Cytoskeleton and cell motility

Cytoskeleton

• the skeletala system that has analogous function

microtubules actin filaments intermediate

cytoskelleton is composed of?

Microtubules

are the largest structural elements of the eukaryotic cytoskeleton.

• they function wide array of cellular processes whose common theme is movement

Cytosolic Microtubules

•Maintain cell shape (structural support) •Provide tracks for intracellular transport (vesicles, organelles, chromosomes) •Form the mitotic spindle during cell division •Involved in cell polarity and the positioning of organelles

Axonemal Microtubules

• •Responsible for the beating motion of cilia and flagella •Enable movement of cells (e.g., sperm cells) •Help move substances across surfaces (e.g., respiratory epithelium)

Singlet Microtubules

•A singlet microtubule is one complete hollow tube.

•It is made of 13 protofilaments.

• Where found: •Cytoskeleton of most cells (cytosolic microtubules) •Mitotic spindle •Axons and dendrites Function: •Transport •Cell shape •Cell division

Doublet microtubules

•A doublet contains two microtubule sub-tubes:

• A-tubule → complete ring with 13 protofilaments

• B-tubule → incomplete ring with 10–11 protofilaments; attached to the A-tubule

• Where found: •Cilia and flagella •Forms the 9+2 axoneme: • 9 doublets arranged in a circle • 2 single singlet microtubules in the center

• Function: •Movement of cilia and flagella •Supports bending and beating motion

Triplet Microtubules

three microtubule sub-tubes:

• A-tubule → complete 13- protofilament tube

• B-tubule → incomplete, shares protofilaments with A

• C-tubule → incomplete, attached to B

• Where found: •Centrioles •Basal bodies (the base of cilia and flagella) •Arranged in a 9 triplet pattern.

• Function: •Organizing centers (MTOCs) •Nucleation of new microtubules •Spindle formation during cell division

Colchicine, Colcemid

• Binds to β-tubulin, preventing microtubule assembly; inhibits mitotic spindle formation, blocking cells in metaphase (used in karyotyping and as anti-gout drugs).

Nocodazole

Binds β-tubulin, inhibiting polymerization; causes depolymerization of microtubules, leading to mitotic arrest.

Vinblastine, Vincristine

• Aggregate tubulin heterodimers, preventing microtubule formation; blocks cell division, leading to apoptosis (used as anti-cancer drugs).

Paclitaxel (Taxol)

• Stabilizes microtubules, preventing their depolymerization; microtubules become too stable to function properly, arresting mitosis (used in chemotherapy).

Cytochalasin D

Prevents addition of actin monomers to plus ends; disrupts actin polymerization, inhibiting cell movement and division.

Latrunculin A

Binds actin monomers, sequestering them; causes depolymerization of actin filaments, leading to loss of cell shape and motility.

Phalloidin

Binds and stabilizes assembled actin filaments; prevents actin depolymerization (toxic, as it locks the cytoskeleton rigid).

Acrylamide

Causes loss of intermediate filament networks; weakens structural support, compromising cell shape and mechanical stability.

Microfilaments

• also known as actin filaments, are the thinnest components of the cytoskeleton in eukaryotic cells, with a diameter of about 7 nm. They are dynamic structures primarily composed of two intertwined strands of the protein actin.

Actin

globular protein (G-actin) that serves as the main structural component of microfilaments, one of the three major parts of the cytoskeleton

Polymerization of Actin Monomers

Individual G-actin proteins join together end-to-end. They assemble into long, thin chains called F-actin (filamentous actin). Two F-actin strands twist around each other to form a helical microfilament

Dynamic Behavior

Microfilaments constantly undergo: Assembly (polymerization) → filament grows Disassembly (depolymerization) → filament shrinks

Intermediate Filaments

are strong, flexible, ropelike fibers that provide mechanical strength to cells that are subjected to physical stress, including neurons, muscle cells, and the epithelial cells that line the body’s cavities.

Class I & II

proteins found in the epithelial cells covering the body surfaces and lining its cavities

Class III

Vimentin is present in connective tissue and other cells derived from nonepithelial cells. Desmin is found in muscle cells, and glial fibrillary acidic protein (GFAP) is characteristic of the glial cells that surround and insulate nerve cells

Class IV

Neurofilament firoteins Commonly found in the nerve cells

Class V

Nuclear Lamins A,B & C form a filamentous scaffold along the inner surface of the nuclear membrane of animal cells

class VI

Neurofilaments found in cells in the embryonic nervous system are made of nestin,

Dynein

is part of the AAA+ (ATPases Associated with various cellular Activities) superfamily of enzymes. The motor domain undergoes a conformational change, particularly in the linker, which is believed to generate the power stroke that pulls the cargo along the microtubule track

Kinesin

• ATP binding causes a conformational change in the motor head, allowing it to detach from the microtubule and swing forward to rebind to a new site, driven by the "power stroke". This process is a cycle of ATP binding, hydrolysis, and release.

Microtubules

are crucial not only for movement within cells but also for the movements of cilia and flagella, the motile appendages of eukaryotic cells.

Myosin

• motor protein that interacts with microfilaments (also known as actin filaments) to generate force and movement within eukaryotic cells. This interaction is powered by the hydrolysis of ATP, which converts chemical energy into mechanical energy.