Cell Biology – Course Pack
Microtubules – 25 nm, Microfilaments (Actin!) – 7-9 nm, Intermediate Filaments – 10 nm
- Polymer of α- β- monomers (55 kDa each).
Found in dimer (110 kDa each and 8nm
long!) because it is more stable.
- Dimers form protofilaments which then
form hollow tubes. 25 nm diameter and up to
100 μ m long.
- α- is (-) end and β- is (+) = polarity! Grow
preferentially from (+) end.
- Both α- β- monomers can bind GTP, however ONLY β-monomer can hydrolyze the
GTP into GDP. GTP creates stronger bonds, once hydrolyzed, easier to disassemble
- Protofilaments can arrange into singlets (13 protofilaments that are relatively
unstable, constantly rearranged) or doublets (13+10, found in cilia and flagella –
axonemel microtubules…nothing to do with axons) or triplets (13+10+10, found in
basal bodies and centrioles).
Centrosome: Is the Microtubule Organizing Center (MTOC) in mammals. Centrosomes
are not found in plants. Made of centrioles which are nine triplets at 90° to each other.
Note that microtubules do not arise directly from centrioles but rather from γ-tubulin
(specifically, γ-TuRC, γ-tubulin ring complex provides nucleating sites) and the protein
- Another type of MTOC are basal bodies at the base of flagella and cilia.
- There is a critical concentration
(Cc) of dimers above which you get
polymerization and below which you
- Also influenced by temperature
- Treadmilling may occur if Cc is
reached at one end but not the other.
However, this is done more so with
actin, fairly rare with microtubules.
Also note that Cc may be different for respective ends (e.g. (-) end has higher Cc,
however, it is usually capped and does not usually undergo growing and shrinking)
- Important, microtubules are dynamic! Constant remodeling in the cell.
Microtubule Disrupting drugs: colchicine depolymerizes and taxol which stabilizes. Both
are bad for the cell (thus why they’re often used as anti-cancer drugs, disrupt cellular
replication and other processes), remember dynamics (growing AND shrinking) are very
important! Microtubule Associated Proteins: Stabilize microtubules. Map2 and Tau. Eb1 is a MAP
which also acts as an attachment site
Microtubule Binding and Severing Protein
(1)Kinesin-13 – a protein that requires energy to disassemble (uses ATP)
(2)Stathmin – does not require energy, binds, and promotes GTP hydrolysis of β-
(3)Katanin (sword) – enzyme (so it requires ATP)
Microtubules as Tracks for Transport – How do we know? Experiments using Squid axon,
it’s large and very long. Inject radiolabeled amino acids into squid axon. The amino acid will
be incorporated in protein. This will allow us to track movement at various time points.
Kinesin most are (+) end directed (anterograde) motor proteins. Made of 2 heavy
chains containing ATPase activity (associate with the microtubules containing head,
neck and stalk) and 2 light chains which are variable in order to bind variable cargo.
Type 1 – Conventional. Moves 16 nm each hydrolyzed ATP (remember, each
dimer is 8 nm long)
Type 2 – Heterotrimeric
Type 5 – Homodimeric – does not carry cargo! Thus, no light chains. Used for
Type 13 – Has head domain, neck domain, but no stalk domain. Cannot bind
cargo, used for end disassembly as previously mentioned.
Dynein (-) directed (retrograde) motor proteins. Dynactin Binding Domain binds
the dynactin heterocomplex whic