Cytoskeleton & Cell Motility

Amanita phalloides (death cap mushroom):
- Contains amatoxins that inhibit RNA polymerase II.
- Contains phallotoxins like phalloidin, which binds to actin filaments.
- Phalloidin is used in cell biology to identify and characterize the actin cytoskeleton in fixed cells.

Autumn crocus (Colchicum autumnale):
- Used to treat joint pain and gout.
- Contains colchicine, which binds to tubulin and prevents microtubule assembly, causing metaphase arrest.
- Useful in karyotype studies to determine the correct number of chromosomes in humans.
- Also used to treat gout.
Taxol (paclitaxel):
- Extracted from the bark of the Pacific yew tree.
- Stabilizes microtubules and prevents depolymerization by binding tightly and specifically to tubulin.
- Used in cell biology labs to study microtubule-associated proteins (MAPs) and microtubule-based motor proteins.
- Used as a chemotherapeutic agent for cancers such as breast, lung, and ovary cancer.

Cytoskeleton:
- Composed of 3 filamentous structures: microtubules, microfilaments, and intermediate filaments
- Dynamic network that supports cells and mediates cell movements.
- Filaments are polymers of protein subunits held together by weak, noncovalent bonds, allowing rapid assembly and disassembly.
- Each element has distinct mechanical properties.
Three types of cytoskeletal elements:
- Microtubules: Hollow tubes made of tubulin subunits.
- Microfilaments: Solid, thinner structures made of actin, often organized into branching networks.
- Intermediate filaments: Ropelike fibers made of related proteins.
Functions of the cytoskeleton:
- Provides structural support and determines cell shape.
- Positions organelles within the cell.
- Acts as a network of tracks for material and organelle movement (e.g., mRNA, vesicles, neurotransmitters).
- Generates force for cell movement (cilia, flagella, pseudopodia).
- Essential for cell division (chromosome separation and cytokinesis).

MAPs are found with microtubules in living tissue, and are a heterogeneous collection of proteins.
Typically have one domain that attaches to the side of the microtubule and another domain that projects outward.
MAPs generally increase the stability of microtubules and promote their assembly by holding tubulin subunits together.
MT-binding activity of some MAPs is controlled by phosphorylation/dephosphorylation.
Alzheimer’s disease (AD):
- An abnormally high phosphorylation of tau (a MAP) is implicated in the development of neurodegenerative diseases, including AD.
- Brain cells of people with these diseases contain neurofibrillary tangles made of excessively phosphorylated tau molecules that cannot bind to microtubules.
- Tau mutations can cause frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTPD-17), indicating tau can become toxic to neurons.

Motor proteins convert chemical energy (ATP) into mechanical energy to generate force or move cellular cargo.
Cellular cargo includes: ribonucleoprotein particles, vesicles, organelles, chromosomes, and other cytoskeletal filaments.
Motor proteins are grouped into three superfamilies: kinesins, dyneins, and myosins.
Kinesins and dyneins move along microtubules (MTs); myosins move along microfilaments (MFs); no motors are known to use intermediate filaments (IFs) as tracks.
Motor proteins move unidirectionally along their cytoskeletal track in a stepwise manner.
Each step of the mechanical cycle is coupled to a step of a chemical (catalytic) cycle that provides energy (ATP binding/hydrolysis).
Molecular-sized motors are greatly influenced by their environment.
Motor proteins have virtually no momentum (inertia) and are subjected to tremendous frictional resistance from their viscous environment.
Motor proteins stop immediately once energy input has ceased.

Move vesicles/organelles from cell body to synaptic knobs along a microtubule track.

Consists of two heavy chains and two light chains.
Heavy chains contain globular domains that bind MTs and hydrolyze ATP, connected to a stalk.
Stalk region dimerizes to form a coiled coil.
Light chains are associated with the tail and bind to cargo.
Plus end-directed.

Two types:
- Cytoplasmic dynein: Moves cargo toward the minus ends of MTs; requires dynactin complex for function.
- Axonemal dyneins: Highly specialized; drive the beating of cilia and flagella.

Structure:
- Composed of actin.
- Two-stranded helical polymers (5-9 nm diameter).
- Flexible; easily bent by thermal fluctuations.
- Organized into linear bundles, 2D networks, and 3D gels.

Very abundant protein in eukaryotic cells.
- Once synthesized, it folds into a globular-shaped molecule (G-actin).
- G-actin molecules polymerize to form long, two-stranded helical polymers called F-actin.
- G-actin monomers bind ATP or ADP; ATP form polymerizes more readily.

All actin monomers in the filament point in the same direction, so microfilaments have polarity.
Minus end: ATP end; plus end: ADP end.
Plus end grows 5-10 times faster than the minus end.
Serve as tracks for myosin motors for various cellular activities like muscle contraction, cell migration, and cytokinesis.