The cytoskeleton
● The cytoskeleton is a very dynamic structure that changes very often.
● It keeps its shape & modifies it in response to environmental cues.
General features of the cytoskeleton
Monomers are not covalently linked.
● Complex network made of 3 different polymers where the distribution of these polymers is different in
the cell:
○ Microtubules
■ Organelle positioning
■ Intracellular transport
■ Facilitate the distribution of components around the cell
○ Intermediate filaments
■ Mechanical strength
■ Give shape & resist high pressure
○ Actin filaments = 7nm
■ Cell shape
■ Organelle shape
■ Cell migration
● Provide for:
○ Shaping of the cell
○ Intracellular movement of
organelles
○ Cell movement
General features of the cytoskeleton
● The actin filaments in the epithelial
cell is surrounding the cell.
● Intermediate filaments give
mechanical structure
● Microtubules, are very important for
positioning, & transport. Ie, bring
vesicles into cells etc
Actin filaments structure
● Twisted chains of monomers of protein actin [G-actin] which constitutes the filaments from
[F-actin].
● Presents structural polarity, the monomers are usually added to the +ve end.
, ● Associated with a large number of actin-binding proteins (ABP)
● There are 3 isoforms of G-actin with different isoelectric point :
○ α-actin found mainly in muscle cells
○ β-actin and γ-actin in non-muscle cells
Actin polymerisation
● Actin filament [f-actin] can grow by addition of actin
monomers [G-actin]
● The addition of new free actin monomers can go on
forever, but only at a specific rate.
● Length of filament is determined by concentration of
G-actin & presence of ABPs.
ABPs
● Formation of filament is controlled by two ABPs.
● One protein is profilin, which binds the monomers to the filaments.
This helps the reaction when it’s binded to the filament.
● Thymosin stops the infinite amount of monomers binding, this is
released which binds to the profilin & stops the actin filament
growing.
Proteins binding to filaments
● 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)
● F-actin severing proteins: break F-actin into smaller
filaments. These can be only broken from one end.
● Motor proteins (Myosin): transport of vesicles and/or
organelles through actin filaments
Actin filaments functions
● Skeletal muscle
○ Arranged in a para-crystalline array integrated with different ABPs
○ Interaction with Myosin motors allow muscle contraction
● Non-muscle cells
○ Cell cortex : form a thin sheath beneath the plasma membrane
● The cytoskeleton is a very dynamic structure that changes very often.
● It keeps its shape & modifies it in response to environmental cues.
General features of the cytoskeleton
Monomers are not covalently linked.
● Complex network made of 3 different polymers where the distribution of these polymers is different in
the cell:
○ Microtubules
■ Organelle positioning
■ Intracellular transport
■ Facilitate the distribution of components around the cell
○ Intermediate filaments
■ Mechanical strength
■ Give shape & resist high pressure
○ Actin filaments = 7nm
■ Cell shape
■ Organelle shape
■ Cell migration
● Provide for:
○ Shaping of the cell
○ Intracellular movement of
organelles
○ Cell movement
General features of the cytoskeleton
● The actin filaments in the epithelial
cell is surrounding the cell.
● Intermediate filaments give
mechanical structure
● Microtubules, are very important for
positioning, & transport. Ie, bring
vesicles into cells etc
Actin filaments structure
● Twisted chains of monomers of protein actin [G-actin] which constitutes the filaments from
[F-actin].
● Presents structural polarity, the monomers are usually added to the +ve end.
, ● Associated with a large number of actin-binding proteins (ABP)
● There are 3 isoforms of G-actin with different isoelectric point :
○ α-actin found mainly in muscle cells
○ β-actin and γ-actin in non-muscle cells
Actin polymerisation
● Actin filament [f-actin] can grow by addition of actin
monomers [G-actin]
● The addition of new free actin monomers can go on
forever, but only at a specific rate.
● Length of filament is determined by concentration of
G-actin & presence of ABPs.
ABPs
● Formation of filament is controlled by two ABPs.
● One protein is profilin, which binds the monomers to the filaments.
This helps the reaction when it’s binded to the filament.
● Thymosin stops the infinite amount of monomers binding, this is
released which binds to the profilin & stops the actin filament
growing.
Proteins binding to filaments
● 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)
● F-actin severing proteins: break F-actin into smaller
filaments. These can be only broken from one end.
● Motor proteins (Myosin): transport of vesicles and/or
organelles through actin filaments
Actin filaments functions
● Skeletal muscle
○ Arranged in a para-crystalline array integrated with different ABPs
○ Interaction with Myosin motors allow muscle contraction
● Non-muscle cells
○ Cell cortex : form a thin sheath beneath the plasma membrane
