Chapter 13- Microtubules

What are microtubules made up of?

Globular protein tubulin

• Tubulin dimers consist of a-tubulin and b-tubulin

• y-tubulin in centrosome helps initiation of microtubule assembly

What do tubulin dimers polymerize to form?

Microtubules: 14 protofilaments around a hollow core. Protofilaments are head-to-tail arrays of tubulin dimers arranged in parallel

• have polarity (- and + ends)

Microtubules assembly and disassembly

GTP bound to b-tubulin is hydrolyzed to GDP shortly after polymerization, weakening binding affinity to tubulin dimers for each other, resulting in rapid depolymerization and loss of tubulin bound to GDP from minus end.

What is necessary for a GTP cap to remain at the plus end of the microtubule?

New GTP-bound tubulin dimers are added more rapidly than GTP is hydrolyzed and microtubule growth continues

What happens if GTP is hydrolyzed more rapidly than new subunits are added?

GDP-bound tubulin at the plus end of the microtubule leads to disassembly and shrinkage

What are vincristine and vinblastine used for?

For cancer chemotherapy because they inhibit microtubule polymerization and affect rapidly dividing cells.

Taxol stabilizes microtubules, which also blocks cell division

What do MAPs (Microtubule-associated proteins) regulate?

Growth and shrinkage of plus ends

Role of CLASP proteins?

Rescue microtubules from catastrophe by stopping disassembly and restarting growth.

In what direction do most microtubules extend in animal cells?

Outwards

• During mitosis, they extend outward from duplicated centrosomes to form the mitotic spindle that controls separation and distribution of chromosomes to daughter cells

What is the role of centrosomes?

To be the microtubule-organizing center

• initiate microtubule growth

• y-tubulin is associated with other proteins in ring-shaped structure called y-tubulin ring complex

• Speeds microtubule growth

What do most animal cell centrosomes have?

A pair of centrioles that form basal bodies of cilia and flagella

• Cylindrical, containing nine triplets of microtubules

How is microtubules stability regulated?

Post-translational modification of tubulin by phosphorylation, acetylation, etc.

What protein is a MAP characteristic of lesions found in the brains of Alzheimer’s patients?

Tau protein

What are the two type of processes supported by stable microtubules in neurons?

Axons: Microtubules have + end towards the tips, associated with tau

Dendrites: Microtubules are oriented in both directions, associated with MAP2

What are the two families of motor proteins responsible for powering microtubule movements?

Kinesins: move along microtubules towards + end

Dyneins: move towards the minus end (Axonemal dynein was the first to be identified)

Major role of microtubules

To transport macromolecules, vesicles, and organelles through the cytoplasm

• Position organelles within the cell

What is the central location of Golgi apparatus?

Due to its interactions with dynein which position it near the centrosome

What are the microtubule-based projections of the plasma membrane?

Cilia and flagella that are responsible for movement of many eukaryotic cells

Cilia

Beat in a coordinated back-and-forth motion, which either moves the cell through a fluid or moves fluid over the surface of the cell.

Flagella

Longer and have a wavelike pattern of beating

Which end of the microtubule is anchored in a basal body?

Minus end, similar in structure to a centriole

• Basal body contains nine triplets of microtubules

Axoneme

Consists of microtubules in a 9+2 pattern: a central pair surrounded by nine outer doublets

• Each doublet is a complete A tubule fused to an incomplete B tubule.

• Nexin links the tubules and two arms of dynein are attached to each A tubule

Mitotic Spindles

Free tubulin subunits are reassembled and interphase microtubule array disassembles

Three types of microtubules that make up mitotic spindles

1. Kinetochore microtubules attach to condensed chromosomes at the centromeres, stabilizing them

2. Interpolar microtubules are no attached to chromosomes but stabilized by overlapping with each other in the center of the cell

3. Astral microtubules extend outward from the centrosomes with plus ends anchored in the cell cortex

Separation of spindle poles results from 2 types of movement

1. Overlapping interpolar microtubules slide past each other to push spindle poles apart, driven by the action of + end directed motor proteins (Between the spindle poles - center of cell)

2. Spindle poles are pulled apart by the astral microtubules, driven by - end directed motors anchored to the cell cortex (towards the cell cortex- edge)

Intermediate Filaments

Diameters intermediate between actin filaments and microtubules

• Provide mechanical strength and a scaffold for localization of cell processes

• Not found in yeast, plants, and some insects

• Made of many types of proteins expressed in different cell types

What are type I and II in epithelial cells?

Keratins

Role of Vimentin?

Forms a network extending out from the nucleus toward the cell periphery (fibroblast, blood cells, etc.)

Role of Desmin?

Expressed in muscle cells where it connects the Z discs of individual contractile elements (muscle cells)

What are Neurofilament (NF) proteins?

The major intermediate filaments of many neurons; provide support for long axons

What are Type V?

The nuclear lamins, which form a meshwork underlying the nuclear membrane

Are intermediates polar?

No, they are apolar and do not possess distinct ends. Share a common structure.

What are desmosomes?

Junctions between adjacent cells (cell-cell contact)

• Keratin filaments attach to dense protein plaques on the cytoplasmic side

• Extracellular side link via cadherins (Desmoglein and desmocollin)

What are Hemidesmosomes?

Junctions between epithelial cells and underlying connective tissue (Cell-ECM contact)

• Keratin filaments are linked to integrins by plectin

Primary role of intermediate filaments

To strengthen the cytoskeleton of cells in the tissues of multicellular organisms

• revealed using transgenic mice with a keratin mutation that disrupted formation of a normal keratin cytoskeleton → severe skin abnormalities