Spirals and paracrystals induced by Vinca alkaloids: evidence that microtubule-associated proteins act as polycations

Formation of the vinca-induced spirals from tubulin and microtubules is dependent upon the presence of microtubule-associated proteins. Formation of paracrystals visualized in thin-sectioned material is also shown to require the presence of microtubule-associated proteins. The polycationic behaviour of the latter in stabilizing microtubules and spirals is demonstrated in experiments where assembly of tubulin in their absence was stimulated by polylysine or high Mg2+ and addition of vincristine caused spirals to form. Spirals were stable to cold, Ca2+, and colchicine, but polyadenylic acid caused their depolymerization.