Question

Problem 1) Katanin is a protein that severs microtubules. Predict what will happen at the site of the newly exposed microtubule ends if the microtubules of a cell are broken in half. Explain your prediction.
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Answer 1

Cytoskeleton is a network of filaments or tubules that is involved in providing a shape and support to the cells.They have major roles in organising the various cell organelles,intracellular transport and cell division. They are found throughout the cytoplasm. Cytoskeleton consists of three filaments Microfilaments,intermediate filaments and Microtubules.

Microtubules are the largest of the three filaments measuring about 23nm .Microtubules are cylindrical hollow rods made up of tubulin subunits.A tubulin subunit contains an alpha tubulin and beta tubulin.These tubulin subunis are joined to form microtubules.

Microtubules have two ends

1.plus end(beta tubulin exposed)

2.minus end.(alpha tubulin exposed)

The plus end is the fast growing and minus end slow growing. The minus end will be anchored to centromere while addition of tubulin subunits will happen at plus end thus leading to the growth.

Each tubulin dimer is tightly bound to a molecule of GTP .Immediately after the addition of new dimer to growing microtubule the GTP is hydrolysed to GDP .GDP is loosely bound to dimers.A microtubule grow by adding GTP carrying subunit to the plus end.When GTP is hydrolysed it will leave a cap of GTP carrying subunit at plus end rest of tubulin subunits carrying GDP which is loosely bound.

So when a molecule like Katanin severe the microtubule in to halves.The newly exposed end will most probably shrink as it will expose the GDP carrying plus end .GDP is loosely bound so it can lead to instability (depolymerisation) and shrinkage of microtubules.

(Katanin is a microtubule severing protein belonging to the family AAA ATPases which can destabilise the microtubules by the energy released by hydrolysis of ATP and can increase the number of microtubules available for polymerization. )