CATION-SELECTIVE PROMOTION OF TUBULIN POLYMERIZATION BY ALKALI-METAL CHLORIDES

A role for charge-based interactions in protein stability at the monom er or dimer level is well known. We show here that such interactions c an also be important for the higher-order structures of microtubule as sembly. Alkali metal chlorides increase the rate of polymerization of pure tubulin driven by either taxol or dimethyl sulfoxide. The effect is cation selective, exhibiting a sequence Na+ > K+ > Li+ > Cs+, with optimal concentrations for Na+ at similar to 160 mM. Hofmeister anion effects are additive with these rate stimulations. Sodium is less pote nt than guanidinium ion stimulation reported previously, but produces a larger fraction of normal microtubules. Alkali metal cations lower t he critical concentration by a factor of similar to 2, produce cold re versible polymers whose formation is sensitive to podophyllotoxin inhi bition, increase the fraction of polymers present as microtubules from similar to 0.9 to 0.99, and reverse or prevent urea-induced depolymer ization of microtubules. In the presence of microtubule-associated pro teins, the promotion of polymerization is no longer cation selective. In the polymerization of tubulin S, in which the acidic C termini of b oth monomers have been cleaved, the cation enhancement is markedly dec reased, although selectivity persists. Because the selectivity sequenc e is similar to that of the coil/helix transition of polyglutamic acid , we suggest that a major part, although not all, of the cation select ive enhancement of polymerization results from shielding of the glutam ate-rich C termini of the tubulin monomers.