Biochemical characterization of tektins from sperm flagellar doublet microtubules

Expression pattern of prohibitin, capping actin protein of muscle Z-line beta subunit and tektin-2 gene in Murrah buffalo sperm and its relationship with sperm motility

Objective

The aim of the current study is to investigate the relationship between prohibitin (PHB), capping actin protein of muscle Z-line beta subunit (CAPZB), and tektin-2 (TEKT2) and sperm motility in Murrah buffalo.

Methods

We collected the high-motility and low-motility semen samples, testis, ovary, muscle, kidney, liver, brain and pituitary from Murrah buffalo, and analysed the expression of PHB, CAPZB, and TEKT2 in mRNA (message RNA) and protein level.

Results

Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) result showed that the expression of PHB was higher and CAPZB, TEKT2 were specifically expressed in testis as compared to the other 6 tissues, and that in testis, the expression of TEKT2 was higher than that of CAPZB and PHB. Immunohistochemistry test revealed that all three genes were located on the convoluted seminiferous tubule and enriched in spermatogenic cells. Both qRT-PCR and Western Blot results showed that the expression levels of PHB, CAPZB, and TEKT2 were significantly lower in the low-motility semen group compared to the high-motility semen group (p<0.05).

Conclusion

The expression levels of PHB, CAPZB, and TEKT2 in Murrah buffalo sperm have a high positive correlation with sperm motility. And the three genes may be potential molecular markers for the decline of buffalo sperm motility.

Insights into the structure and function of ciliary and flagellar doublet microtubules: tektins, Ca2+-binding proteins, and stable protofilaments

Cilia and flagella are conserved, motile, and sensory cell organelles involved in signal transduction and human disease. Their scaffold consists of a 9-fold array of remarkably stable doublet microtubules (DMTs), along which motor proteins transmit force for ciliary motility and intraflagellar transport. DMTs possess Ribbons of three to four hyper-stable protofilaments whose location, organization, and specialized functions have been elusive. We performed a comprehensive analysis of the distribution and structural arrangements of Ribbon proteins from sea urchin sperm flagella, using quantitative immunobiochemistry, proteomics, immuno-cryo-electron microscopy, and tomography. Isolated Ribbons contain acetylated α-tubulin, β-tubulin, conserved protein Rib45, >95% of the axonemal tektins, and >95% of the calcium-binding proteins, Rib74 and Rib85.5, whose human homologues are related to the cause of juvenile myoclonic epilepsy. DMTs contain only one type of Ribbon, corresponding to protofilaments A11-12-13-1 of the A-tubule. Rib74 and Rib85.5 are associated with the Ribbon in the lumen of the A-tubule. Ribbons contain a single ∼5-nm wide filament, composed of equimolar tektins A, B, and C, which interact with the nexin-dynein regulatory complex. A summary of findings is presented, and the functions of Ribbon proteins are discussed in terms of the assembly and stability of DMTs, ciliary motility, and other microtubule systems.

Cryo-electron tomography reveals conserved features of doublet microtubules in flagella

The axoneme forms the essential and conserved core of cilia and flagella. We have used cryo-electron tomography of Chlamydomonas and sea urchin flagella to answer long-standing questions and to provide information about the structure of axonemal doublet microtubules (DMTs). Solving an ongoing controversy, we show that B-tubules of DMTs contain exactly 10 protofilaments (PFs) and that the inner junction (IJ) and outer junction between the A- and B-tubules are fundamentally different. The outer junction, crucial for the initiation of doublet formation, appears to be formed by close interactions between the tubulin subunits of three PFs with unusual tubulin interfaces; other investigators have reported that this junction is weakened by mutations affecting posttranslational modifications of tubulin. The IJ consists of an axially periodic ladder-like structure connecting tubulin PFs of the A- and B-tubules. The recently discovered microtubule inner proteins (MIPs) on the inside of the A- and B-tubules are more complex than previously thought. They are composed of alternating small and large subunits with periodicities of 16 and/or 48 nm. MIP3 forms arches connecting B-tubule PFs, contrary to an earlier report that MIP3 forms the IJ. Finally, the "beak" structures within the B-tubules of Chlamydomonas DMT1, DMT5, and DMT6 are clearly composed of a longitudinal band of proteins repeating with a periodicity of 16 nm. These findings, discussed in relation to genetic and biochemical data, provide a critical foundation for future work on the molecular assembly and stability of the axoneme, as well as its function in motility and sensory transduction.

Sperm flagella: comparative and phylogenetic perspectives of protein components

Sperm motility is necessary for the transport of male DNA to eggs in species with both external and internal fertilization. Flagella comprise several proteins for generating and regulating motility. Central cytoskeletal structures called axonemes have been well conserved through evolution. In mammalian sperm flagella, two accessory structures (outer dense fiber and the fibrous sheath) surround the axoneme. The axonemal bend movement is based on the active sliding of axonemal doublet microtubules by the molecular motor dynein, which is divided into outer and inner arm dyneins according to positioning on the doublet microtubule. Outer and inner arm dyneins play different roles in the production and regulation of flagellar motility. Several regulatory mechanisms are known for both dyneins, which are important in motility activation and chemotaxis at fertilization. Although dynein itself has certain properties that contribute to the formation and propagation of flagellar bending, other axonemal structures-specifically, the radial spoke/central pair apparatus-have essential roles in the regulation of flagellar bending. Recent genetic and proteomic studies have explored several new components of axonemes and shed light on the generation and regulation of sperm motility during fertilization.

