ELE_BC Unit 5 Cytoskeleton

Describe the cytoskeletal system (3)

Highly structured interior
Network of interconnected filaments and tubules
Dynamic and Interchangeable

What are the 6 functions of the cytoskeletal system

Scaffold for structure and shape
Framework for organelle organization
Movement of resources
Binding site for mRNA
Signal Transducer
Important machinery for cell division

What quality of the cytoskeletal system is being described:

These structures continuously assemble (polymerize) and disassemble (depolymerize). This flexibility allows the cell to change shape, crawl, adapt to forces, and divide.

Dynamic nature

What quality of the cytoskeletal system is being described:

Diffusion alone is too slow for large eukaryotic cells. So it acts as a highway network where motor proteins transport organelles and molecular cargo directly to targeted destinations.

Intracellular transport

What quality of the cytoskeletal system is being described:

It provides both static mechanical support (holding shape, anchoring mRNA) and active work (pulling chromosomes apart, moving vesicles).

Scaffolding and Machinery

What cytoskeletal type is being described:

Structural support of surface projections (microvilli), cell motility, cytokinesis

Actin filaments

What cytoskeletal type is being described:

Mechanical strength, tensile scaffold, structural integrity

Intermediate filaments

What cytoskeletal type is being described:

Long-distance organelle/vesicle transport, spatial organelle layout, chromosome segregation

Microtubules

What is the advantage of mRNA anchoring in cytoskeletal system

Allows the cell to produce proteins exactly where they are needed rather than synthesizing randomly and waiting for diffusion

Forms the central contractile ring that pinches the cell into two daughter cells during cytokinesis.

Actin filaments

Forms the mitotic spindle that anchors to and separates chromosomes during division.

Microtubules

What are major structural elements of the cytoskeletal system (4)

Actin filament
Microtubule
Intermediate filaments
Protein networks called Septin

What are 2 classes of microtubules

Cytoplasmic MT
Axonemal MT

Cytoplasmic microtubules are for?

Cell shape
Organelle transport
Spindle formation

Axonemal microtubules are for?

Cillia
Flagella
Basal Bodies

What are the 3 phases of microtubule assembly

Nucleation
Elongation
Plateau

It is a slow lag phase aggregating dimers to form oligomers

Nucleation phase

It is the fast subunit addition phase

Elongation phase

It is the phase at which equilibrium is reached (assembly rate = disassembly rate)

Plateau phase

What is being described: Consists of straight hallow cylinders made up of usually 13 longitudinal arrays called protofilaments

Microtubule

Fix the error. Protofilaments are made up of tubulin dimers (a & b tubulins) and are covalently attached to each other forming the hollow cylinder of the microtubule

Non-covalent bonding of tubulin

What are the two ends of microtubule called

(+) & (-) ends

What do you call the concentration where the amount of tubulin at which MT assembly is equal with MT disassembly

Critical concentration

What is the addition subunits at the plus end and removal at the minus end of microtubules called?

Microtubule Treadmilling

What type of microtubule is being described: Dynamic, individual singlet tubes

Cytoplasmic microtubules

What type of microtubule is being described: Highly stable, organized doublets/triplets

Axonemal Microtubules

Exposed B-tubulin : _____ - Exposed A-tubulin: _____

(+) Plus end, (-) Minus end

Fix the error. The critical concentration at eh plus end is much higher than in the minus end in microtubules

Critical concentration higher on minus end

Fix the error. The growth rate of the plus end is much slower than the minus end

Rapid polymerization at the plus end, slower at the minus end

Is in charge of controlling microtubule number, polarity, protofilament count and timing and location of microtubule assembly

Microtubule Organizing Center (MTOC)

Describe centrosome architecture

Made up of two barrel shaped centrioles surround my pericentriolar material

Fix the error. Similar to humans, plants also possess MTOCs

Plants do not have MTOC

What do you call the caps on the (-) ends attached to the centrosomes

y-Tubulin Ring Complex (y-TuRC)

Describe what happens at High GTP-Tubulin

Addition rate > Hydrolysis rate
Microtubule growth favored

Describe what happens at Low GTP-Tubulin

Addition Rate < Hydrolysis Rate
Rapid shrinkage (catastrophe)

What do you call stabilizes existing microtubule structure and promotes assembly

Microtubule-associated proteins (MAPs)

Where does MAPs bind?

Along the outer longitudinal surface/wall of microtubule

What do you call protects plus ends and regulates growth/shrinkage dynamics

(+)(-) end Interacting Proteins (TIPs)

Where does TIPs binds

Accumulates specifically at the extreme growing plus (+) end

Fix the error. Microtubule generation is dependent on GTP-hydrolysis

Dependent on GTP binding not hydrolysis

Binds directly to the cytoskeletal track and contains ATP binding site

Globular head / Motor domain

Transduces small structural changes triggered by ATP hydrolysis into larger mechanical steps

Lever / Neck

A coiled a-helical domain that bridges the motor domain and cargo-binding tail

Stalk

Binds specifically to cellular cargo like -vesicles organlles protein complex, directly or by adaptor proteins

Tail/Light chains

Are specialized molecular machines that convert chemical energy from ATP hydrolysis into mechanical work, stepping unidirectionally along cytoskeletal tracks to transport intracellular cargo. 

