Biol211

• What are actin-based motor proteins called?

  Myosin
• The actin cytoskeleton is a dynamic structure.
• Actin filaments can grow or shrink in seconds depending on the environment.
• What happens to ATP bound to G-actin during filament assembly?

  It is hydrolyzed
• Most of the actin filament consists of ADP–F-actin.
• If ADP–F-actin is exposed at the ends of a filament, the filament will disassemble.
• What is the protein trimer required for actin polymerization called?

  Actin nucleation core
• Actin polymerization steady state occurs when growth equals depolymerization.
• What is the minimal concentration of actin monomers required for filaments to form called?

  Critical concentration
• The critical concentration is higher at the (-) end of the filament compared to the (+) end.
• Treadmilling occurs when free ATP–G-actin concentration is intermediate between C+c and C−c, causing the (+) end to grow and the (-) end to lose subunits.
• What does profilin enhance in actin dynamics?

  ADP to ATP exchange
• Cofilin destabilizes actin filaments by inducing a change in their twist.
• Thymosin beta-4 sequesters G-actin to prevent its polymerization.
• Which capping protein stabilizes the (-) end of actin filaments?

  Tropomodulin
• CapZ allows actin dynamics only at the (-) end of the filament.
• Formins nucleate the assembly of long actin filaments.
• What are the common domains found in all formin family members?

  FH1 and FH2
• The FH2 domain protects the (+) end of the growing actin filament from capping proteins.
• What are microtubules composed of?

  α- and β-tubulin heterodimers
• Intermediate filaments are formed from proteins such as keratin or lamin.
• Active Rho-GTP binding to formin exposes the FH2 domain and initiates actin filament nucleation.
• What is the characteristic angle formed by the Arp2/3 complex between actin filaments?

  70 degrees
• The Arp2/3 complex requires a nucleation promoting factor to nucleate branched filaments.
• What domain is adjacent to the FH2 domain in all formins?

  FH1
• The Arp2/3 complex comprises seven different proteins
• The shape of the Arp2/3 complex results in a characteristic 70-degree angle between two actin strands.
• What domain of the NPF binds to actin subunits in the Arp2/3 activation process?

  W domain
• Steps in the activation of the Arp2/3 complex

  1️⃣ Binding of actin subunit to W domain of NPF

  2️⃣ Two NPF-actin complexes bind Arp2/3

  3️⃣ Conformational change in Arp2/3

  4️⃣ Arp2/3 binds to the side of an actin filament

  5️⃣ NPFs are released
• Arp2/3-dependent actin assembly during endocytosis is driven by the activation of NPFs
• Cross-linking proteins like fimbrin build bundles of actin filaments with the same polarity.
• Which cross-linking protein forms stabilising links in the leading edge of migrating cells?

  Filamin
• In human erythrocytes, spectrin links to integral membrane proteins through ankyrin
• What is the role of dystrophin in muscle cells?

  Links actin to ECM
• Duchenne muscular dystrophy is caused by mutations in the dystrophin gene on the X chromosome.
• The cytoskeleton can reorganize in less than a minute
• What is one critical role of the cytoskeleton during embryonic development?

  Normal migration
• Actin is encoded by a gene family and is highly conserved across species.
• F-actin is formed from the reversible assembly of G-actin
• What ion is complexed with ATP or ADP in each actin molecule?

  Mg$^{2 + }$