Cytoskeleton and Microtubules Lecture

Introduction

  • Lecture Overview: Grades returned on writing assignments with positive feedback.

  • Upcoming Test: Next exam focuses on cytoskeleton material; scheduled two weeks from now.

  • Preparation: Encourage students to begin studying early for effective preparation.

Writing Assignments

  • Total Number: Clarification that there will be only two writing assignments.

  • Assignment Number Change: Next assignment titled “assignment three” due to initial numbering; miscommunication clarified.

  • Explanation for Fewer Assignments: Length of time required to grade with increased class size.

Cytoskeleton Overview

Introduction to Microtubules

  • Topic Focus: Transition to discuss microtubules and their properties.

  • Comparison to Microfilaments: Encouraged students to consider similarities and differences between microtubules and microfilaments (commonalities in function vs distinctions in structure).

Historical Context of Microtubules

  • Initial Discovery: Awareness of microtubules and tubulin in the 1960s; involvement in cell division recognized.

  • Colchicine Treatment: Chemical that disrupts microtubule formation, demonstrating the effect on cellular structures.

Significance of Microtubules

  • Role in Cell Division: Microtubules form spindle structures, critical in separating chromosomes during mitosis.

Richard Weisenberg's Contributions (1972)

  • First to Reconstitute Microtubules: He discovered that tubulin proteins could spontaneously assemble into microtubules in the presence of GTP and calcium chelators.

  • Polymerization Theories: Connection to polymer science to explain microtubule formation.

Structural Properties of Microtubules

Polymer Definition

  • Definition: Polymers are made of repeated subunits, relevant to microtubules being linear polymers made of tubulin dimers.

Critical Concentration

  • Definition: The concentration of subunits relative to the polymerization process;

    • Above critical concentration: Microtubules elongate.

    • Below critical concentration: Microtubules shrink.

  • Graphical Interpretation: Representation of soluble protein concentration showing various phases of microtubule formation.

Composition of Microtubules

Tubulin Structure

  • Dimers: Microtubules are made from alpha and beta tubulin dimers;

  • Size: Around 55 kDa each; sequence homology about 50% across organisms.

Nucleotide Binding Requirements

  • Involvement of GTP: GTP binds to alpha subunit; not a GTPase, but a GTP binding protein.

  • Beta subunit also has a binding site; the nucleotide state varies between GTP and GDP depending on the assembly process.

Nucleation of Microtubules

  • Nucleation Sites: Microtubules nucleate at specific structures known as the MTOC (microtubule organizing center), typically associated with centrioles.

  • Gamma Tubulin: Acts as a nucleator at the MTOC and is critical for microtubule formation.

Dynamic Instability of Microtubules

Characteristics

  • Dynamic instability: Ongoing change in microtubule length characterized by growth and shrinkage.

  • Critical concentration addressed; comparison made with microfilaments.

Growth and Shrinkage Process

  • GTP Cap Mechanism: Formation of a stable cap encourages microtubule growth; loss of GTP leads to instability and potential breakage.

  • Phenomenon of Rescue and Catastrophe defined.

    • Rescue: Transition from shrinking to growing states, driven by GTP presence.

    • Catastrophe: Transition from growth to shrinkage.

Hypothetical Interaction of Proteins and Microtubules

  • Treadmilling behavior guided by alterations in bound GTP or GDP configurations of the dimers.

  • Factors affecting assembly dynamics: concentration of tubulin, availability of purifying proteins.

Microtubule-Associated Proteins (MAPs)

Role of MAPs

  • MAPs stabilize microtubules; classified as Class I (rescue) and Class II (catastrophe).

  • Specific examples: MAP1A and MAP1B bind catastrophe proteins to prevent microtubule disassembly.

Tau Protein

  • Normal functionality: Stabilizes microtubules by wrapping around them.

  • Hyperphosphorylation: Damages function and integrity, relevant in neurodegenerative diseases (e.g., Alzheimer's).

  • Hyperphosphorylated tau leads to plaque formation.

Class II MAPs

  • Engage in destabilizing microtubules to encourage catastrophe as a functional role.

Cilia and Flagella

Structures and Functions

  • Comparisons made between cilia and flagella (qualitative differences in length and movement capabilities).

  • Axoneme: Central structural element with a nine-plus-two array configuration of microtubules.

  • Dynein interactions lead to bending movements required for motility.

Integration of Microtubules in Movement

  • Dynein motor protein utilizes ATP to generate movement by walking along the microtubule structure, facilitating flagella and cilia movement through coordinated bending.

Conclusion

  • Recap of the importance of understanding microtubule dynamics, interactions, and structural properties for broader biological understanding.

  • Emphasis on the role of motor proteins, and the implications for cellular functions and disease states.