Working at the tail end of tubulin

Prota et al. describe how the enzyme tubulin tyrosine ligase (TTL) specifically recognizes the C terminus of α-tubulin to modulate the behavior of microtubules.

Many enzymes posttranslationally modify the C-terminal tails of the α- and β-tubulin subunits that form microtubules. TTL, for example, adds a tyrosine residue to the C terminus of α-tubulin, resulting in the formation of tyrosinated microtubules that are more dynamic than detyrosinated filaments in cells. Mice lacking TTL die shortly after birth due to neuronal defects, but how TTL recognizes and modifies α- but not β-tubulin is unclear.

Prota et al. determined the crystal structure of TTL in a complex with α- and β-tubulin heterodimers. TTL bound at the interface of the α and β subunits and specifically recognized the dimer’s curved conformation, explaining why TTL is unable to tyrosinate polymerized microtubules, in which the tubulin subunits adopt a straighter configuration. TTL’s tubulin-contacting residues are well conserved, and mutating these amino acids largely abolished TTL’s ability to tyrosinate α-tubulin and restrict neurite outgrowth in cultured neurons. This suggests that the neuronal defects of TTL knockout mice are due to the loss of tubulin tyrosination and not because TTL is required to tyrosinate any other substrates.

TTL’s orientation on tubulin heterodimers placed its catalytic domain near to α-tubulin’s C-terminal tail, which bound to the enzyme’s active site through two glutamate residues missing from β-tubulin’s C terminus. Senior author Michel Steinmetz now wants to obtain the structures of other tubulin-modifying enzymes bound to their substrates in order to determine how their functions differ from TTL.