Assembly of tubulin by classic myelin basic protein isoforms and regulation by post-translational modification.

Myelin basic protein (MBP), a highly cationic protein that maintains the structure of the myelin sheath, associates with tubulin in vivo. The in vitro assembly of tubulin by MBP was examined here using several assays. The unmodified C1 component of 18.5 kDa bovine MBP (bC1) assembled tubulin into microtubules in a dose-dependent manner via filamentous intermediates, and was able simultaneously to promote the formation of microtubule bundles. The critical tubulin concentration in the presence of bC1 was 0.69 +/- 0.05 microM. The effects of post-translational modifications (such as deamidation and phosphorylation) were assayed by comparing the bC1-bC6 components of 18.5 kDa bovine MBP; an increasing level of modification enhanced the ability of MBP to assemble tubulin. The effects of charge reduction via deimination were examined using recombinant murine isoforms emulating the unmodified C1 and deiminated C8 isoforms of 18.5 kDa MBP; both rmC1 and rmC8 exhibited a comparable ability to assemble tubulin. The effects of alternate exon recombination of the classic MBP variants were tested using the recombinant murine 21.5, 17.22, and 14 kDa isoforms. The isoforms containing regions derived from exon II of the classic MBP gene, 21.5 and 17.22 kDa MBP, showed no substantial difference in the extent of tubulin polymerization and bundling when compared to those of 18.5 kDa MBP. The 14 kDa isoform and two terminal deletion mutants of rmC1 were able to induce microtubule polymerization, but not bundling, to the same degree as the longer proteins. Finally, bC1 was shown to disrupt and aggregate planar sheets of crystalline tubulin stabilized by paclitaxel, establishing that these structures are not suitable substrates for the formation of MBP cocrystals.