Tubulin Post-Translational Modifications and Microtubule
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International Journal of Molecular Sciences Review Tubulin Post-Translational Modifications and Microtubule Dynamics Dorota Wloga *, Ewa Joachimiak and Hanna Fabczak Laboratory of Cytoskeleton and Cilia Biology, Department of Cell Biology, Nencki Institute of Experimental Biology of Polish Academy of Sciences, 3 Pasteur Str., 02-093 Warsaw, Poland; [email protected] (E.J.); [email protected] (H.F.) * Correspondence: [email protected]; Tel.: +48-22-589-23-38; Fax: +48-22-822-53-42 Received: 1 September 2017; Accepted: 19 October 2017; Published: 21 October 2017 Abstract: Microtubules are hollow tube-like polymeric structures composed of α,β-tubulin heterodimers. They play an important role in numerous cellular processes, including intracellular transport, cell motility and segregation of the chromosomes during cell division. Moreover, microtubule doublets or triplets form a scaffold of a cilium, centriole and basal body, respectively. To perform such diverse functions microtubules have to differ in their properties. Post-translational modifications are one of the factors that affect the properties of the tubulin polymer. Here we focus on the direct and indirect effects of post-translational modifications of tubulin on microtubule dynamics. Keywords: microtubule dynamics; post-translational modifications; acetylation; glutamylation; glycylation; methylation; phosphorylation; polyamination 1. Introduction Microtubules are an essential component of the cytoskeleton of a eukaryotic cell. They support key cellular functions such as maintenance of cell shape, intracellular transport, cell polarity or cell division. Although microtubules are structurally similar, they may differ in their properties, such as the rate of tubulin subunit turnover. Exposure of cells to cold [1,2] or treatment with microtubule-depolymerizing drugs [3] revealed the existence of the subpopulations of microtubules that differ in their stability. While some microtubules are very labile and disassemble within minutes under such conditions, other microtubules persist for hours or are cold-resistant. Three major factors may contribute to the heterogeneity of microtubule properties: (i) composition of the α- and β-tubulin gene-encoded isotypes that are incorporated into microtubule; (ii) post-translational modifications of tubulin that create a pattern on the microtubule surface known as the “tubulin code” [4] and (iii) interactions with diverse microtubule-interacting proteins (MIPs). It is noteworthy that these three factors can influence each other. For example, specific tubulin isotypes may differ in the extent of their post-translational modifications depending on the presence or absence of modifiable amino acids (for instance, in Caenorhabditis elegans only Mec-12 α-tubulin isotype has the acetylable lysine residue in position 40 [5]). In turn, post-translational modifications can affect binding/interactions of tubulin or microtubules with specific MIPs. Here we focus on the direct and indirect effects of the post-translational modifications of tubulin on microtubule dynamics. 2. Polyamination and Cold-Stable Microtubules During the course of the purification of tubulin from the bovine or porcine brain by cycles of cold-induced microtubule depolymerization and warm-induced repolymerization, it was discovered that some microtubules are cold-resistant [6]. Tubulins in hyper-stable, cold-resistant microtubules, are more basic in charge compared to tubulins from cold-sensitive microtubules [6]. The basic fraction Int. J. Mol. Sci. 2017, 18, 2207; doi:10.3390/ijms18102207 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2017, 18, 2207 2 of 18 Int. J. Mol. Sci. 2017, 18, 2207 2 of 18 are more basic in charge compared to tubulins from cold-sensitive microtubules [6]. The basic fraction of tubulin, abundant in axons of neuronal cells, is at least in part generated by the covalent linkage ofof tubulin,polyamine abundant (polyamination) in axons of[7]. neuronal Polyaminated cells, is attubulin least inis partalso generated abundant by in the testes covalent (possibly linkage in axonemalof polyamine microtubules), (polyamination) but only [7 ].present Polyaminated at low levels tubulin in non-neuronal is also abundant cells [7]. in testesIn vitro, (possibly both free in tubulinaxonemal heterodimers microtubules), and buttubulin only heterodimers present at low incorporated levels in non-neuronal into microtubules, cells [7 ].canIn be vitro modified, both free by transglutaminases.tubulin heterodimers Moreover, and tubulin polyamination heterodimers of tubulin incorporated does not into affect microtubules, its polymerization can be modified [7]. by transglutaminases.The highly conserved Moreover, glutamine