Cytoskeleton

Tubulin and actin have been highly conserved
Microtubules
Centrosome (Microtubule-organizing centre/MTOC)
Contain hundreds of ring-shaped structures formed from another type of tubulin ( tubulin) – serves as a starting point for the growth of microtubule
ß-tubulin bind to -tubulin, the (-) end will embedded in the centrosome and growth occurs at the (+) end
Growing microtubules show dynamic instability, due to the capacity of tubulin molecules to hydrolyze GTP
Chemicals affecting tubulin growth
Colchicine : prevent polimerization of tubulin into microtubules
Taxol : binds tightly to microtubules & prevent them from losing subunits (microtubules can grow but can not shrink, also arrest dividing cells in mitosis)
Motor protein drive intracellular transport
Cytoplasm is in continual motion, mitochondria and small organelles show saltatory movement : move for a short period, stop and then start again
Generated by motor proteins
Actin-based motor protein
Microfilaments
Actin polymerization in vitro proceeds in three steps
Actin monomer binding proteins
Actin filaments allow eucaryotic cells to adopt a variety of shapes and perform a variety of functions
Drugs that affect actin filament and microtubules
Certain toxin can inhibit actin filament functions
Cytochalasin : prevent actin polymerization
Jasplakinolides : promote polymerization
Intermediate filaments
Grouping of intermediate filaments
1) keratin filaments in epithelial cells
2) vimentin and vimentin-related filaments in connective tissue cells, muscle cells, and supporting cells of the nervous system
3) neurofilaments in nerve cells
4) nuclear lamins which strengthen the nuclear
membrane of all animal cells
Keratin filaments
Formed from a mixture of different keratin subunit
Specialized keratins occur in hair, feathers, and claws
Span the interior of epithelial cells
Neurofilaments
Vimentin-like filaments
Filament intermediate
Filament intermediate can be reinforced by accessory proteins such as plectin that link i.f. to microtubules, actin filament and adhesive structures in the desmosome
Lamins : i.f found inside the inner nuclear membrane
* disassemble and re-form at each cell division
(controlled by phosphorilation and
dephosphorilation of lamins by protein kinase)
phosphorilasi filament fall apart
dephosphorilasi lamins reassemble