Motor proteins

Individual MTs switch from growth to shrinkage (happens when rate of addition is slower than the rate of GTP hydrolysis)

Microtubule Catastrophe

A sudden switch back to growth phase (happens when rate of addition is faster than the rate of GTP hydrolysis)

Microtubule Rescue

What is the diirection of Motor proteins along cytoskeletal tracks

Unidirectional in a stepwise manner

What are the two primary Microtubule associated motors:

Kinesin & Dynein

A two-headed molecular motor that walks unidirectionally along microtubule tracks, primarily moving toward the positive (+) end of the filament

Kinesin-I

A large motor protein complex featuring ring-like motor domains that moves along microtubules in the opposite direction—toward the negative (-) end

Cytoplasmic Dynein

Kinesin-I features what type of transport?

Anterograde transport

Cytoplasmic Dynein features what type of transport?

Retrograde transport

A motor protein that moves along actin filaments (microfilaments) rather than microtubules.

Myosin V

What superfamily does kinesins belong to

KLPs (Kinesin-like proteins)

Describe the structure of Kinesin

Tetramer: 2 identical Heavy Chains and 2 identical Light Chains

Fix the error. A single microtubule can only support 1 motor protein at a time

Microtubule acts as a 13 lane (protofilament) highway allowing multiple motorproteins to traverse it

Beyond intracellular cargo transport, dynein is responsible for movement of _____ & _____

Cillia, Flagella

Structural scaffolding proteins that connect the dynactin complex to the cargo membrane

Spectrin / Ankyrin

A large multi-subunit complex required for dynein to attach to cargo and move over long distances.

Dynactin

What class has a 9+2 doublet structure primarily found in cillia and flagella?

Axonemal MT

What class has a dynamic radial network from centrosome primarily found in cell interior and cytoplasm

Cytoplasmic MT

Fix the error. Microtubules possess structural polarity with distinct Plus (+) end (a-tubulin exposed) and Minus (-) end (beta-tubulin exposed)

a (-), b (+)

Massive protein complex featuring a force-generating globular head containing an AAA+-ATPase domain, stalk, lever arm, and intermediate/light chains.

Dynein (Cytoplasmic dynein)

It is the smallest cytoskeletal element, flexible, inextensible double-helical polymer formed by two linear strands of polymerized actin wound around each other

Microfilaments

What are 2 monomers of microfilaments

G-actin (Globular)
F-actin (Filamentous)

What do you call the muscle specific actin isoforms

a-actin

What do you call the non-muscle isoforms of actin

b-actin & y- actin

In microfilaments the barbed end refers to?

(+) end

In microfilaments the pointed end refers to?

(-) end

A fungal metabolite that selectively blocks the Plus ($+$) ends of actin filaments, inhibiting assembly

Cytochalasin D

A toxin that binds tightly to intact actin filaments, preventing their depolymerization and subunit turnover.

Phalloidin

Binds free G-actin monomers, sequestering them to prevent incorporation into microfilaments.

Lantrunculin

Most stable, tough, and least soluble cytoskeletal elements; highly elastic and capable of withstanding high tensile forces.

Intermediate filament

Fix the error. Intermediate filaments can be found in the cytosol of all cellular life

In animal cells only, absent in plants

Fix the error. Intermediate filaments requires nucleotide to stabilize itself

no nucleotide substrate requirement

IF Class 1

Keratin (epithelial)

IF Class 3

Vimentin (mesenchymal) & Desmin (muscles)

IF Class 5

Nuclear lamins (all cell types)

IF Class 6

Nestin (neuron)

Describe intermediate filament structure

Two parallel IF chains form coiled-coil dimer
Two dimers align in antiparallel forming a tetrameric protofilament

(Nucleotide binding) Actin : _____ - Tubulin : _____

ATP, GTP

What do you call third most abundant protein in skeletal muscle. Highly elastic protein extending from the Z line to the M line; prevents the sarcomere from being torn apart during muscular stretching.

Titin

What protein regulates the precise length and number of actin monomers assembled into a thin filament.

Nebulin

It is a complex attached to actin and tropomyosin that acts as a calcium sensor.

Troponin

It is an elongated rod-shaped protein lying in the grooves of the actin helix, blocking myosin-binding sites during rest.

Tropomyosin

It is a double-helical structural spine also called as the thin filaments

Actin

Which band during muscle contraction remains constant

A band

Which band during muscle contraction decreases or narrows

I band & H zone

Which band during muscle contraction are drawn closer to each other

Z lines

ATP-dependent motors moving along actin filaments toward the plus (+) end

Myosin

What do you call the only myosin that moves toward the minus (-) end

Myosin VI

Describe the complete structure of Myosin type II

Hexamer made up of 2 light chains, heavy chains and 2 regulatory light chains as well as two catalytic head domains

What are 5 steps in Excitation-Contraction Coupling

1.      Action potential travels along the plasma membrane and down Transverse (T) tubules into the cell interior.

2.      Voltage change opens calcium channels in the Sarcoplasmic Reticulum (SR), releasing stored Ca²⁺ into the cytosol.

3.  Ca²⁺ binds to Troponin, causing a conformational change.

4.      Troponin shifts Tropomyosin out of the actin grooves, exposing myosin-binding sites.

5.      Myosin heads bind actin to execute cross-bridge cycling.

It is the basic functional unit of myofibril

Sarcomere

It is a dense protein structure that forms the boundary and endpoint of each sarcomere

Z-line

A vertical line located in the exact center of the sarcomere, right in the middle of the H zone and the A band

M-line

It is the region in the center of the sarcomere that spans the entire length of the thick myosin filaments

A-band

Region that contains only thin filaments (actin)

Z-disk

It is a region in the center of the A band that contains only thick filaments and no thing filaments at rest

H-zone