The tektin family of microtubule-stabilizing proteins
© BioMed Central Ltd 2008
Published: 29 July 2008
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© BioMed Central Ltd 2008
Published: 29 July 2008
Tektins are insoluble α-helical proteins essential for the construction of cilia and flagella and are found throughout the eukaryotes apart from higher plants. Being almost universal but still fairly free to mutate, their coding sequences have proved useful for estimating the evolutionary relationships between closely related species. Their protein molecular structure, typically consisting of four coiled-coil rod segments connected by linkers, resembles that of intermediate filament (IF) proteins and lamins. Tektins assemble into continuous rods 2 nm in diameter that are probably equivalent to subfilaments of the 10 nm diameter IFs. Tektin and IF rod sequences both have a repeating pattern of charged amino acids superimposed on the seven-amino-acid hydrophobic pattern of coiled-coil proteins. The length of the repeat segment matches that of tubulin subunits, suggesting that tektins and tubulins may have coevolved, and that lamins and IFs may have emerged later as modified forms of tektin. Unlike IFs, tektin sequences include one copy of a conserved peptide of nine amino acids that may bind tubulin. The 2 nm filaments associate closely with tubulin in doublet and triplet microtubules of axonemes and centrioles, respectively, and help to stabilize these structures. Their supply restricts the assembled lengths of cilia and flagella. In doublet microtubules, the 2 nm filaments may also help to organize the longitudinal spacing of accessory structures, such as groups of inner dynein arms and radial spokes.
Tektins are related to intermediate filament (IF) proteins [1, 5, 31, 32] and nuclear lamins [33–35], whose sequences also show evidence of gene duplication. Within the rod domains of both tektins and IFs, the longitudinal repeating pattern of hydrophobic and charged amino acids suggests that their ancestral protein may have evolved in tandem with tubulin, whose globular monomers polymerize into protofilaments with a 4 nm repeat. This spacing, corresponding to 28 residues along a coiled-coil, would have arisen quite simply in an ancestral tektin as groups of four heptads. However, other coiled-coil proteins do have different patterns of charge, and different superhelix repeats; indeed, the charge pattern of tropomyosin matches the 5.5 nm periodicity of subunits in actin filaments . Thus, it is not clear whether a tubulin-like or a tektin-like protein might have existed first.
Bacteria have a homolog of tubulin, FtsZ (a protein involved in septum formation during cell division), that also forms linear protofilaments with a similar longitudinal spacing, although the spacing is a little longer, approximately 4.3 nm, and the protofilaments do not associate to form microtubules [37, 38]. Is one of FtsZ's protein partners tektin-like? Several of the proteins known to interact with FtsZ [39, 40] appear to form coiled-coil dimers (for example, EzrA, SlmA, and ZapA) but it is difficult to draw parallels with the eukaryotic tubulin-tektin association as FtsZ does not assemble into any long-term stable structure. A coiled-coil protein that forms stable filaments in Caulobacter crescentus has been investigated and shows a basic similarity to IF proteins [41, 42] but this might be coincidental; for example, muscle myosins bundle into filaments but are not considered to be IF-like. The spirochete coiled-coil protein Scc  also forms stable filaments, but the molecules seem to be continuous coiled-coils without any of the breaks or 'stutters' (short interruptions in the repeating pattern of residues) characteristic of IFs. Something in Spirochaeta halophila was found to react with anti-tektin antibodies  but no candidate sequence has been identified in the genome.
Iida et al.  recently discovered that a testis-specific tektin [46, 47] is not located in doublet microtubules but on the surface of structures called dense fibers , which augment the elastic strengths of the sperm tails of many animals, including mammals. Dense fibers do not occur in cilia, or in the flagella of unicellular animals, making it likely that tektin acquired its function in the dense fibers secondarily. If tektin and tubulin evolved together first, lamins/IFs may have evolutionarily 'escaped' in a similar fashion, as a form of tektin that no longer binds to tubulin. The alternative scenario is that the lamin/IF group of coiled-coil proteins evolved first and a modified version of one such protein was subsequently co-opted into axoneme formation, with the length of tubulin becoming adapted to fit the tektin periodicity precisely. In either case, both tektin and tubulin may have adapted to enable a eukaryote ancestor to assemble stable axonemal microtubules. Tubulin could later have found ways of assembling into more dynamic microtubules with the aid of new microtubule-associated proteins (MAPs), some of which may be related to tektins [49, 50].
As already indicated, tektins are essential constituents and specific markers for ciliary and flagellar axonemes (containing doublet microtubules) [1–26] and for basal bodies and centrioles (containing triplet microtubules) [25–29]. In the nematode Caenorhabditis elegans, for example, the expression of tektins correlates spatially with touch receptor cilia . In mammals, tektins occur in testis, brain, retina, and other tissues containing ciliated cells . Of the several types of mammalian tektins, at least two - tektin 2 and tektin 4 - are present in sperm flagella, although tektin 4 is associated with outer dense fibers rather than with outer doublet microtubules [11–20].
