Intermediate Filaments

  1. can be divided into __.
    five major classes
  2. In-depth structure
    - location
    • Radiate through the cytoplasm of a wide variety of animal cells and are often interconnected to other cytoskeletal filaments by thin, wispy cross-bridges
    • -          in many cells, these cross-bridges consists of an elongated dimeric protein called plectin that can exist in numerous isoforms
  3. plectin
    -          each plectin molecule has a binding site for an intermediate filament at one end and, depending on the isoform, a binding site for another intermediate filament, microfilament, or microtubule at the other end
  4. In-depth structure
    - diversity and similarity
    - polypeptides
    • diverse amino acid sequences but similar structural organization that allows them to form similar-looking filaments
    • polypeptides of Ifs contain a central, rod-shaped, alpha helcical domain of similar length and homologous amino acid sequence, which makes the subunits of intermediate filaments very different from the globular tubulin and actin subunits of microtubules and microfilaments
  5. In-depth structure
    - domain
    - rods
    • -          the central fibrous domain is flanked on each side by globular domains of variable size and sequence
    • -          the alpha-helical rods wrap around each other to forma  ropelike dimer approximately 45 nm in length
    • o   the dimer has polarity
  6. Function of IF
    • Provide mechanical strength to cells that are subjected to physical stress, including neurons, muscle cells, and the epithelia cells that are encoded by about 70 different genes
    • Because of its various physical connections, it serves as a scaffold for organizing and maintaining cellular architecture and for absorbing mechanical stresses applied by the extracellular environment
  7. Basic building block
    • basic building block is thought to be a rod-like tetramer formed by two dimers that become aligned side by side in a staggered fashion with their N- and C- termini pointing in opposite directions
    • -          the tetramer itself lacks polarity
    • -          8 tetramers associate with one another side by side to form a filament that is one unit in length
  8. Growth
    • -          subsequent growth of the polymer is accomplished as these unit lengths of filaments associate with one another in an end-to-end fashion to form the highly elongated intermediate filament
    • nothing requires ATP
  9. Characteristics of IFs
    • Less sensitive to chemical agents than other types of cytoskeletal elements and more difficult to solubilized
  10. Types of IF: (3 types)
    - keratin-containing IFs

    - neurofilaments

    - desmin
  11. Keratin-containing IFs
    -          radiate through the cytoplasm, tethered to the nuclear envelope in the center of the cell and anchored at the outer edge of the cell by connections to the cytoplasmic plaques of desmosomes and hemidesmosomes
  12. Neurofilaments
    • -          composed of three distinct proteins: NF-M, NF-H, and NF-L; NF-H and NF-M have sidearms that project outward from the neurofilament
    • -          the sidearms maintain the proper spacing beween the parallel neurofilaments of the axon
    • -          in early stages of differentiation when the axon is growing toward a target cell, it contains few neurofilaments until its nerve cell is fully extended
  13. Desmin
    plays a key structural role in maintaining the alignment of the myofibrils of a muscle cell and its absence makes the cell extremely fragile
Card Set
Intermediate Filaments