Characterization of a novel tektin member, TEKT5, in mouse sperm

Tektins are important components of flagella. Alterations in the expression of or mutations in mouse tektins are correlated with defective sperm motility, a cause of male infertility. Our proteomic studies of flagellar accessory structures previously identified a novel tektin, TEKT5, whose function is unknown. To understand the role of TEKT5 in mouse sperm, we characterized the expression of the mouse Tekt5 gene and the presence of TEKT5 in spermatogenic cells and spermatozoa. A complete cDNA encoding the Tekt5 transcript was assembled following reverse transcription-polymerase chain reaction (RT-PCR) and 3'-rapid amplification of cDNA ends and predicted that TEKT5 is a 62 730-dalton protein with an unusual, long C-terminus. Tekt5 mRNA was highly expressed during late stages of spermiogenesis. Among examined tissues, Tekt5 mRNA was present only in testis and brain, and quantitative RT-PCR showed that the expression level of mRNA in testis was 6.8-fold higher than that in brain. At the protein level, TEKT5 was present in sperm and was enriched in the accessory structures of flagella. Immunofluorescence confirmed that TEKT5 was localized throughout the sperm tail in flagellar accessory structures. The expression pattern suggests that TEKT5 plays an important role in flagella formation during spermiogenesis as well as being implicated in sperm motility.

The tektin family of microtubule-stabilizing proteins

Tektins are insoluble a-helical proteins essential for the construction of cilia and flagella and are found throughout the eukaryotes apart from higher plants.

Axoneme-dependent tubulin modifications in singlet microtubules of the Drosophila sperm tail

Drosophila melanogaster sperm tubulins are posttranslationally glutamylated and glycylated. We show here that axonemes are the substrate for these tubulin C-terminal modifications. Axoneme architecture is required, but full length, motile axonemes are not necessary. Tubulin glutamylation occurs during or shortly after assembly into the axoneme; only glutamylated tubulins are glycylated. Tubulins in other testis microtubules are not modified. Only a small subset of total Drosophila sperm axoneme tubulins have these modifications. Biochemical fractionation of Drosophila sperm showed that central pair and accessory microtubules have the majority of poly-modified tubulins, whereas doublet microtubules have only small amounts of mono- and oligo-modified tubulins. Glutamylation patterns for different beta-tubulins experimentally assembled into axonemes were consistent with utilization of modification sites corresponding to those identified in other organisms, but surrounding sequence context was also important. We compared tubulin modifications in the 9 + 9 + 2 insect sperm tail axonemes of Drosophila with the canonical 9 + 2 axonemes of sperm of the sea urchin Lytichinus pictus and the 9 + 0 motile sperm axonemes of the eel Anguilla japonica. In contrast to Drosophila sperm, L. pictus sperm have equivalent levels of modified tubulins in both doublet and central pair microtubule fractions, whereas the doublets of A. japonica sperm exhibit little glutamylation but extensive glycylation. Tubulin C-terminal modifications are a prevalent feature of motile axonemes, but there is no conserved pattern for placement or amount of these

Functional protofilament numbering of ciliary, flagellar, and centriolar microtubules

This article discusses the current state of knowledge about the evolutionarily conserved structure of ciliary, flagellar and centriolar microtubules, and formally proposes a functional numbering convention for their protofilaments.

Tektin interactions and a model for molecular functions

Tektins from echinoderm flagella were analyzed for microheterogeneity, self-associations and association with tubulin, resulting in a general model of tektin filament structure and function applicable to most eukaryotic cilia and flagella. Using a new antibody to tektin consensus peptide RPNVELCRD, well-characterized chain-specific antibodies and quantitative gel densitometry, tektins A, B and C were found to be present in equimolar amounts in Sarkosyl-urea-stable filaments. In addition, two isoforms of tektin A are present in half-molar ratios to tektins B and C. Cross-linking of AB filaments indicates in situ nearest neighbor associations of tektin A1B and A2B heterodimers, -trimers, -tetramers and higher oligomers. Soluble purified tektin C is cross-linked as homodimers, trimers and tetramers, but not higher oligomers. Tektin filaments associate with both loosely bound and tightly bound tubulin, and with the latter in a 1:1 molar ratio, implying a specific, periodic association of tightly bound tubulin along the tektin axis. Similarly, in tektin-containing Sarkosyl-stable protofilament ribbons, two polypeptides ( approximately 67/73 kDa, homologues of rib72, efhc1 and efhc2) are present in equimolar ratios to each other and to individual tektins, co-fractionating with loosely bound tubulin. These results suggest a super-coiled arrangement of tektin filaments, the organization of which has important implications for the evolution, assembly and functions of cilia and flagella.

Molecular architecture of axonemal microtubule doublets revealed by cryo-electron tomography

The axoneme, which forms the core of eukaryotic flagella and cilia, is one of the largest macromolecular machines, with a structure that is largely conserved from protists to mammals. Microtubule doublets are structural components of axonemes that contain a number of proteins besides tubulin, and are usually found in arrays of nine doublets arranged around two singlet microtubules. Coordinated sliding of adjacent doublets, which involves a host of other proteins in the axoneme, produces periodic beating movements of the axoneme. We have obtained a three-dimensional density map of intact microtubule doublets using cryo-electron tomography and image averaging. Our map, with a resolution of about 3 nm, provides insights into locations of particular proteins within the doublets and the structural features of the doublets that define their mechanical properties. We identify likely candidates for several of these non-tubulin components of the doublets. This work offers insight on how tubulin protofilaments and accessory proteins attach together to form the doublets and provides a structural basis for understanding doublet function in axonemes.

