.beta.2-Tubulin, a form of chordate brain tubulin with lesser reactivity toward an assembly-inhibiting sulfhydryl-directed cross-linking reagent

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Abstract
.beta.1 and .beta.2 are the designations given to 2 forms of .beta.-tubulin that have different electrophoretic mobilities on discontinuous polyacrylamide gels in the presence of sodium dodecyl sulfate. .beta.1 and .beta.2 constitute, respectively, 75 and 25% of the total .beta.-tubulin in bovine brain. Although .beta.1 appears to be ubiquitous in animals, .beta.2 has so far only been found in the brains of cows, pigs, deer, rats, chicks and dogfish but not in squid brain. .beta.2 is not found in bovine kidneys, porcine lungs nor in any nonchordate tubulin that has been examined. When tubulin is reacted with the sulfhydryl-directed reagent N,N''-ethylenebis(iodoacetamide) (EBI), .beta.1, but not .beta.2, is converted to a faster moving form .beta.*. The yield of .beta.2 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 .beta.1 rather than .beta.2. Tubulin molecules that have reacted with EBI to form .beta.* are much less likely to polymerize into microtubules than are molecules that have not formed .beta.*. In view of the observation that only .beta.1, and not .beta.2, can form .beta.*, it is possible that .beta.1 represents a form of tubulin whose assembly may be regulated by a mechanism involving sulfhydryls. .beta.2 may represent a form of tubulin whose assembly is regulated by some other mechanism.