Chapter 17- Cytoskeleton Flashcards
Three types of protein filaments in the cytoskeleton
Intermediate filaments, Microtubules, actin filaments
are the most stable filaments
In termediate filaments (IFs)
organize and interconnect tissues, protect against mechanical stress, serve as scaffolds for signaling molecules
Intermediate filaments
IF subunits form what kind of tetramers
Anti parallel and staggered
In the IF subunit, there’d a conserved rod domain that’s involved in
Assembly
__ tetramers assemble into __ 10 nm filament
8, non-polarized
IFs are relatively __
Stable
most resistant to extraction
of the cytoskeletal filaments
IF
Four major classes of IF
Cytoplasmic: keratin, vimentin, neurofilaments
Nuclear: nuclear lamins
Origin of tumor cells can be identified by
IF type
What class of IF protect epithelial cells from mechanical damage
Keratins and junctions
What IF class are found in other tissues (connective tissue, muscle cells, and glial cells)
Vimentins
What IF class support axons in nerve cells
Neurofilaments
What IF class forms the nuclear lamina that supports nuclear envelope
Lamins
Intermediate filaments protect cells against mechanical stress, most obvious in the
Skin by keratins
form basket-like arrays that bind to desmosomes and
connect neighboring cells
Keratins
Defects in keratins or junction proteins lead to
Cell rupture
In an IF subunit, unstructured domains at__ termini confer specific functions
Amino and carbonyl terminal (2 n and 2 c terminal ends)
intercellular junctions that link the keratin filaments of one cell to those in a neighboring cell
De smosomes
type of skin blistering disease, caused by a defect in keratin expressed in bottom layer of skin
EBS
Nuclear lamins form an IF network __ the nucleus
Inside
Nuclear lamins form a
2D mesh work on nuclear envelope
disassembles the network on nuclear envelope, driving nuclear envelope breakdown during mitosis
Phosphorylation of lamins by kinases
Dephosphorylation of nuclear lamins leads to
Reassembly
You wish to study the assembly of the nuclear lamina in cells. Would the following conditions be expected to enhance or disfavor assembly of the nuclear lamina?
(a) Addition of protein kinase inhibitors
(b) Addition of phosphatase inhibitors
A. Enhance: Phosphorylation of lamins drives disassembly of
the nuclear lamina, so inhibiting the kinases that
phosphorylate lamins will enhance assembly
B. Disfavor: Dephosphorylation of lamins promotes reassembly
of the nuclear lamina, so inhibiting the phosphatases that
dephosphorylate lamins will disfavor assembly
Cadherins inside of the
Desomosome
Desomosome binds our
Cadherins and keratin filaments
hollow tubes with structurally distinct ends, organizing the interior of the cell
Microtubules
The __ is the major MT-organizing center
centrosome
networks are maintained by assembly & disassembly
Microtubules
Growing microtubules exhibit
dynamic instability
Motor proteins drive…
intracellular transport
Cilia & flagella contain MTs moved by
dynein
Largest, most rigid filaments
Microtubules
Primary intracellular railroad track, Primary determinant of cell polarity
Microtubules
The building block of MTs is the…
a/b tubulin heterodimer
MTs are hollow tubes made of __ protofilaments
13
linear chain of tubulin dimers
protofilament
MTs have what kind of ends?
plus (fast-growing) &
minus (slow-growing) ends
In most animal cells, the centrosome organizes an
array of MTs that…
radiates outward through the
cytosol
Centrosomes are duplicated to form the
mitotic spindle poles
Microtubule-organizing centers form __ at the base of cilia and flagella
basal bodies
Microtubule-organizing centers (MTOCs) bind to __ ends of the Microtubules, allowing __ end to extend outward
Minus ends, plus ends
is a MT-organizing center (MTOC)
Centrosome
The centrosome contains __ centrioles and many ____
2, gamma- tubulin ring complexes ( yTURC)
yTURC binds __ ends of tubulin subunits and promotes __ of MTs
minus, assembly
Growing microtubules show
dynamic instability, shrinks independently of its neighbor
enables the cell to continuously monitor its environment
Dynamic instability
Dynamic instability results from
GTP hydrolysis
Addition of GTP-tubulin at the __ end of the MT forms a ____
plus, stable GTP cap
Microtubles with __ tubulin caps depolymerize rapidly
GDP
GDP-tubulin dimers must exchange their __ for __ before they can be
added to a growing MT
GDP for GTP
MT capping proteins stabilize what ends
Plus
__ of MTs can drive changes in cell shape and allow microtubules to form long distance tracks for transport
Selective stabilization
A polarized MT array (minus ends in the cell body; plus ends pointing
towards the axon terminals) provides…
Tracks for transport
Cargoes interact with __, which move them along MTs
motor proteins
__ walk to the MT plus end,
___ walk to the minus end
Kinesins, dyneins
Globular head domains (also called “motor domains”) bind
ATP & MT
Both kinesin and dynein are members of large gene families with __
motor domains & __ tail domains
Conserved, variable
Defects in MTs & motor proteins are linked to
Nerve degeneration
drives extension of the ER network along MT network
Kinesin
places Golgi close to the centrosome
Dynein
Cilia & flagella contain stable MTs moved by
ciliary dynein
move fluid or mucus over the surface of epithelia
move individual cells through fluid
Cilia
Flagella
Cilia and flagella share a conserved structure:
9+2” MT array
Arrangement of ciliary dyneins creates inner and outer __ arms
dynein
Without linkages between neighboring MT doublets, MTs ___
With linkages, MTs __
Slide apart, bend
A disease that results from defects in ciliary dyneins
Kartagener’s syndrome
Will the following scenarios promote microtubule growth?
