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33 Cards in this Set

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  • Back
What are intermediate filaments?
- provide mechanical strength
- made of many different types of proteins
- 8-12 nm in diameter
- no GTP/ATP
- only in soft animals
- made of a two monomers in a parallel dimer which are then combined in an antiparallel tetramer which are then combined into a filament of 8 tetramers
- persistence length = less than 1 micrometer, very easy to bend and hard to break
What are microtubules?
- position the organelles and direct intracellular transport ex. form mitotic spindle
- made of tubulin
- 25 nm in diameter
- GTP binding
- made of tubulin heterodimers each with an alph and beta subunit that creates polarity
- 13 protofilaments make up the microtubule
- persistence length = several millimetres, very stiff
What are actin filaments?
- determine shape of cell surface and are required for whole cell locomotion
- 2 stranged helical polymers of actin
- 7 nm diameter
- ATP binding
- RH helix of 2 protofilaments twisted around each other
- persistence length = 20-40 micrometers
What is treadmilling?
- in actin filaments when T form subunits are added to the plus end and D form subunits are dissociated from the minus end
- this is due to the fact that elongation is faster than hydrolysis at the plus end
Which side of a cell is negative and which is positive?
- Apical is negative
- Basal is positive
What are the three toxins that affect the cytoskeleton and what do they do?
- Phalloidin - binds and stabilizes filaments so they can no longer dissassemble

- Taxol - binds and stabilizes microtubules so cells can no longer form mitotic spindle

- Colchicine - binds microtubule subunits and prevents their polymerization
How are microtubules formed?
- gamma-tubulin for nucleation of microtubule growth
- nucleation starts at MTOC (Microtubule Organizing Centre)
- It is nucleated at the minus end with the plus end growing outward
- gamma-TuRC (gamma-tubulin ring complex) serves as a template at the minus end
What are centrioles?
- 2 in centrosome, at right angle to each other
- short cylinder of microtubules and accessory proteins
- become basal bodies of cilia and flagella in motile cells
- microtubules grow outward into stable astral configuration
What two things regulate actin filament nucleation?
- ARP complex
- formins
What are formins ?
- dimeric proteins
- nucleate growth at plus end of straight, unbranched actin filaments
what did Mizuno et al find out about formins?
- formin stays still while filament is pulled through
- found this using vertical and horizontal fluorescent markers that showed up in an alternating pattern
What is the ARP complex?
ARP = Actin Related Proteins
- nucleates actin filament growth allowing rapid elongation
- ARPs are 45% identical to actin
- helps in interlocking of plant cells
What is the process of the ARP complex?
1. inactive ARP complex - ARP2 and ARP3 held in orientation that prevents them from nucleating a new actin filament

2. An activating factor binds - ARP2 and ARP3 become an active complex that allows for binding of actin monomers which form an actin filament

3. Filaments begin to brnach off of one another at 70 degree angles
What are the different types of actin arrays and how are they held together?
1. Weblike networks - ex cell cortex.
- uses filamin dimers
2. Bundles - ex. contractile bundles in stress fibers or tight parallel bundles in filipodia
- alpha-actinin cross links actin into loose bundles that allow the entry of myosin II
- fimbrin cross links actin into tight bundles that prevents the entry of mysoin II
What did Ehrlicker et al find about binding of proteins to actin?
- some proteins, ex FilGAP only bind to actin web when it is relaxed and others ex. integrin only bind when it is stressed
- tested using Fluorescences Loss After Photoconversion ( a high speed version of FRAP)
What are ERM proteins and how do they work?
- help to attach actin filaments to proteins in the lipid bilayer

1. Inactive folded conformation
2. Phosphorylation or binding of PIP2 switches to active conformation in response to an extracellular signal
3. 2 binding sites - one for actin filament, one for transmembrane protein
What are ERMs directly targeted by?
Receptor Tyrosine Kinases
What do motor proteins do?
- use ATP hydrolysis
- unidirectional
- head recognizes track and tail determines cargo
What are the three types of motor proteins?
1. Myosins
2. Kinesis
3. Dyneins
What is myosin and what does it do?
- an actin in all eukaryotes
- for muscle contraction
- 37 families in humans
- walks toward plus end (all except one type)
What is the make up of myosin II?
- 2 heavy chains, 2000 amino acids each
- 4 light chains
- form bundles of many alpha-helical dimers with heads protruding- head bound to actin only 5% of the time to allow movement
What is the process for myosin II movement?
1. head attached to actin, no ATP (rigor mortis)
2. ATP binds, head is released
3. ATP conversion to ADP causes myosin to flip and move down the actin filament 5 nm
4. Weak binding of the head to the new site causes ADP to be released
5. Myosin goes back to original conformation - flips back (this is called the "power stroke")
What are kinesins?
- similar structure to myosin
- dimer with two heads
- in microtubules
- move toward plus end hand over hand
- for mitotic spindle formation and transport of chromosomes to poles
- 45 types in humans
What is the process for kinesin movement?
1. ATP conversion to ADP in lagging head loosens binding to microtubule
2. ADP conversion to ATP in leading head pulls lagging head over to front
What are dyneins?
- structurally unique
- move on microtubules toward the minus end
- largest (one chain is 5000 amino acids) and fastest (14 micrometers per second)
What are the two main types of dyneins?
1. Cytoplasmic dyneins - homodimers (2 heads) for vesicular traffic
2. Axonemal dyneins - heterodimers and heterotrimers (2 to 3 chains per head) for beating of cilia and flagella
How does dynein movement work?
- have a large dynein head that is a planar ring containing a C-terminal domain and 6 AAA domains, only one of which has ATPase activity
- have long coiled coil stalk with microtubule binding site at the tip
- ATP hydrolysis causes binding (when ATP is bound, stalk is detached)
- release of ADP causes power strong involving rotation of head moving microtubule 8 nm along
What is the lamellipodia?
- made of actin filaments
- for cell motility
What are keratocytes?
- lamellipodia in epidermis of fish and frogs with abundant keratin filaments
What are the two things that help lamellipodia move forward ?
- cofilin - helps depolymerization at the non-growing end
- capping protein - stops elongation in non-forward direction
What are the three members of the Rho protein family and what do they do?
- Rho - activation results in the growth of more parallel stress fibers
- Rac - activation leads to growth of more branched lamellipodia
- Cdc42 - activation leads to filopodia and microspike formation
What does Rac-GTP do?
Rac-GTP is activated causing actin nucleation by ARP complex and cross-linking by filamin to form branched actin in lamellipodia
- positive feedback loop with PI 3-kinase creates more Rac-GTP
What does Rho-GTP do?
- when Rho-GTP is activated it causes nucleation of actin by formins and increased contraction by myosin II - more clustered contractile bundles like stress fibers