Centrin deficiency in Chlamydomonas causes defects in basal body replication, segregation and maturation

Basal body assembly in ciliates: the power of numbers

Centrioles perform the dual functions of organizing both centrosomes and cilia. The biogenesis of nascent centrioles is an essential cellular event that is tightly coupled to the cell cycle so that each cell contains only two or four centrioles at any given point in the cell cycle. The assembly of centrioles and their analogs, basal bodies, is well characterized at the ultrastructural level whereby structural modules are built into a functional organelle. Genetic studies in model organisms combined with proteomic, bioinformatic and identifying ciliary disease gene orthologs have revealed a wealth of molecules requiring further analysis to determine their roles in centriole duplication, assembly and function. Nonetheless, at this stage, our understanding of how molecular components interact to build new centrioles and basal bodies is limited. The ciliates, Tetrahymena and Paramecium, historically have been the subject of cytological and genetic study of basal bodies. Recent advances in the ciliate genetic and molecular toolkit have placed these model organisms in a favorable position to study the molecular mechanisms of centriole and basal body assembly.

Unstable RNAi effects through epigenetic silencing of an inverted repeat transgene in Chlamydomonas reinhardtii

RNA interferences in the unicellular green alga, Chlamydomonas reinhardtii, can be silenced. We have used the silencing of a transgene (aadA) that confers resistance to spectinomycin to investigate the mechanisms responsible for silencing by an artificial inverted repeat (IR) of the aadA gene. The IR construct provided strong silencing, but the RNAi efficiency varied among subclones of a single RNAi-transformed strain with successive cell divisions. Northern blot analyses revealed an inverse correlation between the copy number of the hairpin RNA and the spectinomycin resistance of the subclones. There is an inverse correlation between the efficiency of RNAi and the frequency of methylated CpG (*CpG) in the silenced region. No significant methylated cytosine was observed in the target aadA gene, which suggests the absence of RNA-directed DNA methylation in trans. Several experiments suggest the existence of an intrinsic IR sequence-dependent but a transcription-independent DNA methylation system in C. reinhardtii. The correlation between the *CpG levels and the IR transcript implies the existence of IR DNA-dependent DNA methylation. Treatment of RNAi-induced cells with a histone deacetylase inhibitor, Trichostatin A, rapidly increased the amount of the hairpin RNA and suggests that transcription of the silencer construct was repressed by *CpG-related silencing mechanisms.

Origin and evolution of the centrosome

In this brief account we specifically address the question of how the plasma membrane-associated basal body/axoneme of the unicellular ancestor of eukaryotes has evolved into the centrosome organelle through the several attempts to multicellularity. We propose that the connection between the flagellar apparatus and the nucleus has been a critical feature for leading to the centriole-based centrosome of metazoa, the Spindle Pole Body of fungi, or to the absence of any centrosome in seed plants. We further suggest that the evolution of this connection could be reflected in the evolution of the centrin proteins. We then review evidence showing that the evolution of the centrosome-based tubulin network has been correlated with the evolution of the cortical actin-based cleavage apparatus. Finally we argue that this coevolution had a major impact on the cell individuation process and on the evolution of multicellular organisms. We conclude that only the metazoan lineage evolved multicellularity without loosing the ancestral association of three basic cellular functions of the basal body/axoneme or the derived centrosome organelle, namely sensation, motion and division.

Mechanisms of procentriole formation

The centrosome comprises a pair of centrioles and associated pericentriolar material, and it is the principal microtubule-organizing centre of most animal cells. Like the genetic material, the centrosome is duplicated once and only once during the cell cycle. Despite the fact that both doubling events are crucial for genome integrity, the understanding of the mechanisms governing centrosome duplication has lagged behind the fuller knowledge of DNA replication. Here, we review recent findings that provide important mechanistic insights into how a single procentriole forms next to each centriole once per cell cycle, thus ensuring that one centrosome becomes two.

