The chemical complexity of cellular microtubules: tubulin post-translational modification enzymes and their roles in tuning microtubule functions

Abstract

Cellular microtubules are marked by abundant and evolutionarily conserved post-translational modifications that have the potential to tune their functions. This review focuses on the astonishing chemical complexity introduced in the tubulin heterodimer at the post-translational level and summarizes the recent advances in identifying the enzymes responsible for these modifications and deciphering the consequences of tubulin's chemical diversity on the function of molecular motors and microtubule associated proteins.

Figure 1. Tubulin post-translational modifications

Ribbons representation of the tubulin dimer (PDB 1TUB [Nogales et al. 1998]) with α- and β-tubulin colored green and blue, respectively. The unstructured C-terminal tails were modeled to illustrate their span and are colored red. The location and type of known post-translational modifications is indicated on the structure. Residues colored grey are conserved only in a subset of tubulin isoforms. Phosphorylation occurs at Tyr437 and Ser444 of βIII-tubulin and Ser441 of βVI-tubulin; glycylation occurs on Glu445 of αIIIA/B-tubulin and Glu437 of βIV-tubulin; glutamylation occurs on Glu443 and Glu445 of αIVA-tubulin, and Glu445 of αIA/B-tubulin, Glu435 of βII-tubulin, Glu438 of βIII-tubulin, and Glu433 of βIVa-tubulin. For a more complete list of modifications, see [Redeker 2010].

Figure 2. Chemical structures of poly-Glu and poly-Gly chains added to tubulin

A. Tyrosination B. Branch point created by the addition of a glutamate C. Branching and elongation of a poly-Glu chain D. Branch point created by the addition of a glycine E. Branching and elongation of a poly-Gly chain.

Figure 3. Tubulin post-translation modifications affect the recruitment of motors and MAPs

A. Tyrosinated tubulin is enriched at the microtubule plus-end where it recruits +TIP CAP-Gly domain-containing protein CLIP170 and the depolymerizing kinesin MCAK B. Microtubules in the somatodendritic compartment are tyrosinated while axonal microtubules are detyrosinated. Kinesin-1 KIF5 prefers detyrosinated microtubules to tyrosinated microtubules and is therefore sequestered within the axon. C. Microtubule severing enzyme spastin acts preferentially on poly-glutamylated microtubules D. Poly-glutamylation regulates the interaction between inner-arm dynein and the axonemal B tubule. Left, a cross-sectional view of a “9+2” axoneme. Microtubules are colored blue, inner-arm dyneins, green, outer-arm dyneins, orange, and tubulin tails, red. Right, close up view of the boxed region. Inner-arm dyneins project away from the A-tubule of one microtubule doublet and interact with the B-tubule of an adjacent microtubule doublet using their microtubule interacting domains (colored yellow). Tubulin tails are colored red.

Figure 4. Crystal structure of tubulin tyrosine ligase

Ribbons representation of the TTL crystal structure bound to ATP [Szyk et al. 2011]; N-terminal domain, red; central domain, gold; C-domain, dark blue; ATP shown in ball-and-stick representation.

Figure 5

Monomeric TTL exploits a homo-oligomerization interface common to ATP-grasp enzymes to form a hetero-oligomeric complex with tubulin. Right, structure of the distantly related glutathione S-transferase dimer ([Ji et al. 1992] 1GST.pdb). Left, structure of TTL [Szyk et al. 2011] in the same orientation as the GST monomer with residues delineated as important for tubulin binding from functional studies [Szyk et al. 2011] shown in ball and stick representation.