Different reactivities of brain and erythrocyte tubulins toward a sulfhydryl group-directed reagent that inhibits microtubule assembly

There is considerable evidence that tubulin exists in multiple isotypes, differing in amino acid sequence and tissue distribution. Little is known, however, about the functional significance of these isotypes. Chicken erythrocyte β-tubulin has been shown by peptide mapping to differ significantly from chicken brain β-tubulin. We now find that when that two tubulins, in their native states, are incubated with N,N'-ethylenebis(iodoacetamide) (EBI), a bifunctional sulfhydryl-directed reagent, microtubule assembly by brain tubulin is much more sensitive to inhibition by EBI than is erythrocyte tubulin assembly. The resistance of erythrocyte microtubule assembly to inhibition by EBI is correlated with a low reactivitiy of erythrocyte tubulin with [¹⁴C]EBI. This difference is most marked in the β subunit which reacts 15 and 17% as well, respectively, with [¹⁴C]EBI as do the β₁ and β₂ subunits of brain tubulin. Also, erythrocyte β reacts about 33% as well as does brain β with iodo[¹⁴C]acetamide. These results suggest that a reactive sulfhydryl group, whose oxidation prevents microtubule assembly, is present in brain tubulin but absent or inaccessible in erythrocyte tubulin. Since purified erythrocyte tubulin self-aggregates much more rapidly than does brain tubulin, it is conceivable that erythrocyte and brain tubulin may differ in that the latter may have its assembly subject to a complex regulation, while erythrocyte tubulin assembly may be regulated by a simpler mechanism.