Exam 4 - Lecture 7 Flashcards
The cytoskeleton is a_______and plays a role in _______? Two features are_________.
network of
interconnected filaments and tubules
extending through the cytosol
It plays roles in cell movement and division
It is dynamic and changeable
The major structural
elements of the
cytoskeleton are
Microtubules
Microfilaments
Intermediate
filaments
Microtubules are
composed of
tubulin
subunits and are about
25 nm in diameter
Microfilaments subunits and size
7 nm
wide, are composed of
actin subunits
Intermediate filaments
-size and composition
8–12 nm, are variable in
composition
what is a Mechanically Integrated Structure
cytoskeleton
MTs resist
bending when a cell is compressed
MFs serve as
contractile elements that generate tension
IFs are
elastic and can withstand tensile forces
connect IFs, MFs, and MTs
linker proteins
Example: plectin, found at sites
where intermediate filaments connect to Mts and MTF
the largest structural
elements of the cytoskeleton
Microtubules (MTs)
They are involved in a variety of functions in the
cell
what are the two main types of microtubules
Cytoplasmic microtubules
Axonemal microtubules
Cytoplasmic microtubules
pervade the cytosol and
are responsible for a variety of functions:
Formation of mitotic and meiotic spindles
Maintaining or altering cell shape
Placement and movement of vesicles
Axonemal microtubules
Axonemal microtubules include the organized and
stable microtubules found in structures such as Cilia
and Flagella
Describe the strucutre of MTs
MTs are straight, hollow cylinders of varied length that consist of
longitudinal arrays of polymers called protofilaments
The basic subunit of a protofilament is a heterodimer of tubulin, one
α-tubulin and one β-tubulin
These bind non-covalently to form an αβ-heterodimer, which does
not normally dissociate
Microtubules can form as Singlets, Doublets, or Triplets
Describe Microtubule Assembly Graph
MT formation is slow at first
because the process of
nucleation is slow; this period
is known as the lag phase
The elongation phase is
much faster
Plateau phase: the mass of
MTs reaches a point where the
amount of free tubulin is
diminished.
Describe the lag phase of MT assembly
MT formation is slow at first
because the process of
nucleation is slow; this period
is known as the lag phase
Drugs Affecting Cytoskeleton
Colchicine
Nocodazole
Colchicine
binds to tubulin monomers, inhibiting their assembly into MTs
and promoting MT disassembly
Vinblastine, vincristine are related compounds
Nocodazole
inhibits MT assembly, and its effects are more easily reversed
than those of colchicine
MTs originate from
a microtubule-organizing
center (MTOC)
Many cells have an MTOC called a
centrosome near the nucleus
In animal cells, the centrosome is associated
with
two centrioles surrounded by
pericentriolar material
Centriole walls are formed by
9 pairs of triplet
microtubules
oriented at right angles to each other
involved in basal body formation for cilia and flagella
cells without centrioles have poorly organized mitotic
spindles
describe γ-Tubulin
Centrosomes have large
ring-shaped protein
complexes in them; these
contain γ-tubulin
γ-tubulin is found only in
centrosomes
proteins promote depolarization of MTs
Stathmin/Op18
Catastrophins
Microtubule Stability - MAPs
Cells regulate MTs with great precision
Others regulate MT structure
MAPs, microtubule-associated proteins, bind along a
microtubule wall, allowing for interaction with other cellular
structures and filaments
Some MT-binding proteins use ATP to
to drive vesicle or
organelle transport or to generate sliding forces between MTs
Stathmin/Op18
depolarizes MTs by binding to tubulin heterodimers and prevents
their polymerization
Catastrophins
depolarizes Mts by acting at the ends of MTs and promote the
peeling of subunits from the ends
Microfilaments
1. size
2. best known role
3. 3 main involvements
are the smallest of the
cytoskeletal filaments
Best known for their role in muscle
contraction
Involved in cell migration, amoeboid
movement, and cytoplasmic streaming
What is actin?
What are the various types?
the Protein Building Block of Microfilamens
Actin is a very abundant protein in all
eukaryotic cells
Actins can be broadly divided into
muscle-specific actins (α-actins) and
nonmuscle actins (β- and
γ-actins)
actin assembly
Once synthesized, it folds into
a globular-shaped molecule
that can bind ATP or ADP (G-
actin; globular actin)
G-actin molecules polymerize
to form microfilaments: F-actin
All the actin monomers in the
filament have the same
orientation
Drugs Affecting Microfilaments
Cytochalasins
Latrunculin A
Cytochalasins
are fungal metabolites that prevent
the addition of new monomers to existing MFs
Latrunculin A
is a toxin that sequesters actin
monomers and prevents their addition to MFs
Actin-Binding Proteins role + example
Actin-binding proteins: used
by cells to precisely control
where actin assembles and
the structure of the resulting
network
Example: Capping
proteins bind the ends of a
filament to prevent further
loss or addition of subunits
Microvilli
Actin bundles in microvilli
are the best-studied
examples of ordered actin
structures
Microvilli are prominent
features of intestinal
mucosal cells; they increase
the surface area of the cells
The core of a microvillus
consists of a tight bundle of
MFs with the ends pointed
toward the tip
Intermediate filaments (IFs)
where are they found?
