Tubulin domains probed by limited proteolysis and subunit-specific antibodies

Abstract

The substructure of the tubulin molecule was studied by limited proteolysis and high affinity polyclonal antibodies specific for alpha or beta-tubulin. Brief enzymatic cleavage separates the tubulin monomer into two domains of unequal size. Trypsin splits alpha-tubulin into components with Mr values of 36 X 10(3) and 14 X 10(3), chymotrypsin splits beta-tubulin into 31 X 10(3) Mr and 20 X 10(3) Mr fragments. The cleavage occurs at Arg339 (alpha) and Tyr281 (beta), as determined by sequencing several N-terminal residues of the small domains, i.e. the small domains are the C-terminal parts of the molecules, the large ones are the N-terminal parts. There is a second cleavage site of chymotrypsin within Mr 10(3) to 2 X 10(3) of the C terminus of beta-tubulin. The fragments can be separated only under denaturing conditions. They copolymerize into microtubules and incomplete microtubule walls joined by a wall junction, forming S-shapes and hooks in cross-section. The antibodies were raised against electrophoretically purified tubulin monomers. Those produced with alpha-tubulin are directed predominantly against the large domains; they are either specific for alpha-tubulin or cross-react with the large domain of beta-tubulin. Conversely, antibodies raised against beta-tubulin are directed predominantly against the small domains (beta-specific and beta-cross-reacting fractions). Thus the antibodies discriminate not only between the tubulin chains but also between the domains generated by the proteases. The complementary antigenicity correlates well with the stability of the domains. Potential sites of antigenic determinants are located within the polypeptide chains by comparing theoretical predictions with the pattern of immunoblots. Two epitopes of the alpha-cross-reacting antibodies have been located approximately. One is very close to the C terminus (within about 20 residues), the other is close to the N terminus (within about Mr 8 X 10(3) ). The epitope of the beta-cross-reacting antibody is also located within Mr 12 X 10(3) of the C terminus. The antibodies prevent microtubule assembly and cause disassembly of preformed microtubules. A variety of breakdown products are observed by electron microscopy. They include fibres of about 10 nm width, sheets with undefined substructure, thick tapered fibrous bundles and wispy filaments.(ABSTRACT TRUNCATED AT 400 WORDS)