Cytoskeletal & Microtubules

Question 1

Adhesion & Movement

Answer 1

• cytoskeleton
• adhesion molecules
• extracellular matrix

Question 2

Cytoskeleton Components

Answer 2

Microfilaments – actin (7-9nm)
Microtubules (MT) – αβ-Tubulin dimer (25nm)
Intermediate Filaments (IF) – various (10nm)

Question 3

Microtubule (MT) Structure

Answer 3

• polymer of α and β tubulin –> form protofilament
• have polarity = α one side (-) and β other side (+)
• hollow tubes

Question 4

Dimer Tubulin Subunit

Answer 4

• very stable
• α binds permanently to GTP
• β bound to GTP or GDP b/c can hydrolyze GTP
• growth of polymer

Question 5

Arrangement of MT Protofilaments

Answer 5

Singlet = 13 protofilaments (cytoplasm)

Doublet = 13 protofilaments + one 10 protofilament
(cilia and flagella - axonemal MT)

Triplet = 13 protofilaments + two 10 protofilament
(basal bodies and centrioles)

Question 6

Microtubule Assembly

Answer 6

• dynamic
• centered around MTOC (microtubule organizing center)
• (+) ends grow away from MTOC (sense of direction)

Question 7

Centrosome

Answer 7

• major MTOC

- contain 2 centrioles

Question 8

Centrioles

Answer 8

• 9 triplets in a ring
• do NOT cause polymerization of singlet MT
• in pericentriolar matrix = polymerization of singlet MTs

Question 9

Pericentriolar Matrix

Answer 9

γ-tubulin

• found in MTOC at (-) end
• no growth = capped

augmin

• facilitate microtubule branching
• form mitotic apparatus

Question 10

Cytoskeletal Polymerization

Answer 10

• elongate from single subunits –> form nuclei (time)
• nucleation removes lag
• polymerize above critical concentration

Question 11

Microtubule Disassembly

Answer 11

• disassemble chilled to 4°C

- lag without nuclei

Question 12

Dynamics of Microtubules (4)

Answer 12

Assembly – αβ subunits added to GTP cap
Catastrophe – subunits < critical concentration
Disassembly – depolymerize
Rescue – subunits > critical concentration = polymerize

Question 13

Dynamic Instability

Answer 13

• presence of GTP-β-tubulin cap
• GTP hydrolysis weakens cohesion = fraying
• GTP = adhesion
• GDP = disassembly (high hydrolysis)

Question 14

Microtubule Disrupting Drugs

Answer 14

Colchicine = depolymerize -- no transport
Taxol = stabilize -- prevents depolymerization

Question 15

Microtubule Associated Proteins (MAPs)

Answer 15

• alter microtubule stability = no depolarization

MAP2 = long projection domain (Far spacing)
Tau = short projection domain (Close spacing)

+TIPs = associated with (+) ends of MTs

Question 16

Stability of MT End Proteins

Answer 16

XMAP215 – adhesion, prevent fraying, slow hydrolysis
CLASP – adhesion, prevent fraying, slow hydrolysis

Kinesin-13 – remove dimers with ATP
Stathmin – promote GTP hydrolysis + fraying