Topic 31: Microtubules

What are some common themes for actin filaments and microtubules?

• inherently assembling from soluble subunits

• polarity (+ and - ends)

• dynamic regulation of polymer length coupled to nucleotide hydrolysis

• polymerization stimulated by nucleators, and inhibited by sequestering proteins

• stability & geometry regulated by binding and serving proteins

• serve as substrate for motor proteins

What are tubulin protofilaments assembled from?

• soluble alpha-beta tubulin dimers

alpha beta tubulin dimers

• the basic subunit with bound GTP nucleotides

how do tubulin dimers assemble?

• assemble head to tail to make microtubules with a plus end and a minus end

how many protofilaments make up a microtubule?

• 13 protofilaments associate side to side to form one microtubule

• hollow lumen

• many lateral contract means that MTs are thermally stable, stiff and difficult to bend

what is a seam in a microtubule?

• where alpha and beta are interacting

under polymerizing conditions, growth is more rapid at what end?

• more rapid at the plus end than minus end

what does GTP hydrolyisis favor?

• rapid microtubule depolymerization - dynamic instability

dynamic instability

• when GTP hydrolysis catches up to the plus end, rapid depolymerization results

during rapid depolarization, GTP hydrolysis introduces what?

• curvature in the protofilaments

straight

• gtp bound

curved

• gdp bound

When present, a GTP-tubulin cap at the plus end overrides what?

• the mechanical strain resulting from GTP hydrolysis

the curvature in GDP-tubulin protofilaments is rapidly released upon what?

• upon loss of GTP cap (catastrophe)

Rapid depolymerization can be halted by what?

• are-acquisition of a GTP cap (rescue)

Catastrophe/rescue dynamics appear to predominate over what?

• treadmilling-type dynamics seen with actin

The gamma-tubulin ring complex functions as what?

• a microtubule nucleator

gamma-TuRC functions as what for microtubule polymerization?

• functions as pre-formed platform fro microtubule polymerization

function of the centrosome

• functions as the major microtubule organizing center for animal cells

what does the centrosome contain structurally?

• contains a pair of centrioles surrounded by pericentriolar material rich in gamma-TuRCs

which way do microtubules grow?

• grow plus end out from centrosome

centrioles

• microtubule-based structures with a conserved 9-fold symmetry

• consists of s symmetrical array of 9 microtubule triplets

• organizes the pericentriolar material

what happens to the number of centrosomes in each round of the cell cycle?

• they duplicates; mother & daughter centrioles move apart, and two new daughter centrioles form

what do mother centrosomes serve as?

• basal body of a ciliary axoneme

what do microtubule-binding proteins regulate?

• the geometry of microtubule arrays

what is the packing of the microtubule array regulated by?

• regulated by the spacer arm size of microtubule-binding proteins

  • partially analogous to actin binding proteins alpha-actin and fimbrin)

what does stathmin regulate?

• regulates microtubule polymerization by sequestering tubulin heterodimers

what is stathmin analogous to?

• analogous to thymosin, which sequesters actin monomers

what does phosphorylation cause stathmin to do?

• release tubulin dimers

proteins that bind near microtubule plus ends regulate what?

• microtubule dynamics

Protein EB1

• acts as a plus end tracking protein (+TIP) that recruits other +TIPs to microtubule plus ends

Microtubule (MT)-binding proteins regulate what?

• microtubule stabillity

Kinesin-13 and XMAP215

• two proteins recruited to MT (+) ends

The ratio of kinesin-13 to XMAP215 regulates what?

• how dynamic the microtubule array is.

Katanin

• servers microtubules

What does katanin perform?

• a MT severing function analogous to cofilin severing of actin filaments, but it requires ATP, unlike cofilin

why does katanin requires ATP

• because more bonds must be broken to sever a microtubule than an actin filament, because of all the lateral contacts

What can katanin's functions result in?

• MT destabilization or amplification, depending on local conditions (tubulin-GTP dimer concentration, etc)

kinesins and dyneins are what end-directed?

• plus end directed microtubule motor proteins

which ways do kinesins and dyneins move?

• kinesins move towards the plus end

• dyneins move towards the minus end

1 ATP per step

intracellular cargo can be found by multiple _____ at the same time. What does this allow for?

• motor proteins

• allows for dynamic regulation of the direction of transport

Cells use microtubule (MT) networks and motor proteins to do what?

• position organelles such as ER and Golgi

Where in the cell is ER maintained?

• at cell periphery (towards MT + ends)

Where in the cell is the golgi maintained?

• near MTOC (towards MT - ends)

what are polarized microtubule arrays in nerve cell axons used for?

• for retrograde and anterograde transport

anterograde

• outward

• cargo inside includes mRNAs, mitochondria, material for synapse formation & maintenance

Retrograde

• inward

• cargo includes internalized, activated growth receptors sending survival signals to the cell body (target-dependent trophic support)