Cell Organization and Movement, Part 2: Microtubules and Intermediate Filaments

Understand the traits and functions of the various cytoskeletal elements.
Familiarize with techniques used to study the cytoskeleton.
Explain the dynamic properties of cytoskeletal elements and relate these to their construction and functioning.
Describe the mechanism by which molecular motors move along cytoskeletal elements.
Provide descriptions of structures and functions of specific molecular motors (dyneins, kinesins, myosins).
Examine microtubule organizing centers (MTOCs), their structures, and functions.
Describe the structure and function of cilia and flagella, detailing their similarities and differences.
Elucidate the mechanism of muscle contraction at both cellular and molecular levels.
Understand the existence and importance of non-muscle motility.

αβ-Tubulin
- Assembles into dynamically unstable and polarized microtubules characterized by (+) and (–) ends.
- Microtubules have walls formed from 13 protofilaments: 13, 13+10, and 13+10+10 arrangements.
GTP Hydrolysis
- Assembled tubulin undergoes hydrolysis of GTP.
Microtubule-Associated Proteins (MAPs)
- Mediate the assembly, dynamics, and functional roles of microtubules.
Microtubule Organizing Centers (MTOCs)
- All microtubules are nucleated from these centers, which often remain anchored by their (−) ends.
- Centrosome MTOCs consist of paired centrioles and surrounding pericentriolar material.

Roles of the Cytoskeleton
- Acts as a scaffold providing structural support and maintaining the shape of cells.
- Functions as an internal framework for organizing organelles within the cell.
- Directs cellular locomotion and the intracellular movement of materials.

Microtubules Characteristics
- Hollow, cylindrical structures composed of globular proteins arranged in protofilaments.
- Contains 13 protofilaments with dimers of α- and ß-tubulin subunits; arrangements create tubules with plus and minus ends.
Tubulin Dimer
- Composition: Stably associated α-tubulin and β-tubulin monomers (both 55 kDa).
- Functional Differences:
- α-tubulin: GTP is nonexchangeable.
- β-tubulin: GDP is exchangeable with GTP and can be hydrolyzed.

Structural Polarity
- Dimers align end-to-end into protofilaments, all in the same orientation.
- Protofilaments pack side-by-side with the same subunit polarity forming the microtubule wall.
- Staggered arrangement leads to the interaction of α-tubulin from one protofilament with α-tubulin from adjacent ones at seams with β-tubulin.
- Subunits preferentially add at the (+) end (
  \beta-tubulin exposed).

Singlet Microtubules
- Comprised of 13 protofilaments; primarily found in cytoplasm.
Doublet Microtubules
- Form additional wall with 10 protofilaments found in the outer structure of cilia/flagella.
Triplet Microtubules
- Composed of two additional walls (10 protofilaments) on a 13-protofilament base, found in centrioles and basal bodies.

Microtubule Assembly
- They are initiated from specialized structures known as MTOCs.
Centrosomes in Animal Cells
- Contains two centrioles and surrounding pericentriolar material that facilitates microtubule creation.
- A centriole structure has 9 sets of triplet microtubules (A, B, C).
Contrasting Structures
- Plants lack centrosomes but utilize alternative methods for nucleating microtubule assembly.

Microtubule Nucleation
- Controlled by γ-tubulin found in all MTOCs; critical for MT nucleation.

Dynamic Instability
- Individual microtubules exhibit alternating growth and rapid disassembly phases, depending on GTP-cap or GDP-cap status.
- Energy from GTP hydrolysis is stored and utilized during microtubule disassembly.
- Dynamically unstable microtubules can search out and capture organelles efficiently.
Growth Preference
- Microtubules favor assembly at (+) ends and exhibit rapid dynamic changes in length.

Stabilization and Destabilization
- Side-binding MAPs increase stability.
- (+) end-binding +TIPs modify dynamics or connect cellular components.
- Proteins such as kinesin-13 and Op18/stathmin destabilize the microtubules, enhancing catastrophe frequency.
Microtubule-Associated Proteins (MAPs)
- Comprises diverse protein groups that stabilize and promote microtubule assembly.

Properties and Classes
- IFs are fibrous filaments composed of five classes of IF proteins, four of which show tissue-specific functions.
- Class V lamins provide structural support for nuclei, interact with chromosomes, and connect the cytoskeleton within the cytoplasm.
- IF defects contribute to various human diseases and are less sensitive to chemical stress compared to other cytoskeletal elements.

The cytoskeleton consists of three main structures: microtubules, intermediate filaments, and microfilaments (actin), which are involved in many cellular functions.
Microtubules are hollow tubes 25 nm in diameter, composed of tubulin, and are involved in mitotic spindle formation, centriole structure, and as components of motile cilia/flagella.
Three motor protein families exist – kinesins and dyneins (for microtubules), and myosins (for microfilaments) – generally transport materials in opposite directions along microtubules.
Microtubule nucleation is associated with MTOCs, showcasing dynamic polymerization subject to growth and shrinkage cycles.
Cilia and flagella contain a core axoneme structure of microtubules, essential for their locomotion capabilities.
IFs display structural versatility and assembly composed of various proteins and regulate cellular integrity through coordinated action with other cytoskeletal components.