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.

• Mechanism and Contraction Events of Cardiac Muscle Fibers

  • Calcium in the cytoplasm then binds to cardiac troponin-C, which moves the troponin complex away from the actin binding site.
  • This removal of the troponin complex frees the actin to be bound by myosin and initiates contraction.
  • The myosin head binds to ATP and pulls the actin filaments toward the center of the sarcomere, contracting the muscle.
  • Intracellular calcium is then removed by the sarcoplasmic reticulum, dropping intracellular calcium concentration, returning the troponin complex to its inhibiting position on the active site of actin, and effectively ending contraction as the actin filaments return to their initial position, relaxing the muscle.
  • This animation shows myosin filaments (red) sliding along the actin filaments (pink) to contract a muscle cell.

• 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.

• Rigor Mortis

  • Physiologically, rigor mortis is caused a release of calcium facilitating crossbridges in the sarcomeres; the coupling between myosin and actin cannot be broken, creating a constant state of muscle contraction until enzymatic decomposition eventually removes the crossbridges.
  • Diffusion of the calcium through the pumps occurs, facilitation binding of myosin and actin filaments.
  • Unlike muscular contractions during life, the body after death is unable to complete the cycle and release the coupling between the myosin and actin, creating a state of muscular contraction until the breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition .
  • Diagram showing Actin-Myosin filaments in Smooth muscle.
  • The actin fibers attach to the cell wall and to dense bodies in the cytoplasm.

• Adherens Junctions

  • The actin microfilaments of the cytoskeleton (internal skeleton of the cell). 
  • They link the actin microfilaments to the cadherins.
  • The latter two proteins are what link the cadherin-catenin complex to the cell’s internal skeletal framework (the actin microfilaments).
  • Actin filaments are associated with adherens junctions in addition to several other actin-binding proteins.

• MreB and Determinants of Cell Morphology

  • The conservation of protein structure suggests the common ancestry of the cytoskeletal elements formed by actin, found in eukaryotes, and MreB, found in prokaryotes.
  • Recent studies have found that MreB proteins polymerize to form filaments that are similar to actin microfilaments.
  • MreB is a protein found in bacteria that has been identified as a homologue of actin, as indicated by similarities in tertiary structure and conservation of active site peptide sequence.
  • The conservation of protein structure suggests the common ancestry of the cytoskeletal elements formed by actin and MreB, found in prokaryotes.
  • Indeed, recent studies have found that MreB proteins polymerize to form filaments that are similar to actin microfilaments.MreB controls the width of rod-shaped bacteria, such as Escherichia coli.

• Microscopic Anatomy

  • The two most important proteins within sarcomeres are myosin, which forms a thick, flexible filament, and actin, which forms the thin, more rigid filament.
  • Myosin has a long, fibrous tail and a globular head, which binds to actin.
  • Actin molecules are bound to the Z-disc, which forms the borders of the sarcomere.
  • Together, myosin and actin form myofibrils, the repeating molecular structure of sarcomeres.
  • When ATP binds to myosin, it seperates from the actin of the myofibril, which causes a contraction.

• 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.