Question

You have allowed microtubules to polymerize in vitro under conditions where you observe individual microtubule dynamics (e.g. using fluorescently-labeled tubulin and observing the microtubules on a fluorescence microscope). What happens to the newly exposed ends if you sever a microtubule with a high powered laser (to completely sever the microtubule, not just “nick” or damage it)? Why does this happen?

Answer 1

The newly exposed ends contains tubulin with GTP. The structure of tubulin-GTP subunit is straight. But the structure of tubulin-GDP subunit is curved. The straight tubulin-GTP subunits are located only at the growing (positive) end of the microtubules. They are responsible for the stability of the microtubules. But the tubulin-GDP sub units are curved and the microtubules are very unstable when they are at the growing (positive) end. So, if we sever a microtubule (tubulin) at the positive end, the microtubule will start to shrink.