BCM6229 Cytoskeletal System

Cytoskeletal system

This is a network of interconnected filaments and tubules extending through the cytosol; dynamic and changeable.

1. scaffold for structural support and maintaining cell shape
2. internal framework to organize organelles
3. directs cellular locomotion and movement of materials within the cell
4. provides anchoring site for mRNA
5. signal transducer
6. essential component of the cell's division machinery

What are the 6 main functions of the cytoskeletal system?

1. microtubules
2. microfilaments
3. intermediate filaments
4. septins (protein netowrk)

What are the 4 major structural elements of a eukaryotic cytoskeletal system?

Microtubules

This major structural element of a eukaryotic cytoskeletal system is the largest structural element composed of hollow, cylindrical structures composed of tubulin subunits (25nm in diameter).

Cytoplasmic microtubules

This type of microtubules are used for:

1. organization and maintenance of shape and polarity
2. intracellular transport/trafficking and organelle movement
3. chromosomal movements

Axonemal microtubules

This type of microtubules are organized and stable found in cilia, flagella, and basal bodies for cell motility.

shape

Distribution of microtubules determines the ____ of the cell.

protofilaments

These are straight, hollow cylinders of varied length that consist of (usually 13) longitudinal arrays of polymers that make up microtubules. They're called __________.

Heterodimer of tubulin (1 α-tubulin & 1 β-tubulin)

What is the basic subunit of a protofilament?

non-covalently

Protofilaments bind (covalently, non-covalently) to form an αβ-heterodimer.

True.

Protofilaments are assembled into tubules with plus and minus ends.

T or F?

True.

Plant cells have microtubules but their organization happens without a centralized microtubule-organizing center like centrosomes in animals.

T or F?

γ-tubulin

This is large ring-shaped protein complex in distributed nucleation sites (e.g. nuclear envelope, cell cortex, and around the cell membrane) critical for microtubule nucleation.

γ-tubulin ring complexes (γ-TuRCs)

These complexes nucleate the assembly of new microtubules away from distributed nucleation sites (e.g. nuclear envelope, cell cortex, and around the cell membrane).

microtubules

Loss of γ-TuRCs prevents a cell from nucleating _________.

True.

Newly formed microtubules branch at an angle of pre-existing microtubules.

T or F?

1. rearrangement of existing microtubules
2. disassembly of existing microtubules and reassembly of new ones in different locations

Changes in microtubule spatial organization is a combination of 2 mechanisms, mainly:

GTP

Which nucleoside triphosphate is required for microtubule assembly?

2 GTP molecules

The tubulin heterodimer binds how many GTP molecules?

new GTP

Hydrolysis of GTP leads to a replacement of bound GDP by _______ to "recharge" the tubulin dimer.

plus

The GTP cap at the (plus, minus) end prevents subunit removal in microtubules.

Dynamic instability model

This model states that one population of microtubules grow by polymerization at the plus ends, whereas another population shrinks by depolymerization.

+-TIP proteins (+-end tubulin interacting proteins)

These proteins stabilize microtubules by protecting the growing plus ends and regulating the rate of growth and shrinkage.

High GTP-tubulin

(High, low) GTP-tubulin is quickly added to microtubules, creating a large GTP-tubulin cap.

Low GTP-tubulin

(High, low) GTP-tubulin increases rate of GTP hydrolysis therefore the cap shrinks.

True.

Microtubules can also facilitate movement of vesicles between compartments, as well as act as an agent of intracellular motility by moving organelles, RNA, proteins, etc.

T or F?

Motor proteins

These proteins are molecular motors that convert energy from ATP (couple ATP hydrolysis) into mechanical energy; traverses the microtubular cytoskeleton.

unidirectionally

Motor proteins move (uni-, bi-)directionally along their skeletal track in a stepwise manner.

kinesin and dynein

What are the 2 motor proteins associated with microtubules?

Kinesins

These motor proteins are a tetramer of two identical heavy chains (HC) and two identical light chains (LC).

kinesin-like proteins (KLPs)

Kinesins are a member of a superfamily called?

globular heads

Each kinesin includes a pair of ______, which act as motor domains, connected to a rod-like stalk.

plus

Kinesin is a (plus, minus) end-directed microtubular motor based on its movement.

walking

Kinesin movement along the microtubule looks like ______, with the 2 globular head domains taking turns as the front foot.

directly proportional

Kinesin movement along the microtubule moves along a single protofilament of a microtubule at a velocity (indirectly, directly) proportional to the ATP concentration.

processive

Kinesin movement along the microtubule is _____, motor protein moves along an individual microtubule for a long distance without falling off.

False. KLPs move cargo towards the cell's plasma membrane.

KLPs move cargo away from the cell's plasma membrane.

T or F?

Dynein

This huge motor protein is cytoplasmic with a globular, force-generating head; responsible for cilia and flagella movement.

minus

Dynein is a (plus, minus) end-directed microtubular motor based on its movement.

1. widely distributed throughout the cortex
2. making a single transverse band
3. in the form of a mitotic spindle
4. as a pharmagoplast assisting in the formation of the cell wall of daughter cells

There are 4 distinct arrays of microtubules in a dividing plant cell: