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Three major filaments of the cytoskeleton
- actin filaments
- microtubules
- intermediate filaments
Actin
- A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.
- Function: Shape and Contractility
Microtubules
- Spiral strands of protein molecules that form a tubelike structure
- Function: Organization (organelles, vesicles, chromosomes)
Intermediate Filaments
- Threadlike proteins in the cell's cytoskeleton that are roughly twice as thick as microfilaments
- Function: Mechanical strength
Types of Intermediate Filaments
- Unique to Eukaryotic cells
- Neurofilaments: neuronal cells
- Keratin: Epithelial cells (skin)
- Vimentin: in connective tissue, muscle cells, glial cells (fibroblast and other cells)
- Nuclear lamins: all eukaryotic (animals) cells
Structure of Intermediate Filaments
- Fibrous proteins supercoiled into thicker cables
- Non-polar filament structure
- Structure determines their mechanical strength.
Intermediate Filaments Strength
- Strong and Deformable
- Bend but do not break when force is applied.
- Anchored at cell-cell junction
- Provide mechanical strength to cells and cell layers.
Intermediate Filaments Summary
- Non-polar filaments made of 8 tetramers - like a rope.
- Provide mechanical strength, but are deformable (change shape) due to their structure.
- Heterogenous Family: nuclear lamins, cytoplasmic proteins (keratins, vimentin, neurofilaments, desmin) and, mostly in Eukaryotes.
Nuclear Lamina
- Found on the inner surface of the nuclear membrane and is composed of nuclear lamin proteins.
- Provides mechanical strength to the nucleus
- It provides a scaffold for molecular processes in the nucleus.
Thermodynamic Stability
- Single Protofilaments: thermally Unstable
- Multiple Protofilaments: Thermally Stable
Microtubules Structure
- Composed of 13 aligned protofilaments organized into a hollow tube.
- Stiffest/Straightest filaments in animal cells.
- Basic building blocks: alpha-tubulin and beta-tubulin heteromeric dimer
- Form a network in normal cells and form the spindle in mitotic cells
Microtubules Function
- Organization and long-range transport of organelles.
Centrosome
- Functions as a microtubule organizing center (MTOC).
- Gamma Tubulin links to the alpha-beta tubulin heterodimers to the centrosomes
Plus end of microtubule
- Cells that have a centrosome, microtubules grow rapidly at the plus end
- Most growth and shrinkage occurs at the+ end (away from the MTOC)
Microtubule Growth
- The rate-limiting step is nucleation.
- The cell avoids having to nucleate a filament every time by maintaining the MTOC in the center of the cell, allowing the filaments to grow out from it at their dynamic (+ ends).
- The cell has a specific kind of tubulin (gamma-tubulin) that links the MTs to the MTOC at the - end
Dynamic Instability of Microtubules
- Microtubules require high levels of GTP to grow (and will have a GTP cap)
- Microtubules require a GTP cap to be stable (i.e. they must be actively growing).
- Microtubules shrink in the absence of GTP (in the absence of a GTP cap).
- Microtubules also require MT-binding proteins to maintain their stability.
- In the absence of functional (non-phosphorylated) Tau, MTs are unstable in the brains of Alzheimer's patients.
Microtubule-associated proteins (MAPs)
- Example: Tau
- Function: to increase the stability of microtubules by linking them together.
- Highly expressed in neurons.
Tau
- Protein that stabilize microtubules.
- Binds to microtubules when not phosphorylated.
- Tau does not bind microtubules when phosphorylated by cytoplasmic kinases.
Tau Filaments and Neurodegenerative Diseases
- Tau that is Hyper-phosphorylated (too much phosphorylation)forms tangled clumps of tau proteins inside cells
Microtubules Organization
- Organize location of organelles in the cell.
- Provide tracks for orderly movement of vesicles in the cytoplasm by motor proteins.
Motor Proteins
- Kinesin.
- Dynein
- Myosin
- MT motor proteins carry cargo along MTs
Kinesin
- Plus (+) end-directed.
- ATP-dependent MY motor protein.
- Structure: long and flexible heavy stalk chain. The tail has a light chain and binds to cargo.
- "Walks" along MT
Dynein
- Minus (-) end-directed.
- ATP-dependent motor protein
Myosin
- ATP-dependent actin motor protein.
