A novel 95-kD protein is located in a linker between cytoplasmic microtubules and basal bodies in a green flagellate and forms striated filaments in vitro

Intermediate filament protein evolution and protists

Metazoans evolved from a single protist lineage. While all eukaryotes share a conserved actin and tubulin-based cytoskeleton, it is commonly perceived that intermediate filaments (IFs), including lamin, vimentin or keratin among many others, are restricted to metazoans. Actin and tubulin proteins are conserved enough to be detectable across all eukaryotic genomes using standard phylogenetic methods, but IF proteins, in contrast, are notoriously difficult to identify by such means. Since the 1950s, dozens of cytoskeletal proteins in protists have been identified that seemingly do not belong to any of the IF families described for metazoans, yet, from a structural and functional perspective fit criteria that define metazoan IF proteins. Here, we briefly review IF protein discovery in metazoans and the implications this had for the definition of this protein family. We argue that the many cytoskeletal and filament-forming proteins of protists should be incorporated into a more comprehensive picture of IF evolution by aligning it with the recent identification of lamins across the phylogenetic diversity of eukaryotic supergroups. This then brings forth the question of how the diversity of IF proteins has unfolded. The evolution of IF proteins likely represents an example of convergent evolution, which, in combination with the speed with which these cytoskeletal proteins are evolving, generated their current diversity. IF proteins did not first emerge in metazoa, but in protists. Only the emergence of cytosolic IF proteins that appear to stem from a nuclear lamin is unique to animals and coincided with the emergence of true animal multicellularity.

A SAS-6-like protein suggests that the Toxoplasma conoid complex evolved from flagellar components

SAS-6 is required for centriole biogenesis in diverse eukaryotes. Here, we describe a novel family of SAS-6-like (SAS6L) proteins that share an N-terminal domain with SAS-6 but lack coiled-coil tails. SAS6L proteins are found in a subset of eukaryotes that contain SAS-6, including diverse protozoa and green algae. In the apicomplexan parasite Toxoplasma gondii, SAS-6 localizes to the centriole but SAS6L is found above the conoid, an enigmatic tubulin-containing structure found at the apex of a subset of alveolate organisms. Loss of SAS6L causes reduced fitness in Toxoplasma. The Trypanosoma brucei homolog of SAS6L localizes to the basal-plate region, the site in the axoneme where the central-pair microtubules are nucleated. When endogenous SAS6L is overexpressed in Toxoplasma tachyzoites or Trypanosoma trypomastigotes, it forms prominent filaments that extend through the cell cytoplasm, indicating that it retains a capacity to form higher-order structures despite lacking a coiled-coil domain. We conclude that although SAS6L proteins share a conserved domain with SAS-6, they are a functionally distinct family that predates the last common ancestor of eukaryotes. Moreover, the distinct localization of the SAS6L protein in Trypanosoma and Toxoplasma adds weight to the hypothesis that the conoid complex evolved from flagellar components.

Cell division in Apicomplexan parasites is organized by a homolog of the striated rootlet fiber of algal flagella

Apicomplexan parasites undergo cell division using an evolutionarily conserved mechanism first described in the positioning and assembly of flagella in algae.

