lecture 9 for study guide

Microtubules are the most dynamic component of the cytoskeletal system, characterized by their ability to undergo rapid growth and shrinkage.
They grow quickly, and this dynamic behavior is essential for their function in cellular movement and signaling.
Visualization of microtubules can be done using fluorescently tagged proteins, such as EB1, which binds to the plus end of microtubules. This allows us to observe their growth towards cellular protrusions in real time.

Video observations show microtubules in human cells exhibiting rapid growth towards the leading edge, visible in a two-minute time-lapse where little blue dots (EB1) stream towards protrusions.
Microtubules are described as the "brains" of the cell because the plus ends provide directional cues for cell movement and facilitate cargo transport towards the leading edges.
In static epithelial cells, microtubules orient towards the apical surface, maintaining cellular asymmetry.

Microtubules serve as tracks for the movement of protein cargo, such as vesicles formed in the Golgi apparatus, towards the cell environment.
They deliver potent signaling proteins that regulate cell functions in specific regions.

Actin is enriched around the cell's periphery, creating a structural framework, while the interior has lower actin density which relates to cellular signaling challenges due to cytoplasmic crowding.

Intermediate filaments, such as keratin, are comparatively static and provide structural integrity to cells, particularly in epithelial layers.
Unlike microtubules and actin, intermediate filaments do not exhibit polarity, as they are formed from dimers that align in a staggered manner to create symmetrical structures.

Specific metabolites, like Taxol and colchicine, can destabilize or stabilize microtubule functions.
Taxol stabilizes microtubules, while colchicine promotes disassembly, both impacting cellular behaviors like division and signaling.
The dynamics of microtubules are integral to cell viability, and inhibiting this dynamic behavior can lead to cell death.

Microtubules are polymers made of heterodimers of alpha and beta tubulin, with GTP bound to beta tubulin that influences their stability.
The assembly involves a nucleation phase and ongoing addition of dimers at the plus end, while the minus end is anchored at the microtubule organizing center (MTOC).

Microtubules exhibit dynamic instability, characterized by phases of rapid growth (polymerization) and rapid shrinking (depolymerization).
The transition from stable growth to rapid shrinkage is termed "catastrophe," while a switch from shrinking back to growth is termed "rescue."

Three key proteins regulate microtubule dynamics:
- Kinesin-13: Induces catastrophe by destabilizing the GTP cap at the plus end.
- XMAP215: Enhances growth by promoting the addition of heterodimers at the plus end.
- Stathmin: Binds free heterodimers to reduce their availability for polymerization, effectively inhibiting growth.

Kinesin-1: Motor protein that moves from the minus end to the plus end of microtubules, transporting cargo such as vesicles in the direction of cell growth.
Dynein: Moves in the opposite direction (plus end to minus end) and aids in transporting endocytic vesicles into the cell's interior.

Intermediate filaments provide structural integrity but have limited dynamics compared to actin and microtubules.
Composed of monomers that dimerize into coiled-coil structures, they lose their polarity during assembly resulting in a symmetrical filament that lacks distinct ends.

Plectin connects intermediate filaments with microtubules and other structures, playing a key role in cellular architecture and signaling.
It helps transmit mechanical forces from the cytoskeleton to structures like the nucleus, enhancing spatial organization during migration through constricted environments.

Septins are emerging as a new frontier in cytoskeletal research. They form various structures, including rings and sheets, and function in organizing cellular compartments and forming diffusion barriers.
They are known to regulate material flow during cell division and are abundant at the base of cilia, influencing signaling activities in cells.

Understanding the dynamic behavior and interactions of microtubules, intermediate filaments, and actin is crucial for grasping cellular function, movement, and integrity. Additionally, the role of cross-linking proteins and emerging components like septins showcase the complexity and adaptability of the cytoskeletal systems.