W2 Cytoskeleton Flashcards
Cytoskeleton
Keep shape + modify to environmental clues
Dynamic structure
Monomers not covalently linked
Network of polymers + function
Microtubules, actin/intermediate filaments
For → shape of cell, intracellular movement of organelles + cell movement
Accessory proteins regulate…?
Site/rate of filament formation = nucleation (polymerisation or assembly reactions where the first steps are energetically less favoured than the continuation of growth)
Polymerization/depolymerization
Function
Cross linking of filaments = forming higher order structures
Signalling of intra/extra w/cytoskeleton assembly modules
What are actin filaments
Helical polymers made of actin
Flexible, organised into 2D networks + 3D gels
Cell/organelle shape
Cell migration
Organisation/regulation = ABP (actin binding proteins)
Actin filaments structure
Twisted chain of units (monomers) of the protein actin (G-actin, approx.43 KDa kilodaltons). This chain constitutes the filamentous form (F-actin)
Helical repeat every 37 nm
Thinnest class of the cytoskeleton filaments (7 nm)
Presents structural polarity = binding of myosin subfragment (S1) to filament causes barbed +ve and pointed -ve ends on filament
Associated with a large number of actin-binding proteins (ABP) = variety of organization and function
There are 3 isoforms (different form of a protein) of G-actin with different isoelectric point (pH were molecule carries no net electrical charge):
α-actin found mainly in muscle cells
β-actin and γ-actin in nonmuscle cells
Actin filaments polymerization
Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.
The length of the filament is determined by:
Concentration of G-actin.
Presence of Actin Binding proteins (ABPs)
Capping proteins = control access to free barbed ends of actin filaments as have high affinity for barbed ends + micromolar conc in CT ensures most barbed ends capped
Regulation of G actin levels
Profilin: facilitates actin polymerization
Thymosin β4: prevents the addition of actin monomers to F-actin
Actin bundling proteins
Keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)
Cross-linking proteins
Maintain F-actin in a gel-like meshwork (as seen in the cell cortex, underneath the plasma membrane)
Lamellipodia
Thin protein sheet on actin (propels cell across a substrate)
Filopodia
Thin, tubular protrusions formed at leading edge of motile cells that are composed of linear bundles of actin filaments
Actin in skeletal muscle
Arranged in a paracrystalline array integrated with different ABPs
Interaction with myosin motors allow muscle contraction
Actin in non muscle
Cell cortex = form a thin sheath beneath the plasma membrane
Associated with myosin to form a purse string ring result in cleavage of mitotic cells
Cytokinesis
Actin-myosin ring called contractile forms around equator of cell tightens to form cleavage furrow
Cell migration w/actin
The cell pushes out protrusions at its front (lamellipodia & filopodia)
Actin polymerization
These protrusions adhere to the surface
Integrins (link the actin filaments to the extracellular matrix surrounding the cell)
Cell contraction and retraction of the rear part of the cell
Interaction between actin filaments and myosin
Intermediate filaments structure
Absent in fungi/plants
Toughest of the cytoskeletal filaments (resistant to detergents, high salt etc).
Ropelike with many long strands twisted together and made up of different subunits
Intermediate size (8-12 nm) between actin and microtubules
IF formation starts w/folding of IF proteins into conserved alpha-helical rod shape then series of polymerization + annealing = filaments w/described structure above
Lack polarity as not all interact w/each other
More stable + do not bind nucleotides
Intermediate filament function
Form a network:
Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).
Withstands mechanical stress when cells are stretched
Surrounds nucleus + strengthens the nuclear envelope
Intermediate filament polymerization
Each unit is made of:
N-terminal globular head
C-terminal globular tail
Central elongated rod-like domain
Units (w/alpha helical region) form stable dimers (called coiled-coil dimer)
Every 2 dimer forms a tetramer
Tetramers bind to each other and twist to constitute a rope-like filament
Intermediate filament binding protein
Mainly linkers of IF structure
IFBP stabilize and reinforce IF into 3D networks
Examples of IF binding protein
Filaggrin:
binds keratin filaments into bundles
Synamin and Plectin:
bind desmin and vimentin
Link IF to the other cytoskeleton compounds (i.e. actin and microtubules) as well as to cell-cell contact structures (desmosomes).
Plakins:
Keep the contact between desmosomes (structure by which two adjacent cells attached) of epithelial cells
Intermediate filament function in CT
(Keratins in epithelia, vimentins + neurofilaments)
Tensile strength: this enable the cells to withstand mechanical stress (to stretch)
Structural support by:
Creating a deformable 3D structural framework
Reinforcing cell shape and fix organelle localization
Intermediate filament function in nucleus
Present in all nucleated eukaryotic cells
Form mesh rather than “rope-like” structure
Line in the inner face of the nuclear envelope to:
strengthen it
provide attachment sites for chromatin
Disassemble and reform at each cell division as nuclear envelope disintegrates
i.e. very different from the stable cytoplasmic IFs
process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)
(Lamins) Anchor for chromatin
Microtubules structure
Hollow tubes made up from the protein tubulin
Relatively stiff (25nm), is the thicker of the filaments
Each filament is polarized (i.e. has direction – head/tail or +/-)
It is a dynamic structure
Assemble and disassemble in response to cell needs
tubulin in cell is roughly 50:50 as free or in filament
i.e. very different from the stable cytoplasmic intermediate filaments
Microtubules polymerization
Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts
Centrosome (in the perinuclear region) is the MTOC in most of the cells
Contains γ-tubulin ring that initiates the microtubule growth
Heterodimers of α and β tubulin constitute the microtubule
It is a polarized growth (i.e. there is an end that grows faster (+end) than the other (- end)
Microtubules function
Intracellular transport
Act like railway tracks on which molecular motors run
Different motors for different cargos
Directionality of filaments is vital (each motor only moves in one direction)
Dynein - moves cargo towards minus ends of MT
Kinesin - Typically moves cargo towards plus ends of MT
Organises position of organelles
Provides polarisation of cells
Directionality of filaments is vital
Microtubules in rythmic beating of cilia/flagella
Motile processes, with highly organized microtubule core.
Core consist of 9 pairs of microtubules around 2 central microtubule (axoneme)
Bending of cilia & flagella is driven by the motor protein Dynein
The basal body, at the base of the tubule, controls the assembly of the axoneme
Cilia in the respiratory tract, sweeping mucus and debris from lungs
Flagella on spermatozoa
