Microtubule-associated protein, MAP2, is a calcium-binding protein.

Calcium has been suggested to be an important element in the regulation of microtubule dynamics 'in vivo'. In this report we have analyzed the possibility that microtubule-associated protein 2 (MAP2) binds calcium. MAP2 was blue-stained with the cationic carbocyanine dye 'stains-all' in a similar way to that of calcium-binding proteins and bound 45Ca as estimated from dot-blotting experiments. The calcium-binding characteristics of MAP2, determined by equilibrium dialysis, indicated that MAP2 bound about 3 mol (n = 2.9 +/- 0.4) of calcium per mol of protein (Kd = (0.9 +/- 0.2).10(-5) M). Analysis of the Scatchard plots from equilibrium dialysis and dot-blot assays indicated that MAP2 also presented low-affinity calcium-binding sites (Kd = (0.3 +/- 0.2).10(-4) M). Incubation of nitrocellulose blots of proteolytically digested MAP2 with 45Ca indicated that the calcium-binding sites were located in the region that is not involved in the interaction with tubulin (projection region).