Posttranslational modification of class III β-tubulin

The charge heterogeneity of class III β-tubulin (βIII) during neural development was analyzed by high-resolution isoelectric focusing/two-dimensional polyacrylamide gel electrophoresis in combination with site-specific proteolytic digestion and immunological detection. The number of βIII isoforms (charge variants) gradually increases from one in embryonic brain to seven in adult brain. All of the charge heterogeneity is due to posttranslationally modified sites located within the extreme C-terminal region of the βIII polypeptide. One βIII isoform is present in testis, the only other tissue in which this isotype is expressed. The testis βIII isoform cofocuses with the earliest-appearing embryonic brain βIII charge variant. Our results indicate that the posttranslational modifications of βIII are developmentally regulated, occur at more than one site, and are neuron-specific. The location of these modifications within the extreme C-terminal domain suggests that their function is to modulate the interaction of tubulin with microtubule-associated proteins.