• Cytoskeleton: System of protein filaments in the cytoplasm of a eukaryotic cell that gives
the cell shape and the capacity for directed movement. Its most abundant components are
actin filaments, microtubules, and intermediate filaments.
• Pulls chromosomes apart at mitosis and then splits the dividing cell in 2.
• Drives and guides intracellular traffic of organelles, ferrying materials from one part of
the cell to another.
• Supports the plasma membrane.
• Enables certain cells to swim or crawl.
• Provides machinery for muscle contractions.
• Guides the growth of plant cell walls and controls the amazing diversity of eukaryotic
The Self-Assembly and Dynamic Structure of Cytoskeletal Filaments
• Most animal cells have 3 cytoskeletal filaments that are responsible for the cells’spatial
organization and mechanical properties: Intermediate filaments, microtubules, and actin.
• Intermediate filaments provide mechanical strength. Microtubules determine the
positions of membrane-enclosed organelles and direct intracellular transport.Actin
filaments determine the shape of the cell’s surface and are necessary for whole-cell
• They would be ineffective without accessory proteins. Especially motor proteins.
Cytoskeletal Filaments Are Dynamic andAdaptable
• Individual macromolecular components are in a constant state of flux.
• Microtubules typically near nucleus. They can typically rearrange themselves to form a
bipolar mitotic spindle during cell division. Can also form cilia and flagella.
• Actin filaments underlie the plasma membrane of animal cells, providing strength. Can
also form lamellipodia and filopodia.Actin is the contractile ring that splits cells in 2. Ear
hairs have actin in their surface and microvilli on the intestinal epithelial cells also have
• Intermediate filaments line the inner face of the nuclear envelope, forming a protective
cage for the cell’s DNA.
• Important example of rapid re-organization is during cell division. The interphase
microtubules is reconfigured to form a bipolar mitotic spindle.Actin that usually enables
crawling, is now forms a contractile ring. When division is complete the daughter cells
reassemble into their interphase structures.
• Cells like neutrophils that chase and engulf bacterial and fungal cells use rapid re-
organization to move by crawling.
The Cytoskeleton Can Also Form Stable Structures
• Individual actin filaments remain strikingly dynamic and are continuously remodeled and
replaced on average every 48 hours, even within stable cell surface structures that persist
• Cytoskeleton responsible for cell polarity. • 2 main types of actin filaments within cells, actin cables (long bundles of actin filaments)
and actin patches (small assemblies of filaments associated with the cell cortex, marking
sites of actin-driven endocytosis). These patches arrange themselves in yeast to create
areas of high concentration and polarity.
The Tubulin andActin SubunitsAssemble Head-to-Tail to Create Polar Filaments
• Tubulin: The protein subunit of microtubules.
• Tubulin is a heterodimer containing α-tubulin and β-tubulin bound by non-covalent
• Each monomer has a binding site for GTP. The GTP bound to the α-monomer is
physically trapped at the dimer interface and is never hydrolyzed or exchanged. For the
β-monomer, the nucleotide may be in GTP or GDP form, and is exchangeable.
• Microtubule is a hollow cylindrical structure. The binding energy formed between 2
heterodimers going up is very strong, the adjacent bonds are relatively weak because they
are connected to the same monomer.
• Microtubules are stiff and difficult to bend. It has a polarity with alpha at one end and
beta at the other.
• Actin subunit is a monomer. Each monomer has a binding site forATP orADP.
• Actin assembles head to tail, making it polar.An actin filament can be considered to
consist of 2 parallel protofilaments wrapped around each other.Actin are relatively
flexible.Accessory proteins can bind to make it more stable.
Microtubules and Actin Filaments Have Two Distinct Ends That Grow At Different Rates
• Polarity in microtubules and actin filaments allows for different growth rates on each end.
In the absence ofATP or GTP hydrolysis, the rates of synthesis and loss at each end
should be equal.