Apicomplexa are intracellular parasites that cause important human diseases including malaria and toxoplasmosis. During host cell infection new parasites are formed through a budding process that parcels out nuclei and organelles into multiple daughters. Budding is remarkably flexible in output and can produce two to thousands of progeny cells. How genomes and daughters are counted and coordinated is unknown. Apicomplexa evolved from single celled flagellated algae, but with the exception of the gametes, lack flagella. Here we demonstrate that a structure that in the algal ancestor served as the rootlet of the flagellar basal bodies is required for parasite cell division. Parasite striated fiber assemblins (SFA) polymerize into a dynamic fiber that emerges from the centrosomes immediately after their duplication. The fiber grows in a polarized fashion and daughter cells form at its distal tip. As the daughter cell is further elaborated it remains physically tethered at its apical end, the conoid and polar ring. Genetic experiments in Toxoplasma gondii demonstrate two essential components of the fiber, TgSFA2 and 3. In the absence of either of these proteins cytokinesis is blocked at its earliest point, the initiation of the daughter microtubule organizing center (MTOC). Mitosis remains unimpeded and mutant cells accumulate numerous nuclei but fail to form daughter cells. The SFA fiber provides a robust spatial and temporal organizer of parasite cell division, a process that appears hard-wired to the centrosome by multiple tethers. Our findings have broader evolutionary implications. We propose that Apicomplexa abandoned flagella for most stages yet retained the organizing principle of the flagellar MTOC. Instead of ensuring appropriate numbers of flagella, the system now positions the apical invasion complexes. This suggests that elements of the invasion apparatus may be derived from flagella or flagellum associated structures.

Malaria, toxoplasmosis, and related diseases are caused by infection with unicellular parasites called Apicomplexa. Their name refers to the elaborate invasion machinery that occupies the apical end of the parasite cell. This apparatus allows the parasite to force its way into the cells of its host, and to deliver factors that will manipulate host cell structure, gene expression, and metabolism. Once in the host cell the parasite will begin to grow. The parasite replicates its genome and organelles numerous times and then loads these various elements into numerous daughter cells that will further spread the infection. Here we report a fiber that coordinates the daughter cell budding process. The fiber links the centrosome, which controls the mitotic spindle, and the genome with the microtubule organizing center of the budding daughter. Parasite mutants lacking the proteins that build the fiber fail to form daughter cells at the earliest step. The fiber and its components are remarkably similar to fibers that coordinate flagella in algae. While Apicomplexa are not flagellated (with the exception of certain gamete stages) they evolved from flagellated algae. We propose that elements of the invasion apparatus evolved from the flagellum or flagellum associated structures.

Proteomic analysis of isolated chlamydomonas centrioles reveals orthologs of ciliary-disease genes

BACKGROUND

The centriole is one of the most enigmatic organelles in the cell. Centrioles are cylindrical, microtubule-based barrels found in the core of the centrosome. Centrioles also act as basal bodies during interphase to nucleate the assembly of cilia and flagella. There are currently only a handful of known centriole proteins.

RESULTS

We used mass-spectrometry-based MudPIT (multidimensional protein identification technology) to identify the protein composition of basal bodies (centrioles) isolated from the green alga Chlamydomonas reinhardtii. This analysis detected the majority of known centriole proteins, including centrin, epsilon tubulin, and the cartwheel protein BLD10p. By combining proteomic data with information about gene expression and comparative genomics, we identified 45 cross-validated centriole candidate proteins in two classes. Members of the first class of proteins (BUG1-BUG27) are encoded by genes whose expression correlates with flagellar assembly and which therefore may play a role in ciliogenesis-related functions of basal bodies. Members of the second class (POC1-POC18) are implicated by comparative-genomics and -proteomics studies to be conserved components of the centriole. We confirmed centriolar localization for the human homologs of four candidate proteins. Three of the cross-validated centriole candidate proteins are encoded by orthologs of genes (OFD1, NPHP-4, and PACRG) implicated in mammalian ciliary function and disease, suggesting that oral-facial-digital syndrome and nephronophthisis may involve a dysfunction of centrioles and/or basal bodies.

CONCLUSIONS

By analyzing isolated Chlamydomonas basal bodies, we have been able to obtain the first reported proteomic analysis of the centriole.

