Cytoskeleton
Cytoskeleton - Basic Concepts
Major Differences between Prokaryotes and Eukaryotes
Cytoskeleton Assembly
How we study cytoskeleton and cell structure?
The cytoskeleton is important in structure, movement, and intracellular transportation. There are
three components to the cytoskeleton; microfilaments, intermediate filaments, and microtubules.
The cytoskeleton is built on a framework of three types of protein filaments: intermediate
filaments, microtubules, and actin filaments. Each type
of filament has distinct mechanical properties and is formed from a different protein subunit. A
family of fibrous proteins forms the intermediate
filaments; globular tubulin subunits form microtubules; and globular actin subunits form actin
filaments. In each case, thousands of subunits assemble into fine threads that sometimes extend
across the entire cell.
Microfilaments are the narrowest with a diameter of 7nm made up of linked monomer
proteins called actin combined in a double helix structure. This means microfilaments
are also known as actin filaments. The actin monomers have directionality with two
structurally different ends. They can serve as tracks for the movement of the motor
protein myosin allowing muscle contraction forming sarcomeres which slide past each
Microfilaments other. In animal cell division, the ring made of actin and myosin pinches the cell apart
to generate two new daughter cells. Actin filaments also allow transportation of
protein-containing vesicles within the cell, carried by individual myosin motors which
walk along the actin filament bundles. Mostly the network of actin filaments is found
on the outside of the cell linked to the plasma membrane giving the cell its shape and
structure.
Intermediate Cytoskeleton element made up of multiple strands of fibrous proteins wound together
Filaments with an average diameter of 8-10nm. Intermediate filaments are more permanent and
can grow and disassemble quickly playing a structural role (keratin) in the cell
maintaining the shape of the cell and anchoring organelles into place. They have great
tensile strength with their main function being enabling cells to withstand the
mechanical stress which occurs when they are stretched. When intermediate filaments
Cytoskeleton 1
, are treated with concentrated salt solution and nonionic detergents the intermediate
filaments survive while the rest of the cytoskeleton was destroyed.
Microtubules are the largest type of cytoskeleton fibre with a diameter of 25nm. A
microtubule is made of tubulin proteins arranged to form a hollow straw with each
tubulin protein consisting of two subunits; alpha tubulin and beta tubulin. They are
Microtubules dynamic structures which can grow and shrink and have directionality. They help the
cell resist compression forces. They provide tracks for motor proteins called kinesins
and dyneins. They also form during cell division pulling the sister chromatids/
homologous chromosomes to opposite poles the cell.
Major changes in cell structure occur during differentiation into different cell types. The ultimate
cause is differences in cell structure caused by changes in gene expression. The proximate cause
is changes in internal organisation due to changes in the cytoskeleton. The cytoskeleton is
dynamic, not static.
Cytoskeletal polymers are formed by non covalent protein-protein interactions. There is no
dedicated machinery (polymerase) or factory (ribosome) for assembly. The protein complexes
can be simple dimers or tetramers or more complex macromolecular machines. Head-to-tail
association leads to polymer formation. In a dynamic environment, self assembly occurs where
the constant collisions in the cytoplasm allow proteins to find their partners. They need to meet
in the correct orientation and strong interactions are more kinetically favourable than weaker
interactions.
All three types of filaments are non covalent polymers formed by protein-protein interactions.
The fundamental principles are the same as in protein folding where electrostatic interactions,
hydrophobic interactions, hydrogen bonds, and van der waals attractions. In the filaments the
interactions occur within the proteins. Assembly of a protein polymers is like assembly of any
other multi-protein complexes, except that addition of each new subunit leaves room for yet
another to bind.
Strength of Interactions
The strength of interactions is characterised by the dissociation constant (Kd). There is an
association rate = association rate constant x [A] x [B] = kon [A] [B] Dissociation rate =
dissociation rate constant x [AB] = koff [AB]. At equilibrium, association rate = dissociation
rate; kon [A] [B] = koff [AB]. [A] [B]/[AB] = koff/kon = Kd (dissociation
constant). The (units) are molar. For a small dissociation the value of Kd is small. This means a
tight binding for 10-9M (less molecules disassociated) while a weak binding for 10-3M
(increased concentration means more molecules disassociated).
