The Cytoskeleton Flashcards
General features of the cytoskeleton
Made of 3 polymers: -microtubules
- intermediate filaments
- actin filaments
Useful for:- shaping of the cell
- intracellular movements
- cell movement
Structure of actin filaments
- Twisted chain of units of G-actin which makes up the F-actin
- very thin (7nm)
- plus and minus end means its polar
- associated with a bunch of ABPs
What are the three different isoforms of G-actin and where are they found?
Alpha - mainly in muscle cells
Beta and gamma - mainly in non-muscle cells
Which end of actin filaments does G-actin get added to?
Plus end is more favourable
What two ABPs control the G-actin level?
Profilin facilitates actin polymerisation
Thymosin beta-4 presents the addition of actin monomers to F-actin
What do Actin bundling proteins do?
Keep the F-actin in parallel bundles (like in epithelial cells
What do cross-linking proteins do?
Maintain F-actin in a gel-like meshwork- like in the cell cortex under the plasma membrane
What do F-actin severing proteins do?
Break F-actin into smaller filaments by adding these proteins at the end of the polymers
What do motor proteins (like myosin) do?
Transport vesicles and/or organelles through actin filaments
What are the functions of actin filaments in skeletal muscle cells?
Arranged in a para-crystalline array, integrated with different ABPs
Interaction with myosin allows for muscle contraction
What are the functions of actin filaments in non-muscle cells?
Forms a thin sheath beneath the plasma membrane
Associated with myosin to form a purse sting result in cleavage of mitotic cells (cytokinesis)
What are the steps to cell migration?
- The cell pushes lamellipodia (plural) or filopodia(single) out at its front (actin polymerisation)
- these protrusions adhere to the surface (integrins line the actin filaments to the extra cellular matrix surrounding the cell)
- cell contraction and retraction of the rear part of the cell (interaction between actin filaments and myosin)
What do accessory proteins regulate?
- size and rate of filament formation (nucleation)
- polymerisation/depolymerisation
- function
Structure of intermediate filaments
- tough
- rope-like with many long strands twisted together and made up of many subunits
- medium size (8-12nm)
- they form a network through the cytoplasm, joining up cell-cell junctions (desmosomes)
- withstand mechanical stress when cells are stretched
- they surround and strengthen the nucleus
What is each monomer of the intermediate filaments made up of?
- N-terminal globular head
- C- terminal globular tail
- central elongated rod-like domain
What is a dimer?
Two units of an intermediate filaments monomer that is very stable
What do two dimers form?
Tetramear
What do tetramears look like when they bind to each other?
A rope-like filament
What are the types of cytoplasmic intermediate filament and and their localisation?
- keratin(in epithelia)
- vimentin and vimentin-related (in connective tissue, muscle and neurological cells)
- neurofilaments (in nerve cells)
What is the nuclear intermediate filament and and its localisation?
Nuclear lamins (in all nucleated cells)
What does IFBP stand for?
Intermediate filament binding proteins
Examples of IFBPs
Fillagrin binds keratin filaments into bundles
Synamin and plectin bind desmin and vimentin
Plakins keep the contact between the desmosomes of epithelial cells
What are the functions of the IF in the cytoplasm?
- tensile strength enables the cells to stretch
- provides structural support by creating a deformable 3D structural framework and reinforcing cell shape and fixing the organelle localisation
Functions of the IF in the nucleus
- forms a mesh rather than a rope like structure
- lines the inner face of the nuclear envelope to strengthen it and provides attachment sites for chromatin
- disassembles and reforms at each cell division as the nuclear envelope disintergrates (unstable). This is controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)
