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