Cytoskeleton Structure and Functions

Cytoskeleton

• A network of protein filaments in a cell with a range of functions, including:
  • Connection with Extracellular Matrix (ECM)
  • Maintaining cell shape
  • Intracellular transport
  • Cytokinesis
  • Chromosome separation
  • Cell movement

Composition of Cytoskeleton

• Actin microfilaments:
  • Polymers of actin
  • 7-9 nm diameter
• Intermediate filaments:
  • Tissue-specific proteins
  • 10 nm diameter
• Microtubules:
  • Polymers of tubulin
  • 25 nm diameter

Actin Microfilaments

• Monomer is a globular protein (G-actin) containing an ATP-binding domain
• G-actin polymerizes to microfilament (F-actin) via ATP hydrolysis
• Polymerization involves non-covalent interactions, with monomers in Head-to-Tail orientation
• Each microfilament has a (+) and (-) end, and monomers can be added at both ends
• Dynamic structure, with length dependent on relative rate of loss and gain of G-actin monomers
• Actin microfilaments are usually present as two tightly wound chains
• Comprise approximately 5% of cellular protein

Function of Actin

• Muscle contraction (covered in "Skeletal Muscle" lecture)
• Mechanical support (e.g., in microvilli)
• Maintaining cell shape
• Cell movement

Actin Binding Proteins

• G-actin binding proteins:
  • Thymosin β4: inhibits polymerization
• Cross-linking proteins:
  • Villin: parallel bundles in microvilli
  • Filamin: joining at angles to create a mesh
• Severing:
  • Gelsolin: cuts and binds (+) end; the other part depolymerizes – "gel to sol"

Contraction in Non-Muscle Cells

• Muscle myosin is also present in non-muscle cells
• Interaction between myosin and actin microfilaments allows movement, requiring ATP hydrolysis
• Movement within cells:
  • Cytokinesis: ring of actin forms in the center of the cell, anchors to the plasma membrane, and myosin contraction constricts the cell
• Movement of cells:
  • Lamellipodia-mediated cell movement across the extracellular matrix (ECM)

Intermediate Filaments (IF)

• Polymers of individual IF proteins, 10nm in diameter
• Different IF proteins in different cell types:
  • Epithelial cells: keratin(s) for physical support and external structures
  • Axons: neurofilamin(s) for structural arrangement of axons
  • Universal (nuclear): lamins A, B, C for supporting nuclear structure
• Usually stable and not dynamic, except for lamins during mitosis

Formation of IF Polymer

• 1. Intermediate filament protein (monomer)
• 2. Helical dimer
• 3. Two dimers combine to form a tetramer (the fundamental unit of the IF)
• 4. Tetramers link in a staggered formation and end-to-end to form the filament
• 5. Subunit exchange is slow but occurs throughout the length of the filament

Microtubules

• Tubulin monomer is a heterodimer of α-tubulin and β-tubulin
• Protofilament has a (-) and (+) end
• Monomers can be rapidly added and removed from both (+) and (-) ends
• 13 parallel protofilaments arranged in a hollow tube, approximately 25nm diameter

Microtubule-Organizing Centre (MTOC)

• Usually has one end attached to a MTOC
• One MTOC associated with the nucleus
• Microtubules "grow out" from MTOC until reaching the destination and then are stabilized

Function of Microtubules

• Dynamic scaffold
• Spindle for chromatid separation during mitosis
• Movement of cargo to specific locations in the cell
• Central internal support of cilia
• Stabilize structure of cells (e.g., platelets)
• Organize the structure of organelles (e.g., endoplasmic reticulum)

Mitotic Spindle

• Spindle consists of microtubules
• Spindle formation initiated from the centrosome (a type of Microtubule-Organizing Centre)
• Centrosomes contain a pair of centrioles
• Centrioles contain stable fused microtubules
• Centrosomes form at two poles of the cell
• Kinetochore microtubules attached to the centromere of the chromatid
• Aster microtubules attach the centrosome to the cell membrane

Cilia

• Membrane-bound hair-like extensions
• Microtubules are the central support
• MTOC is called Basal Body, located close to the membrane
• Microtubules facilitate the movement of components up and down within cilia
• All cells have a single primary cilium
• Disassembled during mitosis
• Specialized types:
  • Stereocilia in the inner ear for sound detection
  • Motile cilia in respiratory/lung ciliated epithelia for beating and moving fluid around