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J. Biochem, 1984, Vol. 96, No. 4 959-969
© 1984 Japanese Biochemical Society

research-article

33K Protein—An Inhibitory Factor of Tubulin Polymerization in Porcine Brain1

Susume KOTANI, Hiromu MUROFUSHI, Eisuke NISHIDA and Hikoichi SAKAI

Department of Biophysics and Biochemistry, Faculty of Science, The University of Tokyo Hongo, Bunkyo-ku, Tokyo 113

A factor (33K protein) that modulates tublin polymerization in vitro has been purified to homogeneity from porcine brain by ammonium sulfate fractionation and Whatman DE52, Toyo-pearl HW65C and Bio-Gel A 0.5m column chromatographies. The purified fraction was free of nucleic acids and sugars. The activity of the purified 33K protein is pronase E sensitive but apparently heat- and trypsinresistant though it undergoes tryptic digestion. The 33K protein inhibits polymerization of brain microtubule proteins in a dose-dependent manner and partially depolymerizes preformed microtubules. It also inhibits polymerization of purified starfish tubulin and microtubule elongation involving fragellar outer doublet microtubules and purified porcine brain tubulin. This suggests that the target of the 33K protein is tubulin rather than microtubule-associated proteins. The 33K protein causes incomplete depolymerization of microtubules and a new steady state is quickly attained which is apparently independent of microtubule mass concentration. Divalent cations such as calcium and magnesium do not modulate the inhibitory activity of the 33K protein.

1This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Nos. 57440004, 57380016, 58780176, and 58540454).


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