Lecture 3 - Intermediate Filaments and Microtubules. Flashcards
Out of the three types of filaments, which two show the most overlap?
Microfilaments and intermediate filaments.
5 types of IF proteins?
Keratins (I are acidic & II are basic), Vimentin (III), Neuronal IF proteins (IV) and Lamins (V).
Keratins (types I and II).
- They are found in the outer epithelia. Prominent in skin, hair and nails.
- Heterodimers - always found in pairs, one acidic and one basic.
- Red keratin staining used for their identification.
Vimentin (type III).
- Widely distributed in many different cells.
- Supports cell membranes + keeps organelles correctly positioned.
Neuronal IF proteins (type IV).
- Role in neurotransmission –> they determine the diameter of the axon, which in turn determines the speed of conduction.
Lamins (type V).
- Support the inner nuclear membrane –> determine the structure of the nucleus.
- Play a role in the organisation of different types of chromatin.
IF proteins share a general structure. Describe this.
They have a globular head and a globular tail (N- and C- termini) with a long alpha helical region inbetween.
How do IF filaments assemble?
- 2 IF monomers wrap around each other to form parallel dimers.
- 2 parallel dimers then go head to tail = antiparallel tetramer.
- 2 antiparallel tetramers stack end-on-end = protofilaments.
- 2 protofilaments come together to form a protofibril.
- 4 protofibrils wrap around each other to form the 10nm IF fibre.
Where is the majority of sequence diversity found between IF proteins?
N- and C- termini globular domains. The alpha helical region is highly conserved.
Give an example of a heteropolymer.
Keratins - will always be Type I and Type II.
What determines which proteins become homopolymers and which become heteropolymers?
Spacer sequences in the alpha helical domain.
There is one key difference in the formation of IF fibres in comparison to the formation of MTs and MFs. What is this?
Does not require ATP/GTP.
Keratins and the epidermis.
The skin has several layers of cells. First layer = skin stem cells in the basal epidermal layer. As they differentiate they change keratin expression.
Name two diseases associated with IFs.
Bilstering disease - epidermolysis bullosa simplex (EBS).
Hutchinson Gilford Progeria
Blistering disease - Epidermolysis Bullosa Simplex.
Caused by mutations in the N- and C- termini. Protofilaments cannot form. The basal epidermis cannot adhere properly; epidermis and dermis separate easily.
Hutchinson’s Gilford Progeria.
Caused by mutations in the LMNA gene. It is a premature ageing disease that produces an abnormal form of Lamin A.
Which type of tubulin binds GTP reversibly?
Beta tubulin. Can hydrolyse GTP for GDP. Alpha tubulin binds GTP irreversibly.
Assembly of MTs?
- Tubulin dimers come together in a head-to-tail manner; protofilament formation.
- Protofilaments come together laterally. Form a sheet that curves to form the 25nm tube.
- Have a (+) end and a (-) end. Beta tubulin is found at the (+) end.
- If the rate of addition is greater than the rate of hydrolysation then a GTP cap will form.
- Assembly is concentration dependent - must be above Cc for addition at both ends (greater at the (+) end).
- Once pool is depleted = depolymerisation = greater at the (-) end due to GTP cap.
What does MTOC stand for? Where are they located?
Microtubule Organising Centre. Just outside the nucleus.
Main functions of the MTOC?
They direct the organisation of MTs, the movement of vesicles and the orientation of organelles.
In animal cells, what is the MTOC called?
The centrioles.
Cells with flagellar have an additional MTOC. Name this structure and explain its function.
Basal body. Organises MTs in the flagellar so they can generate the force for movement.
Name the two motor proteins that associate with the MTs for vesicular transport.
Kinesins and dyneins (for MFs it is Type I and V myosins).
Kinesins and dyneins perform movement in opposite directions. Explain this.
Kinesins –> movement AWAY from the MTOC –> Anterograde transport.
Dyneins –> movement towards the MTOC –> Retrograde transport.
MTs and MFs are located in different areas of a cell. How can a single cargo access all areas?
Carry both kinesin and myosin motors, which can be switched.
