Module 6: The Cytoskeleton (Myosin and Actin, Microtubules) Flashcards
What enables the formation of contractile structures through the action of myosin motor proteins?
myosin and actin
What was the first motor protein identified? What does it generate?
- Skeletal muscle myosin
- Force for muscle contraction
- A globular head domain containing the force-generating machine.
- Composed of two heavy chains and two copies of each light chain.
Myosin II
This mediates an amino acid sequence forming an extended coiled-coil.
Myosin II; a process
heavy-chain dimerization
dimerization-joining two identical/similar molecular entities by bonds
Where do the light chains bind in myosin II?
Close to the N-terminal head
How are myosin heads oriented?
In opposite directions
Bind and hydrolyze ATP to walk toward the plus end of an actin filament.
myosin head
What makes the filament efficient at sliding?
opposing orientation of the myosin heads
What results from ATP-driven sliding of highly
organized arrays of actin filaments against arrays of myosin II?
muscle contraction
What uses structural changes in their ATP-binding sites to produce cyclic interactions with a cytoskeletal filament?
motor proteins
Propels motor proteins forward in a single direction to a new binding site along the filament (3)
processes
ATP binding, hydrolysis, and release
What generates each step of movement along actin which generated by swinging it?
it is 8.5 nm-long
lever arm
a piston-like helix which connects movements at the ATP-binding cleft in the head to small rotations
converter domain
Changes in the __ of the myosin are coupled to changes in its __ for actin. Allowing the __ to release its grip on the __ at one point and snatch hold of it again at another
- conformation
- binding affinity
- myosin head
- actin filament
What produces a single step of movement during the mechanochemical cycle? (3)
- Nucleotide binding
- nucleotide hydrolysis
- phosphate release
- ATTACHED: The __ is tightly locked onto an __ in a rigor configuration, which is short-lived in active muscles.
- RELEASED: ATP binds to the __, reducing its affinity for __ and causing it to detach.
- COCKED: ATP binding causes a conformational change, swinging the __ outward. ATP is hydrolyzed, but __ and __ remain bound.
- RE-BINDING AND POWER STROKE: The __ binds weakly to __, releases __, and undergoes a power stroke, losing its bound __.
- FORCE GENERATING: The cycle ends with the __ locked onto a new position in a __ configuration.
The cycle of structural changes used by myosin II to walk along an actin filament.
- myosin head; actin filament
- myosin head; actin
- lever arm; ADP; inorganic phosphate (Pi)
- myosin head; actin; inorganic phosphate (Pi); ADP
- myosin head; rigor
rigor means accurate
What are the steps in the cycle of structural changes used by myosin II to walk along an actin filament? (5)
- Attached
- Released
- Cocked
- Re-binding and Power Stroke
- Force Generating
- are highly specialized for rapid and efficient contraction
- form by the fusion of many separate cells into huge single cells
what type of cells?
muscle cells
What makes up the bulk of the cytoplasm inside muscle cells?
- It is a cylindrical structure 1–2 μm in diameter that is often as long as the muscle cell itself.
Myofibrils
What do myofibrils consist of?
- It is a long, repeated chain of tiny contractile units.
- About 2.2 μm long; gives the vertebrate myofibril its striated appearance
Sarcomeres
Consists of parallel and partly overlapping thin and thick filaments
Sarcomeres
- actin and associated proteins
- attached at their plus ends to a Z disc
- minus ends overlapped with the thick filaments
thin filaments
composed of actin
- To what are the plus ends of thin filaments attached?
- Where do the minus ends of thin filaments overlap?
- Z-disc
- thick filaments
2) myosin
What are thick filaments composed of?
myosin
The arrangement of actin and myosin filaments within the sarcomere is __ with __ filaments evenly spaced between the __ filaments.
- hexagonal lattice
- actin
- myosin
Myosin filaments sliding past the actin thin filaments causes…
sarcomere shortening
Does sarcomere shortening involve changes in the length of the filaments?
No, the length of the thin and thick filaments does not change.