Identification and characterisation of a novel inorganic carbon acquisition gene, CIA7, from an insertional mutant of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a unicellular eukaryotic alga which possesses a CO₂-concentrating mechanism (CCM) that enables it to grow at low CO₂ concentrations. Previously, insertional mutants were generated to enable isolation of inorganic carbon transporters and other proteins that might be essential for a functional CCM. These mutants have an antibiotic resistance gene that encodes a protein that binds to Zeocin inhibiting Zeocin's DNA strand cleavage activity. The DNA flanking the Ble^R insert of one of the high CO₂ requiring strains, named cia7, was cloned with inverse-PCR and sequenced. Sequence analysis showed homology to conserved bacterial proteins of unknown function, but there were no ESTs in this region of the genome. However, the presence of a gene was established by PCR and RLM-RACE. CIA7 was found to have four exons and the Ble^R insert was in the fourth exon. CIA7 encodes a protein of 104 amino acids with a calculated molecular mass of 11.3 kDa. Based on the ChloroP prediction program, the protein is predicted to have a chloroplast targeting signal. Complementation analyses results showed possible partially rescued mutants, and RNAi showed several transformants with a sick on low CO₂ phenotype with reduced expression of CIA7. These results suggest that CIA7 is a gene that facilitates growth in C. reinhardtii under low CO₂ conditions. One possible role of CIA7 would be in the delivery or storage of a metal ion. It may play a potential role as either a domain of a metal transporter or as a metallochaperone.

Functional diversification of centrins and cell morphological complexity

In addition to their key role in the duplication of microtubule organising centres (MTOCs), centrins are major constituents of diverse MTOC-associated contractile arrays. A centrin partner, Sfi1p, has been characterised in yeast as a large protein carrying multiple centrin-binding sites, suggesting a model for centrin-mediated Ca2+-induced contractility and for the duplication of MTOCs. In vivo validation of this model has been obtained in Paramecium, which possesses an extended contractile array - the infraciliary lattice (ICL) - essentially composed of centrins and a huge Sfi1p-like protein, PtCenBP1p, which is essential for ICL assembly and contractility. The high molecular diversity revealed here by the proteomic analysis of the ICL, including ten subfamilies of centrins and two subfamilies of Sf1p-like proteins, led us to address the question of the functional redundancy, either between the centrin-binding proteins or between the centrin subfamilies. We show that all are essential for ICL biogenesis. The two centrin-binding protein subfamilies and nine of the centrin subfamilies are ICL specific and play a role in its molecular and supramolecular architecture. The tenth and most conserved centrin subfamily is present at three cortical locations (ICL, basal bodies and contractile vacuole pores) and might play a role in coordinating duplication and positioning of cortical organelles.

Centrosome biogenesis and function: centrosomics brings new understanding

Centrosomes, which were first described in the late 19th century, are found in most animal cells and undergo duplication once every cell cycle so that their number remains stable, like the genetic material of a cell. However, their function and regulation have remained elusive and controversial. Only recently has some understanding of these fundamental aspects of centrosome function and biogenesis been gained through the concerted application of genomics and proteomics, which we term 'centrosomics'. The identification of new molecules has highlighted the evolutionary conservation of centrosome function and provided a conceptual framework for understanding centrosome behaviour and how it can go awry in human disease.

Centrin1 is required for organelle segregation and cytokinesis in Trypanosoma brucei