The ultrastructure of the Chlamydomonas reinhardtii basal apparatus: identification of an early marker of radial asymmetry inherent in the basal body

The biflagellate unicellular green alga Chlamydomonas reinhardtii is a classic model organism for the analysis of flagella and their organizers, the basal bodies. In this cell, the two flagella-bearing basal bodies, along with two probasal bodies and an array of fibers and microtubules, form a complex organelle called the basal apparatus. The ultrastructure of the basal apparatus was analysed in detail by serial thin-section electron microscopy of isolated cytoskeletons and several newly discovered features are described, including a marker for the rotational asymmetry inherent in the basal bodies and probasal bodies. In addition, the complex three-dimensional basal apparatus ultrastructure is resolved and illustrated, including the attachment sites of all basal apparatus elements to specific microtubular triplets of the basal bodies and probasal bodies. These data will facilitate both the localization of novel basal apparatus proteins and the analysis of mutants and RNA interference cells with only subtle defects in basal apparatus ultrastructure. The early harbinger of radial asymmetry described here could play a crucial role during basal body maturation by orienting the asymmetric attachment of the various associated fibers and therefore might define the orientation of the basal bodies and, ultimately, the direction of flagellar beating.

Foxj1 regulates basal body anchoring to the cytoskeleton of ciliated pulmonary epithelial cells

The forkhead box transcription factor Foxj1 is required for cilia formation and left-right axis determination. To define the role of Foxj1 in ciliogenesis, microarray analysis was performed to identify differentially expressed genes in the pulmonary epithelium of foxj1(+/+) and foxj1(-/-) mice. In the absence of Foxj1, the expression of calpastatin, an inhibitor of the protease calpain, decreased. RNase protection confirmed the decrease in calpastatin expression and decreased calpastatin was detected in the proximal pulmonary epithelium of foxj1(-/-) mice by immunohistochemistry. No change was detected in the expression of calpain 2 in the pulmonary epithelium by western blot or immunohistochemistry. By western blot and immunofluorescence, ezrin, a substrate for calpain, was also found to decrease in the pulmonary epithelium of foxj1(-/-) mice. No change in ezrin gene expression was found by RT-PCR. A decrease in ezrin binding phosphoprotein-50 (EBP-50) was also detected by immunofluorescence in the foxj1(-/-) mouse pulmonary epithelium. Immunoelectron microscopy demonstrated ezrin associated with the basal bodies of cilia in the pulmonary epithelium. Treatment of tracheal explants from foxj1(-/-) mice with a calpain inhibitor resulted in a partial reappearance of cilia observed in these mice. Additionally, following treatment of foxj1(-/-) tracheal explants with calpain inhibitor, basal bodies were observed in an apical location along with relocalization of ezrin and EBP-50. Regulation of calpain activity by calpastatin thus provides a mechanism for regulating the anchoring of basal bodies to the apical cytoskeleton in ciliated cells. In the absence of Foxj1, decreased calpastatin expression with decreased ezrin and EBP-50 results in an inability of basal bodies to anchor to the apical cytoskeleton and subsequent failure of axonemal formation.

Changes in the abundance and distribution of conserved centrosomal, cytoskeletal and ciliary proteins during spermiogenesis in Marsilea vestita

Spermiogenesis in the male gametophytes of the water fern Marsilea vestita is a precise and rapid process resulting in the production of ciliated gametes. Development begins from a single cell within the microspore wall that undergoes nine rapid cell division cycles in distinct planes to produce 32 spermatids that are surrounded by 7 sterile cells. Thereafter, the de novo formation of basal bodies occurs in a discrete cytoplasmic particle known as a blepharoplast, with the subsequent formation of a complex ciliary apparatus in elongating spermatids. The rate and extent of development appear to be controlled at a post-transcriptional level, where the sudden translation of specific stored mRNAs (e.g., centrin) results in the formation of particular structures in the cells (e.g., blepharoplasts). We show here that additional centrosomal and cytoskeletal antigens known as SF assemblin, p95 kDa protein, delta tubulin, gamma tubulin, Xgrip109, Aik, CTR453, RanBPM, BX63, RSP6, and alpha tubulin each exhibit specific localization patterns both on immunoblots of gametophyte protein isolates and in fixed cells. BAp90, PP4, and RLC exhibit specific localization patterns in fixed cells. We show that the antigens exhibit complex patterns of abundance during spermiogenesis. In an attempt to identify regulatory agents involved in spermiogenesis, we employed a RNAi-based screen of 41 randomly selected gametophyte cDNAs on developing populations of synchronously growing gametophytes. The gametophytes treated with each of the RNAi probes exhibited arrest at a specific stage of development. A consequence of anomalous development was the block to assembly of the ciliary apparatus, an effect highlighted by altered staining with anti-centrin, anti-beta-tubulin, and anti-RSP6 antibodies. Our results show that complex, integrated processes of translation and protein partitioning apparently underlie the assembly of the ciliary apparatus during spermiogenesis in male gametophytes of M. vestita.