Rise in cytosolic Ca²⁺ concentration initiates…
muscle contraction
What passes the signal to skeletal muscle?
muscle contraction and signaling
The nerve that stimulates it
What are the two features of muscle cells enabling rapid contraction? (2)
add short description
- Myosin motor heads coupled to ATP binding and hydrolysis
- Specialized membrane system for rapid signal relay
What specialized structures are involved in relaying the signal from the nerves to trigger an action potential in muscle cells? (2)
muscle contraction and signaling
- T tubules (transverse tubules)
- sarcoplasmic reticulum
What does the nerve signal trigger in the muscle cell?
muscle contraction
action potential
Where does the action potential travel in muscle cells?
muscle contraction
T (transverse) tubules
- Ca2+ __ triggers the opening of __ in the __
- Ca2+ flooding into the __ then initiates the contraction
muscle contraction and signaling
- Ca2+ influx
- Ca2+ release channels
- sarcoplasmic reticulum
- cytosol
Ca2+ dependence on muscle contraction is due entirely to a set of __.
specialized accessory proteins
An elongated protein that binds along the groove of the actin filament helix
muscle contraction
tropomyosin
A complex of three polypeptides: T, I, and C
muscle contraction
troponin
Pulls tropomyosin out of its normal binding groove, interfering with myosin head binding
muscle contraction
troponin I-T complex
When Ca²⁺ levels rise,
1. __ binds up to four molecules of Ca²⁺
2. __ releases its hold on actin
muscle contraction
- Troponin C
- Troponin I
What triggers contraction in smooth muscle cells?
Influx of calcium ions (Ca²⁺)
What kind of contractions do smooth muscle cells require? (2)
Slow and sustained contractions
Highly elongated spindle-shaped cells with a single nucleus
muscle contraction
smooth muscle cells
elevated intracellular Ca2+ levels regulate contraction by a mechanism that depends on…
calmodulin
What does Ca²⁺-bound calmodulin activate?
Myosin light-chain kinase (MLCK)
Induces phosphorylation of smooth muscle myosin on one of its two light chains
muscle contraction
Myosin light-chain kinase (MLCK)
- What happens when light chains are phosphorylated?
- What happens when light chains are dephosphorylated?
smooth muscle contraction
- Smooth muscle contracts
- Smooth muscle becomes inactive
Which muscle is the most heavily worked in the body?
heart
What specific isoforms do cardiac muscles express? (2)
- cardiac muscle myosin
- cardiac muscle actin
What condition is a common cause of sudden death in young athletes?
- genetically dominant inherited condition
- heart enlargement, abnormally small coronary vessels, disturbances in heart rhythm
Familial hypertrophic cardiomyopathy
At the molecular level, point mutations in the genes encoding __ or mutations in other genes encoding __
Familial hypertrophic cardiomyopathy
- cardiac β myosin heavy chain
- contractile proteins
Associated with minor missense mutations in the cardiac actin gene
dilated cardiomyopathy
myopathy-diseases affecting muscles that control voluntary movement.
contain small amounts of contractile actin-myosin II bundles
non-muscle cells
How are actin-myosin II bundles regulated?
myosin phosphorylation
What are the functions of actin-myosin II bundles in non-muscle cells? (4)
- Mechanical support
- cytokinesis
- adhesion
- forward motion of migrating cells
Where is myosin I found?
an organism
Acanthamoeba castellanii
What types of myosin are included in the myosin family? (2)
- One-headed myosin
- two-headed myosin
How many distinct myosin families are there?
37
How many myosin genes are included in the human genome?
40
How many myosin genes are expressed in the hair cells of the inner ear? How many mutations are known to cause hereditary deafness?
- 9
- 5
What structure in the inner ear is associated with myosins?
Stereocilia
Intracellular organization, including microvilli and endocytosis
myosin I
- A two-headed myosin with a large step size; used for organelle transport along actin filaments.
- It moves processively along actin filaments without letting go
myosin V
- In this organism, actin cables in the mother cell point toward the bud, where actin is concentrated in patches at sites of cell wall growth
- myosin carry wide range of cargoes
- correct partitioning of organelles between mother and daughter cells
Saccharomyces cerevisiae
- Highly dynamic and play diverse and important roles in the cell
- Polymers of tubulin
- Found in all eukaryotic cells
Microtubules
a heterodimer composed of α-tubulin and β-tubulin
tubulin
What is the binding site in tubulin for?
One molecule of GTP
What condition can result from mutations in the human β-tubulin gene?
condition and what function is lost?
