Stoichiometry of microtubule-associated protein (MAP2):tubulin and the localisation of the phosphorylation and cysteine residues along the MAP2 primary sequence.

The stoichiometry of the dimer between microtubule-associated protein 2 (MAP2) and tubulin has been determined by quantitative dodecylsulphate/polyacrylamide gel electrophoresis to be 1:12 mol X mol-1, a value equal to the number of phosphorylation sites that can be labelled in vitro. The distribution of these sites along the MAP2 primary sequence has been determined by cleaving pre-labelled MAP2 with either alpha-chymotrypsin or at the five cysteine residues with nitrothiocyanobenzoic acid. The phosphorylation sites lie in two clusters: ten within the known tubulin-binding domain at one end of the primary sequence, and a pair midway along the sequence. It is postulated that the tertiary structure of MAP2 is folded to bring all twelve sites into association with the twelve tubulin dimers.