Microtubules, along with microfilaments and intermediate filaments, form the major part of the extensive cytoplasmic network known as the cytoskeleton. The thickest of these filaments are the 20-25 nm microtubules composed of tubulin and several additional microtubuleassociated proteins (MAP). Tubulin is a heterodimer composed of atubulin and b-tubulin. Each subunit is a 55 kD acidic protein. Tubulin assembles into the microtubule system during interphase, then reassembles into the mitotic spindle during cell division.