Elucidation of basal body and centriole functions in Chlamydomonas reinhardtii

In eukaryotic cells, basal bodies and their structural equivalents, centrioles, play essential roles. They are needed for the assembly of flagella or cilia as well as for cell division. Chlamydomonas reinhardtii provides an excellent model organism for the study of the basal body and centrioles. Genes for two new members of the tubulin superfamily are needed for basal body/centriole duplication. In addition, other genes that play roles in the duplication and segregation of basal bodies are discussed.

Integration of the centrosome in cell cycle control, stress response and signal transduction pathways

The identification of cell cycle control and signal transduction components on the centrosome has fostered the idea that the centrosome is more than a microtubule-organizing center. Indeed, recent molecular evidence suggests that the centrosome plays an active role not only in the regulation of microtubule nucleation activity, but also in the coordination of centrosome duplication with cell cycle progression, in stress response and in cell cycle checkpoint control. To achieve these roles, it interacts with a multitude of signal transduction molecules. The specificity of the interactions is mediated through anchoring proteins that bring centrosomal components and regulatory proteins into close proximity. The molecular composition and organization of the centrosome thus reflects its multiple functions.

Centrioles take center stage

Centrioles are among the most beautiful and mysterious of all cell organelles. Although the ultrastructure of centrioles has been studied in great detail ever since the advent of electron microscopy, these studies raised as many questions as they answered, and for a long time both the function and mode of duplication of centrioles remained controversial. It is now clear that centrioles play an important role in cell division, although cells have backup mechanisms for dividing if centrioles are missing. The recent identification of proteins comprising the different ultrastructural features of centrioles has proven that these are not just figments of the imagination but distinct components of a large and complex protein machine. Finally, genetic and biochemical studies have begun to identify the signals that regulate centriole duplication and coordinate the centriole cycle with the cell cycle.

Kinetics and regulation of de novo centriole assembly. Implications for the mechanism of centriole duplication

BACKGROUND

Centriole duplication is a key step in the cell cycle whose mechanism is completely unknown. Why new centrioles always form next to preexisting ones is a fundamental question. The simplest model is that preexisting centrioles nucleate the assembly of new centrioles, and that although centrioles can in some cases form de novo without this nucleation, the de novo assembly mechanism should be too slow to compete with normal duplication in order to maintain fidelity of centriole duplication.

RESULTS

We have measured the rate of de novo centriole assembly in vegetatively dividing cells that normally always contain centrioles. By using mutants of Chlamydomonas that are defective in centriole segregation, we obtained viable centrioleless cells that continue to divide, and find that within a single generation, 50% of these cells reacquire new centrioles by de novo assembly. This suggests that the rate of de novo assembly is approximately half the rate of templated duplication. A mutation in the VFL3 gene causes a complete loss of the templated assembly pathway without eliminating de novo assembly. A mutation in the centrin gene also reduced the rate of templated assembly.

CONCLUSIONS

These results suggest that there are two pathways for centriole assembly, namely a templated pathway that requires preexisting centrioles to nucleate new centriole assembly, and a de novo assembly pathway that is normally turned off when centrioles are present.

How centrioles work: lessons from green yeast

Centrioles are the organizing centers around which centrosomes assemble. Despite a century of study, the molecular details of centriole function and assembly remain largely unknown. Recent work has exploited the unique advantages of unicellular algae to reveal proteins that play central roles in centriole biology.

