Cytoskeleton

Actin filaments are made of actin subunits, observable within cells via fluorescent labeling with antibodies.
Cell images display:
- DNA stained blue.
- Actin network and microtubule network in the top image.
- Microtubules stained green, shown during cell division in the bottom image.
Chromosomes are visible in a condensed form during division, aided by microtubules that pull them apart into two new cells.
Intermediate filaments are stained red, providing structural support to cells, preventing them from becoming shapeless masses.

Actin filaments consist of two intertwined strings of actin subunits, creating a helical structure.
Each actin subunit structure is not critical for understanding how actin operates.
Important functional details include:
- Each subunit can bind ATP (adenosine triphosphate) or ADP (adenosine diphosphate).
- ATP is associated with higher affinity, while ADP, resulting from ATP hydrolysis, has lower binding affinity.

Actin filaments demonstrate polarity with a plus end and a minus end:
- Plus end: where new subunits attach more readily due to higher affinity.
- Minus end: subunit addition occurs, but at a slower rate.
Upon binding, actin subunits undergo conformational changes, enhancing the binding site at the plus end.
Growth primarily happens at the plus end of actin filaments.

When actin filaments form, ATP bound actin can hydrolyze into ADP.
D-actin (ADP form) exhibits weaker affinity for binding partners, leading to disassembly.
The plus end typically remains ATP-rich, while the minus end tends to contain more ADP.
This differential affinity facilitates a phenomenon known as treadmilling:
- Continuous addition of actin to the plus end while the minus end disassembles, producing an apparent movement.
- Treadmilling resembles a treadmill where new actin adds on one end while older actin subtracts on the other.

Multiple proteins are involved in regulating actin assembly and disassembly:
- Some proteins prevent actin assembly, while others promote it.
- The ARP complex enables branching of actin filaments, resulting in non-linear structures which aid in cell movement (e.g., pseudopodia).
- Various proteins are involved in cross-linking filaments, enhancing structural integrity.

Pseudopods are temporary projections of cells allowing movement, driven by actin filament assembly under the membrane.
Amoeboid motility: characterized by drastic shape changes in cells such as neutrophils; relies on dynamic membrane protrusions generated by actin networks.
Actin self-assembles from basic protein building blocks (actin, profilin, capping protein, ARP complex, nucleation promoting factor, etc.) to create these networks.
Mechanism for growth: filaments behave as springs, pushing against membranes via a process known as elastic Brownian ratchet, where random thermal forces enable filament elongation.

Filaments grow against membranes containing signaling molecules, recruiting nucleation-promoting factors (such as WAVE) that accelerate filament formation and elongation.
Capping protein plays a crucial role in branching networks by binding to and terminating filament growth.
Concentration of subunits at the membrane edge facilitates rapid actin polymerization, crucial for pseudopod extension.

Microvilli increase the surface area of cells, particularly in the intestinal tract, enabling nutrient absorption via underlying actin filaments.

Actin filaments play a critical role in muscle contraction, working with myosin motors, although details are not discussed in this context.

Microtubules are larger and stiffer than actin filaments, consisting of alpha and beta tubulin subunits that assemble into hollow tubes.
Like actin filaments, microtubules exhibit polarity with preferential growth at the plus end, which is usually anchored at a microtubule organizing center (e.g., centrosome).

Each microtubule subunit binds to GTP (guanosine triphosphate) instead of ATP, and hydrolysis of GTP to GDP reduces binding affinity, leading to potential disassembly, termed catastrophe.
Microtubules frequently grow and shrink, exhibiting dynamic instability as GTP caps at the plus end protect them from disassembly.

Microtubules primarily grow from microtubule organizing centers (MTOCs), which provide stable gamma tubulin rings to initiate microtubule formation.
The major MTOC in animal cells is the centrosome, which contains several gamma tubulin rings enhancing efficiency of microtubule assembly.

Microtubules serve as transport highways within cells, facilitating intracellular traffic, particularly along long distances in neurons.
Kinesin and Dynein motor proteins operate along microtubules, responsible for delivering cellular cargo to designated locations.

Cilia and flagella, consisting of microtubule arrangements, assist in cell movement through liquid environments.
They achieve movement by bending, regulated through motor proteins connecting adjacent microtubules.

Intermediate filaments provide cellular structure and strength, primarily found in vertebrates and certain soft-bodied animals.
These filaments, resembling ropes, consist of 8 strands of protein subunits, providing considerable tensile strength necessary for resisting stretching or pressure on epithelial cells.
They are different types, tailored to function in varied cell types, e.g., keratin in skin cells.
Notably, intermediate filaments are absent in plants or arthropods, indicating a unique adaptation for support in squishy animals.

Understanding the intricate roles and dynamics of actin filaments, microtubules, and intermediate filaments is crucial for comprehending cellular behavior and movement.
The upcoming exam will assess knowledge based on learning objectives and scientific literature interpretation, especially regarding figures and data analysis.