Flashcards in cytoskeleton Deck (33):
How many types of polymers are there?
what are the types of polymers called?
→microfilaments / actin,
what are microtubules for?
→ organelle positioning
→ intracellular transport
what are intermediate filaments for?
→ mechanical strength
what are actin filaments for?
→ cell shape
→ organelle shape
→ cell migration
give two properties of the polymers that make up the cytoskeleton
→ monomers are very abundant
→ not covalently linkes
what is the structure of actin filaments and what are the isoforms?
→Twisted chain of units (monomers) of the protein actin (G-actin, aprox.43 KDa). This chain constitutes the filamentous form (F-actin).
→Thinnest class of the cytoskeleton filaments (7 nm)
→Presents structural polarity
→Associated with a large number of actin-binding proteins (ABP)
→There are 3 isoforms of G-actin with different isoelectric points :
α-actin found mainly in muscle cells
β-actin and γ-actin in non-muscle cells
how can actin polymerize?
→Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.
what does the length of the polymerized actin filament depend on?
→Concentration of G-actin.
→Presence of Actin Binding proteins (ABPs)
what are G actin levels controlled by?
2 actin binding proteins
what are the two actin binding proteins that regulate G actin and what do they do?
→Profilin: facilitates actin polymerization.
→Thymosin β4: prevents the addition of actin monomers to F-actin.
What are the two actin binding proteins that bind to F actin?
Actin bundling proteins
→ keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)
Cross linking proteins
→ keep actin in gel like mesh work under membrane
what do F actin severing proteins do?
→break F-actin into smaller filaments
what do motor proteins do?
→transport of vesicles and/or organelles along actin filaments.
what proteins break actin apart?
→F actin severing proteins
how do the severing proteins increase the surface area when breaking actin down?
→they work on different points like endo peptidases.
how is actin arranged in skeletal muscle?
→Arranged in a para-crystalline array integrated with different ABPs
→Interaction with Myosin motors allow muscle contraction
what is the function of actin in non-muscle cells?
→Cell cortex : form a thin sheath beneath the plasma membrane
→Associated with myosin form a purse string ring resulting in cleavage of mitotic cells
what is the function of actin in cell migration?
1) Elongation - protrusions (lamellipodia and filopodia) pushed out
2) Adhesion - Integrins link the filaments to the extracellular matrix surrounding the cell
3) Contraction - actin and myosin interaction, contraction and retraction
what are properties of intermediate filaments?
→Toughest of the cytoskeletal filaments (resistant to detergents, high salt etc).
→Ropelike with many long strands twisted together and made up of different subunits.
→Form a network:
Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).
→Withstands mechanical stress when cells are stretched.
And surrounding nucleus
→Strengthens the nuclear envelope.
What is the structure of intermediate filaments?
Each unit is made of:
→N-terminal globular head
→C-terminal globular tail
→Central elongated rod-like domain
→Units form stable dimers
→Every 2 dimers form a tetramear
→Tetramers bind to each other and twist to constitute a rope-like filament
What are the types of intermediate filaments?
1) Keratins (in epithelia, protects from damage/stress)
2) Vimentin/ vimentin related (in connective tissues, muscle cells and neuroglial cells)
3) Neurofilaments (in nerve cells) NUCLEAR:
1) Nuclear lamins (in all nucleated cells)
what do the intermediate filament binding proteins do?
→Mainly linkers of IF structures.
→IFBP stabilize and reinforce IF into 3D networks
what are the intermediate filament binding proteins and their functions?
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).
Keep the contact between desmosomes of epithelial cells.
what are the functions of intermediate filaments in the cytoplasm?
→Tensile strength: this enable the cells to withstand mechanical stress (to stretch!)
→Structural support by:
Creating a deformable 3D structural framework
what are the functions of intermediate filaments in the nucleus?
→present in all nucleated eukaryotic cells
→form mesh rather than “rope-like” structure
line in the inner face of the nuclear envelope to:
→provide attachment sites for chromatin
→disassemble and reform at each cell division as nuclear envelope disintegrates
i.e. very different from the stable cytoplasmic intermediate filaments
→process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)
describe properties of microtubules
→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
what is the MTOC?
Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts.
what is the MTOC in most cells?
Centrosome (in the perinuclear region)
what constitutes the microtubule and what type of growth happens?
→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).
what are the functions of microtubules?
act like railway tracks on which molecular motors run
different motors for different cargoes
→directionality of filaments is vital (each motor only moves in one direction)
→organises position of organelles
hence, provides polarisation of cells
what are the functions of microtubules ( cilia ) ?
→Rhythmic beating of cilia and flagella
→Motile processes, with highly organized microtubule core.
→Bending of cilia & flagella is driven by the motor protein Dynein.