Centrin is a calcium-binding centrosome/basal body-associated protein involved in duplication and segregation of these organelles in eukaryotes. We had shown that disruption of one of the centrin genes (centrin1) in Leishmania amastigotes resulted in failure of both basal body duplication and cytokinesis. Here, we undertook to define the role of centrin1 (TbCen1) in the duplication and segregation of basal body and its associated organelles kinetoplast and Golgi, as well as its role in cytokinesis of the procyclic form of Trypanosoma brucei by depleting its protein using RNA inhibition methodology. TbCen1-depleted cells showed significant reduction in growth compared with control cells. Morphological analysis of these cells showed they were large and pleomorphic with multiple detached flagella. Both immunofluorescence assays using organelle-specific antibodies and electron microscopic analysis showed that TbCen1-deficient cells contained multiple basal bodies, kinetoplasts, Golgi, and nuclei. These multiple organelles were, however, closely clustered together, indicating duplication without segregation in the absence of centrin. This failure in organelle segregation may be the likely cause of inhibition of cytokinesis, suggesting for the first time a new and unique role for centrin in the segregation of organelles without affecting their multiplication in the procyclic form of T. brucei.

Gametogenesis in the Chlamydomonas reinhardtii minus mating type is controlled by two genes, MID and MTD1

In the unicellular algae Chlamydomonas reinhardtii, the plus and minus mating types are controlled by a complex locus, MT, where the dominant MID gene in the MT(-) locus has been shown to be necessary for expression of minus-specific gamete-specific genes in response to nitrogen depletion. We report studies on MID expression patterns during gametogenesis and on a second gene unique to the MT(-) locus, MTD1. Vegetative cells express basal levels of MID. An early activation of MID transcription after nitrogen removal, and its sequence similarity to plant RWP-RK proteins involved in nitrogen-responsive processes, suggest that Mid conformation/activity may be nitrogen sensitive. A second stage of MID upregulation correlates with the acquisition of mating ability in minus gametes. Knockdown of MTD1 by RNAi in minus strains results in a failure to differentiate into gametes of either mating type after nitrogen deprivation. We propose that intermediate Mid levels are sufficient to activate MTD1 transcription and to repress plus gamete-specific genes and that MTD1 expression in turn allows the threshold-level MID expression needed to turn on minus gamete-specific genes. We further propose that an MTD1-equivalent system, utilizing at least one gene product encoded in the MT(+) locus, is operant during plus gametogenesis.

Chlamydomonas reinhardtii hydin is a central pair protein required for flagellar motility

Mutations in Hydin cause hydrocephalus in mice, and HYDIN is a strong candidate for causing hydrocephalus in humans. The gene is conserved in ciliated species, including Chlamydomonas reinhardtii. An antibody raised against C. reinhardtii hydin was specific for an approximately 540-kD flagellar protein that is missing from axonemes of strains that lack the central pair (CP). The antibody specifically decorated the C2 microtubule of the CP apparatus. An 80% knock down of hydin resulted in short flagella lacking the C2b projection of the C2 microtubule; the flagella were arrested at the switch points between the effective and recovery strokes. Biochemical analyses revealed that hydin interacts with the CP proteins CPC1 and kinesin-like protein 1 (KLP1). In conclusion, C. reinhardtii hydin is a CP protein required for flagellar motility and probably involved in the CP-radial spoke control pathway that regulates dynein arm activity. Hydrocephalus caused by mutations in hydin likely involves the malfunctioning of cilia because of a defect in the CP.

The NIT1 promoter allows inducible and reversible silencing of centrin in Chlamydomonas reinhardtii

An inverted repeat corresponding to parts of the centrin gene of Chlamydomonas reinhardtii was placed downstream of the NIT1 promoter, which is induced by ammonium starvation. After induction, transformants developed centrin deficiency as assayed by immunofluorescence, Western blotting, and Northern blotting. The effect was reversible, demonstrating that the NIT1 promoter allowed controlled RNA interference in Chlamydomonas reinhardtii.

The centrosome cycle

Centrosomes are dynamic organelles involved in many aspects of cell function and growth. Centrosomes act as microtubule organizing centers, and provide a site for concerted regulation of cell cycle progression. While there is diversity in microtubule organizing center structure among eukaryotes, many centrosome components, such as centrin, are conserved. Experimental analysis has provided an outline to describe centrosome duplication, and numerous centrosome components have been identified. Even so, more work is needed to provide a detailed understanding of the interactions between centrosome components and their roles in centrosome function and duplication. Precise duplication of centrosomes once during each cell cycle ensures proper mitotic spindle formation and chromosome segregation. Defects in centrosome duplication or function are linked to human diseases including cancer. Here we provide a multifaceted look at centrosomes with a detailed summary of the centrosome cycle.

