Characterization and structural aspects of the enhanced assembly of tubulin after removal of its carboxyl-terminal domain.

Limited subtilisin cleavage of tubulin results in formation of S-tubulin heterodimer and a 4-kDa carboxyl-terminal peptide fragment. This carboxyl-terminal domain constitutes an essential site for MAPs interaction and plays a role in modulating the interactions responsible for tubulin self-assembly into microtubules [Serrano et al. (1984) Proc. Natl Acad. Sci. USA 81, 5989; and Biochemistry 23, 4675]. In the present communication it is shown that addition of the 4-kDa peptide fragment from porcine tubulin to porcine S-tubulin in a molar ratio of about 2:1 does not affect the assembly of the latter. On the other hand, consistent with previous findings on the binding of the 4-kDa peptide by MAP-2, the peptide inhibited MAP-2-induced tubulin assembly (molar ratio of peptide to tubulin, about 2:1; peptide to MAP-2, about 30:1). Comparison of the amino acid composition of the 4-kDa peptide fragment and the C-terminal amino acid residues of S-tubulin with the amino acid sequence of tubulin indicated the subtilisin cleavage site on the tubulin molecule to be between residues Glu417 and Phe418 of the alpha-subunit sequence and between Glu407 and Phe408 of the beta-subunit sequence. The circular dichroism of the 4-kDa fragment in water as solvent is indicative of a molecule with an unordered structure, but when the solvent is changed to a water-trifluoroethanol mixture, the fragment becomes more highly structured. The critical concentration for S-tubulin assembly is not affected by MAPs nor by polylysine, but is decreased by either taxol of dimethylsulfoxide. S-tubulin, with its greater propensity for self-association, has a different conformation from tubulin as shown by a 50% decrease in alpha-helical content, a more hydrophobic environment of at least some of the tryptophan residues as judged from fluorimetry, and a greater compaction indicated by f/f0 = 1.3, as compared to 1.4 for tubulin. The latter point is supported by the observation that the value of the sedimentation coefficient, s20,w = 5.7 S, of the 92-kDa S-tubulin molecule is not significantly different from that of the 100-kDa tubulin, s20,w = 5.8S.