MCB 150 Lecture 24: Finish Actin; Begin Microtubules

Actin & Myosin

• Cells utilize actin and myosin for various functions, including:

  • Cytokinesis: Accomplished via a contractile ring of actin with myosin motors, leading to cleavage furrow formation. This process is essential for cell division in animal cells.

  • Cell Crawling: Involves the extension of the leading edge (lamellipodia) driven by actin polymerization, attachment to the substratum via focal adhesions, and retraction of the trailing edge mediated by myosin II activity.

  • Vesicle Transport: Uses unconventional myosin (classes 1 or 2), which move along actin filaments to transport vesicles to various locations within the cell.

  • Contraction: Muscle contraction, regulated by calcium ions and involving the sliding of actin and myosin filaments.

Myofibril Structure

• The sarcomere as the basic contractile unit.

• Sarcomere contraction: During muscle contraction, the sarcomeres shorten as the actin filaments slide past the myosin filaments.

Microtubules (MTs)

• Functions:

  • Guiding intracellular transport: Microtubules serve as tracks for motor proteins like kinesins and dyneins to transport organelles and vesicles within the cell.

  • Segregating chromosomes during mitosis: The mitotic spindle, composed of microtubules, ensures accurate segregation of chromosomes during cell division.

  • Propulsion or sweeping of fluids over membranes: Cilia and flagella, which contain microtubules, facilitate movement of cells or fluids.

• Structure:

  • Rigid, hollow tubes made of tubulin.

  • Tubulin is a dimer of α-tubulin and β-tubulin. These dimers assemble to form the microtubule structure.

• Polymerization:

  • Tubulin dimers polymerize to form microtubules. This process is dynamic and regulated by various factors.

  • Consist of 13 linear protofilaments surrounding a hollow core. The arrangement provides structural support.

  • Assembly occurs head-to-tail, giving the microtubule polarity. The plus end and minus end have different properties.

  • Polymerization and depolymerization are possible at both ends, but in vivo, most assembly and disassembly occur at the plus end. This dynamic behavior is crucial for microtubule function.

• GTP Binding and Hydrolysis:

  • Both α-tubulin and β-tubulin have GTP binding sites. GTP binding is essential for tubulin assembly.

  • Shortly after dimer addition to a MT, the GTP in the β-tubulin is hydrolyzed. This hydrolysis affects microtubule stability.

  • GTP hydrolysis weakens the affinity for other tubulin subunits. This destabilization can lead to depolymerization.

• Dynamic Instability and the “GTP cap”:

  • GTP-bound tubulin prevents subunits from peeling away, stabilizing the microtubule.

  • Tubulin-GTP favors polymerization, while the absence of GTP (GDP-bound tubulin) favors depolymerization. This balance regulates microtubule length and stability.