Cytoskeleton 2
Cytoskeleton - Basic Concepts
Major Differences between Prokaryotes and Eukaryotes
Cytoskeleton Assembly
How we study cytoskeleton and cell structure?
The cytoskeleton is important in structure, movement, and intracellular transportation. There are
three components to the cytoskeleton; microfilaments, intermediate filaments, and microtubules.
The cytoskeleton is built on a framework of three types of protein filaments: intermediate
filaments, microtubules, and actin filaments. Each type
of filament has distinct mechanical properties and is formed from a different protein subunit. A
family of fibrous proteins forms the intermediate
filaments; globular tubulin subunits form microtubules; and globular actin subunits form actin
filaments. In each case, thousands of subunits assemble into fine threads that sometimes extend
across the entire cell.
Microfilaments are the narrowest with a diameter of 7nm made up of linked monomer
proteins called actin combined in a double helix structure. This means microfilaments
are also known as actin filaments. The actin monomers have directionality with two
structurally different ends. They can serve as tracks for the movement of the motor
protein myosin allowing muscle contraction forming sarcomeres which slide past each
Microfilaments other. In animal cell division, the ring made of actin and myosin pinches the cell apart
to generate two new daughter cells. Actin filaments also allow transportation of
protein-containing vesicles within the cell, carried by individual myosin motors which
walk along the actin filament bundles. Mostly the network of actin filaments is found
on the outside of the cell linked to the plasma membrane giving the cell its shape and
structure.
Intermediate Cytoskeleton element made up of multiple strands of fibrous proteins wound together
Filaments with an average diameter of 8-10nm. Intermediate filaments are more permanent and
can grow and disassemble quickly playing a structural role (keratin) in the cell
maintaining the shape of the cell and anchoring organelles into place. They have great
tensile strength with their main function being enabling cells to withstand the
mechanical stress which occurs when they are stretched. When intermediate filaments
Cytoskeleton 1
, are treated with concentrated salt solution and nonionic detergents the intermediate
filaments survive while the rest of the cytoskeleton was destroyed.
Microtubules are the largest type of cytoskeleton fibre with a diameter of 25nm. A
microtubule is made of tubulin proteins arranged to form a hollow straw with each
tubulin protein consisting of two subunits; alpha tubulin and beta tubulin. They are
Microtubules dynamic structures which can grow and shrink and have directionality. They help the
cell resist compression forces. They provide tracks for motor proteins called kinesins
and dyneins. They also form during cell division pulling the sister chromatids/
homologous chromosomes to opposite poles the cell.
Major changes in cell structure occur during differentiation into different cell types. The ultimate
cause is differences in cell structure caused by changes in gene expression. The proximate cause
is changes in internal organisation due to changes in the cytoskeleton. The cytoskeleton is
dynamic, not static.
Cytoskeletal polymers are formed by non covalent protein-protein interactions. There is no
dedicated machinery (polymerase) or factory (ribosome) for assembly. The protein complexes
can be simple dimers or tetramers or more complex macromolecular machines. Head-to-tail
association leads to polymer formation. In a dynamic environment, self assembly occurs where
the constant collisions in the cytoplasm allow proteins to find their partners. They need to meet
in the correct orientation and strong interactions are more kinetically favourable than weaker
interactions.
All three types of filaments are non covalent polymers formed by protein-protein interactions.
The fundamental principles are the same as in protein folding where electrostatic interactions,
hydrophobic interactions, hydrogen bonds, and van der waals attractions. In the filaments the
interactions occur within the proteins. Assembly of a protein polymers is like assembly of any
other multi-protein complexes, except that addition of each new subunit leaves room for yet
another to bind.
Strength of Interactions
The strength of interactions is characterised by the dissociation constant (Kd). There is an
association rate = association rate constant x [A] x [B] = kon [A] [B] Dissociation rate =
dissociation rate constant x [AB] = koff [AB]. At equilibrium, association rate = dissociation
rate; kon [A] [B] = koff [AB]. [A] [B]/[AB] = koff/kon = Kd (dissociation
constant). The (units) are molar. For a small dissociation the value of Kd is small. This means a
tight binding for 10-9M (less molecules disassociated) while a weak binding for 10-3M
(increased concentration means more molecules disassociated).
Cytoskeleton 2
