β₂-Tubulin, a Form of Chordate Brain Tubulin with Lesser Reactivity toward an Assembly-Inhibiting Sulfhydryl-Directed Cross-Linking Reagent

β₁ and β₂ are the designations given to two forms of (5-tubulin that have different electrophoretic mobilities on discontinuous polyacrylamide gels in the presence of sodium dodecyl sulfate [Little, M. (1979) FEBS Lett. 108, 283-286], (3, and β₂ constitute respectively 75% and 25% of the total β-tubulin in bovine brain. Although β[ appears to be ubiquitous in animals, β₂ has so far only been found in the brains of cows, pigs, deer, rats, chicks, and dogfish but not in squid brain. β₂ is not found in bovine kidneys, in porcine lungs, or in any nonchordate tubulin that has been examined. When tubulin is reacted with the sulfhydryl-directed reagent N,-N-ethylenebis(iodoacetamide) (EBI), βb but not β₂, is converted to a faster moving form, β*. The yield of β₂ in this reaction is not altered by the presence of drugs. When [¹⁴C]EBI is used as a probe, most of the label is incorporated into β₁ rather than β₂. Tubulin molecules that have reacted with EBI to form β* are much less likely to polymerize into microtubules than are molecules that have not formed β*. In view of the observation that only βb and not β₂, can form β*, it is possible that βt represents a form of tubulin whose assembly may be regulated by a mechanism involving sulfhydryls. In contrast, β₂ may represent a form of tubulin whose assembly is regulated by some other mechanism.