Cell Organisation And Movement Flashcards
(36 cards)
Micro filament definition
Sickle cell
Metastasis
Actin polymers organised by actin binding proteins
Plasma membrane integrity and micro villi
Tracks for ATP myosin motor proteins -> muscle contraction
Facilitates endocytosis
Sickle cell- b-globulin haemoglobin chain, aggregation under low O2. Cytoskeleton more rigid
Metastasis- changes in actin regulation allows non motile cells to move.
Micro tubules definition
Colchicine
Taxol
Formed by Tubulin, organised by micro tubule associated protein
Framework for organelles and support cilia and flagella
Structure of the mitotic spindle
Kinesins and dyneins transport
Colchicine- meadow saffron. Inhibits polymerisation of Tubulin. Relieve gout joint pain. Reduces micro tubule dynamics in WBCs so cannot migrate.
Taxol- plant alkaloid. Binds and stabilises micro tubules. Prevents cells from mitosis. Cancer treatment.
Intermediate filaments definition
Support, integrity, barrier. Not transport. A tetra ear is formed by side to side aggregation to form protofibril.
Neurofilaments- axon organisation
Lamins- inner surface nuclear membrane. Mutations cause emery-dreifus muscular dystrophy. Hutchinson-Guildford progeria (accelerated ageing)
Keratin- strength to epithelial cells. Epidermolysis bullosa- skins blisters.
Desmin, GFAP, vimentin
Coordinating the migrating cell cytoskeleton
Motility stimulus
Activation of signal cascade
cdc42 -> polymerisation of actin -> crawling
Also promotes micro tubules to direct secretory vesicles and adhesion molecules and nucleus movement
Structure of actin
F-actin - constructed of G actin monomers. Each actin molecule contains an Mg2+ complexed to ADP or ATP. G actin is divided into 2 lobes and ATP binds to bottom of the central cleft.
Addition of cations promotes formation of filaments
+ end is the joining end
One unit is 28 G actins
Actin polymerisation
Nucleation- ATP-G actin monomers form complexes. Rate limiting step. 3 monomers needed. Critical concentration required.
Elongation- stable nuclei elongate, addition to both ends
Steady state- equilibrium of addition/subtraction
Faster addition to + end as lower critical concentration.
When Pi is released, ATP at + and ADP at -
This then promotes dissociation from the - end
The cofilin cycle (actin polymerisation)
Cofilin binds to f and g actin. Specifically binds to ADP actin at the - end.
Bridges 2 adjacent actin monomers and changes twist -> destabilises
Increases the ADP actin that can be used by Profilin
The profilin cycle (actin polymerisation)
Profilin binds G actin on the opposite side to the ATP binding site
Binds and removes the ADP, monomer free to bind ATP
Profilin-ATP-G actin binds to + end and profilin dissociates
The Thymosin-β4 cycle (actin polymerisation)
Binds to G-ATP actin and inhibits addition. Creates G-actin reserve that can be used in blood clotting
CapZ caps the + end and stabilises, regulated by lipid.
Tropomodulin caps -end, abundant in cells that are stable e.g. RBCs and muscle
Gelsolin- binds to + end, regulated by Ca. Allows binding to side of an actin filament, creating new - end.
Mechanism of actin filament branching
Arp2/3 binds to side of filament (7 subunits)
WASp binds (NPF)-> conformational change allowing complex to bind actin
Filament formation
WASp has an RBD element that binds to Cdc42. Releases the ve terminal domain and activates.
Wiskott Aldrich syndrome
WASp defect. Not enough IgM produced, lack of platelets. X linked.
Listeria motility
Polymerises actin into a comet like tail. ActA is a Arp2/3 activating protein. Leads to actin nucleation of branched filaments -> force
Formins
Assemble unbranched filaments, FH1 FH2 adjacent domains with RBD. The FH1 contains Pro -> profilin binding. Increases concentration of ATP-G actin bound to profilin. Protects + from capping
FH2 inactive when associated to RBD. When bound to GTPase Rho, both domains exposed -> actin nucleation
Actin based cellular structures
Cross linkers
Spectrin
Cross linkers- 2 actin binding sites within polypeptide. Fimbrin is an example, arranges filament with polarity in bundles.
Also formed when 1 actin site dimerises. These are more spaced out, a-actinin is example.
Spectrin- tetramer with 2 sites, forms networks under the plasma membrane. Cross linkers such as filamin serve as springs at leading edges.
Diseases of the cytoskeleton
Arthritis
Osteoporosis- bone loses calcium and becomes thinner
Fibrodysplasia ossificans progressive- fibrous tissue ossified when damaged. Joints become frozen.
How are actin filaments linked to membranes in RBCs
2x14 subunit filaments. Stabilised by tropomyosin and tropomodulin
Linked together by Spectrin
Spectrin links to Ankyrin -> transmembrane protein band 3
Spectrin links to band 4.1 to transmembrane protein glycophorin C
Genetic disorders of actin linking to membrane
Spherocyic anaemia- mutations in Spectrin, band 4.1 and Ankyrin. Cells are rounder and rupture.
Sickle cell- mutation in B globulin. Low oxygen causes vessel occlusion and ischaemia. Actin oxidation stabilises abnormal cell type and exacerbates phenotype.
Structures of cell migration
Filopodia- protrude from leading edge, tight actin bundles formed by formins
Lamellipodium- complex of branched actin formed by Arp2/3
Cortical actin cytoskeleton gives stability to membrane
Stress fibres attached to substratum of cell through focal adhesions
Membrane extension
Leading edge actin is nucleated by Arp2/3 complex, filament grow from + end.
Actin tread milling mediated by cofilin and profilin
Formins and bundling proteins form leading edge
Focal adhesions (migration)
After membrane extension, plasma membrane attached to substratum. Actin bundles anchored to focal adhesions, stops lamella retracting.
As moves forward, adhesions migrate to the back of the cell and are deassembled
Cell body translocation
Bulk contents trans located forward
Organelles squeezed forward by cortical contraction of cytoskeleton
De adhesion and endocytic recycling
Focal adhesions at rear are broken
Integrins are recycled
Freed tail brought forward
Snap forward by stress fibres contracting
Control of actin organisation- injury
TGF-β released from injury sites and activates Rho
Activated platelets release LPA which activates Rho
RBCs release PDGF (platelet derived growth factor) at site of injury, which activates cdc42 and Rac
Activation of Rho
GDP form complexed to GDI in inactive form
Activated by GEF which changes GDP to GTP
Causes localisation to membrane
Rho-GTP then binds effectors that cause changes
Binds to RBD of Formin, profilin binds to FH1 allowing addition at FH2
GAP returns it to the cytoplasm