Lecture 13: Cytoskeleton Part 3: Microtubules

Microtubule Structure

• hollow tubes of 13 pro filaments

• Polar because their subunit is polar

Microtubule Subunit

• Heter dimer of alpha and beta tubulin, connected by IMFS

• Beta tubulin has its GTP hydrolyzed, back & forth while Alpha never does, so beta is the one regulating everything   

• Beta tubulin is on the outside and alpha is huddled on the inside 

TRUE OR FLASE: ACTIN USES ATP and MT’s use GTP

TRUEEEEE

Steps of MT polymerization 

1. Nucleation 

2. Elongation 

3. Steady State 

Main difference between Actin & MT polymerization

MT’s have a MUCH MUCH MUCH higher critical concentration, so they need help to nucleate and receive the help from MTOCs

MTOCs

microtubule organizing center ex; centrosome

True or False: Do MT’s treadmill?

no they have dynamic instability

Dynamic Instability

Mts grow out the centrosome with the plus end coming out but then spontaneously shrink back

• intrinsic and individual to each microtubule 

Catastrophe

Mts switching form growing to shrinking

Rescue

Mts switching from shrinking to growing

Why do we see dynamic instability

because of GTP binding & hydrolysis at the beta subunit

True or False: The GTP Cap stabilizes the filament and allows for polymerization so once the GTP becomes GDP we see shrinkage (rateo of hydrolysis becomes greater than addition)

TRUE 

Microtubule with GDp vs GTP

• with GDP, we see peelign because of the change in confirmation and in turn ctastrope

• with GTP, the confirmation allows for addition of the more subunits 

What causes Mts to shrink 

rate of addition of GTP Actin is slower than the addition and creation of GDP actin 

Two Categories of MT regulating Proteins

1. Promoting Assembly 

2. Disassembly 

Factors that promote MT assembly

1. Centrosome/Gamma Turcs 

2. Augment

3. Plus end polymerizing proteins

4. Stabilizing Proteins  

Factors that promrote MT disassembly

1. kinesis-13 

2. Katanin 

Centrosomes & Gamma TURCS

• centrosomes nucleate MT’s with the plus + end growing off & the minus end staying in

• The gamma turc protein itself is doing the work NOT just the centro

• Gamma turc is pact of the pericentirolar material (proteins inside the centrosome)some on its own.

Gamma Tubulin Ring Complex 

13 gamma tubulin subunits form a ring that templates the new MT

Augmin

• promotes MT polymerization by binding the the side of existing MT’s to create a branched network 

• Recruits a Gamma Tubulin ring complex, creating the new MT to the side

Plus end polymerizing Proteins

• Assembly proteins

• Binds to the Mt’s plus end and promotes polymerization by accelerating the addition of subunits to the plus end 

Stabilizing Proteins

bind to the side of microtubules and stabilize the microtubule lattice, making it less likely for catastrophe to happen

• stabilize end to end and side to side interactions

Kinesin-13

binds to the plus end of Mts and promotes

• it peels the filament apart, having a destabilizing effect

Katanin

Sevrs MT’s right in half Fucntisno of

Microtubules (function)

1. Organize the cell’s interior, by posititong organneles(that one guy on tik-tok )

2. Important for cell division 

True or False: The Centrosome is near the center of the cell, so the plus endss are in the center and the plus ends are farther away from the cell. In turn, Dynein and Kinesin help move cargos to positon organelles

TRUE

TRUE or FALSE: Golgi is deployed to its location usign Dynenin since it’s behidn the nucleus whiel the Er uses Kineiss since it is far away

TRUE