Mice deficient in the axonemal protein Tektin-t exhibit male infertility and immotile-cilium syndrome due to impaired inner arm dynein function

The haploid germ cell-specific Tektin-t protein is a member of the Tektin family of proteins that form filaments in flagellar, ciliary, and axonemal microtubules. To investigate the physiological role of Tektin-t, we generated mice with a mutation in the tektin-t gene. The homozygous mutant males were infertile, while the females were fully fertile. Sperm morphology and function were abnormal, with frequent bending of the sperm flagella and marked defects in motility. In vitro fertilization assays showed that the defective spermatozoa were able to fertilize eggs. Electron microscopic examination showed that the dynein inner arm structure was disrupted in the sperm flagella of tektin-t-deficient mice. Furthermore, homozygous mutant mice had functionally defective tracheal cilia, as evidenced by altered dynein arm morphology. These results indicate that Tektin-t participates in dynein inner arm formation or attachment and that the loss of Tektin-t results in impaired motility of both flagella and cilia. Therefore, the tektin-t gene is one of the causal genes for immotile-cilium syndrome/primary ciliary dyskinesia.

A tektin homologue is decreased in chlamydomonas mutants lacking an axonemal inner-arm dynein

In ciliary and flagellar axonemes, various discrete structures such as inner and outer dynein arms are regularly arranged on the outer doublet microtubules. Little is known about the basis for their regular arrangement. In this study, proteins involved in the attachment of inner-arm dyneins were searched by a microtubule overlay assay on Chlamydomonas mutant axonemes. A 58-kDa protein (p58) was found approximately 80% diminished in the mutants ida6 and pf3, both lacking one (species e) of the seven inner-arm species (a-g). Analysis of its cDNA indicated that p58 is homologous to tektin, a protein that was originally found in sea urchin and thought to be crucial for the longitudinal periodicity of the doublet microtubule. Unlike sea urchin tektin, which is a component of protofilament ribbons that occur after Sarkosyl treatment of axonemes, p58 was not contained in similar Sarkosyl-resistant ribbons from Chlamydomonas axonemes. Immunofluorescence microscopy showed that p58 was localized uniformly along the axoneme and on the basal body. The p58 signal was reduced in ida6 and pf3. These results suggest that a reduced amount of p58 is sufficient for the production of outer doublets, whereas an additional amount of it is involved in inner-arm dynein attachment.

Cloning and characterization of testis-specific tektin in Bombyx mori

A testis-specific cDNA library of Bombyx mori was constructed by an mRNA subtraction technique. Several clones were randomly selected and determined for their nucleotide sequences. One of them, designated as BmTST, contained a 3'-part of an open reading frame homologous to tektin, the protein known to form filamentous polymers in the walls of ciliary and flagellar microtubules. Also isolated was a genomic fragment, which contains the 5'-part of the coding sequence of BmTST and its promoter region. As a whole, the complete open reading frame was found to encode 508 amino acid residues, whose sequence had 28, 28 and 30% identities with the Strongylocentrotus purpuratus tektins A1, B1 and C1, respectively. Expression analysis by reverse transcription polymerase chain reaction with the cDNA and Western blotting with a polyclonal antibody indicated that the BmTST gene was expressed specifically in the testis during sperm maturation. The protein was immunologically detected exclusively in the fraction expected to contain the 9 + 2 flagellar axonemes of sperms. We infer that the BmTst protein is possibly involved in the spermatogenesis of B. mori.

Tektins as structural determinants in basal bodies

Tektins, present as three equimolar 47-55 kDa protein components, form highly insoluble protofilaments that are integral to the junctional region of outer doublet microtubules in cilia and flagella. To identify and quantify tektins in other compound microtubules such as centrioles or basal bodies, a rabbit antiserum was raised against tektin filaments isolated from Spisula solidissima (surf clam) sperm flagellar outer doublets and affinity-purified with nitrocellulose blot strips of tektins resolved by SDS- or SDS-urea-PAGE. These antibodies recognized analogous tektins in axonemes of organisms ranging from ctenophores to higher vertebrates. Quantitative immunoblotting established that outer doublet tektins occur in a 1:17 weight ratio to tubulin. Cilia and basal apparatuses were prepared from scallop gill epithelial cells; cilia and deciliated cells were prepared from rabbit trachea. Tektins were detected by immunoblotting in basal body-enriched preparations while tektins were localized to individual basal bodies by immunofluorescence. Supported by greater fluorescence in basal bodies than in adjacent axonemes in tracheal cells, analysis of basal apparatuses demonstrated both a proportionately greater ratio of tektin to tubulin (approximately 1:13) and two distinct solubility classes of tektins, consistent with tektins comprising the B-C junction of triplets in addition to the A-B junction as in doublets.

Molecular characteristics of the centrosome

As an organizer of the microtubule cytoskeleton in animals, the centrosome has an important function. From the early light microscopic observation of the centrosome to examination by electron microscopy, the centrosome field is now in an era of molecular identification and precise functional analyses. Tables compiling centrosomal proteins and reviews on the centrosome are presented here and demonstrate how active the field is. However, despite this intense research activity, many classical questions are still unanswered. These include those regarding the precise function of centrioles, the mechanism of centrosome duplication and assembly, the origin of the centrosome, and the regulation and mechanism of the centrosomal microtubule nucleation activity. Fortunately, these questions are becoming elucidated based on experimental data discussed here. Given the fact that the centrosome is primarily a site of microtubule nucleation, special focus is placed on the process of microtubule nucleation and on the regulation of centrosomal microtubule nucleation capacity during the cell cycle and in some tissues.