Centrin Deficiency in Paramecium Affects the Geometry of Basal-Body Duplication

BACKGROUND

Ciliary or flagellar basal bodies and centrioles share the same architecture and remarkable property of duplicating once per cell cycle. Duplication is known to proceed by budding of the daugther organelle close to and at right angles to the mother structure, but the molecular basis of this geometry remains unknown. Among the handful of proteins implicated in basal-body/centriole duplication, centrins seem required in all eukaryotes tested, but their mode of action is not clear. We have investigated centrin function in Paramecium, whose cortical organization allows detection of any spatial or temporal alteration in the pattern of basal-body duplication.

RESULTS

We have characterized two pairs of genes, PtCEN2a and PtCEN2b as well as PtCEN3a and PtCEN3b, orthologs of HsCEN2 and HsCEN3, respectively. GFP tags revealed different localization for the two pairs of gene products, at basal bodies or on basal-body-associated filamentous arrays, respectively. Centrin depletion induced by RNAi caused mislocalization of the neoformed basal bodies: abnormal site of budding (PtCen2ap) or absence of separation between mother and daughter organelles (PtCen3ap). Over successive divisions, new basal bodies continued to be assembled, but internalization of the mispositionned basal bodies led to a progressive decrease in the number of cortical basal bodies.

CONCLUSIONS

Our observations show that centrins (1) are required to define the site and polarities of duplication and to sever the mother-daughter links and (2) play no triggering or instrumental role in assembly. Our data underscore the biological importance of the geometry of the duplication process.

Centrin scaffold in Chlamydomonas reinhardtii revealed by immunoelectron microscopy

In the flagellate green alga Chlamydomonas reinhardtii the Ca(2+)-binding EF-hand protein centrin is encoded by a single-copy gene. Previous studies have localized the protein to four distinct structures in the flagellar apparatus: the nucleus-basal body connector, the distal connecting fiber, the flagellar transitional region, and the axoneme. To explain the disjunctive distribution of centrin, the interaction of centrin with as yet unknown specific centrin-binding proteins has been implied. Here, we demonstrate using serial section postembedding immunoelectron microscopy of isolated cytoskeletons that centrin is located in additional structures (transitional fibers and basal body lumen) and that the centrin-containing structures of the basal apparatus are likely part of a continuous filamentous scaffold that extends from the nucleus to the flagellar bases. In addition, we show that centrin is located in the distal lumen of the basal body in a rotationally asymmetric structure, the V-shaped filament system. This novel centrin-containing structure has also been detected near the distal end of the probasal bodies. Taken together, these results suggest a role for a rotationally asymmetric centrin "seed" in the growth and development of the centrin scaffold following replication of the basal apparatus.

RNA silencing in Chlamydomonas: mechanisms and tools

The generation of a comprehensive EST library and the sequencing of its genome set the stage for reverse genetics approaches in Chlamydomonas reinhardtii. However, these also require tools for the specific downregulation of target gene expression. Consequently, a large number of diverse constructs were developed aimed at reducing target gene expression in Chlamydomonas via the stable expression of antisense or inverted repeat-containing RNA. Double-stranded RNA (dsRNA) generated by the annealing of antisense and sense RNAs or by hairpin formation of an inverted repeat, feeds into the RNA silencing pathway. In this pathway, dsRNA is cleaved into approximately 25-bp small interfering RNAs (siRNAs) by the endonuclease Dicer. One of the two complementary strands of a siRNA is then loaded onto an Argonaute-like protein present as core component within larger complexes. Guided by this single-stranded RNA, the Argonaute-like protein either detects homologous transcripts and cleaves these endonucleolytically, or initiates transcriptional gene silencing. This article summarizes current information derived mainly from the Chlamydomonas genome project on components that are assumed to be involved in RNA silencing mechanisms in Chlamydomonas. Furthermore, all approaches employed in Chlamydomonas to date to downregulate target gene expression by antisense or inverted repeat constructs are reviewed and discussed critically.

