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Cell Biology > 5: Cytoskeleton Overview > Flashcards

5: Cytoskeleton Overview Flashcards

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1
Q

what is the cytoskeleton?

A

intricate network of protein filaments that extend throughout the cytoplasm

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2
Q

How do MTs have polarity?

A

one side is alpha (-) end
one side is beta (+) end

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2
Q

MT subunits are

A

alpha and beta tubulin which are 55kDa each. these form a stable dimer of 8nm

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3
Q

diameter of MT?
length of MT?

A

25nm
up to 100s of µm long (project out of cell - flagella)

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4
Q

alpha-beta dimers form

A

protofilaments when B subunit is in GTP subunit

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5
Q

are MTs perfectly circular?

A

no
seam forms

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6
Q

B tubulin is different from a tubulin because

A

B tubulin can be hydrolyzed from GTP to GDP

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7
Q

What are the types of micritubule structures?

A
  1. singlet: most common, found in cytoplasm made of 13 protofilaments
  2. doublet: found in cilia and flagella, A MT (13 PF) + B MT (10 PF)
  3. triplet: basal bodies & centrioles: A MT (13 PF) + B MT (10 PF) + C MT (10 PF) - stable tend to not polymerize and depolymerize
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8
Q

cytoplasmic MT are found in

axonemal MT are found in

A

cytoplasmic: singlet nerve axon cytoplasm MT

axonemal: doublet cilia/flagella MT

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9
Q

assembly of microtubule begina at

A

MT organizing centre (MTOC) which is associated with (-) end and (+) end grows away

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10
Q

What is the centrosome?

A

main cellular MTOC that contains centrioles and pericentriolar matrix

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11
Q

What are other MTOC examples?

A
  • spindle poles in mitotic apparatus
  • basal body: MTOC for cilia/flagella
  • nerves: MT are not continuous (axon + dendrites: orientation varies)
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12
Q

centrioles are found in

A

animals (not plants)

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13
Q

centrioles are

A

2 barrel-like triplet MT that are perpendicular to each other

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14
Q

the pericentriolar matrix is:

A

space around centrioles where proteins gamma tubulin and augmin are found

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15
Q

gamma tubulin and augmin are

A

proteins to make singlet cytoplasmic MTs
gamma tubulin: starts polymerization of singlet MT
augmin: increases polymerization
both form gamma tubulin ring complex

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16
Q

when centrioles replicate…

A

pericentriolar matrix also replicates and they make a mother and daughter MT which are different

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17
Q

gamma tubulin ring complex provides …

A

nucleating sites for MTs = this is the minus end

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18
Q

a nucleating site is…

A

where polymerization of MT can occur on

19
Q

augmin can (rarely) facilitate

A

MT branching

20
Q

to study MT polymerization…

A

Take flagellar nucleus (piece of flagella) which acts as a nucleating site, which is a doublet MT = stable and won’t depolymerize

Add tubulin diners to the nucleus until critical concentration for polymerization is reached

21
Q

Below critical concentration =

A

depolymerization

22
Q

MT – side polymerization and depolymerizes faster because …

A

plus

bc
minus end has gamma tubulin ring complex = capped = blocking polymerization and depolymerization

23
Q

Time of elongation of protofilament depends on

A

nucleus presence

24
Q

What is dynamic instability?

A

MTs are always growing and shrinking independently of each other
Catastrophe: MT stops polymerization and starts depolymerizing (for ex. Below critical conc.)
**Rescue: ** MT stops depolymerizing and starts polymerizing (for example: above critical conc.)

25
Q

dynamic instability depends on

A

presence or absence of GTP-beta-tublin cap

26
Q

GTP-beta-tubulin cap provides

A

lateral cohesion: protofilaments are attached to each other, which increases when alpha and beta tubulin are GTP bound

27
Q

As the protofilament grows, GTP is

A

hydrolyzed to GDP in beta tubulin

28
Q

Most beta tubulin in protofilament that is not on the plus end is

A

in GDP form by hydrolysis = less
lateral cohesion

29
Q

how does MT prevent fraying?

A

minus end has gamma-Tu-RC and + end has GTP-B-tubulin cap which prevents fraying –> fraying = depolymerization

30
Q

how does depolymerization occur?

A

hydrolysis catches up to + end = all B-subunits are in GDP form
* Exact mechanism of assembly is unclear as is role of seam

31
Q

2 requirements for polymerization are

A
  1. above critical concentration
  2. all B-tubulin GTP form
32
Q

Colchicine is

A

MT depolymerizing drug -> remove drug -> see how polymerization occurs

33
Q

Taxol is

A

stabilizes MT -> MT don’t depolymerize -> remove drug -> see how depolymerization occurs
* Anti-cancer drugs: need depoly. of MT in mitosis so taxol can be used to stop depolymerization in cancer cells

34
Q

What are MAPs

A

Microtubule Associated Proteins
* coats MT and stabilizes or destabilizes
* can be regulated or bundle MT

35
Q

Example of MAP regulation

A

Phosphorylation can promote disassembly/destabilization e.g. CDK in cell cycle regulate MAPs

36
Q

MAPs that prevent depolymerization are

A

MAP2 and Tau - binds to MTs

37
Q

Structure of MAPs MAP2 and Tau?

A

MT binding domain and projection domain (sticks out of MT to attach to next MT)

38
Q

difference between Tau and MAP2

A

Large projection domain = MTs are far apart from each other (ex MAP2)
* Small projection domain = MTs are close together (ex. Tau)
* Size of projection domain can regulate MT spacing

39
Q

What are +TIPs?

A

special MAPs associated with the (+) end of MTs that when present helps/stabilizes polymerization and when absent depolymerization occurs

40
Q

example of a +TIP?

A

EB1
* binds to unique structures found on growing (+) end and potentially stabilizes and reducescatastrophe
* EB1 moves along the seam of MT towards (+) end possibly transporting something

41
Q

What are XMAP215 and CLASP?

A
  • MAPs (not +TIPs) found in (+) end and have TOG domains
  • TOG domains bind to growing protofilament at (+) end to help stabilize (+) end to prevent catastrophe by keeping dimers together
42
Q

What does kinesin-13 do?

A
  • with ATP destabilizes MTs by removing terminal dimers
43
Q

What does stathmin do?

A
  • protein that promotes protofilament bending by decreasing lateral cohesion
  • bind tubulin dimers in the curve may promote fraying and GTP hydrolysis
  • stathmin can be inactivated by phosphorylated
44
Q

depolymerization/destabilizing promoting proteins?

A
  • kinesin-13
  • stathmin
45
Q

stabilizing proteins?

A
  • MAP2 and Tau
  • +TIPs (EB1)
  • XMAP215 and CLASP

Cell Biology (4 decks)

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