Paralytic eye-movement disorder due to loss of ocular nerve function
- A hollow cylindrical structure built from 13 parallel protofilaments
- Composed of αβ-tubulin heterodimers stacked head to tail and then folded into a tube
Microtubule
A microtubule is a hollow cylindrical structure built from 13 __. It is composed of __ stacked head to tail and then folded into a tube.
- parallel protofilaments
- αβ-tubulin heterodimers
Protein-protein contacts in microtubules occur along the __; the “top” of the __ molecule forms an interface with the “bottom” of the __ molecule in the adjacent heterodimer; lateral contacts include α–α and β–β.
- longitudinal axis
- β-tubulin
- α-tubulin
Where does the addition and loss of subunits occur in microtubules?
Almost exclusively at the ends
The subunits in each protofilament point in the __ direction; α-tubulins are exposed at the __ end and β-tubulins are exposed at the __ end.
microtubules
- same
- minus
- plus
What influences microtubule dynamics?
binding and hydrolysis of GTP
Where does GTP hydrolysis occur in microtubules?
Only within β-tubulin
What are the two forms of tubulin? (2)
- “T form” (bound GTP)
- “D form” (bound GDP)
In microtubule dynamics, GTP tubulin tends to __, while GDP-tubulin tends to __.
- polymerize
- depolymerize
polymerize-molecules combine to form larger molecules
What determines whether tubulin subunits at the end of a microtubule are in the T or D form? (2)
- relative rates of GTP hydrolysis
- tubulin addition
During a high rate of tubulin addition, the tip of the polymer remains in the __ form, creating a __.
microtubule dynamics
- T
- GTP cap
A high rate of __ or a decrease in __ cause a sudden change from T form to D form in microtubules.
- GTP hydrolysis
- tubulin addition
The rapid interconversion between a growing and shrinking state
microtubule dynamics
dynamic instability
A transition from growth to shrinkage
microtubule dynamics
catastrophe
A transition from shrinkage to growth
microtubule dynamic
rescue
Tubulin subunits with __ bound to the __ produces straight protofilaments that make strong and regular lateral contacts
- GTP
- β-monomer
The hydrolysis of __ to __ in tubulin causes a subtle conformational change in the protein, resulting in __
- GTP to GDP
- curvature/curve
In the microtubule structure, it constrains the curvature of the protofilaments, making the ends appear straight
GTP cap
When the terminal subunits of tubulin have hydrolyzed GTP, the __ is removed, causing the protofilaments to __.
- constraint (GTP cap)
- spring apart
What type of drugs inhibit microtubule functions? (2)
- polymer-stabilizing drugs
- polymer-destabilizing drugs
They cause microtubule depolymerization (2)
Drugs
- colchicine
- nocodazole
- binds to and stabilizes microtubules, increasing tubulin polymerization
- Used to treat cancers of the breast and lung
Drug
taxol
aka paclitaxel
What is a common effect of microtubule-depolymerizing and polymerizing drugs?
kill dividing cells
What is the concentration of tubulin subunits required for spontaneous nucleation of microtubules?
level
very high
Involved in the nucleation of microtubule growth at smaller amounts
γ-tubulin
A specific intracellular location where microtubule nucleation occurs
microtubule-organizing center (MTOC)
- What is the complex where γ-tubulin and two accessory proteins bind to create a spiral ring template for microtubule formation?
- How many protofilaments does a microtubule have, as created by this template?
- γ-tubulin ring complex (γ-TuRC)
- 13
A well-defined microtubule-organizing center (MTOC) located near the nucleus
centrioles
Microtubules are nucleated at their __ ends; __ ends point outward and continuously grow and shrink
centrosomes
- minus
- plus
A pair of cylindrical structures embedded in the centrosome, arranged at right angles in an L-shaped configuration; barrel shape with striking ninefold symmetry
Centrioles
Where does microtubule nucleation take place in the centrosome?
pericentriolar material
A microtubule-organizing center (MTOC) embedded in the nuclear envelope, found in budding yeast, fungi, and diatoms.
spindle pole body
Do fungi or plants have centrioles?
No
What do fungi and plants use to nucleate their microtubules?
