Cytoskeleton Flashcards
What are the three subunits of the cytoskeleton? Where are they each found in the cell?
1: Intermediate Filaments
-emanate from places where cell junctions occur
2: Microtubules
-emanate from the MTOCs (near the nucleus)
3:Actin Filaments
-right below plasma membrane
What are intermediate filaments?
-rope like fibers, 10nm diameter
-forms nuclear lamina, protects axons, and provides mechanical strength for cells
-sturdy, hardy rope
-abundant in epithelial cells to protect our skin from breaking
What are the steps to forming the final structure of an intermediate filament?
1: two alpha helices monomers wrap around to form dimer
2: two dimers come together to form tetramer
-tetramer is staggered and polarity is lost when it forms (no polarity=no motor protein)
3: tetramers come together to make lateral association of 8 tetramers
-the association of 8 coiled-coil tetramers make IFs very strong!
What does it mean that intermediate filaments are thermally stable?
It can’t be broken down naturally by the energy of the environment. It must be disassembled by cellular machinery
What are neurofilaments?
They are intermediate filaments that are found along the length of the axon
-essential for growth and stability of the neuron
What protein can you test for concussions?
Glial Fibrillary Acidic Protein (GFAP)
What is Keratin?
An intermediate filament
-abundant in hair and nails
-make both unusually strong
What is Epidermolysis Bullosa Simplex (EBS)?
-genetic skin disorder resulting from mutations in keratin
-blister formation from mechanical trauma
N/a
They both emanate from the central location of the cell (close to the nucleus)
What are Microtubules and Actin Filaments composed of?
Filaments built of noncovalent bonds (weak bonds)
What is the stability of a single protofilament?
Thermally unstable! Will break down via natural energies in a cell
-this is why Microtubules and actin filaments build upon this
What are the monomers of microtubules?
Tubulin protein monomers!
What are the monomers of Actin filaments?
G-actin protein monomers!
What are some attributes of multiple protofilaments of Microtubule and Actin?
-its thermally stable
-dynamic (can move)
-like a bundle of pencils, can’t break in the middle, but the tip and eraser can be broken off and used
What does microtubule structure resemble?
-drinking straws!
-long, hollow cylinders made of tubulin
-diameter of 25nm
What are microtubules connected to?
The Microtubule Organizing Centers (MTOCs)
What are cilia and flagella composed of?
Microtubules!
What are the two subunits of a microtubule?
Beta-tubulin (plus end) and Alpha-tubulin (minus end)
-polarity is present in these to guide the motor proteins!
Why are intermediate filaments called “intermediate” filaments?
Bc they are intermediate in size compared to the other 2
What is the GTP state of alpha and beta ends of a tubulin protein?
Beta-tubulin = GTP orrr GDP (slow hydrolization is occurring) (tubulin is a slow GTPase)
alpha-tubulin = always GTP (stuck bc a subunit GTP is so cramped it can never hydrolyze)
What is the structure of a microtubule filament?
-hollow cylinder
-13 parallel protofilaments
-polarity/directionality
-addition/loss at ends (but not middle)
Where do microtubules originate?
The Microtubule Organizing Center (MTOC)
Where do the negative and positive ends of the microtubule always go?
Neg ends always point to MTOC and the positive end always faces out into the cell
What is Gamma tubulin?
A special tubulin subunit similar to Alpha and Beta (Y)
-makes ring complex around pericentriolar material that surrounds a pair of centrioles
What happens at the Y-tubulin ring complexes of the centrosome?(Y-TuRC)
Microtubules grow from here!
-this is where the foundation is for 90% of microtubules!
What weak bonds make up filaments of Actin and Microtubules?
Noncovalent bonds!
Tubulin is a_
Slow GTPase!
Where are the nucleating sites for microtubules?
At Y-Tubulin ring complexes at the centrosome (MTOCs)
Where do microtubules grow from?
At the centrosome/MTOCs from Y-Tubulin ring complexes (y-TuRCs)
What is the difference between the plus end and the minus end of microtubules?
The plus end grows more than the minus end!
