Microtubule-assembly inhibitor protein. Its distribution, localization and physicochemical properties

Microtubule-assembly inhibitor protein (MIP) is an acidic protein with Mr 33,000 which inhibits microtubule assembly in vitro [Kotani, S., Murofushi, H., Nishida, E. & Sakai, H. (1984) J. Biochem. (Tokyo) 96, 959-969]. Anti-MIP antibody was affinity-purified from rabbit anti-MIP sera raised against chemically modified MIP. MIP was localized in the nucleus in interphase culture cells as revealed by immunofluorescent light microscopy. Immunoblotting experiments showed that MIP exists in a variety of mammalian cells and tissues. Kidney appeared to be a better source of MIP than brain, the original source. Kidney MIP was isolated by the same procedure as for brain MIP and proved to be indistinguishable from brain MIP in the inhibitory activity of microtubule assembly, molecular mass, immunoreactivity, and one-dimensional peptide mapping. Physico-chemical characteristics of MIP were studied using the kidney protein. It contained 20% aspartic acid and 25% glutamic acid, accounting for its acidic nature. Hydrodynamically, MIP was a monomer with S20,w = 1.9 S and Mr = 30,000. The frictional ratio, f/fo = 1.7, indicated that MIP is not a globular molecule but has either an elongated or an expanded structure. Circular dichroic results showed a low content of alpha-helix or beta-sheet structure for MIP. Proton nuclear magnetic resonance analysis provided evidence that MIP consists mainly of very flexible structures (random-coil-like structures), but still contains a hydrophobic core structure below 60 degrees C.