Controlled proteolysis of tubulin by subtilisin: localization of the site for MAP2 interaction.

The treatment of tubulin with subtilisin resulted in a significant decrease in the ability of tubulin to assemble. The addition of taxol reduced the effect of subtilisin on the assembly of digested protein. Limited proteolysis of tubulin by subtilisin affected simultaneously both alpha- and beta-subunits, and it resulted in the appearance of two major cleavage fragments (32 and 20 kilodaltons) or an alternative pattern yielding two fragments (48 and 4 kilodaltons). The smallest peptide (4 kilodaltons) and also the 20-kilodalton fragment are localized in the C-terminal region of the tubulin alpha-subunit. Digested tubulin can assemble into sheet-shaped polymers, which cannot incorporate MAP2. On the other hand, the isolated C-terminal fragments can bind to MAP2. These results suggest that the carboxyl-terminal domain of the tubulin molecule is the site for the MAP2 interaction.