Why does the plus end of the microtubule grow more than the minus end?
Bc the GTP bound form is added to the growing strand (b end), which is also GTP end facing (an end).
The Minus end is when you add GTP (an end) to old GDP (b ends).
-GTP-GTP bonds much more tightly than GTP-GDP!
-ergo, the minus side is falling apart faster than the plus side, even though both sides are being added onto
How can we tell old tubulin from new tubulin?
If it is hydrolized! Recently added tubulin will have the B end be GTP, but old tubulin B end will have slowly hydrolized into GDP!
What is T-tubulin versus D-tubulin?
T= GTP
D= GDP
What are the words for when microtubules go from growth to shrinking and vice verse?
Growth to shrinking= catastrophe
Shrinking to growth = rescue
What is dynamic instability?
The term used to describe catastrophe and rescue of microtubules
What are microtubule associated proteins (MAPs)?
-proteins that bind to and bundle filaments together by stabilizing and cross linking, to make higher order structures
Ex: Tau
What is Tau?
A MAP (microtubule associated protein) that stabilizes microtubules in neuronal cells
-Tau oligomers are a marker of Alzheimer’s, Dementia, and TBI
Why are microtubule alterations linked to cancer?
Because MTOCS are the centrosome! Everytime we divide, microtubules pull apart sister chromatids. Ergo, if this goes wrong then it leads to cancer!
What is the point/purpose of intermediate filaments?
Structural support!
What is the main point/purpose of having microtubules?
Aid in cell division, vesicular transport, organization of organelles in cells
What is the main point/purpose of having actin?
Cell shape, cell movement, cell division (ring for cytokinesis)
What is the shape of actin?
Helical polymers with a diameter of 8nm, smallest of all 3
What is the structure of Actin?
-G-actin monomers arrange into F-actin
-NOT a heterodiminer! Has a plus end and minus end and binds to ATP
What is the monomer of actin?
G-actin, globular actin!
What does Actin bind to?
ATP!
-can have ADP or ATP bc Actin is a slow GTPase!
What determines whether plus end or minus end of Actin?
Whether or not we have ATP or ADP bound
Which is more flexible, microtubules or actin?
Actin filaments!
What nucleates Actin Filaments?
ARP2/3 complex!
-it is found in cell periphery (not near nucleus)
-can perform branching!
-nucleates actin filaments from minus end to plus end
What is Actin branching?
When the ARP 2/3 complex binds to two filaments at once, forming branchlike structures
-the two filaments must come in at a 70 DEGREE ANGLE
-“Actin filament ends can be capped for stability”
What is Listeriosis and what does it do?
It’s an intracellular parasite
-it recruits the ARP 2/3 complex to its cell surface, which makes actin polymerize and shoot Listeria around like a rocket inside the cell!
What is the point of actin being cross linked?
To form distinct structures!
What are the two types of Actin bundles and what causes them?
1: Contractile Bundle
-loosely bundled
-allows Myosin to enter bundle
-formed when actin filaments pair with A”alpha”-Actinin
2: Parallel Bundle
-tightly bundled
-blocks Myosin from entering
-actin filaments pair with FIMBRIN
What’s an example of the purpose of forming actin bundles?
To give cells peculiar shapes they need in order to perform their intended function!
Ex: actin bundles make a microvilli gut epithelial cell
How does the cytoskeleton as a whole contribute to polarity and stable structures?
They give structure, anchoring cells together and giving mechanical strength
How does the cytoskeleton aid in Mitosis?
During metaphase when sister chromatids are being pulled apart, it is the centrosome and microtubules that are doing the pulling
-actin helps in making to ring that pinches new daughter cells into two distinct units, and then helps in cell movement to pull those new identical daughter cells away from each other
Compare the basic shapes of the three parts of the cytoskeleton with analogies?
Microtubules: like a drinking straw
Actin: a helix
Intermediate filaments: like the yummy strawberry ropey licorice made of individual strings
How does actin play a role in certain heart diseases?
What is Fluid Shear Stress (FSS)?
