Evidence for new C-terminally truncated variants of α- and β-tubulins.

Cellular alpha-tubulin can bear various carboxy-terminal sequences: full-length tubulin arising from gene neosynthesis is tyrosinated, and two truncated variants, corresponding to detyrosinated and Delta 2 alpha-tubulin, result from the sequential cleavage of one or two C-terminal residues, respectively. Here, by using a novel antibody named 3EG that is highly specific to the -EEEG C-terminal sequence, we demonstrate the occurrence in neuronal tissues of a new alpha Delta 3-tubulin variant corresponding to alpha 1A/B-tubulin deleted of its last three residues (EEY). alpha Delta 3-tubulin has a specific distribution pattern: its quantity in the brain is similar to that of alpha Delta 2-tubulin around birth but is much lower in adult tissue. This truncated alpha 1A/B-tubulin variant can be generated from alpha Delta 2-tubulin by the deglutamylases CCP1, CCP4, CCP5, and CCP6 but not by CCP2 and CCP3. Moreover, using 3EG antibody, we identify a C-terminally truncated beta-tubulin form with the same -EEEG C-terminal sequence. Using mass spectrometry, we demonstrate that beta 2A/B-tubulin is modified by truncation of the four C-terminal residues (EDEA). We show that this newly identified beta Delta 4-tubulin is ubiquitously present in cells and tissues and that its level is constant throughout the cell cycle. These new C-terminally truncated alpha- and beta-tubulin variants, both ending with -EEEG sequence, are expected to regulate microtubule physiology. Of interest, the alpha Delta 3-tubulin seems to be related to dynamic microtubules, resembling tyrosinated-tubulin rather than the other truncated variants, and may have critical function(s) in neuronal development.