Cytoskeleton tubulin 1

What are the three primary functions of the cytoskeleton?

It maintains cell shape and organization, and provides support for internal and external movement.

Name the three classes of cytoskeletal filaments.

Microfilaments, microtubules, and intermediate filaments.

What is the primary protein subunit of microfilaments?

Actin.

What is the primary protein subunit of microtubules?

Tubulin.

What is the typical diameter of a microfilament?

7-9 nm.

What is the typical diameter of a microtubule?

25 nm.

What is the typical diameter of an intermediate filament?

10 nm.

What type of protein is the tubulin subunit that forms microtubules?

It is a heterodimer formed from α-tubulin and β-tubulin.

What structural feature gives microtubules their polarity?

The head-to-tail stacking of $\alpha\beta$-tubulin heterodimers, resulting in a distinct plus and minus end.

A microtubule is a hollow tube typically built from how many parallel protofilaments?

13 parallel protofilaments.

In a protofilament, the _ end is terminated by $\alpha$-Tubulin.

(-) end.

In a protofilament, the _ end is terminated by $\beta$-Tubulin.

(+) end.

Which end of the microtubule typically grows and shrinks more rapidly?

The plus (+) end.

What is the term for the initial, slow phase of microtubule assembly where tubulin molecules form a seed?

The lag phase, also known as nucleation.

What are the three phases of microtubule growth kinetics observed in vitro?

Lag phase (nucleation), elongation phase, and plateau phase.

The process where individual microtubules alternate between cycles of growth and shrinkage is called _.

dynamic instability.

What is the role of the GTP cap on a growing microtubule?

The GTP cap, composed of GTP-tubulin dimers, stabilizes the plus end and promotes linear protofilament growth.

What happens to the tubulin subunit conformation after GTP is hydrolyzed to GDP within the microtubule lattice?

Hydrolysis of GTP changes the subunit conformation, forcing the protofilament into a curved shape that destabilizes the microtubule.

In microtubule dynamics, what is a 'catastrophe'?

A catastrophe is the sudden change from a state of microtubule growth to rapid shrinkage.

In microtubule dynamics, what is a 'rescue'?

A rescue is the change from microtubule shrinkage back to a state of growth, often by regaining a GTP cap.

Microtubule nucleation in many cells depends on what complex?

The $\gamma$-tubulin ring complex ($\gamma$-TuRC).

What is the general term for the cellular structure from which microtubules are nucleated?

The microtubule-organizing center (MTOC).

What is the name of the single, well-defined MTOC found in many animal cells?

The centrosome.

What are the two main components of a centrosome?

A pair of centrioles surrounded by pericentriolar material (PCM).

Where within the centrosome is the $\gamma$-tubulin that nucleates microtubules primarily located?

In the pericentriolar material (PCM).

When microtubules grow from the centrosome, which end remains anchored in the MTOC?

The minus (-) end.

What is the general term for proteins that bind to microtubules to modify their properties, such as stability?

Microtubule-associated proteins (MAPs).

Which two MAPs are mentioned as setting the spacing of microtubule bundles, particularly in neurons?

Map2 and Tau.

Which MAP is primarily found in the axons of neurons?

Tau.

Which MAP is primarily found in the dendrites of neurons?

MAP2.

How does microtubule orientation differ between axons and dendrites?

Axons typically have microtubules of uniform polarity (plus ends out), while dendrites have microtubules of mixed polarity.

Mutations in the Tau protein are associated with what category of diseases?

Neurodegenerative diseases, such as Alzheimer's disease.

What are the two main families of microtubule motor proteins?

Kinesins and Dyneins.

What is the primary function of the head (motor) domain of kinesin?

It binds to the microtubule and hydrolyzes ATP to generate motion.

What is the function of the tail domain of a kinesin motor protein?

It binds to the cargo that is being transported.

Classic kinesins typically move cargo toward which end of the microtubule?

Toward the plus (+) end.

What molecule provides the energy for the movement of kinesin and dynein motors?

ATP (adenosine triphosphate).

Describe the structure of a conventional kinesin-1 protein.

It is a tetramer, composed of two heavy chains and two light chains.

How is the movement of kinesin characterized in terms of its stepping motion?

It moves in progressive, regular steps, ensuring at least one head is always attached to the microtubule for long-range transport.

Kinesins belong to a large protein superfamily where the _ is the common element.

motor domain of the heavy chain.

What is the unique function of Kinesin-13 proteins?

They act as microtubule depolymerases (catastrophe factors), inducing depolymerization from both ends of the microtubule.

Which kinesin family is unusual because its members are minus-end directed motors?

Kinesin-14 (e.g., Ncd).

Cytoplasmic dynein moves cargo toward which end of the microtubule?

Toward the minus (-) end.

How does the size and complexity of dynein compare to kinesin?

Dynein is significantly bigger and more complex than kinesin.

What is the force-generating motor domain in dynein called?

The AAA+ ATPase domain (or head).

How is the stepping motion of dynein characterized compared to kinesin?

Dynein takes big but irregular steps.

In a typical cell with a central MTOC, which motor protein would transport vesicles from the Golgi apparatus towards the plasma membrane?

Kinesin, because it moves toward the plus ends which are oriented towards the cell periphery.

In a typical cell, which motor protein would transport cargo from the cell periphery towards the nucleus?

Dynein, as it moves toward the minus ends located at the MTOC near the nucleus.

In axonal transport, movement of mitochondria from the cell body down the axon to the terminus is mediated by which motor protein?

Kinesin.

How can a single vesicle or organelle be transported in both directions along a microtubule?

The vesicle can have both kinesin and dynein motors attached, with their activity being regulated.

The rapid aggregation and dispersal of melanosomes for colour change is an example of transport mediated by _.

microtubule motor proteins (dynein for aggregation, kinesin for dispersal).

What are cilia and flagella?

They are hairlike, microtubule-based appendages on the cell surface that are involved in movement.

What is the name for the core bundle of microtubules and associated proteins found inside a cilium or flagellum?

The axoneme.

What specific type of motor protein is responsible for the bending motion of the axoneme in cilia and flagella?

Axonemal dynein.