Electrophoretic resolution of tubulin and tektin subunits by differential interaction with long-chain alkyl sulfates

Tubulin dimers, formed from globular alpha and beta subunits, and the tektins, three equimolar alpha-helical proteins that form filaments, mutually associate to form the junctional regions of doublet and triplet microtubules. When evaluated by SDS-PAGE, the apparent molecular weights of these proteins can deviate substantially from their sequence molecular weights in a manner sensitive to both the source of SDS and the species of origin. The electrophoretic mobility of sperm tail flagellar tubulins and tektins from an echinoderm and a mollusc were studied systematically using detergent-free stacking and resolving gels with a running buffer containing pure sodium dodecyl sulfate augmented with fixed amounts of C10, C14, C16, or C18 alkyl sulfates. Although having no systematic effect on molecular weight standards, the presence of alkyl sulfates of increasing chain length progressively exaggerated the separation of tubulin subunits, similarly facilitated the separation of two normally comigrating tektins, yet minimally influenced the relative migration of adequately separated tektins. This phenomenon is most likely due to preferential binding of longer chain alkyl sulfates by specific hydrophobic regions of these otherwise similar proteins. The use of binary mixtures of pure alkyl sulfates, required in the running buffer alone, may prove useful for reproducibly separating other proteins that characteristically bind SDS anomalously.

Expression of ciliary tektins in brain and sensory development

Many types of neural tissues and sensory cells possess either motile or primary cilia. We report the first mammalian (murine testis) cDNA for tektin, a protein unique to cilia, flagella, and centrioles, which we have used to identify related proteins and genes in sensory tissues. Comparison with the sequence database reveals that tektins are a gene family, spanning evolution from Caenorhabditis elegans (in which they correlate with touch receptor cilia) and Drosophila melanogaster, to Mus musculus and Homo sapiens (in which they are found in brain, retina, melanocytes, and at least 13 other tissues). The peptide sequence RPNVELCRD, or a variant of it, is a prominent feature of tektins and is likely to form a functionally important protein domain. Using the cDNA as a probe, we determined the onset, relative levels, and locations of tektin expression in mouse for several adult tissues and embryonic stages by Northern blot analysis and in situ hybridization. Tektin expression is significant in adult brain and in the choroid plexus, the forming retina (primitive ependymal zone corresponding to early differentiating photoreceptor cells), and olfactory receptor neurons of stage embryonic day 14 embryos. There is a striking correlation of tektin expression with the known presence of either motile or primary cilia. The evolutionary conservation of tektins and their association with tubulin in cilia and centriole formation make them important and useful molecular targets for the study of neural development.

Expression of ciliary tektins in brain and sensory development

Many types of neural tissues and sensory cells possess either motile or primary cilia. We report the first mammalian (murine testis) cDNA for tektin, a protein unique to cilia, flagella, and centrioles, which we have used to identify related proteins and genes in sensory tissues. Comparison with the sequence database reveals that tektins are a gene family, spanning evolution from Caenorhabditis elegans (in which they correlate with touch receptor cilia) and Drosophila melanogaster, to Mus musculus and Homo sapiens (in which they are found in brain, retina, melanocytes, and at least 13 other tissues). The peptide sequence RPNVELCRD, or a variant of it, is a prominent feature of tektins and is likely to form a functionally important protein domain. Using the cDNA as a probe, we determined the onset, relative levels, and locations of tektin expression in mouse for several adult tissues and embryonic stages by Northern blot analysis and in situ hybridization. Tektin expression is significant in adult brain and in the choroid plexus, the forming retina (primitive ependymal zone corresponding to early differentiating photoreceptor cells), and olfactory receptor neurons of stage embryonic day 14 embryos. There is a striking correlation of tektin expression with the known presence of either motile or primary cilia. The evolutionary conservation of tektins and their association with tubulin in cilia and centriole formation make them important and useful molecular targets for the study of neural development.

The A and B tubules of the outer doublets of sea urchin sperm axonemes are composed of different tubulin variants

The alpha beta-tubulin heterodimer, the structural unit of microtubules, comes in many variants. There are different alpha and beta isotypes encoded by multigene families. Additional heterogeneity is generated by a set of posttranslational modifications. Detyrosination of alpha-tubulin, removal of the carboxy-terminal Glu-Tyr dipeptide of alpha-tubulin, phosphorylation of some tubulins, polyglutamylation, and polyglycylation of alpha- and beta-tubulins all involve the acidic carboxy-terminal region. We have investigated the distribution of tubulin variants in the axonemal microtubules of sea urchin sperm flagella by immunological procedures and by direct sequence and mass spectrometric analysis of the carboxy-terminal peptides. The A and B tubules that comprise the nine outer doublets differ strongly in tubulin variants. A tubules contain over 95% unmodified, tyrosinated alpha beta-tubulin. In B tubules, alpha-tubulin is approximately 65% detyrosinated and both alpha- and beta-tubulin are 40-45% polyglycylated. These results show a segregation of tubulin variants between two different axonemal structures and raise the possibility that posttranslational modifications of tubulins reflect or specify structurally and functionally distinct microtubules.

Selective incorporation of architectural proteins into terminally differentiated molluscan gill cilia

Incubation of excised gills from the bay scallop Aequipecten irradians with 3H-leucine demonstrates that many ciliary structural proteins can attain a degree of labeling approaching that previously reported for sea urchin or surf clam embryos undergoing ciliary turnover or regeneration. This labeling is not a consequence of any predominant incorporation into new cilia at the meristematic growth tips of the gill since tissue regions of varying maturity incorporate label into the same proteins at similar levels, with the most mature region having the highest incorporation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorographic analysis of isolated cilia, separated into detergent-soluble membrane/matrix and detergent-insoluble 9+2 axoneme fractions, reveals that 1) tubulin in the membrane/matrix fraction is labeled whereas tubulin in the axoneme is not; 2) no labeled dynein heavy chains are seen in either fraction; 3) the most heavily labeled axonemal components do not appear to any significant extent in the membrane/matrix fraction; and 4) after thermal depolymerization of the microtubules, nearly all labeled proteins reside in the in-soluble ninefold ciliary remnant, the most prominent being tektin A, an integral component of outer doublet microtubules. Further fractionation of the remnant with sarkosyl-urea to produce tektin filaments demonstrates two solubility classes of tekin A, only the more soluble of which is labeled. Very similar selective architectural protein labeling patterns have been reported for steady-state cilia of sea urchin embryos, and this may indicate a widespread turnover or exchange mechanism characteristic of cilia heretofore considered static.

