Axonal transport

[Ranjita Dutta Roy]

Is it true that microtubules only can transport proteins/organelles in one direction? If so, what's the mechanism behind retrograde transport and protein recycling?

[Steffen]

It's not the microtubules which transport molecules but rather motor proteins "walking" along the tubules. So it depends on the motor protein in which direction proteins/organelles are transported. The two most important ones are kinesin (anterograde) and dynein (retrograde).

[Ranjita Dutta Roy]

Thank you for your reply, Steffen! I'm still dwelling on this topic, but this time I'm thinking more about how the rate of protein transport can be measured. I recently read the chapter about axonal transport in Kandel, and it seemed like in their example they cut up axons into small segments and did western blots for diffferent time points in order to see how far the proteins had travelled through the axons at each time point. I can see how this works for slow transport, but how is this actually related to excoytosis rates? Can we say that ''exocytosis of a specific protein balances transport of that protein'' (rate of travelling at stimulus=rate of exocytosis) or would this be an assumption?

[Steffen]

This would be an assumption which is probably wrong. It is not the case that vesicles for exocytosis are stored in the cell body and when needed rapidly transported down the axon to release chemicals in the end. It's rather like you have a bunch of vesicles in the presynapse from which sometimes (when an action potential arrives) some are released. The axonal transport is steady and provides the presynapse with vesicles all the time, just pushing more and more into the store. Sometimes you'll run out of vesicles (when too much activity is going on) and this shows you that the axonal transport can be slower than exocytosis. The fast axonal transport is appr. 50-400 mm/day which is not nearly enough to provide the presynapse with all needed vesicles from the cell body at the necessary rate.