polyamination 15 (Q15) of in tubulin β-tubulin, does notlocated affect in its close polymerization proximity to [7 the]. GTP pocketThe (Figure highly 1), conserved is the main glutamine modification 15 (Q15) site in butβ-tubulin, additional located putative in close polyamination proximity tosites the were GTP mappedpocket (Figure on both1), the is α the- and main β-tubulins. modification Q128 siteand butQ285 additional of α-tubulin putative are located polyamination within the sites H3 lateral were surfacemapped and on bothM loop, the αrespectively- and β-tubulins. (Figure Q128 1), the and dime Q285r ofinterfacesα-tubulin that are face located the withinheterodimers the H3 lateralof the neighboringsurface and Mprotofilaments loop, respectively [8,9]. (FigureThe Q1331), theand dimer Q256 interfacesresidues are that positioned face the heterodimers near the α-tubulin of the minusneighboring end (Figure protofilaments 1), suggesting [8,9]. possible The Q133 involvement and Q256 in residues the interactions are positioned between near the the α,βα-tubulin-tubulin heterodimerminus end (Figure and γ1-tubulin,), suggesting or with possible heterodimers involvement within in the the interactions same protofilam betweenent. the Thus,α,β -tubulintubulin polyaminationheterodimer and couldγ-tubulin, play a role or with in the heterodimers control of GTP within binding the same or hydrolysis, protofilament. and in Thus, microtubule tubulin latticepolyamination stabilization could [7]. play It is aunknown role in the if in control vivo any of GTP microtubule binding interacting or hydrolysis, proteins and inrecognize microtubule and preferentiallylattice stabilization interact [7 with]. It cold-stable is unknown polyaminated if in vivo any microtubules, microtubule and interacting thus, further proteins modulate recognize their stability.and preferentially interact with cold-stable polyaminated microtubules, and thus, further modulate their stability. Figure 1. The distribution of the polyamination sites in α,β-tubulin heterodimer. Schematic representation Figureof a microtubule, 1. The distribution composed of αof- (green)the polyamination and β- (blue) tubulinsites in heterodimers. α,β-tubulin Red heterodimer. and purple framesSchematic mark representationtubulin heterodimer of a microtubule, with either acomposed visible luminal of α- (green) side (an and inside β- (blue) view) tubulin or outer heterodimers. surface (outside Red view), and purplerespectively. frames An mark inside tubulin and outside heterodimer views of with 3D modeleither ofa visible the tubulin luminal heterodimer side (an withininside theview) microtubule or outer surfacelattice based (outside on 3J6F.pdb view), respectively. (NCBI database); An ainside tubulin and molecule outside is views composed of 3D of 12modelα-helices of the (numbered tubulin heterodimerH1–H12) and within 10 β-strands the microtubule (numbered lattice S1–S10) based linked on by3J6F.pdb loops [8 (NCBI,9]. Putative database); tubulin a tubulin polyamination molecule sites is composedare marked of in 12 orange. α-helicesα-Tubulin (numbered Q128 isH1–H12) located withinand 10 the β-strands H3–S4 loop(numbered and builds S1–S10) the H3 linked surface, by whichloops [8,9].is involved Putative in lateraltubulin interactions polyamination with thesites neighboring are marked protofilament in orange. α (an-Tubulin outside Q128 view, is protofilament located within to thethe left).H3–S4α-Tubulin loop and Q285 builds is located the withinH3 surface, the S7–H9 which loop, isthe involved so-called in M lateral loop, which interactions is also responsible with the neighboringfor lateral interactions protofilament with the(an neighboringoutside view, protofilament protofilament (an to outside the left). view, α protofilament-Tubulin Q285 to is the located right). α within-Tubulin the Q133,S7–H9 located loop, the within so-called the H3–S4 M loop, loop which and Q256 is also located responsible within thefor H8lateral helix, interactions build the minus with endthe neighboringsurface, responsible protofilament for longitudinal (an outside interactions view, protofilament between heterodimers to the right). within theα-Tubulin same protofilaments Q133, located or with γ-tubulin at the nucleation site. Q15 of β-tubulin is located within the H1 helix, positioned near the within the H3–S4 loop and Q256 located within the H8 helix, build the minus end surface, responsible nucleotide binding pocket. for longitudinal interactions between heterodimers within the same protofilaments or with γ-tubulin at the nucleation site. Q15 of β-tubulin is located within the H1 helix, positioned near the nucleotide binding pocket. Int. J. Mol. Sci. 2017, 18, 2207 3 of 18 3. Microtubule Acetylation Acetylation is a highly evolutionarily conserved tubulin modification.