Common example
stability and solubility?
important function
are not
found in cytosol of plant cells but are
abundant in many animal cells
An IF is keratin, an important
component of structures that grow from
skin in animals
IFs are the most stable and least soluble
components of the cytoskeleton
They likely support the entire
cytoskeleton
what is unique about IF proteins
IF Proteins Are Tissue Specific
IFs differ greatly in amino acid composition from tissue to tissue
6 classes of IFs
class 1 - 6
describe class 1
Class I: acidic keratins
describe class 2
Class II: basic or neutral keratins
* Proteins of classes I and II make up the keratins found in epithelial
surfaces covering the body and lining its cavities
class 3
connective tissue, muscle cells, and glial cells
class 4
nerve cells
class 5
inner surface of the nuclear membrane
class 6
nerve cells of embryos
IF assembly
The fundamental subunits of
IF proteins are dimers
IF proteins are fibrous rather
than globular
Each has a homologous
central rodlike domain
Flanking the central helical
domain are N- and C-
terminal domains that differ
greatly among IF proteins
IFs Assembly
Two IF polypeptides
intertwined into a coiled-coil
Two dimers align laterally to
form a tetrameric protofilament
Protofilaments overlap to build
up a filamentous structure
about eight protofilaments thick
Cell motility can be
Movement of a cell or organism through the
environment
Movement of the environment past or through a
cell
Movement of components in the cel
Contractility
used to describe shortening of
muscle cells, is a specialized form of motility
Two Eukaryotic Motility Systems
Microtubule-based
motility
Microfilament-based
motility
Microtubule-based
motility example
examples: fast axonal transport
in neurons; the sliding of MTs in
cilia and flagella
Microfilament-based
motility example
Example: muscle contraction
Microtubule-Based Movement Inside Cell requires what two proteins and whaat is the role of MT
Kinesins and Dyneins
MTs provide a rigid set of tracks for transport of a variety of
organelles and vesicles
Microtubule-associated motor proteins…
Microtubule-associated motor proteins—kinesins and
dyneins—walk along the MTs and provide the force needed for
movemen
Kinesin I
is involved in ATP-dependent transport
toward the plus ends (away from the centrosome),
called anterograde axonal transport
Cytoplasmic dynein
moves particles (cargo) in the
opposite direction, called retrograde axonal transport
Kinesin Movement Along MTs
Kinesin movement looks like “walking,”
with the two globular head domains
taking turns as the front foot
It can move long distances along an MT
before detaching from it by releasing
bound ADP and acquiring a new ATP,
so that the cycle repeats
ATP-dependent motors,
the large superfamily
called
myosins
ATP-dependent motors =
myosins
the large superfamily
myosins, interact
with and exert force on
actin microfilaments
Currently there are 24
known classes of myosins
Myosin Functions
Myosins function in a wide range of cellular events,
including
Muscle contraction
Cell movement
Phagocytosis
Vesicle transport
Muscle contraction
is the most familiar example of mechanical
work mediated by intracellular filaments
Mammals have what types of muscle
skeletal, cardiac, and smooth muscle
Skeletal Muscle Cells contain what two filaments?
muscle fibers contain thin
filaments containing actin
and thick filaments
containing myosin
Thick Filaments
Each thick filament consists of
hundreds of molecules of myosin,
oriented in opposite directions in
the two halves of the filament
The myosin is arranged in
staggered fashion
Protruding heads of myosin
molecules contact the adjacent
thin filaments, forming cross-bridges
Thin Filamentsand the three proteins it contains
Thin filaments interdigitate (interlock) with the thick
filaments
Thin filaments contain three proteins: F-actin,
intertwined with tropomyosin and troponin
The Sliding-Filament Model Explains Muscle
Contraction
The sliding filament model was proposed in 1954
According to the model, muscle contraction is due to thin
filaments sliding past thick filaments, with no change in length
of either
give some functions of MTs, MFS, IF
MT ( varies per type, but Axonemal = cell motility, and cytosoolic varies)
MF - muscle contractions, cell locomotion, cyoplasmic steaming, cell shape, intracellulat transport
IF - structural support, animal cell shape, stregth of nerve cell axons
