Microtubules part 2

Summary

The transcript explores the dynamics of microtubules in cells, detailing critical concentration, growth, and disassembly processes. It introduces concepts like treadmilling and dynamic instability, highlighting how the balance of tubulin subunits influences microtubule behavior. The role of microtubule-associated proteins (MAPs) in stabilizing or destabilizing microtubules is discussed, along with the impact of drugs like colchicine and taxol on microtubule dynamics, which are used in cancer treatment. The script also covers the structure and function of the microtubule organization center (MTOC) and its significance in cellular processes, including organelle positioning and microtubule orientation.

Takeaways

• 😀 Microtubules grow when the concentration of tubulin subunits in the cell exceeds a critical threshold known as the critical concentration (Sisi).
• 😀 When the concentration of tubulin subunits falls below the critical concentration, microtubules disassemble.
• 😀 The phenomenon of 'Treadmilling' occurs when the addition of tubulin at the plus end of the microtubule equals the rate of loss at the minus end, keeping the length constant.
• 😀 Dynamic instability describes the rapid growth and shrinkage of microtubules, where disassembly happens much faster (7 microns per minute) compared to growth (1 micron per minute).
• 😀 The stability of microtubules is influenced by the amount of beta-tubulin bound to GTP at the plus end, with higher levels promoting growth and lower levels leading to disassembly.
• 😀 Microtubule-Associated Proteins (MAPs) regulate the assembly and stability of microtubules, with some stabilizing and others destabilizing them.
• 😀 Some MAPs, like MAP1, help stabilize microtubules, while others, such as tau, can destabilize them by promoting disassembly.
• 😀 Drugs like colchicine and taxol can interfere with microtubule dynamics, making them useful in cancer treatment by halting cell division.
• 😀 MAP2 has longer arms, leading to greater spacing between microtubules compared to the tau protein, which has shorter arms and results in a more compact arrangement.
• 😀 The Microtubule Organizing Center (MTOC) is responsible for organizing microtubules in the cell and controlling their orientation, particularly during mitosis.
• 😀 The MTOC contains centrioles and is associated with gamma-tubulin, which helps in the nucleation and growth of microtubules from the centrosome.

Q & A

What is the critical concentration of tubulin in the context of microtubules?

The critical concentration of tubulin refers to the concentration at which microtubules neither grow nor shrink. When the tubulin concentration is above this value, microtubules will elongate, whereas if it's below, microtubules will disassemble.

What are the two phenomena that occur when the tubulin concentration is around the critical concentration?

The two phenomena that occur are 'treadmilling,' where the addition of subunits at the plus end of the microtubule is equal to the loss of subunits at the minus end, resulting in no change in length, and 'dynamic instability,' where the microtubules alternately grow and shrink rapidly.

Why does microtubule shortening occur faster than elongation?

Microtubule shortening occurs faster because the disassembly (or depolymerization) at the minus end happens at a rate of 7 microns per minute, whereas the growth (polymerization) at the plus end only occurs at 1 micron per minute.

What determines the stability of microtubules?

The stability of microtubules is determined by the amount of beta-tubulin at the plus end that is still bound to GTP. A high concentration of GTP-bound beta-tubulin promotes stability and growth, while low levels lead to rapid disassembly.

What is the role of MAPs (Microtubule-Associated Proteins) in microtubule dynamics?

MAPs play a crucial role in stabilizing or destabilizing microtubules. Some MAPs stabilize microtubules by preventing depolymerization, while others promote disassembly or organize microtubules into specific structures, such as parallel arrays.

How do MAP2 and tau differ in their function regarding microtubules?

MAP2 and tau both interact with microtubules, but MAP2 has longer arms and organizes microtubules into more widely spaced structures, while tau has shorter arms and creates a tighter spacing between microtubules.

What effect do substances like colchicine and taxol have on microtubules?

Colchicine and taxol disrupt microtubule dynamics by preventing microtubules from either growing or shrinking. Colchicine inhibits polymerization, while taxol stabilizes microtubules, preventing depolymerization, which is used in cancer treatments to stop cell division.

What is the Microtubule-Organizing Center (MTOC), and what is its function?

The MTOC is a cellular structure that helps organize microtubules, typically found near the cell's nucleus. It plays a key role in determining the orientation and dynamics of microtubules during cell processes such as mitosis and interphase.

How does the gamma-tubulin ring complex contribute to microtubule formation?

The gamma-tubulin ring complex is located at the MTOC and serves as the nucleation site for microtubule formation. It provides a template for the addition of tubulin subunits, particularly at the minus end of the microtubule.

What is the relationship between tubulin concentration and the dynamics of microtubules?

The concentration of tubulin determines whether microtubules will grow or shrink. At higher concentrations, microtubules polymerize and grow, while at lower concentrations, they depolymerize. This dynamic is crucial for cellular processes like cell division and intracellular transport.