Cell Structure & Dynamics: The Cytoskeleton

Function of Cytoskeleton

• Provides cells with structure and shape

• Transports macromolecules within the cell.

Cytoskeleton (composed of)

Network of polymers in cytoplasm and constantly changing to meet cell needs

Types of support (cytoskeletal)

• Structural support

• Locomotion (must be dynamic)

Structural Elements of the Cytoskeleton

• Microtubules

• Microfilaments

• Intermediate Filaments

Microtubules (Characteristics)

Microtubules are the largest structural elements of the cytoskeleton. They do not easily bend.

Have a tube structure

Microtubules (Functions)

1) Long distance intercellular transport

2) Cellular locomotion (cilia and flagella)

Microtubule (Structure)

Straight, hollow cylinders made of microtubule protofilaments (usually 13 filaments).

• Basic Subunit: Tubulin heterodimer (One alpha (-) and beta (+) tubulin)

Centrosomes (Where)

Microtubule organizing center

Microtubules (Where)

Project away from microtubule organizing centre

Microfilaments (Where)

Cytoplasmic

MTOC

Microtubule organizing center.

MTOC (two major classes)

1) Centrosome

2) Basal Bodies

Centrosome (MTOC)

• Major cytoplasmic MTOC in animals

• Near the nucleus

Basal Bodies (MTOC)

MTOC for flagella/cilia

Centrosomes (structure)

• Two centrioles (animal cells)

• Surrounded by pericentriolar material (made of proteins)

• Minus end is anchored at its base to centrosomes y-tubulin. (Ring shaped complex, only found in centrosomes).

Microtubule dynamics controlled by what

Addition and loss of tubulin dimers. (*addition is called elongation*)

• MTs from by reversible polymerization of tubulin dimers.

Microtubules retracted how

Depolymerization

GTP Hydrolysis (microtubules)

• Tubulin dimers bind GTP

• GTP hydrolyzed to GDP after polymerization

• GDP tubulin: prone to depolymerization

• GTP tubulin at the tip provides a protective cap

When growth stops, MTs become unstable & retract

Kinesin (ATP)

Motor proteins use chemical energy (in the form of ATP) to power mechanical work.

One ATP molecule is required every step taken by kinesin

What is Dynein and kinesin

Motors proteins that move in opposite direction of microtubules (Kinesin moves toward cell wall)(Dynein moves away from call wall)

Microfilaments (Characteristics)

• Smallest of cytoskeletal filaments

• Flexible and strong that control cellular shape and movement (lamellipodia and filopodia) and used for subcellular transportation.

Microfilaments (Composed of)

• Actin: common protein in all eukaryotic cells

• Monomer: globular actin (G-actin) Binds ATP or ADP

• Polymer: filamentous actin (F-actin)

Microfilaments (Polarity/polymerization)

• Polymerization is reversible

• Addition of G-actin is more rapid at the plus end

• G-actin monomers bind ATP, which is hydrolyzed to ADP after assembly onto a microfilament.

Actin-binding proteins control actin organization (Unique aspect of actin)

• Nucleotide-dependent actin binding proteins

• Actin binding proteins generate a wide variety of structures

Actin-Binding proteins (types)

• Filament Formation

• Filament-severing proteins

• Filament-capping proteins

• Filament-crossing linking proteins

• Filament-bunding proteins

Actin in Cell Crawling (Steps)

1) The leading edge extends via polymerization of actin at its tip

2) New adhesions, anchored by actin, form on the undersurface of the lamellipodium

3) Trailing edge (Tail) of the cell detaches, and is drawn forward by contraction of the cell body.

Myosin (Microfilament-based Transport)

• Myosin/actin relationship similar to kinesin/MT

• ATP-dependent motors move along actin fibers or use actin fibers to exert force (Muscle contraction)

Intermediate Filaments (IFs) (Characteristics)

• Provide structural support for entire cytoskeleton (primary role are bearing tension)

• Cytoplasmic IFs are highly cell-type specific

• Most stable component of cytoskeleton.