Which end do microtubules polymerize?

Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer. Therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed. These ends are designated the (−) and (+) ends, respectively.

Where do microtubules polymerize?

Tubulin dimers polymerize to form microtubules, which generally consist of 13 linear protofilaments assembled around a hollow core (Figure 11.37). The protofilaments, which are composed of head-to-tail arrays of tubulin dimers, are arranged in parallel.

Where does nucleation of microtubules occur?

Microtubule nucleation occurs spontaneously in purified tubulin solutions, and molecular intermediates between tubulin dimers and microtubules have been identified. Microtubule nucleation is enhanced in tubulin solutions by the addition of gamma-tubulin or various gamma-tubulin complexes.

Where is the plus end of a microtubule?

Microtubules are ever-changing, with reactions constantly adding and subtracting tubulin dimers at both ends of the filament (Figure 1). The rates of change at either end are not balanced — one end grows more rapidly and is called the plus end, whereas the other end is known as the minus end.

Why do microtubules polymerize?

Microtubules are hollow cylinders of about 25 nm in diameter constructed from the protein tubulin. Tubulin needs to bind GTP in order to polymerize, and hydrolysis of this bound nucleotide is an important factor in the growth and stability of microtubules (for reviews, see 2, 3).

What must happen at the end of the microtubule in order for it to stop shrinking and start growing?

A) What must happen at the end of the microtubule in order for it to stop shrinking and to start growing? GTP-loaded subunits are added quickly enough to cover up the GDP-containing tubulin subunits at the microtubule end, a new GTP cap can form and regrowth is favored.

What is polymerization of microtubules?

Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability.

Which end of the microtubules is bound to the Mtoc?

The minus end of γ-tubulin is anchored near the MTOC, whilst the plus end of γ-tubulin is exposed. This allows elongation to occur from the exposed γ-tubulin through interactions with the minus end of α-tubulin/β-tubulin heterodimers [2].

Which end of a microtubule is embedded in the centrosome?

The minus ends of these microtubules are embedded in the centrosome, while the plus ends terminate near the cell membrane. The plus ends grow and shrink rapidly, a process known as dynamic instability. At the start of cell division, the centrosome replicates and divides in two.

What is the significance of plus ends of microtubules?

At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures.

Why do microtubules grow and shrink during cell division?

When hydrolysis does occur, the constraint is removed and the protofilaments become highly unstable as the stored energy in the lattice is released. This results in rapid shrinking of the microtubule. A typical microtubule will fluctuate every few minutes between growing and shrinking.

What would happen if the solution contained an analog of GTP that Cannot be hydrolyzed?

What would happen if the solution contained an analog of GTP that cannot be hydrolyzed? Microtubules will grow until there is no tubulin left.

What is microtubule polymerization?

In order to understand microtubule polymerization, it is useful to first present some details about the structure of these polymers. Microtubules are hollow cylinders of about 25 nm in diameter constructed from the protein tubulin.

What is the polarity of the protofilaments in a microtubule?

The protofilaments bundle parallel to one another with the same polarity, so, in a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed. While microtubule elongation can occur at both the (+) and (−) ends, it is significantly more rapid at the (+) end.

What happens when GTP is added to the microtubule?

As long as new GTP-bound tubulin molecules are added more rapidly than GTP is hydrolyzed, the microtubule retains a GTP cap at its plus end and microtubule growth continues. However, if the rate of polymerization slows, the GTP bound to tubulin at the plus end of the microtubule will be hydrolyzed to GDP.

What is the difference between (+) and (−) microtubule elongation?

While microtubule elongation can occur at both the (+) and (−) ends, it is significantly more rapid at the (+) end. The lateral association of the protofilaments generates a pseudo-helical structure, with one turn of the helix containing 13 tubulin dimers, each from a different protofilament.