GFP as a tool for the analysis of proteins in the flagellar basal apparatus ofChlamydomonas

Green fluorescent protein (GFP) was used to analyse three proteins in the flagellar basal apparatus of C. reinhardtii: (1) Striated fiber assemblin (SFA), the major component of the striated microtubule-associated fibers; (2) Centrin, present in the nucleus basal body connectors (NBBCs) and the distal connecting fiber (dCF) between the two basal bodies; and (3) DIP13, the Chlamydomonas homologue of human autoantigen NA14. The fusions co-localized with the wild-type proteins when expressed moderately. Overexpression of centrin-GFP and DIP13-GFP resulted in the formation of large aggregates and disturbed the distribution of the respective wild-type proteins. The amount of wild-type DIP13 was significantly reduced in cells overexpressing DIP13-GFP. Moreover, the cells frequently failed to assemble full-length flagella and flagellar regeneration was delayed, indicating a role of DIP13 during flagellar assembly. In contrast, overexpression of GFP-SFA, which retained more wild-type properties than SFA-GFP, increased the size of the striated fibers without altering the cross-shaped pattern. Abnormal patterns were observed in centrin-deficient cells, suggesting that centrin is required for proper localization of SFA. Photobleaching of GFP-SFA fibers indicated that GFP-SFA in the fibers is turned over slowly. Conditionally expressed centrin-GFP was incorporated into NBBCs in regions close to the basal bodies, but underrepresented in the dCF, indicative of a different dynamic of these two centrin fibers. Bending of the NBBCs was observed in vivo during flagellar motion, indicating that the filaments are flexible. In conclusion, in Chlamydomonas GFP-tagging is a useful tool for yielding new insights into the function and properties of the analyzed proteins.

Basal body duplication and maintenance require one member of the Tetrahymena thermophila centrin gene family

Centrins, small calcium binding EF-hand proteins, function in the duplication of a variety of microtubule organizing centers. These include centrioles in humans, basal bodies in green algae, and spindle pole bodies in yeast. The ciliate Tetrahymena thermophila contains at least four centrin genes as determined by sequence homology, and these have distinct localization and expression patterns. CEN1's role at the basal body was examined more closely. The Cen1 protein localizes primarily to two locations: one is the site at the base of the basal body where duplication is initiated. The other is the transition zone between the basal body and axoneme. CEN1 is an essential gene, the deletion of which results in the loss of basal bodies, which is likely due to defects in both basal body duplication and basal body maintenance. Analysis of the three other centrins indicates that two of them function at microtubule-rich structures unique to ciliates, whereas the fourth is not expressed under conditions examined in this study, although when artificially expressed it localizes to basal bodies. This study provides evidence that in addition to its previously known function in the duplication of basal bodies, centrin is also important for the integrity of these organelles.

A mutation in the centriole-associated protein centrin causes genomic instability via increased chromosome loss in Chlamydomonas reinhardtii

BACKGROUND

The role of centrioles in mitotic spindle function remains unclear. One approach to investigate mitotic centriole function is to ask whether mutation of centriole-associated proteins can cause genomic instability.

RESULTS

We addressed the role of the centriole-associated EF-hand protein centrin in genomic stability using a Chlamydomonas reinhardtii centrin mutant that forms acentriolar bipolar spindles and lacks the centrin-based rhizoplast structures that join centrioles to the nucleus. Using a genetic assay for loss of heterozygosity, we found that this centrin mutant showed increased genomic instability compared to wild-type cells, and we determined that the increase in genomic instability was due to a 100-fold increase in chromosome loss rates compared to wild type. Live cell imaging reveals an increased rate in cell death during G1 in haploid cells that is consistent with an elevated rate of chromosome loss, and analysis of cell death versus centriole copy number argues against a role for multipolar spindles in this process.

