Cytoskeleton Notes

Cytoskeleton

Learning Objectives

• Understand the functions of the cytoskeleton.
• Differentiate between the different structures and components of the cytoskeleton.
• Describe the importance of the cytoskeleton.

Introduction

• The cytoskeleton is a network of dynamic and adaptable fibers found throughout the cytoplasm.
• The structure, function, and dynamic behavior of the cytoskeleton can vary depending on the organism and cell type.
• All three cytoskeletal filaments must work together to provide a cell with its structure, strength, shape, and ability to move.
• The cytoskeleton supports the large volume of cytoplasm, which is especially important in animal cells that lack cell walls.
• Cytoskeletal elements interact with cellular membranes.

Cytoskeletal Functions

• Support
  • Maintain cell shape
  • Organize structure
  • Anchor organelles
  • Provide flexibility
• Regulation
  • Regulate biochemical activities
  • Cell signaling
• Motility/Movement
  • Changes in cell location
  • Limited movements of parts of the cell
  • Interactions with motor proteins

Cytoskeleton Functions (Detailed)

1. Gives the cell its shape and mechanical resistance to deformation.
2. Stabilizes tissues through association with extracellular connective tissue and other cells.
3. Contraction can deform the cell and its environment (cell migration).
4. Involved in cell signaling pathways and endocytosis (uptake of extracellular material).
5. In intracellular transport (movement of vesicles and organelles within the cell).
6. Separation of chromosomes and cytokinesis during cell division.
7. Acts as scaffolding to organize the cell's contents in space.
8. Forms specialized structures like flagella and cilia.

Cell Junctions and Cytoskeleton

• Apical
  • Tight junctions seal the gap between epithelial cells.
• Cell-Cell Anchoring Junctions
  • Adherens junctions connect actin filament bundles in one cell with those in the next cell.
  • Desmosomes connect intermediate filaments in one cell to those in the next cell.
• Channel-Forming Junctions
  • Gap junctions allow the passage of small water-soluble molecules from cell to cell.
• Cell-Matrix Anchoring Junctions
  • Actin-linked cell-matrix junctions anchor actin filaments in the cell to the extracellular matrix.
  • Hemidesmosomes anchor intermediate filaments in a cell to the extracellular matrix.
• Basal

Cytoskeleton Components

1. Microtubules
   • Found in the interior of the cell.
   • Play a role in organization and movement.
2. Microfilaments (Actin Filaments)
   • Thicken the cortex around the inner edge of a cell.
   • Resist tension.
3. Intermediate Filaments
   • Found throughout the cell’s cytoplasm and the inner nuclear membrane.
   • Provide mechanical support.

Cytoskeleton Components Dimensions

• Microtubules: ~25 nm in diameter
• Actin Filaments: ~7 nm in diameter
• Intermediate Filaments: ~8-12 nm in diameter

Microtubules

• Straight, hollow tubes of protein.
• Approximately 25 nm in diameter.
• Types: Axonemal and cytoplasmic microtubules.
• Largest of the cytoskeletal elements.
• Important organizing role in eukaryotic cells.

Microtubule Functions

• Intracellular transport (transport of organelles like mitochondria or vesicles; associated with dyneins and kinesins).
• Determines the position of membrane-enclosed organelles.
• Movement: Cilia and flagella.
• Separates chromosomes during cell division (the mitotic spindle).
• Shapes/supports cell.

Axonemal and Cytoplasmic Microtubules

• Axonemal Microtubules
  • Found in subcellular structures associated with cellular movement.
  • Highly organized (bundled together).
  • Stable microtubules (do not show dynamic changes in structure).
  • Examples: Cilia/flagella.
• Cytoplasmic Microtubules
  • Found in the cytoplasm.
  • Loosely organized (show dynamic changes in structure).
  • Provide shape and structural integrity to the cell.
  • Form mitotic/meiotic spindles.
  • Contribute to the movement of organelles/vesicles in the cell.

Microtubule Composition and Assembly

• Composed of tubulin:
  • A globular protein that constructs the microtubule wall.
  • Dimer (made of 2 subunits/polypeptides: α-tubulin and β-tubulin).
• Microtubules grow in length by adding tubulin dimers.
• Dynamic behavior: binding GTP for polymerization.
• Organized by the centrosome.
• Polarity: (- and + ends).
• Grows from the positive end and attaches through the negative end.

Microtubule Organizing Centers

• Centrosome
• Basal body
• Poles of mitotic spindle
• Ciliated cell

Centrosomes

• Microtubules grow out from centrosomes, near the nucleus.
• Centrioles: set pair in a centrosome, nine sets of triplet microtubules arranged in a ring.
• Helps organize microtubule assembly.
• Regulate cell cycle progression.
• Changes size during the cell division process.

Basal Bodies

• Modified centrioles that give rise to cilia and flagella.
• Functions:
  • Provide a template on which the axonemal structure of the cilium can be built.
  • Dictate the position and orientation of the cilium.

Microtubule Formation

• Microtubules can rapidly grow (via polymerization).
• Tubulin dimer with bound GTP (GTP-tubulin).
• GTP-tubulin dimers add to the growing end of the microtubule.
• Addition proceeds faster than GTP hydrolysis by the dimers.
• GTP cap.

Microtubule Dynamics

• Microtubules can shrink (via depolymerization) in size, depending on how many tubulin molecules they contain.
• GTP hydrolysis is faster than the addition of new GTP-tubulin dimers.
• GTP cap lost.
• Protofilaments containing GDP-tubulin peel away from the microtubule wall.
• GDP-tubulin is released to the cytosol.

Flagella

• Extensions projecting from some cells.
• Long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (e.g., sperm cell).
• When present, the cell has just one flagellum or a few flagella.
• Flagella has a 9 + 2 arrangement of microtubules (9 fused pairs of microtubules on the outside of a cylinder, and the 2 unfused microtubules in the center).

Cilia

• Short, hair-like structures.
• Can move entire cells (such as paramecia) or substances along the cell's outer surface (e.g., cilia of cells lining the fallopian tubes, cilia lining the cells of the respiratory tract).
• Cilia has a 9 + 2 arrangement of microtubules (9 fused pairs of microtubules on the outside of a cylinder, and the 2 unfused microtubules in the center).
• Non-motile cilia has 9 + 0 arrangement.

Cilia (Detailed)

• Cilia are short and more numerous than flagella.
• Two types: motile and non-motile.
• Motile cilia have a rhythmic waving or beating motion.
• Non-motile cilia receive sensory information for the cell (extracellular signal molecules).
• Dynein arms attached to the microtubules function as the molecular motors that control the beating (movement) of the cilia and flagella.
• The motion of the cilia and flagella is created by the microtubules sliding past one another (requires ATP).

Microtubule Motor Proteins: Cytoplasmic Dyneins

• Dynein is a protein complex that functions as a molecular motor.
• Converts chemical energy from ATP hydrolysis into the mechanical energy of movement to 'walk' along the microtubule while carrying a vesicle.
• Dyneins bind to microtubules and move or