Purification and characterization of a new, ubiquitously distributed classof microtubule-associated protein with molecular mass 250 kDa

A heat-stable microtubule-associated protein (MAP) with relative molecular mass 250 000: 250-kDa MAP, was purified from bovine adrenal cortex. It is c lassified as a MAP subspecies distinct from MAP1, MAP2, tan, and MAP4, as j udged from its electrophoretic mobility, heat stability and immunoreactivit y. Purified 250-kDa MAP was able to bind to taxol-stabilized microtubules, although it lacked the ability to polymerize purified tubulin into microtub ules. Western-blot analysis showed that this MAP was expressed ubiquitously in mammalian tissues. Immunofluorescence microscopy revealed that polyclon al antibodies raised against 250-kDa MAP stained many punctate structures i n the cytoplasm of cultured cells. Blurry cytosolic staining was also obser ved. Judging from the result of nocodazole treatment, the punctate structur es were associated with the microtubule network throughout the cytoplasm, w hile cytosolic 250-kDa MAP colocalized with free tubulin. Under electron mi croscopy, 250-kDa MAP has the appearance of a hollow sphere of about 12 nm diameter.