Exam 3: lecture 7-cytoskeleton structure

Question 1

What are the 3 types of the cytoskeleton?

Answer 1

1. microfilament
2. intermediate filament
3. microtubule

(in order of size: small->large)

Question 2

What is the connection between bacteria and
eukaryotic cytoskeletal proteins?

Answer 2

bacteria have HOMOLOGUES for eucaryotic cytoskeletal proteins

a) staphylococcus aureus: FtsZ(microtubule-like)
b) e.coli: MreB (actin-like)
c) caulobacter crescentus: crescentin (intermediate filament-like)

Question 3

Technique for visualizing the cytoskeleton: fluorescence microscopy on fixed specimens

Answer 3

fluorescent bind directly
antibodies indirectly label
once dye, cytoskeletal proteins will glow.

ex. fibroblast

Question 4

technique for visualizing the cytoskeleton: live cell fluorescent

Answer 4

fluorescent versions of the proteins are made and put into living cells.
-microscopy/video/cameras are used to view

ex.tubulin

Question 5

technique for visualizing the cytoskeleton: computer-enhanced digital video

Answer 5

high-res images from a vid/camera attached to a microscope are computer processes to increase contrast and remove background features that obscure the image

Question 6

technique for visualizing the cytoskeleton: electron microscopy

Answer 6

revolve individual filaments prepared by thin section, quick-freeze deep-etch

Question 7

microtubules and its functions

Answer 7

the largest of the cytoskeletal component
25nm

functions:
1. move chromosomes around
2.flagella movement
3.”highways” for vesicles

Question 8

microtubules: cytoplasmic

Answer 8

pervade the cytosol

functions:
1. maintaining axons
2. formation of mitotic and meiotic spindles
3. maintaining/altering cell shape
4. placement & movement of vesicles

Question 9

microtubules: axonemal

Answer 9

include organized & stable microtubules found in
-cilia
-flagella
-basal bodies to which cilia + flagella attach

axenome: the central shaft of cillium/ flagellum is highly ordered bundle of MTs

Question 10

what is the order of structure of microtubules?

Answer 10

tubulin dimers (heterodimers)->oilgomers-> protofilament-> sheets of protofilaments-> closing microtubules-> elongating microtubules

Question 11

the kinetics of microtubules in vito

Answer 11

1. lag phase (nucleation): dimers-> oilomers-> protofilaments
2. elongation phase: growing microtubules with - & + ends
3. plateau phase (equilibrium): microtubules with subunits coming on and off (lose and gain the same amount- the size stays the same)

in a cell w/ MTOC (microtubule organizing center)
-anchors the - end= only shrinkage happens at the + end

Question 12

how do new dimers fall off the + end of microtubules?

Answer 12

dimer consists of alpha & beta (tubulin) is bound with GTP

to add: the chain will stay on until the gtp is hydrolyzed

to remove:
1. gtp on B-tubulin hydrolyzes (break off pi) to GDP
2. hydrolysis weakens bonds in polymer

(gtp–>GDP)

3. rapid depolymerization occurs
4. GDP is exchanged for Gtp

Question 13

the dynamic instability model of microtubules

Answer 13

a) growing in length
GTP cap is present at +end to allow polymerization (high tubulin concentration)

b) shrinking in length
GDP cap is at +end to allow rapid depolymerization (low tubulin concentration)

Question 14

Dynamic instability of microtubules: frequency

Answer 14

-plus ends experience larger changes even though both have dynamic instability

Frequency of catastrophe (how often is shrinkage)
- end is shorter in length

frequency of rescue (how often is stabilize)
- end is longer in length (more stable)

Question 15

what regulates microtubule stability in cells?

Answer 15

microtubule-binding proteins
-some use ATP to drive vesicles/ organelle transport/ to generate sliding forces between MTs

-others regulate MT structure

Question 16

Microtubules: what are microtubules -stabilizing/ bundling proteins? + examples

Answer 16

-bind at regular intervals along a microtube wall

Tau: form tight bundles in axons
MAP2: promotes the formation of looser bundles in dendrites (fingerlike)

Question 17

how does MAP2 + Tau promote bundling?

Answer 17

one region binds to the MT wall

another part of the protein extends at right angles for interaction

the length of the extended arm control the spacing

Question 18

how are MTs stabilized?

Answer 18

by +- TIP proteins (tubulin interacting proteins)

capture and protect the growing + ends
-decrease the likelihood that MTS will undergo catastrophic subunit loss

Question 19

How are MTs destabilized? + examples

Answer 19

by microtubule-destabilizing/ severing proteins

-stathmin/OP 18: bind to tubulin heterodimers and PREVENT polymerization

-catastrophins: acts at the end to PROMOTE the peeling of subunits from the + ends (MCAK)

-katnins: SEVER MTs

Question 20

What is “The Mesh”

Answer 20

new organelle
a network of microtubules connectors that stabilizes individual kinetochore fibers of the mitotic spindle (during meiosis/ mitosis) where chromosome attaches

-each mesh holds 2 MTs tgt bc clathrin is in its trimeric state

Question 21

How are kinetochore stabilized in the MEsh

Answer 21

uses clathrin complexes with TACC3 (Transforming Acidic Coiled-Coil) to stabilize fibers where chromosomes attach

1. recruitment of TACC3 and ch-TOG to MTs: regulated by Aurora-A-Kinase-mediated phosphorylation on Ser558
2. Clathrin recruitment: clathrin binds to TACC3 or TACC3/ch-TOG sub-complexes on MTs
3. complex accumulation: clathrin binds more than one TACC3 on adjacent MTs-> locking the complex= bridges between adjacent parallel