Module 4 - The cytoskeleton and molecular motors Flashcards
Microtubules: what are they and what do their plus and minus sides do?
Unbranches cylinders of approximately 25nm diameter that assembled from tubulin heterodimers
Plus side - grows quickly with an exposed b-tubulin
Minus side - grows slowly if at all
How are microtubules made within cells/within test-tubes
Tubulin concentration is too low for spontaneous polymerisation so cells use a template of gamma-tubulin and other proteins (nucleation)
Tubulin, GTP, and Mg²⁺ left at 37°C can manufacture microtubules
How do microtubules grow?
Grow at the plus end of the tubulin ring complexes of centrosomes
Microtubles
They are dynamic and grow and shrink independently of its neighbours
What does slow hydrolysis give rise to?
On/off switches as GDP dissociating is a slow process and GTP associating is a fast process
Why does tubulin need GTP?
GTP tubulin can polymerise with other dimers, meanwhile, GDP tubulin cannot
How do microtubules have dynamic instability?
The GTP cap on the minus end of the microtubule will determine whether growth or shrinkage occurs (no GTP cap - shrinkage)
How can microtubules be stabilised?
By the binding of microtubule-associated proteins (MAPs) along the microtubule, drugs (taxol), or capping complexes (proteins at the cell membrane attach to the microtubules and stabilise them)
Can microtubules be depolymerised?
Yes, by putting cells on ice or using drugs that bind free tubulin dimers (Nocodazole, Colchicine, and Colcemid)
Actin filaments: Subunit, filaments, drugs, and motor proteins?
Monomeric actin, binding ATP
2-stranded 7nm flexible helix
Phalloid (stabilises filaments), Cytochalasin (caps polymerisation from ends), and Latrunculin (binds to actin filaments, preventing polymerisation)
Myosin
Microtubules filaments: Subunit, filaments, drugs, and motor proteins?
Tubulin dimers, binding GTP
Fairly rigid hollow 25nm tube
Taxol (stabilising), Nocodazole (destabilising)
Dyneins, kinesins
Where are actin filaments found?
Sarcomeres, contractile fibres in other cells (dividing cells, stress fibres), and are found in some non-contractile parts of cells (microvilli)
Actin polymerisation: how does it work?
The plus end of the actin filament is built up by ATP-bound actin monomers and the minus end of the filament loses the ADP-bound actin filaments (this allows the actin filament to almost always have the energy bound for use(?))
Actin filaments: can they be stabilised and how would experimental manufacture of actin filaments work?
Yes, by a protein cap attaching
Actin, ATP, and Mg²⁺ left at 37°C can manufacture microtubules
Why does actin not immediately polymerise and form filaments in the cell?
5% of the cell’s proteins are actin. In a test tube, they would all bind and form actin filaments; however, many proteins in the cells prevent this from occurring until the actin filaments are required
Lamellipodia: where is it located and how does actin interact with it?
The end of the cell that is moving in that direction
Actin polymerises at the lamellipodia and causes an increase in length, causing the other side to move, beginning contraction
Arp: what is it and what does it do?
Arp 2/3 complex -
Actin-related proteins which bind to existing actin sites, nucleating actin assembly, and preventing the minus-end disassembly
Filopodia: what are they and what do they do?
The protrusions at the leading edge of migrating cells, extended by actin polymerisation at the cell membrane that are anchored by proteins
They guide the cell by probing the environment and reacting with surrounding ECM
Mechanisms of animal cell migration
1 - Cell pushes out filopodia
2 - Cell adheres to surroundings
3 - contraction
The adhering of animal cells
Focal contacts contain trans-membrane plasma proteins (integrins) as well as contractile actin bundles (stress fibres) adheres the cell
Contraction of the cell
Myosin II (motor protein) causes movement
Intermediate filaments: which organisms have them and what types are there?
Found in animals but not unicellular organisms, plants, or fungi
Cytoplasmic filaments
Nuclear filaments
Cytoplasmic filaments: what are they, what animals have them present?
Keratin filaments - epithelial cells
Vimentin filaments - in connective cells
Neurofilaments - nerve cells
All animals except arthropoda and hydra
Nuclear filaments: what are they, what animals have them present, and where are they located?
Nuclear laments
All animals
Just under the nuclear envelope, forming the nuclear lamina
Progeria: what is it and how does it occur?
A premature ageing syndrome
Mutations in nuclear lamins
Intermediate filaments: Subunit, filaments, drugs, and motor proteins?
Various that do not use ATP/GTP
8-stranded flexible helix (10nm)
None
N/A
Properties of intermediate filaments
- Connected by desmosomes and strengthen cells
- Stable but may disassemble during replication (by phosphorylation)
Keratin mutations: what do they cause?
In a keratin mutation, the cells rupture between the nucleus and hemidesmosomes and cause damage to underlying basal tissue, causing blistering
Desmin mutations: where is desmin found and what do the mutations cause?
Desmin is found within every type of muscle and a mutation may lead to muscular dystrophy or cardiac myopathy
Neurofilaments
Strengthen neurons
Intermediate filaments assembly
Symmetrical filaments organised in an antiparallel manner which causes there to be no polarity
What are the two main motor proteins and what movement do they do?
Kinesins - move cargo towards microtubule plus ends (cell periphery)
Dyneins - move cargo towards microtubule minus ends (cell centre)
The two types of dyneins
Cytoplasmic dynein
Axonal dynein
What are the differences between dyneins
All have the same dynein motor base, but each attaches to a different dynactin (adaptor protein)
Viral entry into the cell
Enter using cytoplasmic dynein either within endosomes or as viral capsids
Types of sliding
- Anti-parallel sliding (ie mitosis/meiosis)
- Parallel sliding (ie cilia/flagella)
What are the axoneme structure in cilia/flagella and what purpose?
Ring of microtubule doublets with a two single microtubule centre with dynein arms attached
Used for cilial/flagellar beating
Myosins: what uses them and what is the more well-known myosin?
Plants. algae and many Fungi use myosins along with actin filaments for transport
Myosin II is found in most cell types, not just muscle (also found in stress fibres)
What allows myosin filaments to remain stable and what happens if they are not preset?
Desmin intermediate filaments form a scaffold which stabilises muscle Z discs
Muscle/cardiac dystrophy
Myosin I: what does it do and what are its key features?
It helps reshape the membrane by pulling on underlying actin filaments and it also facilitates short organelle movement
Only has one head
