On the identification of alpha- and beta-tubulin subunits

Confusion appears to have arisen in the literature regarding the designation of alpha- and beta-tubulin in polyacrylamide gels. The presence or absence of 8 M-urea in sodium dodecyl sulfate (SDS) polyacrylamide gels leads to different patterns for unalkylated tubulin subunits (and other proteins), making difficult the designation of the alpha and beta subunits by original definition using electrophoretic mobility in the molecular weight dimension. The specific biochemical property of posttranslational tyrosylation of the alpha subunit has been used to identify further this subunit. Under all conditions tested, the beta subunit has been found to be more acidic than the alpha subunit, with isoelectric point differences that agree with theoretical and published values. If the tubulin subunits are reduced and alkylated, the beta subunit migrates more rapidly in SDS polyacrylamide gels, with or without urea present. However, unalkylated tubulin subunits can comigrate or even reverse their relative mobility if 8 M-urea-SDS polyacrylamide gels are used for subunit separation. The results also confirm the earlier reports that the post-translational tyrosylation of protein appears exclusively restricted to alpha-tubulin and can be demonstrated in an in vivo situation. In addition, the results suggest that only the alpha 2 subunit of tubulin is tyrosylated.