Function of Microtubules?

Microtubules are conveyer belts inside the cells.

*They move vesicles, granules, organelles like

mitochondria, and chromosomes via special attachment proteins

 (hint: micotubules--> tubes--> transferring)

(ves/gran/org/chrom)

2 motor proteins for Microtubules

– Kinesin

– Dynein

hint: DK- Donkey Kong+ Barrow (tubules)

4 specific responsibilities of microtubules?

1. Beating of cilia and flagella

2. Transport of membrane vesicles 

3. Extension of neuronal growth cone

4 Formation of mitotic spindle

Protofilaments

linear polymers of tubulin which are actually globular protein.

are Microtubules polarized?

Yes, they have a Plus and minus ends giving them directionality!!!

What controlls microtubule stability?

Stability controlled by microtubule-associated proteins (MAPs)

Microtubule organizing center

(centrosome or basal body)=(MTOC) functions in what?

"Orientation of microtubules is determined

and fixed by MTOC"

Microtubules are made up of what?

Tubulin dimers

knowing that Tubulin is a GTPase, which binds

and hydrolyses GTP. How many GTP binds per tubulin?

What is the difference between the differing locations of GTP binding?

β and one at the α subunit

• β subunit --> GTP can be exchanged for GDP  

   

   

• α subunit---> Irreversible  

Protofilaments can be arranged into

what 3 kinds of microtubules, what specific functions

does each serve?

1. singlet- intracellular microtubule
2. doublet- flagella, cilia
3. triplet- basal body and centrioles

How are microtubules assembled?

Microtubules assemble by polymerization

of α and β subunits

Which state does tubulin assembly favor, GTP bound or GDP bound?

Assembly of GTP bound tubulin is favored over GDP bound

GTP>GDP

heterodimer

and alpha and beta subunit combined.

During nucleation of Microtubule, what is first necessary?

Nucleartion requires tubulin, Mg++, and GTP

1. tubues

2. Mg

3. GTP

after all the 3 components are present and nucleation progresses, what happens first?

A Heterodimer is produced (alpha +b tublin)

After the Heterodimer forms what is the next step in

Microtubule assembly?

protofilaments

When tubulins join to Microtubules what happens to GTP?

When tulin molecule adds to the microtubule, the GTP is hydrolyzed to GDP

GTP-----(hydrolyzed)-----> GDP

assembly of microtubles is preferred at which end of the microtubule?

@ which end is the rate greater?

Preferred at the (+) end, rate is higher at this end as well because the critical concentration is higher at this end

Dynamic instability means that Microtubules are in a constant state of assembly and disassembly, and the rate of ass/dissass is controlled by the needs of the cell, such as:

***Why does Dynamic Instability occur?

Dynamic instability occurs because the rate of polermization at the growing end is greater then the rate of hydrolysis of GTP in microtubules.

 The cell can provide a GTP cap on the growing end of a microtubule to regulate further growth.Thus, the microtubule becomes stable and does not depolymerize. It may also be encouraged to continue growing.

Microtubule depolarization occurs more rapidly at which end, the one with GDP or GTP?

depolarization occurs at the end with GDP, @ the GTP end GTP favors growth

tubules grow untill it can grow no longer which is when it shrinks, It shrinks untill cannot shrink any longer at which point grows

GTP cap encourages what?

Growth, loss of cap causes depolarization

What is the function of MAPs

(Microtubule Associated Proteins), how do they go about doing these functions?

Regulate microtubule assembly and

structure by 1. stabalizing and destabilizing.

– or 2. sever microtubules

– Also 3. cross link microtubules with plasma

membrane or other cytoskeletal filament

What are the 3 different MAPs?

1.Tau family

2. MAP2
3. MAP4

What is the mechanism of MAPs that allows it to serve it's function?

• Bind along the end or cap a microtubule

• One domain binds to tubulin polymers or

unpolymerized tubulin.

How does MAPs binding to the ends of tubulin assist in stabalizing microtubues.  

*This speeds up polymerization which will facilitates assembly and stabilizes the microtubules.

– The other end projects out and will bind to vesicles or

granules, IF or other MT.

Where can MAP 2 be found? What is it's specific function?

Where can MAP 4 be found? What is it's specific *function?

Location: Neuronal and non-neuronal cells

function: Regulates microtubule stability during mitosis

**only one with different specific function

 Where can "Tau" be found? What is it's specific function?

location: Axons and dendrites

Function: Cross-links microtubule into intermediate filaments    (similar to MAP 2 function)

2 Destabalizing MAPs?

1. Op18/stathmin

2. Katanin

Mechanism for destabalizing MAP "Op18/stathmin"?

Op18/stathmin

– Increases depolymerization

by binding tubulin dimers

– *Enhances dynamic instability

Mechanism for destabilizing MAP "Katanin"? 

Katanin

Function of Taxol and Vinblastine?

function: Drugs that disturb microtubule polymerization

-  Stabilize microtubules

– Block cell division

2 Microtubule motor protein families?

1. kinesins

2. dyneins

characteristics and functions of motor families

1.  Both move along microtubules

2. Both have heads (motor) and tail (effector) domains

3. Both transport vesicles and organelles

4. Dyneins also power eukaryotic cilia and flagella and chromosomes

5. Both bind and use ATP to do the work

Only special characteristics that is different from any other motor protein in #3.

Special function of Dynein motor proteins?

Dyneins powers eukaryotic cilia and flagella

and chromosomes

**Differences between Dyneins and

Kinesins

- Dyneins walk toward the - end

- Kinesins walk toward + end

Function of Kinesin in neurons?

function: It is responsible for fast axonal flow, in which organelles and vesicles are carried from near the centrosome in the cell body to axon endings.

refresher:

move toward +

Needs ATP

Where does ATP bind to on Kinesin?

Where does Microtubule binding occur?

Where does Vesicle binding occur

Both ATP and microtubule binding domains are on the heads 

Tail binds to vesicles

What is the function of the "hinge" region on stalk of motor proteins?

coiled coil is interrupted by a few hinge regions that give flexibility to the otherwise stiff stalk domain.

special characteristic of kinesin motor protein?

One head is  always attached to microtubule

What is the Mechanism for kinesin movement?

1. detached at first (ADP) state

2. releasing of ADP causes attachment of foward head to microtubule

3. ATP attachment to FWD head causes kickin of back legs to the front

4. New fwd head releases it's ADP- new cycle  

What do Kinesin and Myosin have in common?

1. Kinesins and Myosins both transport vesicles

2. They both have similar structures except *kinesin walks on microtubules

***Differences between Kinesin and Myosin Motor proteins?

3

2. kinesin walks on microtubule

3. the molecular mechanism is different from myosin

Difference between Kinesin and myosin reaction (movement) cycle?

hint:  mainly ATP binding causes different effect

• Kinesin

– Rigor-like tight state with ATP bound

– Each kinesin motor domain binds tightly to a microtubule when it has bound ATP.

• Myosin

– rigor tight state with

no ATP

– Myosin dissociates from actin upon binding ATP.