Week 10 - Filament Formation and Motor Proteins Flashcards
What is Kinesin and what does it do?
A motor protein that walks along microtubules to carry vesicles.
What are the 2 types of Axonal Vesicular Transport?
- Dynein movement
2. Kinesin movement
What direction does Kinesin move in?
Moves towards the axon terminus/ microtubule PLUS end (Beta-tubulin).
What direction does Dynein move in?
Moves towards the cell body/microtubule MINUS end (alpha-tubulin).
How are some organelles transported?
Via microtubule walking of motor proteins.
What organelle is closer to the PLUS end of the microtubule?
The endoplasmic reticulum.
What organelle is closer to the MINUS end of the microtubule?
The Golgi apparatus.
Which motor protein walks from the Golgi apparatus to the ER?
Kinesin; walks towards PLUS end.
Which motor protein walks from the ER to the Golgi apparatus?
Dynein; walks towards MINUS end.
The Golgi apparatus is different from the centrosome, but still an example of what?
Example of a MTOC.
What 3 things are microfilaments/actin filaments involved in?
- Cell motility (crawling)
- Contractile activity
- Cytokinesis
What are actin filaments made of?
Actin monomers; helical filaments (2 protofilaments twisted in a helix)
What motor protein acts on actin filaments?
Myosin.
Are actin filaments polar?
Yes; due to the regular orientation of actin monomers in each protofilament.
Free actin monomers are bound to what?
Bound to ATP; at the centre of the protein.
Actin is an ATPase that hydrolyzes what?
Hydrolyzes ATP; ADP remains bound
Growth of actin filaments is faster at which end?
Faster at the plus end.
Actin filaments have what kind of cap?
An ATP cap.
Actin filaments are polar; what is at both ends?
One end has N and C terminus, the other end doesn’t.
What is at the plus end of an actin filament?
The plus end of an actin monomer has an N and C termini.
What is at the minus end of an actin filament?
No terminus.
What happens when all actin monomers are d-form at the cap?
Disassembly of the monomer occurs when all ATP is hydrolyzed to ADP.
Differences in Microtubules and Actin Filaments?
Microtubules Actin-
Filaments
-heterodimers -monomers
-GTP or GDP -ATP or ADP
-GTP cap -ATP cap
-T and D-form -T and D-form
heterodimers monomers
-13 parallel -2 protofilaments protofilaments twisted
forming hollow around each
cylinder other -dynein/kinesin -myosin
3 steps of actin polymerization in vitro?
- Nucleation (everything monomers)
- Elongation (growth phase)
- Steady state (equilibrium phase)
What is the Critical concentration during actin polymerization in vitro?
CC: Critical concentration
- not everything in the filament
- to keep this requires a continual supply of actin bound to ATP
- monomers coming on/off (filament doesn’t change in length)
Which end of actin filaments has a higher affinity for actin monomers?
The plus end.
Although actin filaments look stable, there is a constant exchange of what?
There is a constant exchange of monomers art the ends.
What is actin filament tread-milling?
As there is a constant exchange of actin monomers, a pulse of labelled actin monomers will “move down” or “treadmill” an actin filament; until all monomers are eventually replaced.
What proteins are involved in actin filament assembly?
- Profilin: inhibits nucleation, speeds elongation
2. Cofilin; accelerated disassembly.
What is the structure of Myosin?
- 2 heavy and 4 light chains
- tails of the 2 heavy chains are organized in a coiled-coil
- 2 heads associated with 2 light chains each (4 total)
- ATP hydrolyzed at myosin head.
Which Myosin activity on actin filaments causes muscles to contract?
Mysoin II motor activity.
When a muscle contracts, what is Myosin II doing?
Myosin II shortens and pulls actin filaments closer together (stretches them) to contract muscle fibres.
What are common mechanisms of all motor proteins?
- they all couple ATP hydrolysis with conformational changes to generate force
- all move in a specific direction along filaments with polarity.
What do Intermediate Filaments provide?
Structural support.
Intermediate filaments have 3 propeties…
- Tough
- Flexible
- Extensible; unlike actin and microtubules, they ARE elastic!
Intermediate filament are not found where?
- NOT in plant cells
- not in ALL animal cells
Intermediate filaments are prominent in what kind of cells?
Cells subjected to mechanicla stress; i.e. epithelial calls (keratin), neurons (neurofilament proteins), muscle cells (desmin).
What is the structure of an Intermediate Filament?
A coiled-coild dimer forms a STAGGERED antiparallel tetramer.
Are intermediate proteins polar?
NO; there is no filament polarity. Despite it’s monomer starting with polarity, after dimers form and come together to form a tetramer, polarity is lost.
Do intermediate filaments have motor proteins?
No; motor proteins require polarity.
Intermediate filaments pack together into what structure?
Pack into rope-like filaments.
Where are Keratin Filaments anchored in epithelial cells?
Anchored at the sites of cell to cell contact by desmosomes; held by mechanical strength.
Keratin filaments from neighbouring cells anchor at the cell boundary but do not do what?
They anchor at the cell boundary but DO NOT cross boundaries.
Keratin filaments can also anchor to what?
Can also anchor to hemi-desmosomes.
Where are hemi-desmosomes anchored?
Anchored to the extracellular matrix (ECM).
