Structural basis for α-tubulin-specific and modification state-dependent glutamylation

Area of Science:

• Cell Biology
• Structural Biology
• Biochemistry

Background:

• Microtubules exhibit complex posttranslational modification patterns.
• Tubulin tyrosine ligase-like (TTLL) enzymes are key regulators of these modifications.
• The substrate specificity and pattern generation mechanisms of TTLLs remain poorly understood.

Purpose of the Study:

• To elucidate the molecular mechanisms by which TTLL6 recognizes its substrates and generates specific glutamylation patterns on microtubules.
• To understand the role of TTLL6 in ciliopathies.

Main Methods:

• Cryo-electron microscopy
• Kinetic analysis
• Single-molecule biochemistry

Main Results:

• TTLL6 employs a quadrivalent recognition mechanism, simultaneously assessing microtubule geometry and existing posttranslational modifications.
• TTLL6 binds to a β-tubulin subunit to modify the α-tail of an adjacent tubulin dimer.
• Recognition spans two tubulin dimers along and across protofilaments, ensuring high fidelity.
• TTLL6 is stimulated by glutamylation on the β-tail of a laterally adjacent tubulin dimer, creating a positive feedback loop.
• This mechanism generates localized microtubule glutamylation patterns.

Conclusions:

• TTLL6 utilizes a sophisticated recognition strategy to ensure precise modification of microtubules.
• The enzyme's mechanism provides insights into generating tubulin chemical and topographic complexity.
• Understanding TTLL6 function is crucial for deciphering its role in ciliopathies.