Transcriptional control of tektin A mRNA correlates with cilia development and length determination during sea urchin embryogenesis

Previous studies have shown that tektin A, one of three integral filamentous protein components of outer doublet microtubules, is synthesized in sea urchins in an amount correlating to the length of embryonic cilia initially assembled or experimentally regenerated. To investigate further the molecular mechanism for the regulation of tektin synthesis, tektin cDNA clones were used to assess mRNA levels during ciliogenesis, zinc-induced animalization, deciliation-induced regeneration and theophylline-induced elongation. Possibly involved in centriole replication, low, near-constant levels of mRNA for all three tektins are present in the unfertilized egg and during cleavage stages. Preceded by new synthesis of tektin B and C mRNAs, tektin A mRNA is up-regulated during ciliogenesis, but only tektin A mRNA levels correlate directly with ciliary length in animalized embryos; the others augment larger, non-limiting pools of tektins B and C. Tektin mRNAs decrease to steady-state levels after ciliogenesis, but are up-regulated again when the embryos are deciliated, correlating with the length of cilia to be deployed. In a species where a 3-fold ciliary length increase can be induced by theophylline treatment of zinc-arrested embryos, the mRNAs accumulate to proportionately higher levels during arrest but are not translated until induction, whereupon they decrease inversely with ciliary elongation. This suggests transcriptional control with respect to mRNA amounts but post-transcriptional control with respect to the expression of this phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Ciliogenesis in sea urchin embryos--a subroutine in the program of development

One major milestone in the development of the sea urchin embryo is the assembly of a single cilium on each blastomere just before hatching. These cilia are constructed both from pre-existing protein building blocks, such as tubulin and dynein, and from a number of 9 + 2 architectural elements that are synthesized de novo at ciliogenesis. The finite or quantal synthesis of certain key architectural proteins is coincident with ciliary elongation and proportional to ciliary length. Upon deciliation, the synthesis of architectural proteins occurs anew, a new cilium grows, and the stores of various building blocks are replenished. This routine of coordinated ciliary gene expression may be replayed experimentally many times without delaying normal development. The ability to regenerate cilia has allowed elucidation of these various protein synthetic relationships and has led to the discovery of the pathways by which membrane-associated tubulin and axoneme-associated architectural proteins are conveyed into the highly compartmentalized growing cilium. The sea urchin embryo thus provides a very convenient model system for studies of ciliary assembly and maintenance, coordinate gene expression and membrane dynamics.

At least one of the protofilaments in flagellar microtubules is not composed of tubulin

BACKGROUND

The core of the eukaryotic flagellum is the axoneme, a complex motile organelle composed of approximately 200 different polypeptides. The most prominent components of the axoneme are the central pair and nine outer doublet microtubules. Each doublet microtubule contains an A and a B tubule; these are composed, respectively, of 13 and 10-11 protofilaments, all of which are thought to be made of tubulin. The mechanisms that control the assembly of the doublet microtubules and establish the periodic spacings of associated proteins, such as dynein arms and radial spokes, are unknown. Tektins, a set of microtubule-associated proteins, are present in the axoneme as stable filaments that remain after the extraction of doublet microtubules; they are localized near to where the B tubule attaches to the A tubule and near to the binding sites for radial spokes, inner dynein arms and nexin links. Tektin filaments may contribute in an interesting way to the structural properties of axonemes.

RESULTS

We have fractionated doublet microtubules from sea urchin sperm flagella into ribbons of stable protofilaments, which can be shown to originate from the A tubule. Using cryo-electron microscopy, conventional electron microscopy, scanning transmission electron microscopy, three-dimensional reconstruction and kinesin decoration, we have found that one protofilament in the ribbon is not composed of tubulin. This protofilament is an integral protofilament of the A tubule wall, has less mass per unit length than tubulin and does not bind kinesin.

CONCLUSION

Contrary to what is generally assumed, at least one protofilament in the wall of the A tubule is not composed of tubulin. Our data suggest that this nontubulin protofilament is primarily composed of tektins, proteins that show some structural similarity to intermediate filament proteins. A 480 A axial periodicity within these ribbons, revealed by scanning transmission electron microscopy, can be related to the structure of tektin, and may determine the large-scale structure of the axoneme in terms of the binding of dynein, nexin and radial spokes to the doublet microtubule.

Do prokaryotes contain microtubules?