A cDNA from the green alga Spermatozopsis similis encodes a protein with homology to the newly discovered Roadblock/LC7 family of dynein-associated proteins

A clone, designated as B15, was isolated from a cDNA library of the unicellular green alga Spermatozopsis similis and characterised. The deduced amino acid sequence of its open reading frame exhibits high homology to members of the recently discovered roadblock/LC7 protein family (robl/LC7) of dynein-associated proteins. Homologies were highest to a robl/LC7-member from human testis (86%, identity 56%) and to the roadblock protein of Drosophila (88%, identity 52%). Data bank analyses revealed no homologies to known higher plant proteins. B15 is a single copy gene in the genome of Sperm-latozopsis and its transcript was detectable throughout the cell cycle.

Evidence for a direct role of nascent basal bodies during spindle pole initiation in the green alga Spermatozopsis similis

Basal body replication in the naked biflagellate green alga Spermatozopsis similis was analyzed using standard electron microscopy and immunogold localization of centrin, an ubiquitous centrosomal protein, and p210, a recently characterized basal apparatus component of S. similis. Fibrous disks representing probasal bodies appear at the proximal end of parental basal bodies at the end of interphase and development proceeds via a ring of nine singlet microtubules. Nascent basal bodies dock early to the plasma membrane but p210, usually present in basal body-membrane-linkers of S. similis, was already present on the cytosolic basal body precursors. In addition to the distal connecting fiber and the nuclear basal body connectors (NBBC) of the parental basal bodies, centrin was present on the fibrous probasal bodies, in a linker between probasal bodies and the basal apparatus, in the connecting fiber between nascent basal bodies and their corresponding parent, and, finally, a fiber linking the nascent basal bodies to the nucleus. This NBBC probably is present only in mitotic cells. During elongation a cartwheel of up to seven layers is formed, protruding from the proximal end of nascent basal bodies. Microtubules develop on the cartwheel indicating that it temporarily functions as a microtubule organizing center (MTOC). These microtubules and probably the cartwheels, touch the nuclear envelope at both sides of a nuclear projection. We propose that spindle assembly is initiated at these attachment sites. During metaphase, the spindle poles were close to thylakoid-free lobes of the chloroplast, and the basal bodies were not in the spindle axis. The role of nascent basal bodies during the initial steps of spindle assembly is discussed.

A 210 kDa protein is located in a membrane-microtubule linker at the distal end of mature and nascent basal bodies

A monoclonal antibody raised against purified flagellar basal apparatuses from the green flagellate Spermatozopsis similis reacted with a protein of 210 kDa (p210) in western blots. The protein was partially cloned by immunoscreening of a cDNA library. The sequence encoded a novel protein rich in alanine (25%) and proline (20%), which contained regions similar to proteins of comparable amino acid composition such as extracellular matrix components or the membrane-cytoskeletal linker synapsin. Using a polyclonal antibody (anti-p210) raised against the C-terminal part of p210, it was shown that the protein was highly enriched in the basal apparatuses. Immunogold electron microscopy of isolated cytoskeletons or whole cells revealed that p210 was located in the flagellar transition region. The protein was part of the Y-shaped fibrous linkers between the doublet microtubules and the flagellar membrane, as indicated by statistical analysis of post-labeled sections using anti-centrin and anti-tubulin as controls. In premitotic cells p210 was located in a fibrous layer at the distal end of nascent basal bodies, which was perforated by the outgrowing axoneme. During deflagellation the protein remained at the basal body but we observed changes in its distribution, indicating that p210 partially moved to the tip of the basal body. p210 can be used as a marker to determine basal body position, orientation (parallel or antiparallel) and number in S. similis by indirect immunofluorescence. We suppose that p210 is involved in linking basal bodies to the plasma membrane, which is an important step during ciliogenesis.