The force that is exerted on the cells of our arteries as blood is being pumped through
-high FSS = often linear arteries, less chance for plaque buildup
-low FSS = if an artery split, more chance for atherosclerosis
What happens to actin exposed to high FSS sheer stress?
The cell and actin Aline’s with the direction of stress/bloodflow
What are the two microtubule motor proteins?
Kinesin (the little walking guy) and Dynein
-both rely on directionality of polarity on microtubule
What is the one motor protein on Actin?
Myosin
How does Kinesin work?
little walking guy
-motor domain =N-terminus
-cargo binding site = C-terminus
-always walks towards the + end of the microtubule, uses lots of ATP to “walk” via cyclical binding
What is the dynactin complex?
The accessory protein that links Dyneins with vesicles bc they cannot link directly themselves
-an adaptor for Dynein, has nothing to do with Actin or anything like that!
What does Dynein do?
They “walk” towards the minus end of the microtubules (MTOCs/Y-TRCs)
-they also hydrolyze ATO in a cyclic manor, but movement is called “the drunken sailor”
-they are really large and fast, require many accessory proteins to bind to its cargo
What pathways are Dynein utilized in?
Endocytosis (Clathrin) and retrograde (retrieval) pathways
What do Dyneins and Kinesins work together to do?
Localize organelles/hold them in place
What does Myosin do?
It moved towards the + end of actin
How does Myosin move?
It does a power stroke, where the head (N-terminus) dissociates and scoops up to bind with actin at the + site
What makes up skeletal muscles?
Myofibrils < muscle fibers < muscles
-muscle fibers = single large cells 50 micrometers in diameter
What are myofibrils?
-cylindrical bundles found in cytoplasm made up of 2 types of filaments: thick (15nm) and thin (7nm)
-made up of sarcomeres
What are sarcomeres?
-“the contractile unit of muscle cells”
-The space between 1 Z-disc (anchor, + end of actin here) and another
What causes muscle contraction? (As a whole)
-coordinated movement of myosins force actin filaments to come closer together
What causes muscle contraction? (Specifically)
Ca2+ comes in and allows Myosin to walk to the + end of actin. Its anchored so can’t go anywhere just pulls the Z-disks closer together (interlocking fingers example)
How does Ca2+ allow muscle contraction to occur?
Usually Troponin and Tropomyosin are blocking the Myosin binding site. When calcium comes in, it bind to Troponin and changes the configuration of Troponin and Tropomyosin.
-this allows for the Myosin-binding site to be exposed and for muscle contraction to occur
What motor protein allows for cell movement?
Myosin!
How are cels able to move along a substrate?
1: Actin polymerization at the plus end/front protrudes the front end of the cell (creating a Lamellopodium)
2: Myosin contraction at the back of the cell “pushes” the cell forward
What is a Lamellopodium?
When acting polymerizes at the front of the cell
-this is made possible by ARP 2/3 which branches actin at the front of the cell to “push”
What connects the nucleus and the cytosol?
-they are mechanically indirectly coupled through the cytoskeleton!
-this helps coordinate cellular events
-KASH domain proteins going from cytoplasm -> perinuclear space and SUN proteins going from nucleus -> perinuclear space facilitate this
How can we target cancer by targeting microtubules?
No microtubules = no cel division!
-Taxol is a chemotherapy drug that targets all fast dividing cells by stabilizing microtubules and preventing cell division
What cytoskeleton subunits do what role in mitosis?
Microtubules: form the mitotic spindle and pull sister chromatids apart
Actin and myosin: form the contractile ring that separates daughter cells, then they move apart from each other
What is responsible for giving a cell its shape?
Actin!
What are the 2 ways ATP is used in muscle contractions?
1: for myosin movement
2: for the ATP pump to relax
How areY-TRCs and ARP 2/3 similar?
They are both nucleators! They both nucleate the minus end so the plus end can grow!
What are differences between Y-TURC and ARP 2/3?
Y: located at the MTOC, cannot branch
ARP: located at cell periphery, can branch!
At what end are microtubules nucleated?
At the minus end!
