Beta-tubulin folding is modulated by the isotype-specific carboxy-terminal domain

To investigate the contribution of the carboxy-terminal domain in the process of tubulin folding and dimer formation, we constructed a beta 1-beta 3 tubulin chimaera and two truncated carboxy-terminal beta 3-tubulins. The capacity of these altered polypeptides to incorporate into dimers and into microtubules was tested by non-denaturing electrophoresis and co-assembly experiments. The chimaera and the truncated protein with a deletion encompassing the last 12 amino acid residues (beta 3 delta C12) were incorporated into dimers and microtubules, though the level of incorporation was diminished compared to wild-type beta 3-tubulin. However, the level of incorporation of beta 3 delta C12 into subtilisin-digested dimers was similar to the incorporation of wild-type beta 3-tubulin. Since subtilisin deletes the carboxy-terminal region, these results suggest a regulatory role of the carboxy-terminal region in the folding process itself and not in the formation of the dimer.