Cell Biology: Microtubules and Intermediate Filaments part 3

Microtubules

  • Largest diameter of the three filament types.
  • Stained green in microscopy; DNA stained blue.
  • Located throughout the cytoplasm of cells not undergoing mitosis (interphase).
  • Involved in maintaining cell shape and organelle movement.

Microtubule Structure

  • Composed of subunits that form large, hollow, rigid tubes.
  • Subunits are not covalently attached, allowing for rapid assembly and disassembly.

Centrosome

  • Microtubule organizing center (MTOC).
  • In animal cells, contains centrioles with a specific architecture: nine triplets of microtubules.
  • Centrioles are found at right angles to each other within the centrosome.
  • The centrosome nucleates and organizes microtubule growth.

Microtubules in Cells

  • Microtubules extend throughout the cytoplasm in interphase cells.
  • They form the architecture of cilia.
  • Cilia also contain connecting proteins and motor proteins for movement.

Microtubules as Railroad Tracks

  • Microtubules facilitate directed cargo movement within the cell.
  • Adapter proteins called kinesins bind to cargo (e.g., vesicles, organelles) and move them along microtubules.
  • The direction of movement is determined by the motor protein.
    • Kinesins move towards the plus end.
    • Dyneins move towards the minus end.
  • The plus end of a microtubule polymerizes and depolymerizes faster than the minus end.

Motor Proteins

  • ATP hydrolysis drives conformational changes in motor proteins, enabling movement along microtubules.
  • Kinesin and dynein are examples of motor proteins that move cargo along microtubule tracks.
  • ATP binding, hydrolysis, and release cause the motor protein to undergo a series of conformational changes, leading to processive movement along the microtubule track.
  • The globular heads of kinesin attach to the microtubule track, and ATP hydrolysis facilitates their stepping motion.

Cargo Transport

  • Vesicles and other cellular components are transported along microtubule tracks by motor proteins.
  • Organelles can have both kinesin and dynein motors attached, allowing them to switch directions.

Dynamic Instability

  • Microtubules exhibit dynamic instability, with subunits rapidly assembling and disassembling as needed by the cell.

Intermediate Filaments

  • Diverse group of proteins with varying functions.
  • Help cells resist changes in shape.
  • Anchor organelles.
  • Form a meshwork under the nuclear membrane (nuclear lamina).

Intermediate Filaments in Cells

  • Found throughout the cytoplasm in various cell types.
  • The nuclear lamina, composed of intermediate filaments, supports the nuclear envelope and provides attachment sites for chromosomes.

Cell-Cell Connections

  • Tight junctions: Seal cells together to prevent the passage of molecules between them.
  • Desmosomes (spot welds): Connect the cytoskeletons of adjacent cells, providing strength and preventing tearing.
  • Gap junctions: Channels that allow for communication between adjacent cells, enabling the passage of ions and small molecules.

Cell Junctions and Communication

  • Epithelial cells rely on tight junctions to create a barrier against pathogens.
  • Desmosomes provide mechanical strength to tissues, such as heart muscle.
  • Gap junctions allow for electrical continuity and coordinated movements in tissues like heart muscle.

Plant Cell Communication

  • Plant cells communicate through plasmodesmata, channels that perforate the cell walls and allow for the exchange of materials between adjacent cells.