Transcript No Slide Title

The Cytoskeleton
Assembly and Dynamic Structure
of Cytoskeletal Filaments
Pages 965-991
The Cytoskeleton
Green = Microtubules
Red = Actin
Blue =
Coomassie
Blue stain
Functions of the Cytoskeleton
-Pull chromosomes apart at mitosis and then splits the dividing
cell into two
-intracellular trafficing of organelles and other proteins
-support of the plasma membrane
-allows the cell to bear stresses and strains
-allows cells to swim (sperm) or crawl (fibroblasts)
-provides machinery for muscle contraction
-allows neurons to extend axons and dendrites
Types of Cytoskeletal Filaments
1. Actin Filaments
-determine the shape of the cell’s surface
-necessary for whole-cell locomotion
-diameter: 5-9 nm
2. Intermediate Filaments
-provide mechanical strength and resistance shear stress
-diameter: 10 nm
3. Microtubules
-determine the positions of membrane-enclosed organelles
-directs intracellular transport
-diameter: 25 nm
Changes in the
Cytoskeleton
Microtubules
Red = Actin
Green = Microtubules
Actin
Actin and Microtubules in
Drosophila Embryos
Actin – Red
Microtubules – Green
Division every 10 min
Neutrophil Movement
Assembly and disassembly of the actin filaments allows the
cell to change directions
Similarities between the
Types of Filaments
1. They form as helical assemblies of subunits
2. They all self-associate using a combination of endto-end and side-to-side protein contacts
3. Assembly and disassembly can occur rapidly
4. Accessory proteins regulate the spatial distribution
and dynamic behavior of the filaments
Cytoskeletal Structure
-Assymetric
-Characteristic Shape
Polarity of Actin in Yeast
Rapid Reorganization of
the Cytoskeleton
Protofilament Formation
Protofilament – long linear strings of subunits joined end-to-end
Composed of globular subunits that make a similar number
of longitudinal and lateral bonds
Actin and Microtubule Formation
Intermediate Filaments
Composed of fibrous subunits that make more lateral bonds
than longitudinal bonds
3 Stages of
Polymerization
Nucleation – Subunits must
assemble into an initial
aggregate that is stabilized
by many subunit-subunit
interactions
-This is the rate-limiting step
in polymerization
-Special proteins catalyze
nucleation at specific sites
Microtubule Structure
GTP/GDP
Tightly
bound
13 protofilaments
Actin Filament Structure
2 protofilaments
Preferential Growth of Microtubules
-Polarity of Microtubules and
Actin Filaments is created by
the parallel orientation of its
subunits
-Alpha units are exposed at +
end and beta subunits at the –
end
-The + end is more dynamic,
growing and shrinking faster,
while the – end is slower
Treadmilling in a Living Cell
Microtubules
Treadmilling of Actin
T form contains ATP
D form contains ADP
Treadmilling – Subunits are added at the
+ end while being removed at the - end
Dynamic Instability of Microtubules
Cc(D) > Cc(T)
-The D form leans
more readily toward
disassembly
-The T form leans
more readily towards
assembly
Dynamic Instability – the rapid interconversion between a growing
and shrinking state at a uniform free subunit concentration
GTP Hydrolysis
Causes Structural
Changes
GTP- straight filaments
GDP – curved filaments
Structural Changes
in Microtubules
Curved filaments (GDP)
doesn’t allow for as many
interactions between
protofilaments
Dynamic Instability in a Living Cell
Treadmilling and dynamic instability large amounts of
energy are used, but it gives the cell spatial and temporal
flexibility in response to its environment
FtsZ, a Tubulin
Homologue in Procaryotes
-Essential in cell division, a band of
FtsZ protein forms at the site of
separation, where the new cell wall is
to form
-Constriction and disassembly of the
FtsZ band through GTP hydrolysis
helps to pinch the two daughter cells
apart
Actin Interactions
-Actin is found in all
eucaryotic cells
-humans have 6 different
actin genes, alpha actin is
expressed in only muscle
cells while beta and
gamma are in almost all
nonmuscle cells
-Tubulin is also found in
all eucaryotic cells and
there are also multiple
forms of the subunits
Intermediate Filament Subunits
-no overall polarity
Intermediate Filament Construction
-They are easily bent but extremely difficult to break
-Protein phosphorylation probably regulates their disassembly
Mechanical Properties of
Cytoskeletal Filaments
Keratin Filaments in Epithelial Cells
-About 20 different types found
in human epithelial cells and
about 10 more that are specific
to hair and nails
-Keratin filaments are made of
equal numbers of type I
(acidic) and type II
(neutral/basic) chains
-Usually they are crosslinked
by disulfide bonds
Blistering of the Skin
due to Mutated Keratin
Epidermolysis bullosa simplex – the skin blisters in response to
even slight mechanical stress, which ruptures the basal cells