PURIFICATION OF MICROTUBULE-ASSOCIATED PROTEIN MAP1B FROM BOVINE BRAIN - MAP1B BINDS TO MICROTUBULES BUT NOT TO MICROFILAMENTS

A simple procedure for the purification of MAP1B from bovine brain is described. The procedure requires two ion-exchange chromatographic ste ps and results in >95% pure MAP1B with a typical recovery of about 25- 30 mg/kg of brain tissue. SDS-PAGE analysis of the purified protein sh ows that it is composed of a high molecular mass (330kDa) heavy chain and two low molecular mass (32kDa and 18kDa) associated light chains. The estimated stoichiometry of heavy chain:light chain is 1:2 and 1:0. 2 mole/mole protein for the 32kDa and 18kDa light chains respectively. Western blotting, using monospecific monoclonal antibodies, shows tha t only the heavy chain is recognised by the anti-MAP1B antibody and is not immunostained by either the MAP1A or MAP2 monoclonal antibodies. Purified MAP1B binds efficiently to both unpolymerised tubulin and pol ymerised tubulin and co-sediments with taxol-stabilised microtubules. Go-incubation experiments show that MAP2 can compete with MAP1B bindin g to microtubules, indicating common or overlapping sites. However, MA P1B binds to neither G-actin nor F-actin nor co-sediments with F-actin , suggesting that it is not an actin-binding protein. (C) 1995 Wiley-L iss, Inc.