L3: Microtubules

Question 1

Microtubules basic structure and function

Answer 1

• alpha and beta-tubulin heterodimer assembled into hollow, tube-like cylinders
• Provide structural support
• Cell division
• Intracellular transport
• Cell motility
• Help generate cell polarity

Question 2

Drugs related to m.tubules, related diseases

Answer 2

• Colchicine (prevents assembly)
• Taxol (cancer drug, prevents disassembly)
• Gout, skin/joint conditions, cancer

Question 3

GTP and tubulin

Answer 3

• Whilst both types can bind GTP, only beta can hydrolyse the bound GTP to GDP (after m.tubule assembly), and exchange GDP for GTP (after subunit disassembles)
• GTP on alpha subunit is static

Question 4

Multiplicity in m.tubules

Answer 4

• Singlet
• Doublet (cilia, flagella)
• Triplet (basal bodies, centrioles)

Question 5

MTOCs/centrosomes

Answer 5

• Nucleate and organize m.tubules
• Most m.tubules start from a MTOC; (-) ends remain associated

Question 6

M.tubule polarity

Answer 6

• Heterodimers arranged head-to-tail, w/ crown of alpha at (-) and crown of beta at (+) end
• (-) end typically associated w/ MTOCs in non-polarized cells w/ (+) ends extending towards periphery
• Motors read and utilise polarity
• Ends have Cc and are able to treadmill in the same fashion as actin filaments

Question 7

Gamma-tubulin

Answer 7

• Most m.tubs are nucleated by gamma-tubulin ring complexes which are located in pericentriolar material of an MTOC
• It provides a BS for the dimer, and ring complex appears to provide a template for nucleating m.tub formation

Question 8

GTP cap model

Answer 8

• Explains dynamic instability in m.tubs
• GTP-tub can add to end of growing m.tub; eventually GT hyd. to GDP leaving GDP-tubulin (bulk of m.tub)
• GTP-tub only present at ends, cap stabilising whole m.tub
  -> if cap lost, m.tub shortens either until it disappears or until GTP-cap restored

Question 9

MAPs (microtubule associated proteins)

Answer 9

• Bind to m.tubs and promote assembly, increase stability and sometimes crosslink m.tubs into bundles
  e.g. MAP2, Tau

Question 10

End-binding m.tubule proteins

Answer 10

• +TIPs (plus end tracking proteins); influence assembly and disassembly
  e.g. EB1
• Kinesin13 helps disassembly
• Stathmin binds to curved protofilaments and enhances their disassembly

Question 11

EB1

Answer 11

Stimulates spontaneous nucleation and growth of m.tubs
Promotes catastrophes (transitions from growth to shrinkage)
Promotes rescues (reverse event)