Microtubules and Actin Filaments

Intermediate filaments - Provide mechanical strength
Microtubules - Determine the positions of membrane enclosed organelles and direct intracellular transport.
Actin filaments - Determine the shape of the cells surface and are necessary for whole cell locomotion.

Accessory proteins bring together filaments and cell components for binding. Necessary for assembly of filaments, motor proteins included.

Heterodimer formed of globular proteins called alpha tubulin and beta tubulin.

Nucleated at their minus end, the plus end grows outward from each MTOC. 2 accessory proteins bind directly to gamma-tubulin to create a gamma-tubulin ring complex.

Binds to free tubulin subunits and adds them to the + end of a microtubule.

Links the + end of microtubules to membrane.

Binds to 2 tubulin heterodimers and prevents addition onto ends of microtubules.

Causes microtubules to form tight bundles

Motor maps Dynein and Kinesin
Nonmotor maps:
Tau MAPs  - causes MT to form tight bundles (found in axons)
MAP2 - causes the formation of loose bundles (found in dendrites)
XMAP215 – stabilizes plus ends, accelerates assembly
Kinesin 13 – enhances catastrophe at the plus end
Augmin – binds to the sides of MT for branching
Stathmin slows binding of microtubule subunits

ARP is the nucleator
Tropomisin stabilizes the filament
Cofilin destabilizes filaments
Profilin promotes adding to filaments
Thymosin slows addition of subunits

Causes formation of loose bundles