Cell Cytoskeleton Flashcards
Which of the stages of microfilament formation does the cell need to overcome?
The nucleation stage- formation of the nuclei of the microtubules by the individual subunits
Overcome in Vigo using s microtubule organising centre (MTOC) imbedded in centrosomes, they grow form gamma tubulin ring complexes
What are microtubules made from?
Tubulin
A diner made from alpha and beta subunits
Each bind s GTP molecule but only in the beta sub unit is this hydrolysed to GDP
The tubulin molecules bind to form a protofilament
13 protofilaments form a microtubule
What are MAP proteins and how do they affect microtubule structure?
Bind to and stabilises microtubules
Speeds up polymerisation
Also connect to vesicles, actin or other microtubules
Often regulated by phosphorylation
Describe actin?
Globular monomeric protein
Has three subunits; alpha beta and gamma
Each can bind a molecules of ATP or ADP
Subunits are polar and assemble head to tail
What do actin binding proteins (ABP) do?
Can for cross links between fibres
Or from fibres to the plasma membrane
What is the structure of intermediate filaments?
Alpha helix monomer
Coiled-coil dimer
The dimers then form a rope like structure by further polymerising
Which filaments have a motor function?
Actin microfilaments- need ATP and myosin
Microtubules- need ATP and kinesins or dyesins
What are the names and functions of the three domains in myosin?
Head domain- binds to F-actin and used ATP hydrolysis to generate force
Neck domain- acts a linker and as a lever to transducer the force generated by the head
Tail domain- mediates interaction with cargo molecules or other myosin tail molecules
Describe muscle myosin?
Myosin 2, conventional myosin
Made from 6 polypeptides; two identical heavy chains
Two pairs of light chains
Moved toward the plus/barbed end of F-actin
Summarise the myosin cross bridge cycle
Myosin tightly bound to actin & ATP site is empty
ATP binds to myosin head & causes s conformational change to unbind it from actin
ATP hydrolysis & neck undergoes conformational change to displace it along the filament
Weak binding to actin causes the release of Pi & then tight binding to actin
Release of Pi causes power stroke & ADP release & head region returns to original conformation
What is the difference between processive and non processive motors?
Non processive- net movement results from uncoordinated myosin heads cycling independently eg. Myosin 2
Processive- net movement results from myosin heads cycling in a coordinated manner eg. Myosin 5
How is (non muscle) myosin 2 regulated?
Myosin light chain kinase (MLCK)
Phosphorylation of MLCK causes myosin 2 to take on an extended active state exposing it’s actin binding sites
Describe kinesins motor cycle
Lagging head has ATP bound & locked strongly to the MT
Leading head is randomly searching for the next binding site and weakly docks & the lagging strand hydrolyses the ATP to ADP
Release of Pi from the lagging head weakens it’s binding to MT & the leading head rebinds ATP and locks down onto the MT
The leading head locking down throws the lagging head forward and begins the cycle again
What are the differences between myosin and kinesin cycling
Myosin uses ATP binding to release actin binding & kinesin uses ATP for strong binding
Myosin used ATP hydrolysis to reset the neck region & kinesin uses ATP hydrolysis to get Pi
Myosin uses Pi for strong binding & kinesin uses Pi to weaken MT binding
Myosin uses ADP release for the power stroke & kinesin uses the release of ADP to strengthen MT binding
Describe the motor cycle of dyne ins
ATP hydrolysis causes stalk attachment to MT
Release of ADP and Pi causes power stroke
Cycle repeats with other dyne in head
Name all the cell-cell junctions and their associated filaments
Tight junctions- actin
Adherents junctions- actin
Desmosomes- keratin intermediate filaments
Gap junctions
Name the cell-matrix junctions and associated filaments
Hemidesmosomes- keratin intermediate filaments
Focal adhesions- actin
What is pemphigus foliaceus?
Desmoglein 3 (upper epidermis des isomers) pathogenic antibodies Epidermis mainly intact- good prognosis
What is pemphigus vulgaris?
Desmoglein 3 (lower epidermis des isomers) pathogenic antibodies Epidermis severely compromised- fatal if untreated
What is epidermolysis bullosa simplex?
Separation in basal layer due to a mutation in the hemidesmosome causing blisters
What is junctional epidermolysis bullosa?
Separation in the basement membrane due to a mutation in laminin/BPAG2/alpha6/beta4 subunits
What is dystrophic epidermolysis bullosa?
Separation in upper dermis due to a mutation in collagen 7
What are the minimum requirements for actin based movement?
Nucleation of new actin filaments
Capping of old filaments
Recycling of monomers from old filaments
Name the complex used to accelerate actin nucleation?
Arp2/3 complex
Describe actin based movement
Protrusion- filopodia form testing the environment then lamellipodia form commiting to that direction
Translocation- formation of beefcake adhesions via integrins to the ECM generating traction
Actin/myosin contractility
Detachment- protease degrade contacts
What happens to a cell if there is no expression of E-cadherin?
No adherens junctions Changes in cell morphology Loss of polarity Increased motility Increased invasiveness Cancerous characteristics
How are actins on focal adhesions switched from inactive to active conformations?
Outside-in signalling
When cells come into contact with basement membrane integrins bind to ECM proteins so chains switch to their active conformations
Inside-out signalling
Platelets express alpha11b-beta3 integrin
When platelets come into contact with thrombin takin is activated which activates the integrin do it has an increased affinity for fibrinogen and leads to the formation of a playlet plug at the site of the wound
How are collagen fibres formed?
Assembly of alpha chain triple helix I. ER then a secretory vesicles transports to the ECM where the helicase into fibrils and then aggregation into fibres
Describe the formation of an elastic fibre
Tropoelastin formed in the ER
Translocated to the plasma membrane, secreted and cross linked to elastin and then cross linking with fibrillin microfilaments to form elastic fibres
Name some phospholipids
Phosphatidylethanolamine Phosphatidylserine Phosphatidylcholine Sphingomyelin Sphingosine
Name drugs inhibit microtubule polymerisation?
Colchicine
Vinblastine
Nocidazole
Name a drug that favours microtubule polymerisation
Taxol
Name drugs that inhibit actin polymerisation
Phalloidin
Cytochalasin D
Latranculin
