Lecture 23 - Microtubules

Microtubules

Function: Tracks for transport and organization within the cell

Discovered by: Keith R Porter

Microtubule structure

Made of alpha-beta tubulin dimers

Protofilaments: head-to-tail assembly of dimers

Microtubule: 13 protofilaments in a hollow cylinder

Plus and minus ends based on beta (+) and alpha (-) subunits

Growth occurs at the plus end

Alpha and beta subunits bind GTP, but only beta hydrolyzes it

TIRF microscopy

measures growth rates using fluorescence

Dynamic instability

constant switching between growth and shrinkage phases

Growth (+): Assembly

Shrinkage (-): Catastrophe

Rescue: Spontaneous regrowth

GTP hydrolysis

GTP favors assembly, GDP favors disassembly

Balance between hydrolysis and assembly determines growth or shrinkage

Microtubule-associated proteins (MAPs)

Modify microtubule behavior according to cellular needs

Examples:MAPs for nucleation and anchoring: y-tubulin ring complex (at centrosomes)MAPs for sequestration: promote catastropheMAPs for destabilization: destabilize dimers with ATPase functionMAPs for growth acceleration: XMAP215, CKAP2 (increase polymers)MAPs for stabilization: Tau, MAP2MAPs for crosslinking: connect microtubules to other cellular components

Microtubule severing enzymes

Cut microtubules in the middle (e.g., katanin)

Regulated by Aurora B kinase

Microtubule motors

Move cargo along microtubule tracks

Kinesin-1 (plus-end motor): uses ATP for movement

Structure: head (binds ATP), light chains (attach to cargo)

Movement: head-over-head stepping with ATP hydrolysis

Dynein (minus-end motor): uses ATP for power strokes

Mitotic spindle

Made of microtubules during cell division

Kinetochores: attach chromosomes to spindle fibers

Aurora B kinase

regulates chromosome separation

Cohesins

hold sister chromatids together until Aurora B is activated

Spindle assembly

Nucleators (y-TuRC): initiate microtubule growth at centrosomes

Growth factors (XMAP215, CKAP2): promote proper spindle formation

Destabilizers (kinesin-13, katanin, spastin): control microtubule length

Spindle motors

Dynein: anchors spindle

Kinesin-5: slides microtubules for chromosome movement