Overview
This lecture covers the structure, components, and functions of the cytoskeleton, focusing on microfilaments, microtubules, and intermediate filaments, and their roles in maintaining cell shape and enabling cell motility.
Historical Background & Discovery
- Early theories described the cytoskeleton as a fibrous network or structured scaffold in the cytoplasm.
- Advances in microscopy in the 1960s led to the identification of dense fibrillar structures, now known as the cytoskeleton.
Cytoskeleton Overview
- The cytoskeleton is a dynamic protein filament network that maintains cell shape, structure, and internal organization.
- It enables cell division, movement, and the formation of cellular extensions.
- The three main components are microfilaments, microtubules, and intermediate filaments.
Microfilaments (Actin Filaments)
- Microfilaments are actin polymers with ~6 nm diameter, supporting cell shape and specialized structures like microvilli.
- Concentrated under the plasma membrane in the cortex, they help exclude organelles from the cell periphery and support cell movement.
- Actin monomers bind and hydrolyze ATP, allowing polymerization into polar filaments with distinct plus and minus ends.
- Polymerization involves lag, growth, and linking phases, regulated by accessory proteins.
- Actin dynamics drive cell motility, such as locomotion and the formation of protrusions.
Microtubules
- Microtubules are hollow cylinders (~25 nm diameter) made of alpha and beta tubulin heterodimers, assembled into 13 protofilaments.
- They are rigid, have defined polarity (plus and minus ends), and grow/shrink faster at the plus end.
- Originate from the microtubule organizing center (centrosome), dictating geometry and polarity within the cell.
- Motor proteins (kinesins move to plus end, dyneins to minus end) transport vesicles and organelles along microtubules.
- Microtubules form the backbone of structures like cilia, flagella, and centrosomes.
Intermediate Filaments
- Intermediate filaments (~10 nm diameter) provide mechanical stability and resistance to deformation.
- Less dynamic than actin or microtubules, abundant in cells under mechanical stress.
- Subtypes include keratins (epithelia), vimentin (connective tissue), neurofilaments (neurons), and lamins (nucleus).
- Built from coiled-coil dimers forming stable, staggered polymers; regulated by phosphorylation and proteolysis.
- Accessory proteins link intermediate filaments to other cytoskeletal elements, creating a flexible, resilient network.
Key Terms & Definitions
- Cytoskeleton — Network of protein filaments providing structural support and facilitating movement in cells.
- Microfilaments (Actin filaments) — Thin filaments composed of actin, crucial for cell shape and movement.
- Microtubules — Rigid, hollow tubes made from tubulin, involved in intracellular transport and structural organization.
- Intermediate filaments — Rope-like protein fibers giving cells mechanical strength.
- Centrosome — Main microtubule-organizing center in animal cells.
- Motor proteins — Proteins (kinesin, dynein) that move along microtubules transporting cellular cargo.
- Cortex — Dense actin network beneath plasma membrane.
Action Items / Next Steps
- Review upcoming videos/lectures covering detailed mechanisms of transport and motility involving myosin, kinesin, and dynein.
- Write summaries on the roles of cytoskeletal elements in various cell types.
- Prepare questions on centrosome structure and cilia/flagella function for deeper study.