• Since it is polar, the rate constants are different for each end.
• Plus end: End of a microtubule or actin filament at which addition of monomers occurs
most readily; the “fast-growing” end of a microtubule or actin filament. The plus end of
an actin filament is also known as the barbed end.
• Minus end: End of a microtubule or actin filament at which the addition of monomers
occurs least readily; the “slow-growing” end of the microtubule or actin filament. The
minus end of an actin filament is also known as the pointed end.
• If free subunit concentration drops below a critical concentration, the fast growing end
• Microtubules expose the alpha subunit on the minus end and β on the plus end.
• On actin, theATP binding cleft point towards the minus end.
• Acell can use free energy released during polymerization or depolymerisation to do
mechanical work (like push or pull a load).
Filament Treadmilling and Dynamic InstabilityAre Consequences of Nucleotide Hydrolysis by
Tubulin and Actin
• Actin and microtubule filaments are both enzymes that can catalyze the hydrolysis of a
nucleotide (GTP or ATP). Free subunits do this slowly, when in a filament, it is rapid.
• T-form is when bound to ATP or GTP, D-form is when bound toADP or GDP.
• When hydrolyzed, the free energy released is stored in the polymer lattice. T form
polymers grow, while D-form polymers shrink. • Free subunits often in T form. Older T-forms get hydrolyzed first. Newer ones formATP
caps or GTP caps.
• Since D-forms have a higher critical concentration, they often disassemble.
• Treadmilling: Process by which a polymeric protein filament is maintained at constant
length by addition of protein subunits at one end and loss of subunits at the other end.
• At a certain concentration, subunit addition and loss are equal. This is ‘steady-state
• There is a chance that subunit addition can catch up to the other end, transforming that
end to the D-form.
• Dynamic instability: Sudden conversion from growth to shrinkage, and vice versa, in a
protein filament such as a microtubule or actin filament.
• The change from growth to shrinkage on one end is called a catastrophe and from
shrinkage to growth is called a rescue.
• In microtubules, GTP bound is straight, GDP bound starts to bend.
• Actin filaments undergo length fluctuations on a much smaller scale.
• Dynamic instability is thought to pre-dominate in microtubules, whereas treadmillling is
thought to pre-dominate in actin.
How Cells Regulate Their Cytoskeletal Filaments
• Most of regulation of filaments is performed by accessory proteins.
AProtein Complex Containing γ-Tubulin Nucleates Microtubules
• Microtubule-organizing centre (MTOC): Region in a cell, such as a centrosome or a
basal body, from which microtubules grow.
• γ- tubulin is present in smaller amounts and is involved in the nucleation of microtubule
growth in organisms.
• γ-tubulin ring complex (γ-TuRC): Protein complex containing γ-tubulin and other
proteins that is an efficient nucleator of microtubules and caps their minus end.
• γ-tubulin thought to serve as a template that creates a microtubule with 13 protofilaments.
Microtubules Emanate from the Centrosome inAnimal Cells
• Centrosome: Centrally located organelle of animal cells that is the primary microtubule-
organizing centre and acts as a spindle pole during mitosis. In most animals cells it
contains a pair of centrioles.
• Most animal cells have a single MTOC, the centrosome. It is from here that microtubules
• The minus ends are near the centrosome, plus ends away from it.
• Acentrosome contains a centrosome matrix with multiple copies of γ -TuRC.
• Centriole: Short cylindrical array of microtubules, closely similar in structure to a basal
body. A pair of centrioles is usually found at the center of a centrosome in animal cells.
• Centrioles form almost an L-shape in the centrosome.
• Centrioles duplicate and form the mitotic spindles during replication.
• Acentriole consists of short cylinders of microtubules and accessory proteins.
• Neither fungi nor plants cells contain centrioles, yet all contain γ-tubulin.
• Centrosome uses microtubules to help center it in the cell by pushing against the walls. • Even without centr