Cytoskeleton part 2

• Cytoskeleton includes Microtubules, also known as intermediate filaments

Microtubules

• Microtubules are polymers of the protein tubulin and are components of the cytoskeleton
• Microtubules are major components of the cytoskeleton
• Tubulin protein is a superfamily of globular proteins
• Alpha- and beta-tubulins polymerize into microtubules
• Microtubules function as structural support, aid intracellular transport, and assist in DNA segregation

Microtubules - Structure

• Microtubules are hollow tubes, approximately 25 nm in diameter
• Microtubules comprised of tubulin dimers, with alpha and beta subunits
• Organized into 13 parallel protofilaments

Microtubules – Dynamic Instability

• Microtubules exhibits dynamic behavior, binding GTP for polymerization
• Dynamic assembly and turnover allow the cell to rapidly reorganize its cytoskeleton
• GTP hydrolysis plays an important role in this process
• Alpha- and beta-tubulin dimers bind to GTP to form microtubules
• GTP-dimers assemble onto the (+) ends of microtubules
• Beta-tubulin is exposed on the (+) end, while alpha-tubulin is exposed on the (-) end
• After incorporation into the microtubule, GTP on beta-tubulin hydrolyzes into GDP
• The stability of the dimer influenced by whether beta-tubulin is bound to GTP or GDP
• Dimers bound to GTP tend to assemble into microtubules, leading to growth
• Dimers bound to GDP tend to disassemble, leading to shrinkage

Microtubules - Formation

• Microtubule growth requires nucleation
• Nucleation initiates de novo formation of microtubules
• Microtubules form at distinct structures called Centrosomes
• A centrosome is an organelle that serves as the main microtubule organizing center (MTOC)
• Microtubules emerge from the centrosome structure
• Centrosomes regulate cell-cycle progression
• MTOCs function in the organization of eukaryotic flagella and cilia
• MTOCs function in the organization of mitotic and meiotic spindle apparatus
• Mitotic and meiotic spindle separates the chromosomes during cell division

Microtubules - Function

• Microtubules actively transport organelles within the cytoplasm.
• Functions include Endocytosis, Exocytosis, Mitochondria movement, and Protein transport between the ER and Golgi

Microtubules and Motor Proteins

• Motor proteins generate force and 'walk' along microtubules
• Kinesins move toward the (+) end
• Dyneins move toward the (-) end
• The 'head' domain of these proteins binds to the microtubule, whilst the tail domain binds to membrane or cargo

Kinesin

• Motor proteins function in concert with microtubules
• Motor provide transport for organelles and vesicles

Microtubule Drugs

• Tubulin-binding drugs kill cancerous cells by inhibiting microtubule dynamics, which are required for DNA segregation and cell division
• Agents which stabilize microtubules (e.g., Paclitaxel (Taxol®)) used in Ovarian and breast cancers
• These agents prevent dissociation of tubulin subunits as well as block mitosis, stabilizing GDP-bound tubulin

Microtubule-Disrupting Drugs

• Agents destabilizing microtubules include Colchicine
• Colchicine treats inflammatory conditions (Arthritis, gout)
• Colchicine promotes disassembly of microtubules, leading to dramatic change in organelle location
• Colchicine blocks migration of white blood cells, reducing inflammation.

Intermediate Filaments

• Intermediate filaments are of various proteins, depending on the type of cell that they are in.
• Averaging 10 nm in diameter
• More stable than actin filaments
• Heterogeneous component of the cytoskeleton
• Frequently bound to plasma membrane at cell-cell junctions and also found within the nucleus.
• Impart resistance to stress

Intermediate Filament Assembly

• Two polypeptides wind around each other in a coiled-coil structure to form dimers
• Dimers associate in a staggered antiparallel fashion to form tetramers
• Tetramers associate end to end to form protofilaments and laterally to form filaments
• Each filament contains approximately eight protofilaments wound around each other in a ropelike structure

Intermediate Filaments Function

• Toughest of cytoskeletal components
• Further stabilized though interaction with accessory proteins
• Plectin links the cytoskeleton to the plasma membrane and connects cells
• Impart resistance to stress
• Function in the maintenance of cell-shape by bearing tension (during mitosis and during the positioning of the centrosome)
• Resist compression

Classes of Intermediate Filaments

• Whereas actin filaments and microtubules are polymers of single types of proteins, intermediate filaments are composed of a variety of proteins that are expressed in different types of cells
• There are 4 classes of intermediate filaments
• Type I : keratins are found in hair and nails
• Type II: vimentin within cellular membranes and connective tissue and muscle.
• Type III: neurofilaments
• Type IV: nuclear lamins within the lumen of the nucleus, and is involved in dissolution of nuclear membrane in mitosis

Intermediate Filament Dysfunction

• Abnormalities of neurofilaments leads to diseases in motor neurons, specifically amyotrophic lateral sclerosis (ALS).
• ALS (Lou Gehrig's disease; motor neurone disease) leads to progressive loss of motor neurons
• This leads to muscle atrophy, paralysis, and eventual death
• ALS and other types of motor neuron disease are characterized by the accumulation and abnormal assembly of neurofilaments
• The resulting intermediate filament supports the axons of nerve cells.