actin

Examples of actin in the following topics:

• ATP and Muscle Contraction

  • ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction.
  • As myosin expends the energy, it moves through the "power stroke," pulling the actin filament toward the M-line.
  • When the actin is pulled approximately 10 nm toward the M-line, the sarcomere shortens and the muscle contracts.
  • The cross-bridge muscle contraction cycle, which is triggered by Ca2+ binding to the actin active site, is shown.
  • With each contraction cycle, actin moves relative to myosin.

• Regulatory Proteins

  • Tropomyosin and troponin prevent myosin from binding to actin while the muscle is in a resting state.
  • The binding of the myosin heads to the muscle actin is a highly-regulated process.
  • When a muscle is in a resting state, actin and myosin are separated.
  • The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule.
  • Describe how calcium, tropomyosin, and the troponin complex regulate the binding of actin by myosin

• Microfilaments

  • They function in cellular movement, have a diameter of about 7 nm, and are made of two intertwined strands of a globular protein called actin .
  • For this reason, microfilaments are also known as actin filaments.
  • Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin.
  • Actin and myosin are plentiful in muscle cells.
  • When your actin and myosin filaments slide past each other, your muscles contract.

• Sliding Filament Model of Contraction

  • Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules.
  • Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments.
  • Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments.
  • Another protein, nebulin, is thought to perform a similar role for actin myofilaments.
  • During contraction myosin ratchets along actin myofilaments compressing the I and H bands.

• Control of Muscle Tension

  • Neural control initiates the formation of actin–myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction .
  • The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce.
  • Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin.
  • If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.
  • This results in fewer myosin heads pulling on actin and less muscle tension.

• Skeletal Muscle Fibers

  • Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
  • Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins.
  • Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
  • Thin filaments are composed of actin, tropomyosin, and troponin.
  • The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.

• Intermediate Filaments and Microtubules

  • Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin (microfilaments), and that of 25 nm microtubules, although they were initially designated 'intermediate' because their average diameter is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells.

• The Mitotic Phase and the G0 Phase

  • A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate.
  • The actin filaments pull the equator of the cell inward, forming a fissure.
  • The furrow deepens as the actin ring contracts; eventually the membrane is cleaved in two .
  • During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate.

• Muscle Tissues and Nervous Tissues

  • Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins, actin and myosin, that run throughout the muscle fiber .
  • Skeletal muscle cells can contract by the attachment of myosin to actin filaments in the muscle, which then ratchets the actin filaments toward the center of the cells.

• Malpighian Tubules of Insects

  • They contain actin for support.