Cytoskeleton and Cellular Architecture Flashcards by Caleb Yockey

What are the 3 types of cytoskeleton filaments?

Intermediate filaments MIDDLE

Microtubules LARGEST

Actin filaments SMALLEST

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What is the strongest and most flexible of the 3 filaments?

Intermediate filaments

many thin, parallel strands that are strong but can flex

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What are the 3 domains of the intermediate filament?

1 - N head NOT ALPHA HELICAL

2 - central rod domain

3 - C tail NOT ALPHA HELICAL

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Are high energy nucleotides required for assembly of IF? What about the other filaments?

NO they arent

they are for the others

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What are the 4 steps of assembly of a IF? Which one is the fundamental subunit?

1 - alpha helical monomer

coiled-coil dimer

3 - STAGGERED tetromer of two coiled-coil dimers

4 - Unit length filament - 8 laterally associated tetramers (32 total IF proteins)

5 - final IF

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Are the tetramers of IF polar?

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Are the interactions between the dimers, tetramers, and ULFs of the IFs strong or weak? What is the advantage of this:

They are weak allow them to deform to handle stress by sliding past one another

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Does the IF still stretch even at low stress? Why?

YES due to the H-bonds severing because of the streching of the alpha helices

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What are all of the cytoplasmic IFs? What about nuclear?

Cytoplasmic - desmin filament (mucles), keratin filaments (epithelial), vimentin (connective-tissues, muscles, and glial cells), neurofilaments (nerves)

Nuclear - nuclear lamins in all animal cells

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Why is keratin a special type of IF?

There are more than 50 different kinds

they are usually heterodimers while other IFs are homodimers

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What is the function of the nuclear lamina? How is it different from other IFs?

This is a mesh of lamin proteins that support the nuclear envelope (lipids) to give its strength and shape

nuclear lamins completely disassemble during mitosis in each cell cycle due to phosphorylation and reassemble in interphase when dephosphorylated

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What is progeria?

A disease where children show signs of aging, though to be caused by a defective nuclear lamin gene.

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What are desmosomes?

These are contact points between cells that lock them together formed by the IFs of each cell radiating out and linking together forming a network that extends through many cells

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What is EBS (epidermolysis bullosa simplex)?

An inherited defect of keratin or keratin-associated proteins that causes blistering of the skin with no trauma.

Mutant keratin renders it susceptible to collapse.

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How did they find out EBS was inherited?

By injecting embryos with the mutant plasmid and placing them in a mother mouse, some of her offspring expressed the mutant keratin.

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What is the least flexible filament therefore the most rigid?

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What is required to assemble MT?

GTP (energy)

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What is the main function of the MT? Does this require ATP?

to move cargo through the cell

YES

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What are three synonymous terms for MTOC (microtubule organizing center)?

Centrosome - during interphase

Spindle poles - mitotic cell

Basal body - ciliated cell

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What is the function of MTs from centrosome?

to give size and shape of interphase cell

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What is the function of the spindle poles?

drives chromosome positioning and segregation during mitosis

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What is the function of the MT cytoskeleton?

Scaffold for plasma membrane and large organelles

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Describe the structure of the centrosome. What binds the microtubules to the centrosome?

These are made up of microtubule triplets that form a ring, pericentriolar material is around it.

gamma-tubulin anchors and stabilizes mircrotubules bound the the centrosome or other MTOC

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Describe the building block of a MT.

Made of alpha and beta tubulin bound together forming a heterodimer, both of which are bound to a GTP molecule

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Which end of the MT can the GTP be cleaved to GDP?

The Head + end

Describe the hollow tube structure of alpha and beta tubulin.

Made of 13 protofilaments that are made of alpha/beta tubulin dimers and make a rigid hollow tube.

What end of the microtubule attaches to the centrosome?

The - tail end

Describe the dynamic instability of the MT.

This occurs of the + head end, when GTP is bound to the fundamental units they are added to the + end and when GDP is bound to them they are lost from the + head end, this is called catastophe.

Why does dynamic instability only occur at the + end usually?

Because the gamma-tubulin at the - end stabilized the - tail.

Describe the GTP cap of a growing MT

This is when the addition of GTP to the + end is faster than the GTP hydrolysis creating a growing GTP cap.

