Motors, Actin, Cell Motility

Microtubules are critical components of the cytoskeleton in eukaryotic cells.
They are hollow tubes made from tubulin dimers (alpha tubulin and beta tubulin).
Functions:
- Maintain cell shape and resist compression.
- Role in cell division:
- Centrioles and centrosomes are involved in mitosis and meiosis.
- Form the mitotic spindle vital for chromosome separation.
- Cilia and flagella, which aid in movement and sensory functions.
Motor proteins associated with microtubules:
- Kinesin: moves cargo towards the plus end (outer part) of the microtubule.
- Dynein: moves cargo towards the minus end (inner part) of the microtubule.

The cytoskeleton is a dynamic network that consists of three major components: microtubules, intermediate filaments, and microfilaments.
Main functions of the cytoskeleton:
- Provides structural support and shape to the cell.
- Facilitates intracellular transport.
- Plays a role in cell motility and division.

Comprised mainly of actin, forming a double helix structure of actin monomers.
Functions:
- Determine cell shape and enable movement (e.g., lamellipodia and filopodia).
- Vital for muscle contraction and cell division (cytokinesis).
- Microvilli increase surface area for absorption in epithelial cells.

Composed of various protein subunits, including keratin and vimentin; they have a strong, rope-like structure.
Functions:
- Provide mechanical strength and rigidity.
- Anchor the nucleus and other organelles.
- Help cells resist shear forces.
Distributions:
- Located beneath the cell membrane and along the lateral borders.

Microtubule-associated proteins (MAPs) such as tau and MAP2 do not transport cargo but play structural roles.
MAP2 is found in dendrites, while tau is enriched in axons.
Abnormal tau can lead to neurofibrillary tangles, disrupting neuronal structure and stability, linked to Alzheimer's disease.

Motor proteins facilitate cargo transport along microtubules and microfilaments:
- Kinesin: moves cargo towards the plus end of microtubules.
- Dynein: moves cargo towards the minus end of microtubules.
- Myosin: moves along actin filaments, typically towards the positive end.
Mechanism of kinesin and dynein transport involves ATP hydrolysis for energy.

The centrosome is a primary MTOC; organizes microtubules for processes like cell division.
Basal bodies, another form of MTOC, are associated with the formation of cilia and flagella.

Myosins operate primarily along microfilaments (actin) and come in conventional and unconventional types.
- Conventional (Myosin II): responsible for muscle contraction.
- Unconventional: involved in vesicle transport and other cellular processes.
Function:
- Myosins facilitate contraction that pulls the trailing edge of cells and push the leading edge through actin polymerization.

Actin monomers exist as G-actin (globular) and can polymerize into F-actin (filamentous).
Actin filaments are dynamic, constantly assembling and disassembling at their ends, particularly at the plus end.
Polymerization dynamics are regulated by various actin-binding proteins (e.g., ARP2/3 complex for branching, capping proteins for growth regulation).

Microtubules: Transport and structural support; involved in vesicle transport and chromosome segregation.
Microfilaments: Provide tensile strength; critical for muscle contraction, movement, and maintaining cell shape.
Intermediate Filaments: Offer mechanical stability and rigidity, primarily resist stretching forces.

Example: Zebrafish embryos use actin-myosin dynamics for camouflage by redistributing melanin granules in response to light conditions.