C

Chapter 19 - Microfilaments and Intermediate Filaments

19.1: Actin Structures

  • Actin is a huge part of the cytoskeleton and is abundantly found in eukaryotic cells
  • Though the isoforms of actin have minor differences, they all have very different functions
  • Globular G-actin and F-actin are both subunits oriented in the same direction
  • The (-) end of actin filaments is polarized and has an exposed ATP binding site
  • The filaments of actin are organized into networks and bundles divided by cross-linked proteins
  • The CH domain family is the main housing of many actin cross-linked proteins

19.2: The Dynamics of Actin Assembly

  • The actin cytoskeleton is dynamic within cells and the filaments can grow and shrink at a rapid rate
  • Polymerization of G-actin and nucleation occur at the same time in cells, and once polymerization stabilizes the rates for the addition and loss of subunits becomes equal
  • Critical concentration (Cc) is the equilibrium of actin monomers and actin filaments
  • When G-actin is above Cc, filaments grow and when it is below Cc, then filaments depolymerize
  • The (+) side of actin grows faster than the (-) side, and there is more Cc monomer on the (-) side than there is on the (+) side
  • Specialized actin building proteins control the length, assembly, and stability of actin filaments
  • Bacteria and viruses can be moved and the shape of a cell can be changed through the regulated polymerization of actin

19.3: Myosin-Powerd Cell Movements

  • Actin filaments interact with myosin isoforms through the actins head domains, though their cellular roles can differ depending on their tail domains
  • The sliding filament assay can monitor the movement of actin filaments by myosin
  • Intracellular translocation of various membrane limited vesicles along actin filaments are powered by myosins I, V, and VI
  • Contractile bundles with primitive sarcomere like organization can be formed by actin filaments and myosin II in nonmuscle cells
  • A transient bundle of myosin II and actin, known as the contractile ring, is formed through the dividing of the cell, which later pinches into two halves through cytokinesis
  • Sarcomeres are what the organization of actin thin filaments and myosin thick filaments in skeletal muscle cells are called

19.4: Cell Locomotion

  • Events of migrating cells
    • Pseudopodium or lamellipodium extension
    • Extended leading edges adhesion to the substratum
    • Streaming of the cytosol; forward flow
    • Retraction of cells body
  • Cell locomotion occurs through actin polymerization and branching generated movement, adhesion structures assembly, and myosin II-mediated cortical contraction
  • The organization and assembly of the cytoskeleton can be induced through external signals, and the internal of both G proteins and calcium can be induced as well
  • When these are all polarized, the result is cell locomotion

19.5: Intermediate Filaments

  • Only cells that show multicellular organization have intermediate filaments in them
  • Intermediate filaments are grouped into IF proteins, which based on their sequences and tissue distribution, are organized into four different types
  • Intermediate structures proceed with the assembly of intermediate filaments
  • Intermediate filaments organization is mediated by various IFAPs and provide cells with structural stability