Evolutionary divergence of enzymatic mechanisms for posttranslational polyglycylation.

Polyglycylation is a posttranslational modification that generates glycine side chains on proteins. Here we identify a family of evolutionarily conserved glycine ligases that modify tubulin using different enzymatic mechanisms. In mammals, two distinct enzyme types catalyze the initiation and elongation steps of polyglycylation, whereas Drosophila glycylases are bifunctional. We further show that the human elongating glycylase has lost enzymatic activity due to two amino acid changes, suggesting that the functions of protein glycylation could be sufficiently fulfilled by monoglycylation. Depletion of a glycylase in Drosophila using RNA interference results in adult flies with strongly decreased total glycylation levels and male sterility associated with defects in sperm individualization and axonemal maintenance. A more severe RNAi depletion is lethal at early developmental stages, indicating that protein glycylation is essential. Together with the observation that multiple proteins are glycylated, our functional data point towards a general role of glycylation in protein functions.

Glutamylation on alpha-tubulin is not essential but affects the assembly and functions of a subset of microtubules in Tetrahymena thermophila.

Tubulin undergoes glutamylation, a conserved posttranslational modification of poorly understood function. We show here that in the ciliate Tetrahymena, most of the microtubule arrays contain glutamylated tubulin. However, the length of the polyglutamyl side chain is spatially regulated, with the longest side chains present on ciliary and basal body microtubules. We focused our efforts on the function of glutamylation on the alpha-tubulin subunit. By site-directed mutagenesis, we show that all six glutamates of the C-terminal tail domain of alpha-tubulin that provide potential sites for glutamylation are not essential but are needed for normal rates of cell multiplication and cilium-based functions (phagocytosis and cell motility). By comparative phylogeny and biochemical assays, we identify two conserved tubulin tyrosine ligase (TTL) domain proteins, Ttll1p and Ttll9p, as alpha-tubulin-preferring glutamyl ligase enzymes. In an in vitro microtubule glutamylation assay, Ttll1p showed a chain-initiating activity while Ttll9p had primarily a chain-elongating activity. GFP-Ttll1p localized mainly to basal bodies, while GFP-Ttll9p localized to cilia. Disruption of the TTLL1 and TTLL9 genes decreased the rates of cell multiplication and phagocytosis. Cells lacking both genes had fewer cortical microtubules and showed defects in the maturation of basal bodies. We conclude that glutamylation on alpha-tubulin is not essential but is required for efficiency of assembly and function of a subset of microtubule-based organelles. Furthermore, the spatial restriction of modifying enzymes appears to be a major mechanism that drives differential glutamylation at the subcellular level.

TTLL3 Is a tubulin glycine ligase that regulates the assembly of cilia.

In most ciliated cell types, tubulin is modified by glycylation, a posttranslational modification of unknown function. We show that the TTLL3 proteins act as tubulin glycine ligases with chain-initiating activity. In Tetrahymena, deletion of TTLL3 shortened axonemes and increased their resistance to paclitaxel-mediated microtubule stabilization. In zebrafish, depletion of TTLL3 led to either shortening or loss of cilia in several organs, including the Kupffer's vesicle and olfactory placode. We also show that, in vivo, glutamic acid and glycine ligases oppose each other, likely by competing for shared modification sites on tubulin. We propose that tubulin glycylation regulates the assembly and dynamics of axonemal microtubules and acts either directly or indirectly by inhibiting tubulin glutamylation.

Tubulin glycylation and glutamylation deficiencies in unconventional insect axonemes.

Though the 9+2 axonemal organization has generally been conserved throughout metazoan evolution, insect spermatozoa possess a substantial variety in axoneme ultrastructure, displaying different axonemal patterns. Therefore, insects provide a wide range of models that may be useful for the study of the mechanisms of axoneme assembly. We have used antibodies specific for glutamylated, monoglycylated, and polyglycylated tubulin to investigate the tubulin isoform content expressed in the unorthodox sperm axonemes of four insect species belonging to both of the superorders Palaeoptera and Neoptera. Each one of these axonemal models exhibits distinctive structural features, either showing the typical radial organization endowed with a ninefold symmetry or consisting of an helical arrangement with up to 200 microtubular doublets, but in all cases these axonemes share the absence of a microtubule central pair. Our results showed that all these atypical patterns are characterized by a dramatic decrease in both tubulin glycylation and glutamylation levels or even lack of both polymodifications. These data provide the first examples of a simultaneous extreme reduction or even absence of both polymodifications in axonemal tubulin. Given the unrelated positions of the analyzed species in the insect phylogenetic tree, this common feature is probably not due to evolutionary relationships. Therefore, our findings support the hypothesis of the existence of a correlation between the low level of polymodifications and the lack of a microtubule central pair in these peculiar insect flagellar axonemes, similarly as was previously proposed for cilia of Tetrahymena glycylation site mutants.

