Chapter 17: Cytoskeleton
1.What are the three cytoskeleton components? Describe each comparing relative sizes and
where they are primarily located in the cell. 2. How are they each formed?
1. MICROTUBULES: Hollow cylinders with a diameter of about 25 nm. They
extend outward through the cell interior, creating a system of tracks in which
organelles, transport vesicles, and other components can be transported.
Microtubules are composed of a & b tubulin subunits. Main function is cell
transportation and organization.
INTERMEDIATE FILAMENTS: Ropelike fibers with a diameter of about 10 nm.
Intermediate filaments are constructed from fibrous intermediate filament proteins;
a. keratin filaments in epithelial tissue, b. vimentin and vimentin related filaments in
connective tissue, muscle cells, and glial cells, c. neurofilaments in nerve cells, d.
nuclear lamins in the nucleus. Intermediate filaments are found in the cytoplasm,
where they form a network surrounding the nucleus and extending throughout the
cell periphery. They are also found inside the cell’s nucleus, where they form the
nuclear lamina, which strengthens the nuclear envelope. Main function is cell
support (tensile strength).
ACTIN FILAMENTS (MICROFILAMENTS): Helical polymers with a diameter of
about 7 nm. Microfilaments are composed of actin proteins and are found mostly in
the cell cortex (the layer of cytoplasm just beneath the plasma membrane). Main
function is cell movement.
2. Intermediate filaments are formed via polymerization of intermediate filament protein
subunits. An a-helical monomer entwines with another of the same monomer, forming a
parallel coiled coil dimer. Two dimers entwine, creating a staggered antiparallel tetramer. 8
tetramers join, creating the subunit that will be added to the growing intermediate
filament. Intermediate filaments do NOT show polarity at their ends, unlike microfilaments
and microtubules.
Monomer---coiled coil dimer----staggered, antiparallel tetramer---8 tetramers-----add to
end of growing intermediate filament.
Microtubules grow out of organizing centers: a. in nondividing cells the organizing center is
the centrosome b. in dividing cells the organizing center is the mitotic spindle c. in ciliated
cells the organizing center is the basal body. Microtubules are formed from a & b tubulin
subunits that stack together. 13 protofilaments interact to form a microtubule.
Microtubules do show structural polarity, with the b tubulin subunits on the + end and the
a subunits on the – end, with the - end embedded in the centrosome. Inside the centrosome,
the y-tubulin ring complex serves as the nucleation, or starting point, for microtubule
growth.
Actin filaments consists of a twisted chain of actin monomers. Actin filaments have
structural polarity, with a + and – end. Actin filaments can grow by the addition of the
actin monomers to either end, but growth is faster at the + end. Each actin monomer
carries a molecule of ATP that is hydrolyzed when the monomer is added to the growing
microfilament. The hydrolysis of ATP promotes depolymerization. Actin filaments undergo
“treadmilling” where the addition of monomers at the + end is equal to the loss of
monomers at the – end, therefore the filament will stay at the same length.
, In which cytoskeleton component are the following proteins found: vimentin, keratin, -tubulin,
actin, -tubulin, lamin, neurofilament, -tubulin.
Intermediate Filaments: vimentin, keratin, lamin, neurofilament
Actin Filaments: actin
Microtubules: b-tubulin, y-tubulin, a-tubulin
What are the four major classes of intermediate filaments? In what types of cells are they found?
1. Keratin filaments: epithelial cells
2. Vimentin and vimentin related filaments: connective, muscle, and glial cells
3. Neurofilaments: nerve cells
4. Nuclear lamins: nucleus
Keratin filaments, vimentin & vimentin related filaments, and neurofilaments are
cytoplasmic. Nuclear lamins are found inside the nucleus.
How do keratin fibers support epithelial cells? What happens if there are mutations in keratin
genes?
Keratin filaments are the most diverse class of intermediate filaments and are found in
every kind of epithelium. Keratin filaments interact cell to cell through desmosomes and
connect to the basal lamina through hemidesmosomes, distributing the stress put onto the
cells when they are stretched. Those with the disease epidermolysis bullosa simplex have
skin that is vulnerable to mechanical damage due to mutations that affect keratin filament
formation.
What is plectin?
Plectin is an accessory protein that bundles intermediate filaments to other cytoskeletal
elements throughout the cell.
What is the nuclear lamina? What is it composed of? What are its functions? What is
Hutchinson-Gilford Progeria? What are the nuclear defects in this disease?
The nuclear lamina is composed of intermediate filaments called lamins. The nuclear
lamina lies directly beneath the inner nuclear membrane and serves to strengthen the
nuclear envelope. Defects in the nuclear lamina cause progeria, a disease that causes
premature aging. The nuclear instability caused by the disruption of the nuclear lamina
leads to impaired cell division, increased cell death, altered chromosome movement, and
diminished cell repair.
What are the KASH-family and SUN-family proteins and what do they do?
KASH and SUN proteins link the cytoplasmic cytoskeleton to the nuclear interior,
including chromosomes and the nuclear lamina.
