cytoskeleton Flashcards

1
Q

Structure of actin filaments

A

-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

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2
Q

Function of actin filaments

A

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
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3
Q

Actin filaments in cell migration

A
  • 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
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4
Q

Actin polymerisation

A
  • 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
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5
Q

ABP proteins binding to monomers

A

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

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6
Q

How can the cell structure filaments ?

A

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

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7
Q

How are cells able to control length of filament severing them?

A
  1. F actin severing proteins - break f actin into smaller filaments - if a cell request quick depolymerisation of the filaments
  2. Motor proteins (myosin) - transport of vesicles and or organelles through actin filaments
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8
Q

What is the cytoskeleton

A

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.

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9
Q

Function of cytoskeleton

A
  • shape cell , intercellular movement of organelles and vesicles , cell movement
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10
Q

Cytoskeleton general features

A
Made of 3 different polymers: 
1. Microtubules
 2. Intermediate filaments 
3. Actin filaments 
(all three structures spread throughout the entirety of the cell)
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11
Q

How si the cytoskeleton dynamic?

A

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

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12
Q

Structure of intermediate filaments

A

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

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13
Q

Intermediate filaments polymerisation

A

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

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14
Q

Types of intermediate filaments

A

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
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15
Q

Intermediate filaments binding proteins IFBP

A
  • 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
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16
Q

Function of the intermediate filaments in the cytoplasm

A
  • 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
17
Q

Functions of intermediate filaments in the nucleus

A
  • 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)
18
Q

Structure of microtubules

A

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

19
Q

Polymerisation of microtubules

A

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 )

20
Q

Functions of microtubules

A

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

21
Q

Organises position of membrane enclosed organelles in correct orientation for example..,

A

-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

22
Q

Cilia and flagella

A

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