CONCLUSION

The increased chromosome loss rates observed in a centrin mutant that forms acentriolar spindles suggests a role for centrin protein, and possibly centrioles, in mitotic fidelity.

Calcium and Magnesium Binding to Human Centrin 3 and Interaction with Target Peptides

There are four isoforms of centrin in mammals, with variable sequence, tissue expression, and functional properties. We have recently characterized a number of structural, ion, and target binding properties of human centrin isoform HsCen2. This paper reports a similar characterization of HsCen3, overexpressed in Escherichia coli and purified by phase-reversed chromatography. Equilibrium and dynamic binding studies revealed that HsCen3 has one mixed Ca(2+)/Mg(2+) binding site of high affinity (K(d) = 3 and 10 microM for Ca(2+) and Mg(2+), respectively) and two Ca(2+)-specific sites of low affinity (K(d) = 140 microM). The metal-free protein is fragmented by an unidentified protease into a polypeptide segment of 11 kDa, which was purified by HPLC, and identified by mass spectrometry as the segment of residues 21-112. Similarly, controlled trypsinolysis on Ca(2+)-bound HsCen3 yielded a mixture of segments of residues 1-124 and 1-125. The Ca(2+)/Mg(2+) site could be assigned to this segment and thus resides in the N-terminal half of HsCen3. Temperature denaturation experiments, circular dichroism, and utilization of fluorescence hydrophobic probes allowed us to propose that the metal-free protein has molten globule characteristics and that the dication-bound forms are compact with a polar surface for the Mg(2+) form and a hydrophobic exposed surface for the Ca(2+) form. Thus, HsCen3 could be classified as a Ca(2+) sensor protein. In addition, it is able to bind strongly to a model target peptide (melittin), as well as to peptides derived from the protein XPC and Kar1p, with a moderate Ca(2+) dependence.

Destruction of maternal centrioles during fertilization of the brown alga, Scytosiphon lomentaria (Scytosiphonales, Phaeophyceae)

In brown algal fertilization, a pair of centrioles is derived from the male gamete, irrespective of the sexual reproduction pattern, i.e., isogamy, anisogamy, or oogamy. In this study, the manner in which the maternal centriole structure is destroyed in early zygotes of the isogamous brown alga Scytosiphon lomentaria was examined by electron microscopy. At fertilization, the zygote had two pairs of centrioles (flagellar basal bodies) derived from motile male and female gametes, and there was no morphological difference between the two pairs. The flagellar basal plate and the axonemal microtubules were still connected with the distal end of centrioles. Ultrastructural observations showed that the integrity of maternal-derived centrioles began to degenerate even in the 1-h-old zygote. At that time, the cylinder of triplet microtubules of the maternal centrioles became shorter from the distal end, and a section passing through the centrioles indicated that a part of the nine triplets of microtubules changed into doublet or singlet microtubules by degeneration of B and/or C tubules. In 2-h-old zygote, there was no trace of maternal centrioles ultrastructurally, and only the paternal centrioles remained. Further, reduction of centrin accompanying destruction of the maternal centrioles was examined in immunofluorescence microscopy. Centrin localized at the paternal and the maternal centrioles had the same fluorescence intensity in the early zygotes. At 4-6 h after fertilization, two spots indicating centrin localization showed different fluorescence intensity. Later, the weaker spot disappeared completely. These results showed that there is a difference in time between the destruction of the centriolar cylinders and the reduction of centrin molecules around them.