γ-tubulin
Dynamic plus ends point outward toward the cell periphery, and stable minus ends are collected near the nucleus.
what configuration is this?
aster-like configuration
It has the ability to find the center of the cell, establishing a general coordinate system used to position many organelles within the cell.
microtubule cytoskeleton
Microtubules in cells exhibit a much higher __ rate, a greater __ frequency, and extended pauses in microtubule growth.
- polymerization
- catastrophe
modulate filament dynamics and organization.
microtubule
Microtubule-binding proteins
- proteins that bind to microtubules, stabilizing them against disassembly and mediating interactions with other cell components.
- prominent in neurons, specifically in axons and dendrites extending from the cell body.
- have at least one domain that binds to the microtubule surface and another that projects outward.
- targets of several protein kinases
microtubule-associated proteins (MAPs)
__ has a long projecting domain that forms bundles of stable microtubules that are widely spaced, while __ has a shorter projecting domain that forms bundles of more closely packed microtubules.
- MAP2
- tau
Proteins that bind the __ of microtubules influence the stability and dynamics of microtubules.
ends
- The frequency of __ refers to the transition from a growing to a shrinking state.
- The frequency of __ refers to the transition from a shrinking to a growing state.
- catastrophe
- rescues
bind to microtubule ends and prying protofilaments apart, promoting disassembly.
Catastrophe factors (kinesin-13)
protects microtubule minus ends from the effects of catastrophe factors.
Nezha/Patronin
enriched at microtubule plus ends, binds free tubulin subunits, and delivers them to the plus end, promoting polymerization and counteracting catastrophe factor activity.
XMAP215
stabilized by association with a capping protein or the centrosome and serve as depolymerization sites.
microtubules
minus ends
explore and probe the entire cell space.
microtubules
plus ends
Accumulate at the active plus (+) ends of microtubules and appear to move around the cell as passengers on rapidly growing microtubules, dissociating when microtubules shrink.
plus-end tracking proteins (+TIPs)
They modulate the growth and shrinkage of microtubules and control their positioning. (2)
- kinesin-related catastrophe factors
- XMAP215
a small dimeric protein that attaches to the plus end of microtubules, allowing the cell to harness the energy of polymerization for positioning spindles, chromosomes, or organelles.
EB1
Cells __ unpolymerized tubulin subunits to maintain a pool of active subunits.
sequester
isolate or hide away
- binds to two tubulin heterodimers and prevents their addition to the ends of microtubules, decreasing the effective concentration of tubulin subunits.
- Phosphorylation of this inhibits its binding to tubulin.
- expressed in the neurons of the amygdala.
microtubule dynamics
stathmin (Op18)
For a microtubule to be severed, __ longitudinal bonds must be broken, __ for each protofilament.
how many
- thirteen
- one
meaning “sword,” is made up of two subunits where the smaller one hydrolyzes ATP to perform the actual severing, while the larger subunit directs this to the centrosome.
microtubule dynamics
katanin
transport cargo and perform various functions along microtubules.
Motor proteins
What are the two main types of motor proteins associated with microtubules? (2)
- Kinesins
- Dyneins
- carries membrane-enclosed organelles away from the cell body toward the axon terminal by walking toward the plus end of the microtubule.
- Similar to myosin II; both have two heavy chains per active motor, and the motor domain is the common element.
- have 14 distinct families
- uses ATP hydrolysis
- have a binding site in the tail for another microtubule.
kinesin-1 (conventional kinesin)
How many distinct families are there in the kinesin superfamily?
14
Where is the motor domain located in kinesins, and what direction do they move? (2)
microtubule dynamics
- motor domain - N-terminus of the heavy chain
- kinesins move toward the plus end
In kinesin-1 function, __ is used to facilitate the movement and depolymerization of microtubule ends.
a reaction
ATP hydrolysis
- In kinesin-1 function, small movements at the nucleotide-binding site regulate the (1)__ and (2)__ of the motor head domain to a long linker region.
- Kinesins move 8 nm toward the (3)__ end, with cycles of linker (1)__ and (2)__ allowing for hand-over-hand stepwise movement.
- docking
- undocking
- plus
In kinesins, these cycles are closely coordinated to ensure efficient movement along the microtubule.
nucleotide-hydrolysis cycles
- minus-end directed motors
- consists of one, two, or three heavy chains and a large variable number of associated intermediate, light-intermediate, and light chains.
