Question

1. Compare alpha and beta tubulin in terms of nucleoside binding and hydrolysis.

2. What is the underlying basis for microtubule polarity?

3. What is the effect of temperature on microtubule assembly? How does the presence of short microtubule fragments affect tubulin assembly?

Answer 1

Answer 1 - The α-tubulin and β-tubulin monomers each bind one molecule of GTP, but they differ in their ability to exchange the nucleotide. The nucleotide bound to α-tubulin, at the so-called N-site, cannot be exchanged. The nucleotide bound to β-tubulin, at the E-site, can be exchanged. GTP is required at the E-site in order for tubulin to polymerize. Upon polymerization, however, this nucleotide is hydrolyzed and can no longer be exchanged.

Answer 2 -

The structural polarity of the microtubule  results from the fact that the tubulin  subunits that comprise the microtubule are heterodimers of alpha and beta tubulin . The polarity  of the microtubule exists not only at the two ends of the filament, but all along the length of its lattice.

Answer 3-

Microtubules assemble by polymerization of alpha and beta tubulin  dimers. Once microtubules have assembled, their stability is temperature-dependent. For instance, if microtubules are cooled to 4 °C, they depolymerize into stable αβ-tubulin dimers . When warmed to 37 °C in the presence of GTP, the tubulin dimers polymerize into microtubules.

The addition of fragments of other microtubules to a solution of αβ-tubulin accelerates the initial polymerization rate by acting as nucleation sites.