cytoskeleton Flashcards
Structure of actin filaments
-Twisted chain of monomers of the proteins actin known as G (globular) actin. This chain makes the filamentous form F- actin
- Thinnest of the cytoskeletal filaments ( 7nm) diameter
- Presents structural polarity - meaning they have an end where monomers are added
known as plus end and end where addition of monomers is less favourable known as
the minus end.
- Associated with large number of actin binding proteins (ABP) - they influence its
organisation and its function
- 3 isoforms of G- actin with different isoelectric points: alpha actin found mainly in
muscle cells. Beta actin and gamma actin = non muscle cells
Function of actin filaments
Skeletal muscle :
- Arranged in para crystalline array integrated with different ABPs
- Interaction with myosin motors allow muscle contraction
- Non muscle cells : actin filament can participate in 4 different aspects of the cell - 1.
Microvilli 2. Contractile bundles 3. Lamellipodia filopodia - involved in cell motility 4.
Contractile ring - needed in mitosis cytokinesis stage - Cell cortex : form thin sheath beneath the plasma membrane
- Associated with myosin form a pure string ring results in cleavage of mitotic cells
Actin filaments in cell migration
- Cell pushes out protrusions at its front to identify which root to take we have filopodia
- long extensions which then lead to formation of lamellipodia- big wave of
membrane and here there is an accumulation actin filaments in particular shapes -
in case of filopodia they form bundles and for lamellipodia they form mesh structure - Protutions then adhere to the desired surface - to do that they interact with a
collection of proteins known as integrins which link the actin to the extracellular
matrix surrounding the cell - Cell concentration and retraction of rear parts of the cell to facilitate movement
contraction involved interaction of actin filaments and myosin
Actin polymerisation
- Actin filaments ( F- actin) can grow by addition of G- actin at either end
- Length of filament determined by:
1. Concentration and availability of free G - actin monomers
2. Presence of ABPs
ABP proteins binding to monomers
G- actin levels controlled mainly by 2ABPs:
- Profilin: facilitates actin polymerization by joining to the momonmer making it more
likely and accessible to join to the plus end
- thymosin beta4 : prevents addition of actin monomers to f actin. These two proteins help regulate the polymerisation process
How can the cell structure filaments ?
Once filaments formed the cell can structure them in two different ways because of
two other ABPs
1. Actin bundling proteins - keep F- actin in parallel bundles ( similar to microvilli
placement)
2. Cross - linking proteins - maintain F-actin in a gel like meshwork (similar to
cell cortex underneath plasma membrane) - providing mechanical strength
How are cells able to control length of filament severing them?
- F actin severing proteins - break f actin into smaller filaments - if a cell request quick depolymerisation of the filaments
- Motor proteins (myosin) - transport of vesicles and or organelles through actin filaments
What is the cytoskeleton
skeleton of the cell with the function of keeping cells shape and modified in response to environmental cues - the second function different to regular animal skeletons making it a dynamic structure.
Function of cytoskeleton
- shape cell , intercellular movement of organelles and vesicles , cell movement
Cytoskeleton general features
Made of 3 different polymers: 1. Microtubules 2. Intermediate filaments 3. Actin filaments (all three structures spread throughout the entirety of the cell)
How si the cytoskeleton dynamic?
