Cell Movement, Actin Function, and Endocytosis

Actin Movement and Function
  • Actin filaments play a crucial role in cellular movement and shape changes.
  • Myosin interacts with actin to create movement by hydrolyzing ATP, which propels the filaments forward.
  • This movement generates thrust, assisting cytosol and membrane transport.
Pseudopod Movement
  • Pseudopods, extensions of the cytoplasm, form in response to stimuli and are involved in movement towards a target (like food or pathogens).
  • The alignment of actin filaments helps in the forward thrust, allowing for effective transport within cells.
Types of Cellular Transport
  • Eukaryotes exhibit complex transport mechanisms:
    • Active Transport: Energy-dependent movement across the membrane.
    • Passive Transport: Movement without energy investment.
  • Unique to eukaryotes is endocytosis, where the membrane engulfs particles.
  • Subcategories of endocytosis:
    • Phagocytosis: Engulfing large particles (e.g., bacteria by white blood cells).
    • Receptor-Mediated Endocytosis: Involves receptors for specific particles that trigger internalization when occupied.
  • Active filaments facilitate the engulfing process by aiding in membrane deformation and vesicle transport within the cell.
Cytoskeleton and Evolutionary Perspective
  • The cytoskeleton is not exclusive to eukaryotes; bacteria have analogous proteins:
    • MREB and PARM: Involved in maintaining shape and partitioning plasmids, capable of filament formation and ATP hydrolysis.
    • FTSZ: Forms a ring structure involved in septation, hydrolyzes GTP.
  • These proteins are presumed predecessors of eukaryotic actin and tubulin, indicating an evolutionary link.
Pathogenic Mechanism of Listeria
  • Listeria is a pathogenic bacterium that employs eukaryotic mechanisms for cell infection:
    • It enters eukaryotic cells via endocytosis.
    • After engulfment, Listeria escapes the vesicle and polymerizes actin around itself.
    • The polymerized actin acts like a catapult, propelling the bacterium into adjacent cells, allowing rapid spread and proliferation.
White Blood Cell Response to Chemokines
  • Chemokines act as signals for white blood cells, guiding their movement:
    • White blood cells change behavior in response to chemokines, allowing them to squeeze through blood vessels towards infection sites.
    • This responsiveness illustrates the dynamic interaction between immune cells and their environment, emphasizing the importance of actin in immune response.