CSF ch 16 Microtubules

Microtubules (MTOC)

Largest diameter, hollow and stiff

dimer of a/b tubulin

Dynamic of MT

rapidly assemble and dissemble in location

2 classes of motor proteins: dynein and kinesin

Function of MT

cell polarity, cytoplasmic organization

Organelle/axonal transport

Mitotic spindle chromosome segregation

cilia/flagella by permanent MT assemblies

MT= ER network anyways

MT structure

a/b dimers (tubulins)

• A and B bind GTP

• only B hydrolyze GTP

Linear polar protofilament

13 protofilament

Microtubules are polar

a tubulin: (-) end (slow)

b tubulin: (+) end (fast)

ab tubulin heterodimer

contains 2 GTP, 1 each

only B hydrolyze

MT rings join laterally→ polymers

Linking proteins

bend conformation

MTOC: y-tubylin

nucleates microtubules at centrosome

(-) stability

(+) end instability and polymerization/depolymerization

Basal body and centriole: 9+0

Axoneme cilia and flagella: 9+2

Microtuble GTP hydrolysis

GTP cap stabilize and grows

• with GTP, no disassembly

GDP: polymerization and less stable

Dynamic Instability- MT

Near Cc

Slow polymerization gtp-tubulin at (+) end

rapid dissociation GDP present (+) end

Dynamic instability caps

GTP: grow/rescue

GTP loss: catastrophe

y-tubulin- mt protein

nucleates at (-) end

Plectin

links IFs

MAPS-MT

Bind and stabilize at (+) end

Kinesin 13-MT

catastrophe/disassembly

Parallel bundling and cross link-MT

MAP2 and Tau

Branching and Nucleating

Augmin

+TIPS

Bind (+) end

• MAP215: grow (+) end

• Kinesin 13: catastrophe (+) end

• EB1: track (+) end, affinity GTP cap

Catastrophies

Catastrophin: enhance catastrophe

XMAP215: suppress catastrophe (stabilize (+) end)

Stathmin

bind 2 microtubule dimers to decrease subunit availability

• regulate dynamic instability

• fearless mice (neurons)

• equal to thymosin

sequester and sever tubulin

Stathmin and Katanin

Katanin

Sever MT

Slice 13 bonded surface

2 complexes

• Small ATP severing

• Large at (-) end, sever mitosis

Katanin mutants

MT increase in knockouts

replace with GTP cap to grow

Actin microfilaments and mictrotubulues

destabilize from nuc. hydrolysis

Microtubule specific proteins

ATP hydrolysis and bi-directional

Unidirectional motor proteins

Kinesin: (+) ended (13/14 exceptions)

• anterograde axon, outward er, segregation

Dynein: (-) ended

• retrograde axon, inward golgi, cilia/flagella

Structure of motor proteins

microtubule binding, atp binding/activity, movement by head

2 headed from dimerization

Kinesin types

n-term (+) ended: kinesin 1 and 5

c-term reversed: kinesin 14

central motor: pull subunit, kinesin 13

Kinesin activity

lagging head bind ATP

Leading head bind ADP

Forward displacement rear motor domain by disassociating ADP and bind ATP in leading head

• neck liner: ATP bind and moved it to forward pointing

Pulls rear head forward

• PI ejected

Kinesin vs Myosin

Kinesin

• Hand over hand/ 8mn

• Processivity: hundreds

• (+) end MT

• ATp binding: swing trailing head

Myosin

• Ratchet movement/ 5mn

• processivity: 1-2 cycles

• (+) end actin

• atp binding: disassociates

Dynein- MT motor protein

(-) end

2 types

• Cytoplasmic: dynactin

• Ciliary/axonemal: bridges

Dynein strucyure

Contains Motor head domain

Cytoplasm dynein (mt (-) end)

Transport vesicles/organelles along MT

link by dynactin complex

• dynactin: dynein + arp1

Ciliary/axonal dynein (mt (-) end)

bind doublet MT by tail and globular head

form bridges (power stroke towards - end)

Axoneme=MT

MT core of cilia/flagella (9+2 arrangement)

• contains nexin, ABmt, dynein, radial spoke

Linking proteins

cilia and flagella, prevent sliding by bending

ATP and linking proteins shift and bend

Basal bodies

MTOC and 9+0 arrangements

Anchor MT of axoneme at surface cell

Cilia= signaling functions

vision/smell

Co-opt centriole as basal body

(-) dynein

(+) kinesin/myosin