Lecture 17 - Cytoplasmic trafficking and movement of organelles/vesicles Flashcards
What are the differences between actin and microtubules in regulating membrane trafficking?
Actin is found at the cell periphery. It forms the tracks for vesicular transport to the plasma membrane.
Microtubules form tracks for fast vesicular transport using kinesin (+ end directed; remember K+ potassium ion) or dyneine (- end directed) motors.
What is the MTOC?
The point where microtubules converge. This is rich in gamma tubulin.
What are the structural properties of microtubules?
Microtubules are formed of alpha and beta tubulin. They assemble in a head to tail conformation of alternating units. Microtubules are always found in tubular structures. Sideways the tubulin components are not exactly aligned and instead form spirals.
Alpha tubulin is always bound to GTP. Beta tubulin can bind to GTP which can be transformed to GDP by a beta tubulin dependent reaction. When this happens the affinity of the binding becomes lower. HIgh concentration of GTP bound tubulin molecules results in stable conformation. GTP eventually transforms to GDP and it becomes prone to dissociation. High amount of GTP concentration results in growing microtubule. As microtubule grows it can reach a level where the tubulin bound to GTP concentration is low and it causes catastrophe and as a result it breaks. When the opposite happens rescue occurs. As a result microtubules are very dynamic.
What are the characteristics of microtubules that allows them to translocate?
microtubule is polarized with a negative end (toward cytosol) and a positive end (towards MTOC)
Dissociation and association are faster at positive end compared to the negative end.
How does cargo travel long distances (through axons for example) if microtubule size is so limited?
Microtubules link to each other via accessory proteins to allow transport of long distances.
What are the steps in determining the rate of movement of cargo in an axon?
1) Radioactive amino acids injected into dorsal root ganglia near spinal chord.
2) Animals were killed at various times.
3) Sciatic nerve was removed and cut into small fragments for analysis.
4) Speed measured
What is the structure of kinesin that interacts with the vesicle? What is the direction of movement of kinesin.
Kinesin receptor interacts with the vesicle. Kinesin moves from -ve end to +ve end.
What is the function of the neck region of kinesin?
Neck region enables mobility of the heads.
What is the size of each step of kinesin?
8nm
How much ATP is consumed each step?
1 ATP
What are the domains found on kinesin heads?
Each head contains a catalytic core and a neck linker.
What is the sequence of events that kinesin uses to translocate along microtubules?
1) ADP bound heads have high affinity; head attaches releasing ADP.
2) ATP enters the nucleotide binding site to replace ADP. This results in neck linker zipping onto catalytic core.
3) The zipping motion “throws” the next head in front of the previous head.
4) Next head attaches releasing ADP and the process begins again.
How are motor proteins able to attach to different organelles?
Different tail and light chains allow different organelle specificity
What are different about motor proteins of the same subfamilies?
They differ in their tail and light chains which directs their function to different organelles.
What is the direction of kinesin movement affected by?\
Direction of movement of kinesin can be altered by a mutation in the neck region.
*Polarity of the neck region is essential for correct directional movement of motor protein.
