Biochemical characterization of mapmodulin, a protein that binds microtubule-associated proteins

Mapmodulin is a 31-kDa protein that stimulates the microtubule- and dynein-dependent localization of Golgi complexes in semi-intact Chinese hamster ovary cells, We have shown previously that it binds the microtubule binding domains of the microtubule associated proteins, MAP2, MAP4, and tau, We also showed that mapmodulin is identical to a protein named PHAPI (Vaesen, M., Barnikol-Watanabe, S., Gotz, H., Awni, L.A., Cole, T., Zimmermann, B., Kratzin, H.D. and Hilschmann, N. (1994) Biol. Chem. Hoppe-Seyler 375, 113-126). We report here that mapmodulin is a phosphoprotein that is predominantly cytosolic but is also found peripherally associated with endoplasmic reticulum and Golgi membranes in mammalian cells, The protein occurs as a trimer in cytosol, and phosphorylation is required for its microtubule-associated protein-binding activity, Heat treatment of nonphosphorylated mapmodulin can render it competent for binding to microtubule-associated proteins, suggesting that phosphorylation induces a conformational change in mapmodulin. Finally, despite identity in polypeptide sequence with a protein reported to act as an inhibitor of protein phosphatase 2A, native mapmodulin was not able to inhibit protein phosphatase 2A in Chinese hamster ovary cell cytosol.