Centrosomes are the primary site of nucleation of microtubules in animal cells. As the tubulin subunits polymerize they can be covalently modified by acetylation of a lysine and a detyrosination of a tyrosine residue from the c-terminus. MAPs bind to microtubules and mediate interaction with other cell components. 55-60 kD. MAPs have two domains, one binds to microtubules and the other to cell components. MAPs speed up nucleation.
MAPs stabilize microtubules by inhibiting the dissociation of tubulin from microtubules.
MAPs also help create differentiated cytoplasm by being distributed differently in different cell types. The nucleating structure is 3 actin monomers. Once nucleated, actin monomers are added faster at the barbed end. Actin filaments can undergo treadmilling , where actin monomers are added constantly to the plus end and lost from the minus end. Actin filaments are depolymerized by cytochalasins .