This document goes into the details of the cytoskeleton, including the network of the three polymers, the features of the cytoskeleton and cell migration and the role of cytoskeleton in this function. Has a key table for summary at the end including all the concise points.
● 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
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