Assembly of brain microtubule proteins isolated from the Atlantic cod, Gadus morhua, was found to be much less sensitive to colchicine than assembly of bovine brain microtubules, which was completely inhibited by low colchicine concentrations (10 microM). The degree of disassembly by colchicine was also less for cod microtubules. The lack of colchicine effect was not caused by a lower affinity of colchicine to cod tubulin, as colchicine bound to cod tubulin with a dissociation constant, Kd, and a binding ratio close to that of bovine tubulin. Cod brain tubulin was highly acetylated and mainly detyrosinated, as opposed to bovine tubulin. When cod tubulin, purified by means of phosphocellulose chromatography, was assembled by addition of DMSO in the absence of microtubule-associated proteins (MAPs), the microtubules became sensitive to low concentrations of colchicine. They were, however, slightly more stable to disassembly, indicating that posttranslational modifications induce a somewhat increased stability to colchicine. The stability was mainly MAPs dependent, as it increased markedly in the presence of MAPs. The stability was not caused by an extremely large amount of cod MAPs, since there were slightly less MAPs in cod than in bovine microtubules. When "hybrid" microtubules were assembled from cod tubulin and bovine MAPs, these microtubules became less sensitive to colchicine. This was not a general effect of MAPs, since bovine MAPs did not induce a colchicine stability of microtubules assembled from bovine tubulin. We can therefore conclude that MAPs can induce colchicine stability of colchicine labile acetylated tubulin.

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