In eukaryotic cells, microtubules are 24-nm-diameter tubular structures composed of a class of conserved proteins called tubulin. They are involved in numerous cell functions including ciliary motility, nerve cell elongation, pigment migration, centrosome formation, and chromosome movement. Although cytoplasmic tubules and fibers have been observed in bacteria, some with diameters similar to those of eukaryotes, no homologies to eukaryotic microtubules have been established. Certain groups of bacteria including azotobacters, cyanobacteria, enteric bacteria, and spirochetes have been frequently observed to possess microtubule-like structures, and others, including archaebacteria, have been shown to be sensitive to drugs that inhibit the polymerization of microtubules. Although little biochemical or molecular biological information is available, the differences observed among these prokaryotic structures suggest that their composition generally differs among themselves as well as from that of eukaryotes. We review the distribution of cytoplasmic tubules in prokaryotes, even though, in all cases, their functions remain unknown. At least some tend to occur in cells that are large, elongate, and motile, suggesting that they may be involved in cytoskeletal functions, intracellular motility, or transport activities comparable to those performed by eukaryotic microtubules. In Escherichia coli, the FtsZ protein is associated with the formation of a ring in the division zone between the newly forming offspring cells. Like tubulin, FtsZ is a GTPase and shares with tubulin a 7-amino-acid motif, making it a promising candidate in which to seek the origin of tubulins.

Centrosomal components immunologically related to tektins from ciliary and flagellar microtubules

Centrosomes are critical for the nucleation and organization of the microtubule cytoskeleton during both interphase and cell division. Using antibodies raised against sea urchin sperm flagellar microtubule proteins, we characterize here the presence and behavior of certain components associated with centrosomes of the surf clam Spisula solidissima and cultured mammalian cells. A Sarkosyl detergent-resistant fraction of axonemal microtubules was isolated from sea urchin sperm flagella and used to produce monoclonal antibodies, 16 of which were specific- or cross-specific for the major polypeptides associated with this microtubule fraction: tektins A, B and C, acetylated alpha-tubulin, and 77 and 83 kDa polypeptides. By 2-D isoelectric focussing/SDS polyacrylamide gel electrophoresis the tektins separate into several polypeptide spots. Identical spots were recognized by monoclonal and polyclonal antibodies against a given tektin, indicating that the different polypeptide spots are isoforms or modified versions of the same protein. Four independently derived monoclonal anti-tektins were found to stain centrosomes of S. solidissima oocytes and CHO and HeLa cells, by immunofluorescence microscopy. In particular, the centrosome staining of one monoclonal antibody specific for tektin B (tekB3) was cell-cycle-dependent for CHO cells, i.e. staining was observed only from early prometaphase until late anaphase. By immuno-electron microscopy tekB3 specifically labeled material surrounding the centrosome, whereas a polyclonal anti-tektin B recognized centrioles as well as the centrosomal material throughout the cell cycle. Finally, by immunoblot analysis tekB3 stained polypeptides of 48–50 kDa in isolated spindles and centrosomes from CHO cells.

Tubulin and tektin in sea urchin embryonic cilia: pathways of protein incorporation during turnover and regeneration

Axonemal precursor tubulin is the major protein component of the detergent-soluble membrane/matrix fraction of sea urchin embryonic cilia. Its unusual abundance may reflect the rapid turnover of these cilia, a process that is further documented here. However, whether during induced regeneration or normal turnover and growth, most other newly synthesized axonemal proteins are not detectable in the membrane/matrix fraction, raising the question of how non-tubulin precursors transit the growing cilium to the distal tip where assembly is generally thought to occur. Three potential explanations were considered: (1) the assembly of these components is proximal; (2) their relative concentration is too low to detect; or (3) tubulin alone is conveyed via a membrane/matrix pathway while most other axonemal proteins are transported in association with the axoneme. Light microscope autoradiography of axonemes pulse-chase labeled with [3H]leucine showed relatively uniform labeling, with no evidence for proximal incorporation. Fully grown cilia and cilia at early stages of regeneration were isolated from labeled embryos, fractionated into membrane/matrix, axonemal tubulin and architectural remnant components, and their labeled protein compositions were compared. Heavily labeled axonemal proteins, most notably the integral microtubule doublet component tektin-A, were not detected in the membrane/matrix fraction of emerging cilia, even though nearly half of the total ciliary tubulin appeared in that fraction, arguing against membrane-associated or soluble matrix transit for the architectural proteins at low concentrations. However, after thermal fractionation of axonemes from growing cilia, labeled proteins characteristic of the architectural remnant dominated the solubilized microtubule fraction, supporting axoneme-associated transport of the non-tubulin proteins during growth, in contrast to a membrane/matrix pathway for tubulin.

Tektin B1 from ciliary microtubules: primary structure as deduced from the cDNA sequence and comparison with tektin A1

Tektins are a class of proteins that form filamentous polymers in the walls of ciliary and flagellar microtubules, and they may also be present in centrioles, centrosomes and mitotic spindles. We report here the cloning and sequencing of a cDNA for ciliary tektin B1. Comparison of the predicted amino acid sequence of tektin B1 with the previously published sequence for tektin A1 reveals several features that better define this class of proteins. Like tektin A1, the central region of the tektin B1 polypeptide chain is predicted to form a coiled-coil rod, consisting of four major alpha-helical regions that are separated by non-helical linkers. Between the central rod domains of tektins A and B there is a 34%/20% amino acid sequence identity/similarity, including equivalent 50-residue segments containing 36 identities, and a high probability of long-range structural homology. The tektin polypeptide chains are divided into two major segments that have significant sequence homology to each other, both within a given tektin chain and between tektins A and B, indicative of gene duplication events. The tektins have a secondary structure and molecular design similar to, but a low primary sequence homology with, intermediate filament proteins. Unlike tektin A1, tektin B1 lacks any part of the C-terminal IFP consensus sequence.

Stabilization of sea urchin flagellar microtubules by histone H1

Complex microtubule assemblies are essential components of eukaryotic cilia and flagella. They are extremely stable and are not affected by agents that normally induce polymer disassembly. The molecular basis of this microtubular stability is unknown, and it is not related to any feature of the constitutive tubulin. In sea urchin sperm flagella, axonemal microtubules are found to be stabilized by a protein identical to histone H1, a result that defines a new role for this histone and provides evidence for a concerted evolution of chromatin and microtubular structures.

