Lecture 17 & 18 - Microtubules Flashcards
What is the main purpose of the cytoskeleton?
it maintains cell shape, organization, and provides support for internal and external movement
What are the 3 classes of cytoskeletal filaments? What proteins make them?
- microfilaments: actin
- microtubules: tubulin
- intermediate filaments: various proteins
What is a tubulin subunit?
a heterodimer made of two closely related globular proteins called alpha-tubulin and beta-tubulin
A microtubule is built from 13 ________ _________
parallel protofilaments; each composed of alpha/beta-tubulin heterodimers stacked head to tail and then folded into a tube
Why is the helical shape advantageous?
the helical microtubule lattice makes them stiff and hard to bend
What gives microtubules polarity?
the orientation of the subunits: the microtubule plus end grows and shrinks more rapidly than its minus end
What are the 3 phases of microtubule growth, starting with individual dimers?
- Lag phase - nucleation
- Elongation phase
- Plateau phase
What is microtubule nucleation?
the process in which several tubulin molecules interact to form a microtubule seed
- tubulin dimers assemble into protofilaments
- laterally associated linear protofilaments
- slow process: must form out of nothing
Dynamic instability
a process in which individual microtubules alternate between cycles of growth and shrinkage
What does the addition of GTP-tubulin to the plus end of a protofilament do?
GTP-tubulin = alpha tubulin
it causes the end to grow in a linear conformation that assembles into the cylindrical wall of the microtubule
What happens to GTP in the microtubule?
at the (+) end it stabilizes the microtubule by creating a GTP cap, this allows the microtubule to grow
GTP gets hydrolyzed once its in the microtubule - this region is unstable and will not stand by itself without the GTP cap
if all GTP gets hydrolyzed, the microtubule begins to shrink
Catastrophe vs rescue
Catastrophe is the change from growth to shrinkage
Rescue is the change from shrinkage to growth
How do scientists think catastrophe works?
hydrolysis of GTP after assembly changes the conformation of the subunits and tends to force the protofilament into a curved shape that is less able to pack into the microtubule wall
What complex does nucleation depend on?
gamma-tubulin ring complex
- anchored to accessory proteins
- prevents nucleation from being very slow by providing support for tubulins to grow
- forms helical pitch so tubulin subunits can join to that instead of forming out of nowhere
Where do microtubules generally nucleate from?
the microtubule-organizing center (MTOC) where gamma-tubulin is most enriched
- many animals possess a single, well-defined MTOC called the centrosome
Centrosomes
composed of 2 centrioles and are surrounded by a dense mass of protein termed the pericentriolar material
- gamma-tubulin is in the pericentriolar material
What structure is gamma-tubulin located in? When is it not there?
the centromere; when the cell is dividing
Microtubule associated proteins (MAPs)
proteins that bind and stabilize microtubules
- help microtubules interact with other things
Map2 and Tau
MAPs
- set the spacing of the microtubule bundles
- Map2 is a long protein => larger spacing of bundles
- Tau is a shorter protein => smaller spacing of bundles
What can cause neurodegenerative diseases?
Tau mutations
- affects how neurons communicate => remove Tau => spacing is too tight (or microtubules are not bundled) => vesicles cannot travel through
What are classic kinesins?
motors that move towards the (+) end of microtubules
- moves from center (nucleus) to plasma membrane
Kinesin
- a tetramer protein
- two heavy chains & two light chains
- heavy chain has many domains
- head splits ATP and converts the energy into motion
- tail is cargo-binding
Movement of kinesins
- at least one kinesin head is always attached to the microtubule
- progressive steps (all the same size)
- for long-range
What is the common element in the kinesin family?
the motor domain of the heavy chain
Kinesin-13 proteins
(microtubule depolymerases)
- induce depolymerization uniquely from both ends of the microtubule
- incapable of movement
- regulate microtubule dynamics to control spindle assembly
Kinesin-14
unusual: it moves from (+) end to (-) end in motility assays
- its tail can bind microtubules and allows it to organize microtubule bundles
- parallel microtubules
- antiparallel microtubules
- clustering microtubules
- overlap microtubules
What is another microtubule motor protein, besides kinesin?
cytoplasmic dynein
Dynein
- much larger and more complex than kinesin
- head is force generating motor (AAA = ATPase domain)
- stalk contains the microtubule-binding site at its tip
- tail binds cargo
- ATP changes the conformational structure to disassociate microtubule binding
Dynein steps and direction
big but irregular; moves towards minus ends
Compare kinesin and dynein
Kinesin:
- small
- moves towards plus end
- ATP driven
- regular steps
Dynein:
- big
- moves towards minus end
- ATP driven
- irregular movements
Microtubule motors move _______ in the secretory pathway
vesicles; kinesin-mediated and dynein-mediated transport of vesicles, vesicular-tubular structures, and organelles
If both motor proteins are attached to a vesicle, how does it get transported in either direction?
kinesin must be inhibited for minus-end transport
- kinesin-binding proteins prevent kinesin-microtubule binding
Cilia and flagella
hairlike cell appendages that have a bundle of microtubules at their core
- the core is called the axoneme, which is composed of microtubules and their associated proteins
- axonemal dynein bends the axoneme which moves the cilium and flagellum
Melanosomes
rapid movement is mediated by dynein (or dynamin??) and microtubules
- melanosome is an organelle that synthesizes and stores melanin
- coordinated movement because microtubules are uniformly polarized
