Cytoskeleton (Exam 1) Flashcards
What are the 3 specific kinds of protein structures that make up the cytoskeleton
Microfilaments (actin) (7nm thick)
Intermediate filaments (8-10 nm thick)
Microtubules (25nm in diameter)
Actin filaments are highly _______ among eukaryotes
conserved
how tick are actin filaments
7nm thck
how long may actin filaments be
may be up to 7 micrometers long
actin filaments are organized into ______ and _____ networks
Bundles, 3-D
Actin filaments bind to specific transmembrane proteins either ____ or ____
directly, indirectly
Actin filaments exist in what two forms
Monomers (G-actin) and long chains (F-actin)
What are the three major varieties of actin filaments
alpha actin (found in muscle tissue) Beta actin (non-muscle actin) Gamma actin (non-muscle actin)
each actin monomer (G actin) has a binding site for ____, which binds tightly to G-actin
ATP
Each actin monomer can bind tightly with two other actin monomers to form what
Filamentous actin (F-actin)
Actin monomers are oriented in the same direction and thus display
Polarity
Actin _______ is reversible
Polymerization
The rate at which actin monomers are aded to the growing filament is proportional to what
cytosolic concentration of actin filaments
what type of actin dissociates more readily from filaments than ATP-actin
ADP-actin
what is the first step in actin polymerization
Nucleation: a trimer is formed
additional actin monomers can be added to either end
ATP -actin associates with which end of actin
The growing (plus or barbed) ends
The ATP is hydrolyzed to _____ following polymerization
ADP
The barbed end grows how much faster than the pointed end
5-10 times faster
Very low concentration of G-actin favor the ____ of actin filaments
disassembly
Intermediate concentrations of G-actin favor
Treadmilling
- dynamic equilibrium between the minus end and the plus end
- results in zero net growth
Actin microfilaments consist of a ____ _______ chain of G-actin subunits
Double helical
name two drugs that affect actin polymerization
cytochalasin and Phalloidin
Cytochalasin role in actin polymerization
drug that inhibits elongation
can inhibit movements (cell division)
Binds to the barbed ends of actin
What is Phalloidin’s role in actin polymerization
Binds to actin filaments and prevents dissociation
can be labeled with fluorescent dyes to allow visualization of actin filaments
prevents depolymerization by binding to actin filaments
what determines the different functions of actin
Actin-binding proteins and not actin itself
Spectrin
is an actin binding protein
found in RBCs
binds cortical cytoskeleton to the plasma membrane
Dystrophin
Actin binding protein
binds cortical cytoskeleton to the plasma membrane
(defects can lead to muscular dystrophy)
Villain and Fimbrin
Actin binding proteins
cross-link actin filaments in microvilli
Calmodulin and Myosin I
Actin binding proteins
Cross-links actin to plasma membrane in microvilli
alpha actin
actin binding protein
cross-links stress fibers and connects actin to protein-plasma membrane complexes
Filamin
Actin binding protein
Cross-links actin at wide angles to form screen-like gels
Thymosin
Actin binding protein that controls treadmilling
captures actin monomers and prevents actin monomers from being polymerized
Profilin
Actin binding protein that controls treadmilling
binds to actin monomers
facilitates exchange of bound ADP for ATP- which favors polymerization
increases actin growth
Gelsolin
Actin binding protein that controls treadmilling
Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin
in presence of calcium ion, fragments actin filaments and remains bound to plus end
Cofilin
Actin binding protein that controls treadmilling
Triggers depolymerization of ADP- bound Actin at the minus end
Arp 2/3
Actin binding protein that controls treadmilling
initiates growth of F-actin from sides of existing filament- causes branching
Latrunculins
Actin binding protein that controls treadmilling
binds to G-Actin and induces F-actin depolymerization
how thick are intermediate filaments
8-10nm
where are intermediate filaments most abundant
in cells that are subject to mechanical stress
What is the function of intermediate filaments
Provide tensile strength in cells such as neurons and muscle
strengthen epithelial cells as desmosomes and hemidesmosomes
form a cytoplasmic network in most cells
associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell
What structural component do all intermediate filaments have
A common monomer consisting of a central alpha-helical rod flanked by head and tail domains
what determines the specific functions of intermediate filaments
Head and tail domains
Describe intermediate filament structure
Central rod of two polypeptides form a cold dimer
- Rods are aligned tail-to-tail and head-to-head
-dimers associated in a staggered antiparallel fashion to form tetramers
tetramers assemble end to end to form protofilaments
pairs of protofilaments associate laterally to form a protofibril
4 protofibrils are wound together to form filaments (8 protofilaments)
Which are more stable actin filaments or intermediate filaments
intermediate filaments
because dimers align in antiparallel fashion, polymerized filaments do not have distinct ends (no treadmilling)
Type I Intermediate filaments
Acidic Keratins
Type II intermediate filaments
Neutral to basic keratins
Type III intermediate filaments
Vimentin
Desmin
Glial fibrillary acidic protein
Peripherin
Type IV Intermediate filaments
Neurofilaments
Type V intermediate filaments
Nuclear lamins (associated with nuclear membrane and assembly of nuclear membrane)
Type VI intermediate filaments
Nestin
How large is the diameter of microtubules
25 micrometers
What is a structural characteristic across microtubules
alpha and beta tubulin
Microtubules consist of how many protofilaments
13
how are the 13 protofilaments arranged
parallel to form a cylinder with a hollow core
How are microtubules similar to actin
they have a + and - end (display polarity)
What tubule dimers with GTP bound associate with the growing end of a microtubule
Beta tubulin
The plus end of a microtubule grows more rapidly in the presence of what
low calcium ion concentration
what destabilizes a microtubule
hydrolysis of GTP to GDP
which Tubulin subunit faces the plus end
The beta tubulin
Factors that inhibit microtubule polymerization
Colchicines (originally used to arrest mitotic cells, would arrest in metaphase)
Colcemid
Vincristine (anticancer drug)
Vinblastin (anticancer drug used to treat Hodgkin’s lymphoma)
Nocodazole
Factors that stabilize microtubules
Taxol (anticancer drug- used to treat ovarian cancer)
blocks mitotic division by not allowing formation of mitotic spindle
Functions of the cytoskeleton
cell movement support and strength for the cell phagocytosis Mitotic spindle formation Cytokinesis Cell-to-Cell and cell-to-extracellular matrix adherence changes in cell shape
Anterograde
Transports cargo along microtubule
mediated by Kinesin (toward the plus end)
Retrograde transport
Dynein
runs toward minus end or back to axon hillock
what does myosin I bind to
Actin and cell membrane
how many heads does myosin I have
one
What does the tail of myosin I bind to
Vesicles
How many heads does Myosin V have
Two heads
What does the heads of myosin V bind to
Actin
What do the tails of myosin V bind to
Vesicles
How many heads does myosin II have
Two heads
what does the tails of myosin II bind to
Myosin
What do the heads of myosin II bind to
Actin