(a ) Addition of a drug that inhibits hydrolysis of the GTP carried out
by tubulin dimers
(b) Addition of a drug that inhibits exchange of GDP for GTP by
tubulin dimers
(c) Addition of a drug that increases the affinity of GDP-tubulin
dimers for other tubulin dimers
A. Ye s – Inhibition of GTP hydrolysis promotes the formation of a GTP cap. Microtubules with GTP caps will continually elongate.
B. No – Tubulin dimers must exchange their GDP for GTP before being added to a microtubule plus end.
C. Yes – Increasing the affinity of GDP-tubulin for other tubulin dimers will prevent the dissociation of GDP-tubulin dimers from microtubule plus ends in the event of loss of the GTP cap.
filaments are thin and flexible
Actin
are cross-linked to form stable bundles that support
membranes (microvilli in the gut, stereocilia in the inner ear)
Actin
Actin filaments are structurally
asymmetric and…
polar
Actin filaments have what end
plus end
(fast assembly) and a minus end
(slow assembly)
ATP-actin adds preferentially to the
plus end
Actin hydrolyzes its bound ATP soon after being
incorporated into the filament
Actin monomers add to the __ end faster than ATP is hydrolyzed, so the…
plus, plus end grows
At the __ end, ATP is hydrolyzed faster than new monomers can be added,
so the __ end depolymerizes (In actin)
minus
How does actin and microtubules differ with plus and minus ends
Microtubules can grow and shrink at the plus end, but in actin the plus end only grows
Cells initiate and control actin filament assembly with
nucleating factors (e.g. formin, ARP complex)
In cells, the actin concentration is very
high
What uses “monomer-sequestering” proteins (e.g. profilin, thymosin) that bind to __ monomers and prevent them from adding to the ends of actin filaments.
actin
Often thicker & more complex than the cortex of red blood cells
actin-rich cortex
Cell locomotion depends on _ different actin
networks
three
actin polymerization in the lamellipodium (i.e. the “leading edge”)
Dynamic actin network
actin arrays making focal contacts with the extracellular matrix through integrins
Stabilized actin network
actin networks pulling up the rear of the cell
Contractile
Forward movement of the lamellipodium depends on
branched actin polymerization
As filaments “age”, actin undergoes
ATP hydrolysis
ADP-actin is disassembled at the
minus end
Actin monomers add to filament _ ends
plus, pushing membrane forward
binds to the side of an existing actin filament and nucleates a
new filament, which grows out at an angle, making a “branch”
ARP complex
assemble actin filaments found in filopodia
Formins
Formin promotes the assembly of ___ filaments
straight, unbranched
Formins bind to the actin filament __ end
plus
are transmembrane proteins that bind to the extracellular
matrix and connect it to the actin cytoskeleton at focal contacts
Integrins
Actin attachment to substrate by
focal contacts
Contractile filaments bring up the…
rear of the cell
Stable actin filaments interact with
myosin II
a motor protein that slides actin filaments to generate contraction
myosin II
Extracellular signals control the arrangement of actin
filaments using
monomeric GTPases
promotes assembly of contractile networks
Rho
promotes assembly of branched actin networks (using ARP complex)
Rac
promotes assembly of straight actin filaments (by activating formin)
Cdc42
Myosin I found in
Membrane binding
Myosin II found in
muscle & contractile fibers
Myosin II is a
dimer
Myosin heads walk towards the _ end of actin filaments
plus
Muscle contraction depends on _
bundles of actin & myosin II filaments
organized
Muscle contraction is triggered by a sudden rise in the intracellular
Ca2+ concentration
a string of sarcomeres
myofibril
The cytoplasm of muscle cells is filled with organized
arrays of contractile proteins called
myofibrils
the basic unit of contraction
sarcomere
Motor nerve releases
acetylcholine
T-tubules extend inward from plasma membrane and contact the
sarcoplasmic reticulum
VG channels of calcium are in the __ and release channel in __
T tubule, SR
In the absence of Ca2+, __ binds along the actin filaments & blocks
myosin from binding
tropomyosin
In contraction sarcomere shortens by
1 micrometer
Which of the items below doesn’t describe something similar about the polymerization mechanisms of actin and microtubules?
A. The rate of subunit addition is faster at the plus end than at the minus end
B. Depolymerization initiates at the plus ends of filaments
C. Nucleotide hydrolysis promotes the depolymerization of filaments
D. Free subunits (actin and tubulin) bind nucleoside triphosphates
B (actin doesn’t depolymerize)