Primary structure of tektin A1: comparison with intermediate-filament proteins and a model for its association with tubulin

Tektins are proteins that form filamentous polymers in the walls of ciliary and flagellar microtubules and that have biochemical and immunological properties similar to those of intermediate-filament proteins. We report here the sequence of a cDNA for tektin A1, one of the main tektins from Strongylocentrotus purpuratus sea urchin embryos. By hybridization analysis, tektin A mRNA appears maximally at ciliogenesis. The predicted structure of tektin A1 (M(r) 52,955) is a series of alpha-helical rod segments separated by nonhelical linkers. The two halves of the rod appear homologous and are probably related by gene duplication. Comparison of tektin A1 with intermediate-filament proteins, including nuclear lamins, reveals a low amino acid homology but similar molecular motif, i.e., pattern of helical and nonhelical domains. This study indicates that tektins are unique proteins but may be evolutionarily related to intermediate-filament proteins, and suggests a structural basis for the interaction of tektins and tubulin in microtubules.

Identification of a tektin-like protein associated with neurofilaments in the developing chick nervous system

A 160-kD polypeptide, which is recognized by an affinity-purified polyclonal antibody to the 55-kD tektin-A polypeptide from sea urchin sperm flagellar microtubules, is associated with neurofilaments in embryonic chick nerve cells. Antibodies to tektin-A and monoclonal antibodies to the neurofilament triplet proteins colocalize to filaments in cultured nerve cells and to filaments in extracts of chick spinal cord, using indirect immunofluorescence microscopy and immunogold electron microscopy. The antigen reacting with anti-tektin-A in chick brain and spinal cord extracts has been identified as a 160-kD polypeptide by SDS-PAGE and has been shown to be distinct from the known neurofilament-triplet proteins by two-dimensional immunoblot analysis. These data suggest that a unique protein with limited sequence homology to tektin-A is a component of the neuronal cytoskeleton and is incorporated into or associated with neurofilaments.

Search for eukaryotic motility proteins in spirochetes: immunological detection of a tektin-like protein in Spirochaeta halophila

The serial endosymbiotic theory (SET) for the spirochete origin of undulipodia (cilia and eukaryotic flagella) predicts that a greater number of axonemal proteins will show homology to spirochete than to other bacterial proteins. To continue testing, the SET proteins associated with eukaryotic motility (tektin, centrin and calmodulin) were sought in Spirochaeta halophila. Western blot immunological detection techniques (for tektin and centrin) and two-dimensional gel analysis (for calmodulin) were used. Tektins are filamentous proteins extending the length of the axoneme in sperm tails and other undulipodia. Whole cell extracts of S. halophila were probed with antibodies made against three sea urchin (Lytechinus pictus) sperm axonemal tektins (tektins A, B, and C). In the spirochetes, one tektin-like protein was detected as a band on Western blots (a C tektin.) An antibody made against Lytechinus pictus sperm tail axonemes, affinity-purified against the C tektin of another sea urchin, Stronglyocentrotus purpuratus, bound to a 30 kDa protein from Spirochaeta halophila. The C tektin epitope was not detected in Escherichia coli. Both the poly- and monoclonal anti-centrin antibodies cross-reacted with multiple proteins in the control alga (Tetraselmis striata) and in the putatively negative control bacterium E. coli. No cross reaction was seen between any anti-centrin antibody and S. halophila. Neither did a two-dimensional gel analysis reveal the presence of calmodulin in these spirochetes or in the two other prokaryotes tested (Spiroplasma citri, Acholeplasma laidlawii). Although neither centrin nor calmodulin were detected, a 30 kDa tektin-like protein apparently is present in these spirochetes.

Localization of the sea urchin Spec3 protein to cilia and Golgi complexes of embryonic ectoderm cells

Expression of the Spec3 gene of Strongylocentrotus purpuratus is associated with ectodermal ciliogenesis. An antiserum was raised against the amino terminus of the deduced Spec3 amino acid sequence and used for immunofluorescent staining. Cilia and an apical structure at the base of the stained cilium of each ectodermal cell stained intensely in gastrula and later stage embryos. Microtubule-depolymerizing agents dispersed the concentrated spot of apical staining, suggesting a localization of Spec3 antigen to the Golgi complex. Immunogold electron microscopy confirmed the localization of Spec3 antigen on cilia and in the Golgi complex. Spec3 antigen showed a diffuse punctate staining pattern in the ectodermal cytoplasm of hatching blastula when Spec3 transcripts are most prevalent, suggesting that after synthesis, Spec3 is sequestered in the Golgi complex before appearing on cilia. Whereas the predicted Mr of the Spec3 protein is 21,600, immunoblotting with S. purpuratus proteins indicated that a Spec3 antigen was concentrated in cilia and migrated as an SDS-resistant aggregate of Mr approximately 350,000. Spec3 is also concentrated in cilia of Lytechinus pictus but the protein migrated with an Mr approximately 23,000 in this species. The S. purpuratus Spec3 antigen remains associated with the ciliary axoneme after extraction of membrane proteins.

