Lecture 4 The Cytoskeleton

Microfilaments / Actin

Globular protein, arranged in a helix shape. 7nm in diameter. Responsible for contractile forces within muscle cells. G-actin.

Arranged in bundles, forming meshed networks and is also responsible for structure of cells.

Eg, experiment with wild type fruit fly, eye, mutation due to actin

Intermediate filaments

Structural protein in eukaryotic cells

8-12 nm in diameter, provide support against mechanical stress in cells. Provides stability when stretched aka under stress.

Microtubules

Tube like structure, made of tubulin monomers, alpha and beta tubulin. Beta tubulin is where polymerisation and elongation occurs the fastest, the plus end.

Large around 25nm in diameter.

Main part of the mitotic spindle, aids in the segregation of chromosomes during cell division

Function of Myosin I

Transport of vesicles

Function of Myosin II

Generates contractile force, a motor protein that interacts with F actin

Where do microtubules originate from?

They originate from the MTOCs, eg centrosome is an example of this. The centrosome itself becomes duplicated during mitosis.

• A significant proportion of cancers have abnormal centrosomes..

Microtubules are the main component in:

cellular cilia

How is microtubule stability regulated?

Regulated by proteins associated with the microtubules: tau, MAP2.

Tau stabilises proteins found in the axons, a deficiency of tau can lead to neurodegenerative disorders like Alzeihmer’s

MAP2 is found in dendrites.

Actin based structures within motile structures

Stress fibres: actin-myosin bundles within a cell’s cytoplasm

Lamellipodium: thin, sheet like extensions that contain bundles of actin filaments

Filopodia: finger like protrusions that contain bundles of actin filaments, used for sensing the environment around.

A mutation in keratin can lead to a condition called:

Epidermolysis bullosa (ED)

Can cause the skin to separate and be extremely fragile.

Easy to get sick, pathogen entrance and be dehydrated, water can evaporate quite easily.

Classes of intermediate filaments:

Cytoplasmic: Keratin, Vimentin (connective tissue, muscles), neurofilaments (neurons)

Nuclear: nuclear lamins, found in all animal cells.

Role of nuclear lamins:

Helps maintain the structure of the nucleus and aids in DNA replication, anchors chromatin during gene expression

What are neurofilaments are what can they be used to measure in the body?

Neurofilaments found in neurons, are released from neurons when they get damaged. Volume of these filaments can be measured and used as a biomarker to detect neurodegenerative diseases earlier than other techniques…

MTOCs?

Microtubule organsing centres

eg, centrosomes

How are microtubules composed?

dimers of alpha and beta tubulins, polymerisation is quicker at the beta tubulins. These go on to form eukaryotic cells.

Role of actin-myosin ring?

Squeezes apart cells in cell division, forming 2 cells.

A sarcomere

a basic unit of contractile muscle fibres

A form of dynein allows for …?

movement!

eg cilia in sperm cells

beating of cilia to remove mucus from airways

A su

What is Nocodazole?

A synthetic drug used in the treatment of cancer, chemotherapy. Affects the formation of microtubules.

What other things apart from drugs can affect the formation of microtubules?

PROTEINS, which are destabilising:

Kinesin-13 and Katanin

Kinesin-13 attaches to the microtubule, uses ATP to remove tubulin sub units, shortening the microtubule

Katanin severs the microtubule, making longer ones shorter

Syndrome which is caused by mutations in the ciliary dyneins?

Bardet-Biedl syndrome, rare genetic disease

Can cause obesity, lead to kidney abnormalities and can affect rods and cones in the retina

Motor proteins are dependent on what?

ATP or GTP

They hydrolyse these molecules to release energy which is then used for cellular activities like transporting things like proteins and vesicles from each of the ends.

Polymerisation of actin and its structure?

G actin binds to each other using ATP forming a nucleus (nucleation). Elongation then occurs, forming an insoluble F-actin (fibrous). F actin is a helical structure made of G actin monomers.

Polymerisation happens quicker at the barbed end (plus end)

Actin migrates from plus to minus end, ATP is hydrolysed to ADP as actin is added onto the polymer chain.

Properties of alpha tubulin?

Made of alpha helices, has a nucleotide binding site which is vital for microtubule assembly

Properties of beta tubulin?

Formed from beta pleated sheets and has a site where GTP can be hydrolysed to GDP.

What do kinesins and dyneins transport?

Vesicles, proteins, mRNA, organelles

From what end to what end? Kinesin and dynein

Kinesin: from the minus to the plus end

Dynein: from the plus to the minus end

Minus end is called the what and plus end is the what?

Minus end= pointed end

Plus end= barbed end