While it is clear that tektins are in or next to the partition of outer doublet microtubules (Figure 3a-f), some questions remain about their exact locations and functions. Electron microscope (EM) tomography of sea urchin tubules  has revealed a longitudinally continuous thin filament (at the tip of the arrow in Figure 3e) associated with the middle tubulin protofilament of the partition, which would be a good position to provide a central stabilizing element for a sliding and bending doublet microtubule, and is consistent with the proposed role of tektin in regulating the length of an axoneme through a limited supply of one of the tektins [59, 60]. However, this thin filament is distanced from the sites of attachment of radial spokes, dynein arms and the regulatory complexes, where the long periodicities inherent in a tektin filament (Figure 3g-h) might serve another useful purpose, as a molecular 'ruler'. Schemes employing both the 32 nm length of tektin AB molecules and 48 nm or 32 nm spaced tektin C molecules (Figure 3i) have been proposed to account for the 96 nm repeating series of accessory proteins on sea urchin doublet microtubules [7, 9, 10, 52]. In species with only one type of tektin, filaments assembled from 32 nm or 48 nm long molecules could still interact with a series of accessory structures to produce a 96 nm repeat. However, there are likely to be length-measuring proteins other than tektins in the axonemes of all species.
Linck has proposed that tektins bundle to form one of the protofilaments close to the inner junction between tubules A and B [10, 52, 61], which would be consistent with evidence that tektins are stably connected to the accessory structures . However, the EM tomographic image (Figure 3c-f) does not indicate any protofilament with a radically different internal composition. In contrast, the unique thin filament on the partition has the appearance expected for a simple tektin AB polymer, such as that seen by Pirner and Linck  and modeled in Figure 3i,j, and the long sideways projections reaching out as far as the junctions might explain the association of tektin with dynein. These long strands projecting sideways from the thin filament may be amino-terminal domains, for example, from tektin A (see Figure 3g), or could be separate coiled-coil proteins (possibly tektin C or related to the Chlamydomonas 'rib' proteins [54–56]). The additional proteins that co-purify with the insoluble tektins are presumably associated with the partition, rather than with regions of the A- and B-tubules that disintegrate early (see Figure 3b); in addition to the continuous filament and associated projections on the A-tubule side of the partition, the tomogram (Figure 3c-f) shows a considerable amount of material on the B-tubule side.
It is also possible that tektins can form more than a single filament; the crosslinking experiments [10, 52] proved the existence of tektin AB heterodimers and continuous polymers, and tektin C homodimers and tetramers, but not necessarily complexes of all three proteins. For example, the partition filament might be tektin AB while tektin C tetramers could associate with accessory attachment sites (Figure 3a,e). Alternatively, there could be more than one heteropolymeric filament per doublet, if the reported quantitation  turns out to be accurate. Data for Chlamydomas flagella (which apparently contain a soluble tektin that is not retained in the insoluble ribbon fraction ) also suggest two separate roles and sites for tektin in the doublet. The flagella of mutants lacking inner dynein arms contain only 20% of the normal amount of this tektin, suggesting that the other 80% may co-assemble with inner dynein arms. Thus, in species making only one type of tektin, one protein might occupy both types of sites, forming a continuous filament on the partition and a more soluble complex at the base of the inner dynein arms or radial spokes.
Many details remain to be resolved regarding the structural arrangement of tektins, ribs and other proteins that co-purify with the stable ribbons of axonemal doublet microtubules. Filaments from a range of sources other than sea urchin sperm  and Chlamydomonas [54–56] flagella need to be isolated to investigate their compositions and structural characteristics. Similarly, there is more to be learned from three-dimensional EM cryo-tomography , including images to be reconstructed with 48 nm or 96 nm rather than 16 nm longitudinal averaging. The possibilities of identifying different proteins in sea urchin axonemes by labeling are limited (antibodies are unlikely to reach sites located inside the doublets) but better methods are available for microorganisms such as Chlamydomonas and Tetrahymena, which can be genetically modified to add labels or remove components. Initially, it will probably be rewarding to compare tomograms of wild-type Chlamydomonas and the mutants mentioned above .
The precise function of the tektin signature sequence, RPNVELCRD, remains to be determined. This question may be approached using peptides or small segments of tektin produced by recombinant expression systems. It may be possible to determine whether the conserved loop binds directly to tubulin and, if so, what types of mutations eliminate binding. A related question is why mammalian tektin 4 locates to dense fibers rather than to doublet tubules , even though it has the standard signature sequence. Is there any tubulin in the outer dense fibers?
It would also be interesting to know what makes some tektins insoluble after the assembly of doublet tubules, although, presumably, only soluble complexes are transported into the flagellum. Is there a post-translational modification, similar to the phosphorylation that allows vimentin to remain soluble until it is assembled into IFs and allows it to be resolubilized during disassembly ? As tektins are unlikely to be reused [59, 60], they might be phosphorylated immediately after translation, dephosphorylated in the course of axoneme assembly but then degraded by proteolysis during flagellar retraction. Such events will probably be most conveniently studied in Chlamydomonas or Tetrahymena. The cause of differential solubility of tektins that are assumed to be in different locations in triplet microtubules  might also be investigated.
A continued search for prokaryotic ancestors of tektins and IF proteins is expected. The Escherichia coli protein SlmA  is of possible interest because it apparently supports FtsZ assembly (possible tektin-like behavior) and also associates with the bacterial nucleoid (possible lamin-like behavior), although its coiled-coil is so short as to correspond to just one of the Strongylocentrotus purpuratus (sea urchin) tektin coiled-coil segments in Figure 2. However, there may be a related protein in other bacterial species that has grown longer through gene duplication.
It is likely that many such questions will be answered as new researchers take an interest in tektins. After many years of being regarded as an obscure group of specialized proteins, they have become important, as related genes are found in every newly sequenced eukaryotic genome. Tektins will increasingly be used in phylogenetic studies [21–23, 30] and may turn out to vary even among human beings and be useful, for example, in tracking population movements.
I thank Dick Linck for introducing me to tektins long ago and for reading this review and making helpful comments.