Cytoskeletal Systems in Vesicle Transport: Actin, Microtubules, and Motor Proteins

What are the two main cytoskeletal systems involved in vesicle transport?

Actin filaments and microtubules.

What are actin filaments composed of?

G-actin subunits that assemble into filamentous F-actin.

What is the polarity of actin filaments?

They have a (+) fast-growing end and a (-) slow-growing end.

What is actin treadmilling?

A dynamic process where actin adds at the (+) end and disassembles at the (-) end.

How does actin contribute to vesicle transport?

It provides tracks for short-range vesicle movement and assists in endocytosis.

What proteins promote actin filament formation near vesicles?

Nucleation-promoting factors such as WASP that activate the Arp2/3 complex.

What does the Arp2/3 complex do?

It nucleates new actin filaments and creates branched actin networks.

At what angle does Arp2/3 branch actin filaments?

Approximately 70°.

How is WASP activated?

By binding to active Cdc42-GTP, which exposes its WCA domain to stimulate Arp2/3.

What is the role of actin polymerisation in endocytosis?

It provides force to move clathrin-coated vesicles away from the plasma membrane.

What pathogen uses actin for intracellular motility?

Listeria monocytogenes.

How does Listeria move inside cells?

Using its surface protein ActA to activate Arp2/3 and polymerise actin into a comet tail.

What is the function of cofilin in actin dynamics?

It enhances depolymerisation at the (-) end to recycle actin monomers.

What is the role of myosin motor proteins?

They move vesicles along actin filaments or tether them to the actin cytoskeleton.

What is the general structure of myosin motors?

An actin-binding head, an ATP-hydrolysing motor domain, a lever arm, and a cargo-binding tail.

How do myosins generate movement?

ATP hydrolysis in the head domain produces conformational changes that create a power stroke.

What direction do most myosins move toward?

The (+) end of actin filaments.

What is the function of myosin V?

It transports organelles or vesicles toward the cell periphery and supports actin-based vesicle transport.

What is the microtubule network used for?

Long-range and high-speed vesicle and organelle transport.

What are the two main microtubule motor proteins?

Kinesins and dyneins.

Which direction do kinesins move?

Toward the (+) end of microtubules (typically outward from the cell center).

Which direction do dyneins move?

Toward the (-) end of microtubules (toward the cell center).

How do microtubules help maintain organelle structure?

By positioning organelles such as the Golgi and ER through motor-protein transport.

What is the role of Golgins?

Large coiled-coil proteins at the Golgi that tether vesicles and interact with Rab GTPases and microtubule motors.

What is the dynein complex composed of?

Two identical heavy chains, intermediate and light chains, and the accessory complex dynactin.

What is the function of dynactin?

It links dynein to cargo and regulates dynein activity.

How does dynein move along microtubules?

Through ATP-driven conformational changes in its AAA motor domains, generating an 8 nm step per cycle.

What happens to lysosome positioning when dynein is lost?

Lysosomes disperse throughout the cytoplasm instead of clustering near the nucleus.

How is bidirectional vesicle transport achieved?

By alternating activity between dynein (inward) and kinesin (outward) motors.

What determines whether vesicles move inward or outward?

Regulation of motor activity and adaptor protein interactions.

What are melanophores?

Pigment cells containing melanosomes used to study bidirectional microtubule transport.

How do melanosomes move during dispersion?

By kinesin-2 motors transporting them outward along microtubules, aided by myosin-V tethering.

How do melanosomes move during aggregation?

By dynein-dynactin motors moving them inward toward the cell center.

What signalling molecule regulates pigment dispersion and aggregation?

cAMP, through PKA-mediated control of motor activity.

Why is cytoskeletal regulation important for vesicle transport?

It ensures correct cargo delivery, organelle positioning, and cell polarity.