Characteristics of tubulin aggregation by tubulin-binding proteins from brain and by synthetic polycations

Promotion of microtubule assembly by oligocations: cooperativity between charged groups

The rate and, to a lesser degree, the extent of microtubule assembly from rat brain tubulin is enhanced by oligocations such as polyamines, melittin, polybasic drugs, oligolysines, and oligoarginines. The effect is cooperative for degrees of polymerization up to seven for oligolysines and up to five for oligoarginines and is interpreted as an interaction with up to seven closely spaced anionic charges. Microtubules so formed appear to be normal by electron microscopy, and by salt, colchicine, and cold sensitivities. Lysyl residues in excess of seven (or five for arginine) in larger oligomers interact nearly noncooperatively. Separation of lysyl charges by intercalation of alanyl residues reduced assembly promoting potency for hexalysines. The cooperative portion of the response is most likely associated with the highly acidic extreme C termini of tubulin because their removal with limited subtilisin treatment markedly reduces oligolysine potency. However, some cooperative interactions with oligocations can also occur with more widely spaced anionic charges elsewhere in tubulin. The potential role of oligocations in the intracellular regulation of microtubule assembly is discussed.

Effect of polyanions and polycations on detyrosination of tubulin and microtubules at steady state

Microtubule protein preparations purified from rat brain were used to study the effect of polycations and polyanions on the release of the COOH-terminal tyrosine of the alpha-chain of tubulin catalyzed by tubulin carboxypeptidase. (1) Most of the polycations and polyanions tested, independently of the ionogenic group, inhibited the reaction in a concentration-dependent fashion. Under steady-state conditions, detyrosination of the microtubule pool was inhibited to the same degree as occurred with the non-assembled tubulin pool, except in the case of chondroitin sulphate. This compound inhibited detyrosination of the non-assembled tubulin pool, but not that of microtubules. (2) Heparin, the most potent inhibitor tested, produced the dissociation of the carboxypeptidase from microtubules. Many, but not all, of the other microtubule-associated polypeptides were also dissociated by heparin. (3) Polylysine counteracted the inhibitory and dissociating effects of heparin. (4) Heparin protected tubulin carboxypeptidase against inactivation. Our results and previous reports describing, in nervous tissue, the presence of proteoglycans, RNA and basic proteins that inhibit detyrosination, suggest that tubulin carboxypeptidase might be physiologically modulated by electrically charged macromolecules.

Kainate reversibly aggregates brain tubulin in vitro

The effect of kainate, an heterocyclic analogue of glutamate on tubulin polymerization was studied. We demonstrate that kainate induces a dose-dependent aggregation of rat brain tubulin either purified by DEAE-Sephadex A-50 or in the presence of MAPs into a mesh-like structure. Such polymer is cold-, CaCl2- and colchicine-insensitive. Removal of kainate from the incubation medium yields free tubulin competent for polymerizing into normal microtubules in the presence of GTP.