lecture 8

Study Tips for Exams

  • Encourage collaboration among peers to share successful study strategies.

  • Understand the material rather than relying solely on exams as a knowledge assessment.

  • Engage actively with the study materials, rephrasing key concepts in your own words.

Active Binding Proteins

  • Proteins play a critical role in shaping the cytoskeleton.

  • Example: Actin monomers (G-actin) polymerize to form filaments (F-actin) at appropriate times and places.

Critical Concentration and Treadmilling

  • Definition: The concentration of actin monomers at which the filament can either maintain its length or undergo shrinkage.

  • If above the critical concentration, filaments grow; if below, they shrink.

  • The plus end of the filament generally grows faster than the minus end.

Polarity of Filaments

  • Actin filaments are polarized, having distinct plus and minus ends.

  • The difference in growth rates is attributed to the chemical differences at each end.

  • At the plus end (rapid growth), the addition of actin monomers can keep pace with ATP hydrolysis.

  • At the minus end (slower growth), ADP actin leads to loss before new monomers can be added.

Actin Binding Proteins

  • Profilin: Promotes actin filament assembly by delivering G-actin to the growing plus end.

  • Thymosin: Acts as a brake to inhibit actin polymerization by binding to G-actin.

  • Cofilin: Binds to ADP actin at the minus end, causing disassembly and recycling of monomers.

Nucleating Proteins

  • Formin: Facilitates the formation of long, straight actin filaments in stress fibers.

  • Arp2/3 Complex: Nucleates branched actin networks important for lamellipodia and cellular protrusions.

Mechanism of Action

  • Actin nucleators speed up the assembly by providing stable oligomer structures, reducing lag time in filament initiation.

  • Arp2/3 nucleates branched networks; Formin assists linear filament growth.

Bundling Proteins

  • Fimbrin: Creates tightly bundled actin filaments, important for filopodia structure.

  • Alpha-Actinin: Forms loosely bundled actin filaments allowing myosin-driven contraction.

Myosin II and Muscle Contraction

  • Myosin II is an actin-binding motor protein that converts ATP into mechanical work.

  • Phosphorylation of regulatory light chains activates myosin, enabling it to slide actin filaments past one another, generating force and contraction.

  • Contraction involves a repetitive cycle of ATP binding, hydrolysis, head movement, and filament interaction.

Cellular Functions of Actin Networks

  • Actin dynamics play crucial roles in cell migration, division, and morphology.

  • Understanding the interplay between actin-binding proteins aids in comprehending cellular behavior.

  • The study of actin networks is essential for understanding development, especially in processes like heart formation.