TTLL13P

Model summary

A soluble cytosolic enzyme that installs glutamate side chains onto tubulin tails, thereby writing a polyglutamylation mark that tunes microtubule stability and the recruitment of associated proteins. By catalyzing ATP-dependent glutamyl addition to tubulin, it modulates cytoskeletal dynamics and contributes to orderly cell cycle transitions. Its soluble architecture supports cytosolic residency with transient engagement of microtubule-rich assemblies to coordinate post-translational control of the cytoskeleton.

Predicted Gene Ontology terms

1. 01

   I begin with the domain architecture.

   The protein contains a single, centrally placed InterPro signature IPR004344 (Tubulin-tyrosine ligase/Tubulin polyglutamylase family) spanning residues 85–430. This family defines the TTL/TTLL enzyme superfamily that catalyzes ATP-dependent post-translational modifications on tubulin. The breadth of the IPR004344 coverage across the catalytic core indicates a soluble enzyme that binds ATP and glutamate to form isopeptide linkages on target proteins. The absence of transmembrane segments and the soluble enzyme fold argue for a cytosolic enzyme that transiently associates with cytoskeletal assemblies.

2. 02

   This TTL/TTLL core dictates the molecular function: it catalyzes ligation of glutamate onto tubulin tails, i.e., tubulin-glutamic acid ligase activity (GO:0070740).

   The chemistry of this family proceeds by activating glutamate and forming a γ-glutamyl isopeptide bond to the exposed C-terminal tails of tubulin, a reaction class that directly establishes and extends glutamyl side chains. That enzymatic role causally places the protein in the biological process of protein polyglutamylation (GO:0018095), because repeated addition of glutamate units modulates microtubule behavior and the recruitment of microtubule-associated proteins.

3. 03

   From this enzymology and process, I infer cellular context.

   Enzymes of the TTL/TTLL family operate in the soluble phase of the cell and engage microtubules through transient binding; thus a cytosolic localization is expected and aligns with GO:0005829 cytosol. The soluble catalytic architecture and lack of targeting signals support a cytosolic pool that can shuttle to microtubule-rich regions when needed.

4. 04

   Mechanistically, the enzyme’s single TTL/TTLL catalytic module (residues 85–430) binds ATP and glutamate and recognizes acidic tubulin tails, installing glutamate units that tune microtubule dynamics and MAP/catastrophe-rescue factor binding.

   This modification pathway explains how the enzyme influences cytoskeletal regulation and cell cycle transitions: polyglutamylation modulates microtubule stability and trafficking, which feed forward into mitotic spindle function and checkpoint timing.

5. 05

   Given this mechanism, I hypothesize interaction partners that enable targeting and function.

   The primary substrates and transient cofactors are α/β-tubulin dimers and assembled microtubules. The enzyme likely forms transient complexes with tubulin-assembly factors and microtubule-associated proteins that read or compete for glutamyl marks. It may also coordinate with cytosolic chaperonins that maintain tubulin folding and with cell-cycle regulators that synchronize modification with mitosis. Together, these interactions position the enzyme as a cytosolic modifier that writes a glutamyl code on tubulin to orchestrate cytoskeletal behavior and cell cycle progression.