they way it’s organised allows it to change quickly from polymers to monomers and vice versa
- High abundance of monomers - meaning quickly able to form filaments when needed
- the way the cells forms these polymers = is by making them with weak bonds ( not covalently bonded) allowing this process to be reversible (monomer to filamentous polymer - filamountous polymer to monomer)
- Why this process is vital for the cell - example : signal such as nutrient source received by cell where the cell needs to move towards that signal
- Cell needs to disassemble the filamentous polymer - possible because of weak non covalent bonds - rapidly redistribute the monomers to the necessary areas
- Reassemble filaments at the desired site
-there are accessory proteins that regulate : 1. Site and rate of filament formation 2. Polymerisation and depolymerisation 3. Function of the filaments
Structure of intermediate filaments
Toughest of the cytoskeletal filaments - resistant to detergents and high salts
- Ropelike structure - many long strands twisted together , also made of different
subunits - opposite to actin filaments and microtubules which are made up of only
one type of protein monomer
- Intermediate size of 8-12nm between actin filaments and microtubules - there is a
range because of its composition of different types of proteins
- Form network throughout the cytoplasm joining up to cell-cell junctions
(desmosomes) - main role = withstand mechanical stress when cells are stretched
- Form network around surrounding nucleus - main role = strengthens nuclear
envelope
Intermediate filaments polymerisation
Each unit is made of : N- terminal globular head ( NH2) , c- terminal globular tail (COOH) previous structures connected via central elongated rod like domain
- Units polymerise to then form stable dimers
- Every 2 dimers form a tetramer
- Tetrameres bind to each other twisting to form a rope like filament. Constituting final
intermediate filament
Types of intermediate filaments
Based on type of proteins that make up the intermediate filament and where they are localized we can distinguish the groups in which the intermediate filament fall into:
- Cytoplasmic intermediate filaments - classified in three main groups: 1. Keratins - found in the epithelial cells, hair and nails 2. Vimentin and vimentin-related - found in connective tissue, muscle cells and neuroglial cells - provide strength 3. Neurofilaments - found in nerve cells
- Nuclear intermediate filaments - group of proteins all belonging to same family known as nuclear lamins - found in all nucleated cells
Intermediate filaments binding proteins IFBP
- Main function of IFBP is to link IF structures
- IFBP stabilise and reinforce IF into 3D networks Example of IFBP:
- Fillagrin - binds keratin filaments into bundles
- Synamin and plecrin - bind desmin and vimentin - also link IF to other cytoskeleton
components ( i.e actin and microtubules) as well as to desmosomes - these
structures facilitate contact between cells - Plakins - keep the contact between two different desmosomes of the two different
epithelial cells
Function of the intermediate filaments in the cytoplasm
- Provide tensile strength - enables cells to withstand mechanical stress , allowing them to stretch essentially
- Provide structural support - create deformable 3D structural framework - also provide structural support by reinforcing cell shape and fix organelle localization
Functions of intermediate filaments in the nucleus
- Form mesh rather than rope like structure - known as lamins
- Line in inner face of nuclear envelope to strengthen it and provide attachment sites
for chromatin - Lamins Disassemble and reform at each cell division as nuclear envelope
disintegrates - very different from stable cytoplasmic IFs - this process is controlled by post translational modifications ( phosphorylation and dephosphorylation)
Structure of microtubules
Hollow tubes made up from protein called tubulin - similar to actin as it is only made of this one protein
- Stiff (25nm) thickest of the filaments - low level flexibility
- Each filament is polarized like actin - where there is a positive end where
polymerisation occurs and a negative end where is is less favourable
- Dynamic structure - assembles and disassembles in response to cell needs and
signals received by the cell
- Tubulin in the cell can either be free or in a filament ( usually 50-50 split)
- Microtubules different to the stable cytoplasmic intermediate filaments
Polymerisation of microtubules
Microtubules organizing center (MTOC) are specialised protein complexes where the assembly of tubulin units start - MTOC work as a primer for the polymerisation process
- Centrosome which is a perinuclear region (cytoplasmic region just around the nucleus.) is the location of MTOC in most of the cells
- The tubulin can exist in three different isoforms: alpha beta and gamma tubulins
- Gamma tubulins along with other accessory proteins constitutes the MTOC which
initiates microtubule growth
- Heterodimers of alpha and beta tubulin constitute the microtubule
- It is a polarized growth - meaning there is an end that grows faster(+ve end ) than the
other end (-ve end )
Functions of microtubules
Intracellular transport of vesicles and organelles - act like railway tracks where molecular motors run ( protein motors use ATP)
- Different motors for different cargoes
- Directionality of filaments is vital ( each motor only moves in one direction in one
direction)
- Motors that move cargoes towards the plus end of the microtubules belong to a
family known as kinesin ( some kinesin motors also move towards negative end )
- Motors that move cargoes towards the negative end are known as dynein
Organises position of membrane enclosed organelles in correct orientation for example..,
-This provides polarisation of cells
- Directionality of filaments is vital
- Microtubules important during cell division - microtubules form parallel bundles
mitotic spindles allowing the cell to disrepute chromosomes uniformly
Cilia and flagella
Motile processes ,with highly organized microtubules core
- Core consist of 9 pairs of microtubules around 2 central microtubule (axoneme)
- Bending of cilia and flagella ( bending because they are involved in motile process)
is driven by the interaction of the microtubules with the motor protein dynein - The basal body at the base of the tubule controls the assembly of the axoneme
- Example of cilia in the respiratory tract - sweeping mucus and debris from lungs
- Example of flagella on spermatozoa - facilitating and helping the spermatozoa to
swim
