microtubules

Question 1

what is the structure of microtubules?

Answer 1

they are long hollow cylinders made of the protein tubules. They are more rigid than actin

Question 2

how are microtubules formed? where are they formed?

Answer 2

They are made of protein tubule dimers (always beta plus alpha minus) that bind together in protofilaments which then bind to form a cylinder. They usually originate from some kind of microtubule-organizing center (MTOC) such as the centrosome, spindle pole, and basal body

Question 3

what is the difference between alpha and beta tubulin?

Answer 3

both can bind GTP but only beta can hydrolyze. beta is plus end and alpha is minus end

Question 4

what are the two microtubule structures that can exist?

Answer 4

T-form: bound to GTP

D-form: bound to GDP

Question 5

what is a GTP cap?

Answer 5

when subunits are added at a high rate, the GTP has not had time to by hydrolyzed and the tip of the polymer is in T form. This T form tip is a GTP cap. The cap stabilizes the plus end and keeps it straight.

Question 6

why is the GTP cap stabilizing?

Answer 6

the protofilaments are straight when they contain GTP and they can form more stable lateral bonds with neighboring protofilaments. hydrolysis to GDP curves the protofilament and destabilizes the microtubule

Question 7

what is dynamic instability?

Answer 7

individual microtubules alternate between a period of slow growth and a period of rapid disassembly. Each microtubule grows/shrinks independently of others.
catastrophe is the change from growth too shrinkage.
rescue is the change from shrinkage to growth

Question 8

what is cholchicine?

Answer 8

toxic drug that depolymerizes microtubules. comes from Coleus houseplant

Question 9

what is Paclitaxel?

Answer 9

a microtubule polymerizing drug that binds to beta subunit and stabilizes microtubule. comes from the Pacific Yew.
can be cancer treatment by interfering with growth of cancer cells by blocking mitosis

Question 10

how are microtubules nucleated?

Answer 10

by gamma-TURCs found on the centrosome. They have a ring structure that makes a good template for tubular microtubule formation.

Question 11

what is a centrosome? what role does it play in microtubules?

Answer 11

the centrosome is a major microtubule organizing center in animal cells. It is non membrane bound and made of dense mass of protein, always containing two centrioles. Tubulin polymerizes from gamma-TURCs in centrosome

Question 12

what is self centering?

Answer 12

microtubules can grow, push into the walls of their container, and eventually center the centrosome. This is important for establishing a coordinate system (positioning organelles/chromosomes)

Question 13

what are the nucleation sites of cilia or flagella?

Answer 13

Basal bodies: modified centrioles, one (not a pair) is found at the base of cilium/flagellum

Question 14

what are the types of accessory proteins for microtubules? (MAPs)

Answer 14

polymerizing
nucleating
capping
severing and depolymerizing
bundling
motor proteins

Question 15

what are polymerizing proteins? (MAP)

Answer 15

they stabilize microtubules

Question 16

what are nucleating proteins? (MAP) example

Answer 16

they start assembly (nucleate assembly). gamma-TURC

Question 17

what are capping proteins? (MAP)

Answer 17

they localize microtubules within cells and may anchor them to bilayers. this can polarize a cell

Question 18

What are severing and depolymerizing proteins? (MAP) example

Answer 18

they destabilize microtubules. Stathmin sequesters free tubulin dimers and causes microtubule shrinkage

Question 19

what are bundling proteins? (MAP)

Answer 19

they bind to microtubules and determine the spacing between them. ensures uniform spacing

Question 20

what are molecular motors?

Answer 20

enzymes that generate force and walk along microtubules toward either plus or minus end.

Question 21

what are the domains of motor proteins?

Answer 21

head domain: binds microtubules and generates force

tail domain: binds bilayers or other cargo

Question 22

What is kinesin? how does it function?

Answer 22

a dimer of two motor domains connected by coiled tail. Its rear head detaches, passes over other head, and rebinds. When motor domain is ATP bound it binds to binding site strongly, when hydrolyzed to ADP it is loose. One head is always bound so it doesn’t let go of microtubule track

Question 23

What are the two motor proteins we discussed and which end do they walk towards?

Answer 23

Kinesin: walks toward plus end
Dynein: walks toward minus end
cargo don’t necessarily bind to just one of these proteins, a tug of war or coordinate motor activity can occur

Question 24

what is cytoplasmic Dynein and axonemal Dynein? how do they differ from other motor proteins?

Answer 24

cytoplasmic Dynein: have 2 larger head domains. used similarly to other motor proteins
axonemal Dynein: have three large head domains. found in cilia and flagella, specialized for sliding movements of mictrotubules that drive beating of cilia/flagella

Question 25

how does intracellular cargo become loaded onto the correct motor protein?

Answer 25

the motor protein’s tail domain mediates cargo binding. adaptor proteins associate with motor proteins to regulate motor activity and to link motor proteins to appropriate cargo

Question 26

how are microtubules arranged in flagellum and cilium?

Answer 26

one basal body is found at the base of the cilium/flagellum and serves as the template for formation of axoneme (microtubules). there is a 9 +2 arrangement of Microtubules in the axoneme

Question 27

what are the structural elements of the axoneme that allow for bending?

Answer 27

there are outer doublet held in place by nexin to prevent sliding and Dynein is walking along neighboring microtubules. the walking of Dynein leads to bending of the axoneme and flagellum

Question 28

what are the motions used by flagella and cilia to move?

Answer 28

flagella: continuous sinusoidal wave
cilia: power stroke followed by recovery stroke

Question 29

what is kartageners syndrome?

Answer 29

the flagella and cilia can’t move. results in inversion of normal asymmetry of organs, male sterility, and susceptibility to lung infections

Question 30

How are microtubules able to transport cargo in long axons and dendrites?

Answer 30

microtubules can transport cargo long distances within cells. microtubules in axons and dendrites have their plus ends and minus ends oriented in both directions relative to soma. fibroblasts are only oriented with plus end away from soma