Problem 5 - Cell and Molecular Biology: Concepts and Experiments

Question

Microtubules can be formed in vitro from tubulin that is bound to GTP analogues that (unlike GTP) cannot be hydrolyzed. What properties would you expect these microtubules to possess?

Step-by-Step Solution

Answer

They would be more stable and likely grow continuously without depolymerization.

Step 1: Understanding Microtubule Dynamics

Microtubules are made of tubulin dimers that bind to GTP, which is later hydrolyzed to GDP. This hydrolysis is crucial as it causes structural instability and provides dynamic instability to microtubules, leading them to either grow or shrink.

Step 2: Impact of Non-Hydrolyzable GTP

When tubulin is bound to non-hydrolyzable GTP analogues, the hydrolysis step is blocked, preventing the conversion of GTP to GDP. This leads to microtubules that cannot undergo the usual dynamic instability because they do not experience the GDP-induced depolymerization.

Step 3: Expected Properties of Microtubules

Microtubules formed with non-hydrolyzable GTP analogues are expected to be more stable because they lack the GDP-bound tubulin, which causes destabilization. Thus, they would likely grow continuously without the rapid depolymerization normally seen in natural microtubules.

Key Concepts

Tubulin-GTP Binding

Microtubules are dynamic structures composed of tubulin proteins. Tubulin dimers are the building blocks of microtubules. They bind with GTP (Guanosine Triphosphate) to initiate stability. This binding is crucial as it stabilizes tubulin dimers, enabling them to incorporate into the growing end of microtubules.

• GTP-bound tubulin adds to the growing microtubule end.
• The presence of GTP allows microtubules to extend.

Tubulin-GTP binding ensures that the assembly of microtubules is smooth and continuous. In a typical environment, tubulin bound to GTP adds structural integrity and lengthens the microtubule by forming a cap that protects it from disassembly. Without this critical binding, the dynamics of microtubule growth would not be possible.

GTP Hydrolysis

The process of GTP hydrolysis is fundamental to the behavior of microtubules. After a tubulin dimer is added to a microtubule, the GTP bound to the dimer is hydrolyzed to GDP (Guanosine Diphosphate). This step is key for microtubule dynamics because it brings about a change in the structure and behavior of the microtubules.

• Hydrolysis converts GTP to GDP after tubulin dimer incorporation.
• GDP-bound tubulin is less stable, leading to potential disassembly or shrinkage.

GTP hydrolysis induces dynamic instability, a hallmark of microtubule function. This instability allows microtubules to alternate between phases of growth and shrinkage, essential for their roles in cell division, intracellular transport, and cell structure maintenance.

Microtubule Stability

Microtubule stability is directly influenced by the binding and hydrolysis of GTP. When tubulin binds to non-hydrolyzable GTP analogues, the hydrolysis step is blocked. These analogues mimic GTP but do not convert to GDP, which impacts microtubule behavior dramatically.

• Lack of GDP conversion leads to continuous microtubule growth.
• Without the destabilizing effect of GDP, microtubules become more stable.

In typical scenarios, GDP-bound tubulins cause microtubule destabilization, leading to depolymerization. However, when GTP hydrolysis is prevented, as in the case with non-hydrolyzable GTP analogues, microtubules exhibit enhanced stability. This stability alters their dynamics, allowing them to persist without the usual rapid changes between growth and shrinkage seen in natural microtubule assemblies.