the actin microfilament bundles, sheets and matrices. In addition, gelsolin acts to fragment actin filaments that rapidly disassemble. Actin microfilaments form a dynamic and flexible mechanical system with which a cell can interact and adjust to its external physical environment.
Myosin, a Molecular Motor Moving on Actin Microfilaments
Force can be applied to actin filaments by ATP-powered motor proteins. The most common example of actin motor proteins are the myosins. Myosins are a pair of intertwined proteins that bind to actin microfilaments with two flexible protein “legs.” These legs use ATP hydrolysis to go through a cycle of protein conformational changes that literally walk the myosin along actin microfilaments. Myosins attached back to back will use ATP to pull actin microfilaments together. Organized constriction of actin microfilament in the cortical region below the cytoplasmic membrane is responsible for the purse string-like furrowing of dividing cells.
Muscle Contraction by Concatenated, Ordered Actin and Myosin
Muscle cells are gigantic and contain multiple nuclei on the surface of a highly organized series of bundles of actin and myosin myofibrils in the form of a single muscle fiber. The muscle fibers appear striated in microscopy, because they consist of light bands of actin microfilaments that extend into dark bands of myosin. In muscle contraction, the ATP-powered sliding of the myosin on the actin is initiated by the nerve-triggered release of calcium into the muscle cells that activates the myosin motors.
Actin Moves the Cytoskeleton
Microtubules provide a stable cytoskeletal array radiating from the center of the cell to the cytoplasmic membrane. Intermediate filaments give the cell strength. The actin microfilaments are the thinest and most dynamic cytoskeletal filaments and they provide the extension and contraction that makes cells move and change shape.
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