Can other proteins stabilize the + end of a MT?

YES, like capping proteins on the PM.

What are MAPs and TIPs of MTs?

MAPs - such as tau protein that binds to the sides of the MT and stabilize the + end TIPs - mediate + end dynamics and attachments

What is taus role in alzheimers?

this is when neurofibrilary tangles form due to tau not being bound to MT along with dissociation of the MT these aggregates

What are the two types of motors on MTs that move cargo, which direction do they travel?

dyneins - towards the - end Kinesins - mostly + end directed

Describe the basic structure of the MT motors

Heterotetramer 2 head domains and 2 stalk domains (heavy chain) 2 tail domains that attach to the cargo (vesicles, organelles, other MTs) LIGHT CHAIN

How do the motors of MTs move the cargo?

via ATP hydrolysis

What is the 9+2 structure of cilia and flagella MTs?

9 microtubule doublets that form a ring around 2 single MTs in the middle

What is the function of a primary cilium?

This is ONE cilium on a cell that is nonmotile and functions in signaling

What is the function of plectin?

To link IFs, MTs, and actin

Which filament is the most dynamic and highly regulated?

actin filaments

Which filament has the smallest diameter and intermediate flexibility?

actin filaments

Are aactin filaments polar?

YES - pointed end + barbed end

What is another term for AFs?

microfilaments

Are AFs held together by weak longitudinal and lateral bonds to form a double helix (F-actin)?

YES

Describe each ends of the actin filament in regards to which tend to lose monomers and which tend to gain monomers. What does this lead to?

- pointed end - tends to lose monomers + barbed end - tends to gain monomers this leads to a treadmilling effect due to polarity

What is the difference of F-actin and G-actin?

G-actin is soluble while F-actin is filamentous

What are the 5 classes of proteins that regulate actin?

monomer-sequestering proteins nucleating protein capping protein severing protein cross-linking and bundling proteins

What is an example of a mono-binding protein of actin filaments?

PROFILIN - this binds G-actin and prevents its addition to the - end to only allow it to attach to the barbed + end by introducing it to nucleating proteins

What are some examples of nucleating proteins of actin filaments?

Formins and Ena/VASP - stimulate polymerization at + end ARP2/3 - initiates branching to form networks of actin filaments

What is an example of a capping protein of AF?

F-actin capping protein (FACP) - binds to positive end and stabilizes it, blocking the addition or loss of subunits on that end This competes with nucleating proteins to allow for growth in specific regions of the filament to form networks,

What are some examples of severing/depolymerizing proteins of AFs?

ADF/cofilin - stabilizes spontaneous breaks in filament by severing it producing new ends Gelsolin - severs filaments but remains associated with + end and stabilizes it

Describe some cross-linking/bundling proteins of AFs.

Bundling - fibrin and alpha actin - fibrin forms tightly packed bundles (filopodia and alpha-actin forms loose bundles (plasma membrane attachment and special role in muscle) cross-linking - filamin - organizes actin filaments into gel-like networks like in the cell cortex

LOOK AT SLIDE 42 FOR SOME FUNCTIONS OF ACTIN

DO IT

Describe the cell cortex.

This underlies the plasma membrane and gives cell shape, size, and mechanical properties mesh formed by filamin, to reshape the cell the cortex must be reshaped.

Does actin act in unidirectional growth from the PM?

YES like the growth cone that is specific to neurons

What are some functions of the myosin I and II motor on AFs?

Myosin I - moves things towards the + end AND can be attached to PM to move AFs along plasma membrane (myosin moves towards positive end pushing filament in opposite direction) Myosin II - these have tail domains that interact with one another and pull towards each other SEE SLIDE 46

LOOK AT SLIDE 47 ON CELL MIGRATION

DO IT

What do many sarcomeres make? What do many myofibrils make?

Many sarcomeres make myofibrils Many myofibrils make a muscle cell

In muscle contraction, which way does the myosin head move?

Towards the positive end of the AF, bringing the AF towards the bare region of myosinII filament during contraction. relaxation is opposite.

Describe how calcium regulates muscle contractions.

neuron signals release of Ca2+ in myocyte calcium binds to troponin and changes its conformation causing tropomyosin fiber to shift allowing myosin and actin to associate by exposing the binding site