Mutations of tubulin glycylation sites reveal cross-talk between the C termini of alpha- and beta-tubulin and affect the ciliary matrix in Tetrahymena.

Two types of polymeric post-translational modifications of alpha/beta-tubulin, glycylation and glutamylation, occur widely in cilia and flagella. Their respective cellular functions are poorly understood. Mass spectrometry and immunoblotting showed that two closely related species, the ciliates Tetrahymena and Paramecium, have dramatically different compositions of tubulin post-translational modifications in structurally identical axonemes. Whereas the axonemal tubulin of Paramecium is highly glycylated and has a very low glutamylation content, the axonemal tubulin of Tetrahymena is glycylated and extensively glutamylated. In addition, only the alpha-tubulin of Tetrahymena undergoes detyrosination. Mutations of the known glycylation sites in Tetrahymena tubulin affected the level of each polymeric modification type in both the mutated and nonmutated subunits, revealing cross-talk between alpha- and beta-tubulin. Ultrastructural analyses of glycylation site mutants uncovered defects in the doublet B-subfiber of axonemes and revealed an accumulation of dense material in the ciliary matrix, reminiscent of intraflagellar transport particles seen by others in Chlamydomonas. We propose that polyglycylation and/or polyglutamylation stabilize the B-subfiber of outer doublets and regulate the intraflagellar transport.

Glutamylated tubulin: diversity of expression and distribution of isoforms.

Glutamylation of alpha and beta tubulin isotypes is a major posttranslational modification giving rise to diversified isoforms occurring mainly in neurotubules, centrioles, and axonemes. Monoglutamylated tubulin isoforms can be differentially recognized by two mAbs, B3 and GT335, which both recognize either polyglutamylated isoforms. In the present study, immunoelectron microscopy and immunofluorescence analyses were performed with these two mAbs to determine the expression and distribution of glutamylated tubulin isoforms in selected biological models whose tubulin isotypes are characterized. In mouse spermatozoa, microtubules of the flagellum contain polyglutamylated isoforms except in the tip where only monoglutamylated isoforms are detected. In spermatids, only a subset of manchette microtubules contain monoglutamylated tubulin isoforms. Cytoplasmic microtubules of Sertoli cells are monoglutamylated. Mitotic and meiotic spindles of germ cells are monoglutamylated whereas the HeLa cell mitotic spindle is polyglutamylated. Three models of axonemes are demonstrated as a function of the degree and extent of tubulin glutamylation. In lung ciliated cells, axonemes are uniformly polyglutamylated. In sea urchin sperm and Chlamydomonas, flagellar microtubules are polyglutamylated in their proximal part and monoglutamylated in their distal part. In Paramecium, cilia are bi- or monoglutamylated only at their base. In all cells, centrioles or basal bodies are polyglutamylated. These new data emphasize the importance of glutamylation in all types of microtubules and strengthen the hypothesis of its role in the regulation of the intracellular traffic and flagellar motility.

Glutamylated and glycylated tubulin isoforms in the aberrant sperm axoneme of the gall-midge fly, Asphondylia ruebsaameni.

The axonemal organization expressed in the sperm flagella of the cecidomyiid dipteran Asphondylia ruebsaameni is unconventional, being characterized by the presence of an exceedingly high number of microtubular doublets and by the absence of both the inner dynein arms and the central pair/radial spoke complex. Consequently, its motility, both in vivo and in vitro, is also peculiar. Using monoclonal antibodies directed against posttranslational modifications, we have analyzed the presence and distribution of glutamylated and glycylated tubulin isoforms in this aberrant axonemal structure, and compared them with those of a reference insect species (Apis mellifera), endowed with a conventional axoneme. Our results have shown that the unorthodox structure and motility of the Asphondylia axoneme are concomitant with: (1). a very low glutamylation extent in the alpha-tubulin subunit, (2). a high level of glutamylation in the beta-subunit, (3). an extremely low total extent of glycylation, with regard to both monoglycylated and polyglycylated sites, either in alpha- or in beta-tubulin, (4). the presence of a strong labeling of glutamylated tubulin isoforms at the proximal end of the axoneme, and (5). a uniform distribution of glutamylated as well as glycylated isoforms along the rest of the axoneme. Thus, our data indicate that tubulin molecular heterogeneity is much lower in the Asphondylia axoneme than in the conventional 9+2 axoneme with regard to both isoform content and isoform distribution along the axoneme.