1.What are the three cytoskeleton components? Describe each comparing relative sizes and
where they are primarily located in the cell. 2. How are they each formed?
1. MICROTUBULES: Hollow cylinders with a diameter of about 25 nm. They
extend outward through the cell interior, creating a system of tracks in which
organelles, transport vesicles, and other components can be transported.
Microtubules are composed of a & b tubulin subunits. Main function is cell
transportation and organization.
INTERMEDIATE FILAMENTS: Ropelike fibers with a diameter of about 10 nm.
Intermediate filaments are constructed from fibrous intermediate filament proteins;
a. keratin filaments in epithelial tissue, b. vimentin and vimentin related filaments in
connective tissue, muscle cells, and glial cells, c. neurofilaments in nerve cells, d.
nuclear lamins in the nucleus. Intermediate filaments are found in the cytoplasm,
where they form a network surrounding the nucleus and extending throughout the
cell periphery. They are also found inside the cell’s nucleus, where they form the
nuclear lamina, which strengthens the nuclear envelope. Main function is cell
support (tensile strength).
ACTIN FILAMENTS (MICROFILAMENTS): Helical polymers with a diameter of
about 7 nm. Microfilaments are composed of actin proteins and are found mostly in
the cell cortex (the layer of cytoplasm just beneath the plasma membrane). Main
function is cell movement.
2. Intermediate filaments are formed via polymerization of intermediate filament protein
subunits. An a-helical monomer entwines with another of the same monomer, forming a
parallel coiled coil dimer. Two dimers entwine, creating a staggered antiparallel tetramer. 8
tetramers join, creating the subunit that will be added to the growing intermediate
filament. Intermediate filaments do NOT show polarity at their ends, unlike microfilaments
and microtubules.
Monomer---coiled coil dimer----staggered, antiparallel tetramer---8 tetramers-----add to
end of growing intermediate filament.
Microtubules grow out of organizing centers: a. in nondividing cells the organizing center is
the centrosome b. in dividing cells the organizing center is the mitotic spindle c. in ciliated
cells the organizing center is the basal body. Microtubules are formed from a & b tubulin
subunits that stack together. 13 protofilaments interact to form a microtubule.
Microtubules do show structural polarity, with the b tubulin subunits on the + end and the
a subunits on the – end, with the - end embedded in the centrosome. Inside the centrosome,
the y-tubulin ring complex serves as the nucleation, or starting point, for microtubule
growth.
Actin filaments consists of a twisted chain of actin monomers. Actin filaments have
structural polarity, with a + and – end. Actin filaments can grow by the addition of the
actin monomers to either end, but growth is faster at the + end. Each actin monomer
carries a molecule of ATP that is hydrolyzed when the monomer is added to the growing
microfilament. The hydrolysis of ATP promotes depolymerization. Actin filaments undergo
“treadmilling” where the addition of monomers at the + end is equal to the loss of
monomers at the – end, therefore the filament will stay at the same length.
, In which cytoskeleton component are the following proteins found: vimentin, keratin, -tubulin,
actin, -tubulin, lamin, neurofilament, -tubulin.
Intermediate Filaments: vimentin, keratin, lamin, neurofilament
Actin Filaments: actin
Microtubules: b-tubulin, y-tubulin, a-tubulin
What are the four major classes of intermediate filaments? In what types of cells are they found?
1. Keratin filaments: epithelial cells
2. Vimentin and vimentin related filaments: connective, muscle, and glial cells
3. Neurofilaments: nerve cells
4. Nuclear lamins: nucleus
Keratin filaments, vimentin & vimentin related filaments, and neurofilaments are
cytoplasmic. Nuclear lamins are found inside the nucleus.
How do keratin fibers support epithelial cells? What happens if there are mutations in keratin
genes?
Keratin filaments are the most diverse class of intermediate filaments and are found in
every kind of epithelium. Keratin filaments interact cell to cell through desmosomes and
connect to the basal lamina through hemidesmosomes, distributing the stress put onto the
cells when they are stretched. Those with the disease epidermolysis bullosa simplex have
skin that is vulnerable to mechanical damage due to mutations that affect keratin filament
formation.
What is plectin?
Plectin is an accessory protein that bundles intermediate filaments to other cytoskeletal
elements throughout the cell.
What is the nuclear lamina? What is it composed of? What are its functions? What is
Hutchinson-Gilford Progeria? What are the nuclear defects in this disease?
The nuclear lamina is composed of intermediate filaments called lamins. The nuclear
lamina lies directly beneath the inner nuclear membrane and serves to strengthen the
nuclear envelope. Defects in the nuclear lamina cause progeria, a disease that causes
premature aging. The nuclear instability caused by the disruption of the nuclear lamina
leads to impaired cell division, increased cell death, altered chromosome movement, and
diminished cell repair.
What are the KASH-family and SUN-family proteins and what do they do?
KASH and SUN proteins link the cytoplasmic cytoskeleton to the nuclear interior,
including chromosomes and the nuclear lamina.
