Question

The dynamic instability of microtubules is absolutely required for their function. Describe the process of “Catastrophe”. When does it occur? Why does it occur?

Answer 1

Microtubules are a part of the cytoskeleton, they are made up of polymers of tubulin. These are responsible for providing stable structure and protection from mechanical injury to the cell membrane and organelles. Dimers of two globular proteins alpha and beta tubulin, polymerize to form microtubules. They extend upto 50 micrometers and are very dynamic. Thus facilitate transportation of materials within cells.

Nucleation is the first event in the formation of a microtubule. It starts with MTOC ( microtubule organizing centers). There is third type of tubulin is present inside MTOC, that is gamma tubulin. It binds with other proteins and form a complex. Here starts the polymerization, where other alpha n beta tubulin add up. This end is called - minus end and growth occurs towards + plus end.

Now occurs dynamic instability, which is assembly and disassembly at the ends. GTP is needed for assembly of dimers. Instability occurs due to growing and shrinking of dimers at the ends. GTP bound beta tubulin is stable however, it shortly hydrolyses to GDP after assembly. This becomes unstable, tends to fall off. Thus a GTP bound tubulin cap is needed so as to prevent the disassembly. This act as a rescue system. When hydrolysis occurs, depolymerization starts and shrinking of microtubule, this process is called as catastrophe. A microtubule has half life of 5-10 mins. Some can remain for few hours too.