Explain how a microtubule is assembled from alpha and beta s

Explain how a microtubule is assembled from alpha and beta subunits, and how this creates a polarity in the microtubule. Which end is the plus end, and which is the minus end? Why is this polarity important? Which end does the microtubule grow from?

Solution

Microtubules are like hollow pipes and are built with 13 parallel protofilaments. Microtubules contain a globular protein called tubulin . Tubulin protein consists of alpha and a beta tubulin subunits. These subunits stack on each other, with alternating alpha and beta tubulin forming a protofilament containing beta at one end and an alpha tubulin at the other end. The beta end is called the plus end and the alpha end is called the minus end. All the thirteen protofilaments are arranged in such a way that all of plus ends occur on the same side. This gives the overall microtubule a directionality or polarity.

Microtubules are polar structures with two distinct ends containing a fast-growing plus end and a slow-growing minus end. This polarity plays an important role in determining the direction of movement along microtubules. Microtubules grow by addition of tubulin subunits at the plus end.

Microtubules are known to undergo treadmilling. It is a dynamic behavior in which tubulin molecules bound to GDP are continually lost from the minus end. The lost tubulin subunits are replaced by the addition of tubulin molecules bound to GTP to the plus end of the same microtubule. The GTP hydrolysis in microtubules is known as dynamic instability. In this phenomenon, the individual microtubules alternate between cycles of growth and shrinkage.