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Post-translational modification of tubulin tails

The tubulin C-termini are prone to intense modification and regulation by different
biochemical pathways inside neurons. In mammalian cells, both α- and β-tubulin
occur as seven to eight different genetic variants, which also undergo numerous
post-translational modifications (Luduena, 1998; McRae, 1997; Rosenbaum,
2000). The main control of the microtubule function is achieved via covalent
modifications (Banerjee, 2002). Indeed in organisms such as the protists that
express identical α- and β-tubulins (Silflow, 1991), post-translational
modifications provide the only source of variation. Modifications such as
acetylation, palmitoylation, phosphorylation and polyglutamylation are post-
translational modifications found on other proteins; others such as detyrosination
and polyglycylation appear to be tubulin specific (McKean et al., 2001).

I. Tyrosination/detyrosination of α-tubulin

Tyrosination-detyrosination is one of the major posttranslational modifications in
which the C-terminal tyrosine residue in α-tubulin is added or removed reversibly
(Banerjee, 2002). The tubulin tyrosination cycle involves the enzymatic removal
of the C-terminal tyrosine residue present on some α-tubulin isotypes by a
specific carboxy-peptidase, and its subsequent restoration by a tubulin-tyrosine
ligase (Idriss, 2000). Although the functional relevance of this modification is not
always clear, highly stable microtubules such as those of the axoneme are
detyrosinated, and this appears to reflect the length of time the individual
α-tubulin substrate molecule has spent in a microtubule. Although tyrosination
does not alter the assembly activity of tubulin in vitro, these two forms of tubulin
have been found to be distributed differently in vivo and are also correlated with
microtubule stability (Gundersen et al., 1984). Recent evidence indicates that
detyrosination of tubulin can regulate interaction of microtubules with vimentin
intermediate filaments by a kinesin-dependent mechanism (Kreitzer et al., 1999).

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