Sea urchin oocytes possess elaborate cortical arrays of microfilaments, microtubules, and intermediate filaments

Extensive arrays of microfilaments, microtubules and cytokeratin-type intermediate filaments were detected in the cortex of Strongylocentrotus droebachiensis oocytes using fluorescently labeled antibodies on both cortex and whole mount preparations. All three filament systems undergo dramatic structural reorganization during meiotic maturation of the egg. Microfilaments form a dense meshwork within the cortex of the oocyte. After meiosis, the filaments rearrange and shorten, resulting in a more loosely organized network. Both cortical microtubules and microtubules associated with a microtubule-organizing center are observed within the oocyte. After meiosis, the number and length of the cortical microtubules gradually diminish. A microtubule organizing center is found situated between the germinal vesicle and the plasma membrane in many oocytes. A network of filaments extends from the microtubule organizing center and radiates peripherally toward the germinal vesicle, presumably marking the animal pole. Cytokeratin-like intermediate filaments form a reticular network within the oocyte cortex, then solubilize during meiosis. In whole mounts of oocytes there is a single focal center of cytokeratin staining from which filaments radiate. Indirect immunofluorescence experiments, using anti-tubulin and anti-cytokeratin antibodies simultaneously, reveal the intermediate filament focal center to be localized within the microtubule organizing center. These results demonstrate the presence of a complex cortical cytoskeleton in premeiotic eggs of the sea urchin, Strongylocentrotus droebachiensis.

Relationship between tektins and intermediate filament proteins: an immunological study

Affinity-purified antibodies raised against three flagellar tektins (tektin A, B, and C) from each of two sea urchin species (Lytechinus pictus and Strongylocentrotus purpuratus) were used to study the immunological relationship between tektins and intermediate filament proteins. By immunofluorescence microscopy, several antitektins revealed a staining of intermediate filament-like arrays in three vertebrate cell lines tested. Immunoelectron microscopy substantiated the cross reaction of antitektins with intermediate filaments. When the cells were treated with cytochalasin B, the arrangement of the filaments recognized by anti-(Lp)-tektin B was altered; the alteration observed is typical for keratin filaments. By immunoblot, it was found that anti-(Lp)-tektin B cross reacted with two isoforms or different proteins of approximately 54 kD with pIs of 6.1 and 6.2 in human carcinoma epithelia (HeLa) cells and with two isoforms or different proteins of approximately 55 kD with pIs of 6.1 and 6.3 in pig kidney epithelia (LLC-PK1) cells. Furthermore, when antitektin antibodies were affinity purified with the 54 kD HeLa keratin, these keratin-specific antibodies again restained the original tektins on immunoblots. From these observations, it can be concluded that tektins and keratins are to a certain extent immunologically related. To determine the degree of the immunological relationship, tektin filaments and purified intermediate filaments from HeLa cells were cleaved with alpha-chymotrypsin and examined by quantitative immunoblot analysis. On immunoblots of digested tektins from L. pictus, anti-(Lp)-tektin B recognized several cleavage products in the range of 20 kD to 46 kD. However, when immunoblots of digested intermediate filaments from HeLa cells were probed, the cross reaction of anti-(Lp)-tektin B with HeLa keratins was eliminated by more than 98% within 2 min, suggesting that tektins have epitopes in common with the end domains of certain keratins.

The role of axonemal components in ciliary motility

1. The axoneme is the detergent-insoluble cytoskeleton of the cilium. 2. All axonemes generate movement by the same fundamental mechanism: microtubule sliding utilizing ATP hydrolysis during a mechanochemical cycling of dynein arms on the axonemal doublets. 3. Structure, fundamental biochemistry and physiology of the axoneme are conserved evolutionarily, but the phenotypes of beating movements and the responses to specific cytoplasmic signals differ greatly from organism to organism. 4. A model of asynchronous dynein arm activity--the switch point hypothesis--has been proposed to account for cyclic beating in the face of unidirectional sliding. The model suggests that the diversity of beat phenotype may be explicable by changes in the timing of switching between active and inactive states of doublet arm activity. Evidence of axonemal splitting in arrested axonemes provides new support for the hypothesis.

Separation of tubulin subunits by reversed-phase high-performance liquid chromatography

When properly solubilized with trifluoroacetic acid (TFA), alpha- and beta-tubulin subunits from a variety of sources may be resolved at high yield by reversed-phase high-performance liquid chromatography (HPLC), using a Waters muBondapak C18 column and simple linear aqueous acetonitrile gradients containing TFA. The tubulin subunits are typically the most non-polar proteins present, with the beta-tubulin subunit eluting before the alpha. Column temperature above ambient improve both the resolution and the yield; less polar solvent systems do not. Tubulins not freely soluble in aqueous TFA may be solubilized in 6 M guanidine-hydrochloric acid with no change in retention time. Other columns with shorter carbon chain lengths and larger pore size produce a single, unresolved tubulin peak. Reversed-phase HPLC analysis provides an independent comparative evaluation of organelle-specific tubulins, with characteristic retention time differences observed between homologous ciliary and flagellar outer doublet tubulin subunits and also between them and their cytoplasmic counterparts.

Evidence for tektins in centrioles and axonemal microtubules

Affinity purified, polyclonal antibodies were prepared against three tektins (tektins A, B, and C) isolated from sea urchin sperm axonemal microtubules. These antibodies (anti-tektins) were used to localize tektins in axonemes, basal bodies, and centrioles. By immunofluorescence microscopy it could be demonstrated that in sperm tails from Lytechinus pictus all three anti-tektins stain all nine axonemal doublet microtubules and A-tubule extensions along their entire length. In addition to staining doublet microtubules, anti-tektin C weakly labeled the central-pair microtubules in sperm tails from Patiria miniata. The anti-tektin staining revealed also a clear cross-reaction with basal bodies of sea urchin sperm and with centrioles of cells from hamsters, humans, and pigs. These data provide evidence of tektin or tektin-like proteins in basal bodies and centrioles and suggest that centriole microtubules are constructed according to the same principles as flagellar microtubules.