Lecture 2

Question 1

Cytoskeleton organization

Answer 1

• Microtubules transport vesicles and proteins to different ends of the cell
• Polarized actin define cell shape and behavior
• Intermediate filaments contribute to cell polarity
• Interphase: Microtubules radiate from cell center, Actin enriched at cell cortex
• Mitosis: Microtubules form mitotic spindle, Actin at cell cortex disassembles
• Cytokinesis: Microtubules keep cell components seperate, Actin form contractile ring

Question 2

Microtubules

Answer 2

• Alpha(-) and Beta(+) tubulin form dimers, hydrolyze GTP
• Assemble head to tail to make polarized protofilaments
• 13 protofilaments form hollow microtubule
• Rapid loss and gain of GTP cap cause dynamic instability. Alpha tubulin GTP bound
• y-tubulin interact with alpha-tubulin at (-) end. Nucleates/stabilizes (-) end and protect from depolymerization
• y-tubulin found near centrioles
• kinesin go toward (+) end, dynein go toward (-) end. Use ATP hydrolysis for energy

Question 3

Actin Filament

Answer 3

• Actin monomers asymmetric, hydrolyze ATP. actin monomers assemble into polarized actin filaments(two strand twist together)
• ARP2/3 complex nucleate (-) end of actin filaments. Undergo treadmilling
• Growing actin network pushes leading end forward, actin/myosin contract to bring lagging edge forward
• Integrin heterodimer directly bind extracellular matrix proteins, indirectly interact with actin filaments
• Myosin use ATP hydrolysis for energy -> walk toward (+) end
• Rac-GTP push cell forward, Rho-GTP push back of the cell

Question 4

Cell polarity

Answer 4

• C.elegans: sperm entry define posterior
• Trigger cytoskeleton polarization at fertilized egg
• Actin organized by Rho-GTP activity
• Microtubule organized by centrosomes where sperm enter