- the largest of known molecular motors and the fastest.
microtubule dynamics
Dyneins
homodimers made up of two heavy chains.
microtubule dynamics
cytoplasmic dynein
involved in organelle and mRNA trafficking, positioning the centrosome and nucleus during cell migration, and constructing the microtubule spindle.
microtubule dynamics
cytoplasmic dynein I
transports material from the tip to the base of cilia.
microtubule dynamics
Cytoplasmic dynein II
Specialized for rapid and efficient sliding movements of microtubules that drive the beating of cilia and flagella.
microtubule dynamics
Axonemal dyneins (ciliary dyneins)
The general rule regarding nucleotide hydrolysis in __, there is a coupling of nucleotide hydrolysis to microtubule binding and unbinding, as well as to a force-generating conformational change.
microtubule dynamics
dyneins
In the interphase of cells, they are responsible for the transport and positioning of membrane-enclosed organelles.
microtubule dynamics
cytoskeletal motors
- Kinesins are responsible for fast __, moving materials toward the cell’s periphery.
- Cytoplasmic dyneins are responsible for __, moving materials toward the cell center.
microtubule dynamics
- antegrade axonal transport
- retrograde axonal transport
antegrade - forward; retrograde - backward
What type of movements require the action of minus-end directed cytoplasmic dynein?
microtubule dynamics
Centripetal movements
What type of movements require kinesin motors? (toward the periphery require plus-end directed kinesin motors)
microtubule dynamics
Centrifugal movements
centrifugal - moving away from the center
- The endoplasmic reticulum (ER) is located at the __ of the cell.
- The Golgi apparatus is located near the __.
- edge
- cell center
- a large protein complex associated with cytoplasmic dynein that helps translocate organelles effectively.
- a short, actin-like filament that forms the actin-related protein Arp1.
microtubule
Dynactin
Defects in microtubule-based transport have been linked to neurological diseases such as __, where cells fail to migrate to the cerebral cortex.
- smooth brain (lissencephaly)
a dynein-binding protein required for nuclear migration in several species; its absence leads to nuclear-migration defects as migrating neurons fail to attach to dynein.
microtubule
Lis1
regulate their activity, resulting in changes in the position of organelles or whole cell movements.
microtubule
motor proteins
contain large pigment granules that can alter their location in response to neuronal or hormonal stimulation.
microtubule; a cell
Fish melanocytes
a structure that forms during cell division, allowing the segregation of chromosomes; its assembly depends on the reorganization of the interphase array of microtubules to form a bipolar array.
mitotic spindle
In __, microtubules have mixed polarities, while in __, the minus end points back toward the cell body, and the plus end points toward the axon terminals.
- dendrites
- axons
What structures are filled with bundles of microtubules in neurons? (2)
- Axons
- dendrites
specialized motility structures made from microtubules and dynein, with a bundle of microtubules at their core. (2)
- Cilia
- Flagella
__ enable cells to swim through liquid media with an undulating motion, while __ beat with a whiplike motion resembling the breaststroke.
- Flagella
- Cilia
undulating-resembling the motion of waves
- the core structure of cilia and flagella, composed of microtubules and their associated proteins, arranged in a distinctive pattern.
- consists of nine doublet microtubules arranged in a ring around a pair of single microtubules in the center.
axoneme
forms bridges between neighboring doublet microtubules around the circumference of the axoneme, facilitating the bending motion necessary for ciliary and flagellar movement.
Axonemal dynein
What condition is caused by hereditary defects in axonemal dynein?
- Symptoms include sinus inversus due to disrupted fluid flow in the embryo, male sterility due to immotile sperm, and a high susceptibility to lung infections due to paralyzed cilia.
sinus inversus-organs in chest & abdomen positioned in a mirror image
Kartagener’s syndrome (a type of primary ciliary dyskinesia (PCD))
What protein is found in bacterial flagella?
flagellin
- nonmotile counterpart of cilia and flagella
- specialized cellular compartments or organelles
- shares structural features with motile cilia
primary cilium
consists of nine groups of fused triplet microtubules arranged in a cartwheel structure.
centriole
In the nasal epithelium, primary cilia are involved in __ and __.
- odorant reception
- signal amplification
In rod and cone cells, primary cilia are responsible for converting __ signals into __ signals.
- light
- neural
