Identification of two new members of the tubulin family.

Analysis of the delta- and epsilon-tubulin sequences indicates that they both consist of two structural domains of which the N-terminal domain can bind to alpha/beta heterodimers while the C-terminal domain probably binds to a non-tubulin protein. Both additional tubulins probably bind GTP but lack GTPase activity, while their synthesis requires the TCP1 chaperonine but is not autoregulated. Although these properties resemble those of gamma-tubulin, the low sequence identity (Table I) demonstrates that the gamma-, delta-, and epsilon-proteins should be classed as different members of the tubulin family. The identification of these additional members is unexpected. Examination of the cellular expression and distribution of the delta- and epsilon-tubulins, and whether other organisms contain homologous genes, may reveal further features of the eukaryotic cytoskeleton.