The Chlamydomonas reinhardtii proteins Ccp1 and Ccp2 are required for long-term growth, but are not necessary for efficient photosynthesis, in a low-CO2 environment

The unicellular green alga Chlamydomonas reinhardtii acclimates to a low-CO2 environment by modifying the expression of a number of messages. Many of the genes that increase in abundance during acclimation to low-CO2 are under the control of the putative transcription factor Cia5. C. reinhardtii mutants null for cia5 do not express several of the known low-CO2 inducible genes and do not grow in a low-CO2 environment. Two of the genes under the control of Cia5, Ccp1 and Ccp2 , encode polypeptides that are localized to the chloroplast envelope and have a high degree of similarity to members of the mitochondrial carrier family of proteins. Since their discovery, Ccp1/2 have been candidates for bicarbonate uptake proteins of the chloroplast envelope membrane. In this report, RNA interference was successful in dramatically decreasing the abundance of the mRNAs for Ccp1 and Ccp2 . The abundance of the Ccp1 and Ccp2 proteins were also reduced in the RNAi strains. The RNAi strains grew slower than WT in a low-CO2 environment, but did not exhibit a mutant carbon concentrating phenotype as determined by the cells' apparent affinity for dissolved inorganic carbon. Possible explanations of this RNAi phenotype are discussed.

Lack of the Rhesus protein Rh1 impairs growth of the green alga Chlamydomonas reinhardtii at high CO2

Although Rhesus (Rh) proteins are best known as antigens on human red blood cells, they are not restricted to red cells or to mammals, and hence their primary biochemical functions can be studied in more tractable organisms. We previously established that the Rh1 protein of the green alga Chlamydomonas reinhardtii is highly expressed in cultures bubbled with air containing high CO(2) (3%), conditions under which Chlamydomonas grows rapidly. By RNA interference, we have now obtained Chlamydomonas rh mutants (epigenetic), which are among the first in nonhuman cells. These mutants have essentially no mRNA or protein for RH1 and grow slowly at high CO(2), apparently because they fail to equilibrate this gas rapidly. They grow as well as their parental strain in air and on acetate plus air. However, during growth on acetate, rh1 mutants fail to express three proteins that are known to be down-regulated by high CO(2): periplasmic and mitochondrial carbonic anhydrases and a chloroplast envelope protein. This effect is parsimoniously rationalized if the small amounts of Rh1 protein present in acetate-grown cells of the parental strain facilitate leakage of CO(2) generated internally. Together, these results support our hypothesis that the Rh1 protein is a bidirectional channel for the gas CO(2). Our previous studies in a variety of organisms indicate that the only other members of the Rh superfamily, the ammonium/methylammonium transport proteins, are bidirectional channels for the gas NH(3). Physiologically, both types of gas channels can apparently function in acquisition of nutrients and/or waste disposal.

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.

Centrin gene disruption impairs stage-specific basal body duplication and cell cycle progression in Leishmania

Centrin is a calcium-binding cytoskeletal protein involved in the duplication of centrosomes in higher eukaryotes. To explore the role of centrin in the protozoan parasite Leishmania, we created Leishmania deficient in the centrin gene (LdCEN). Remarkably, centrin null mutants (LdCEN(-/-)) showed selective growth arrest as axenic amastigotes but not as promastigotes. Flow cytometry analysis confirmed that the mutant axenic amastigotes have a cell cycle arrest at the G(2)/M stage. The axenic amastigotes also showed failure of basal body duplication and failure of cytokinesis resulting in multinucleated "large" cells. Increased terminal deoxy uridine triphosphate nick end labeling positivity was observed in centrin mutant axenic amastigotes compared with wild type cells, suggesting the activation of a programmed cell death pathway. Growth of LdCEN(-/-) amastigotes in infected macrophages in vitro was inhibited and also resulted in large multinucleated parasites. Normal basal body duplication and cell division in the LdCEN knockout promastigote is unique and surprising. Further, this is the first report where disruption of a centrin gene displays stage-specific/cell type-specific failure in cell division in a eukaryote. The centrin null mutant defective in amastigote growth could be useful as a vaccine candidate against leishmaniasis.

Cilia, primary ciliary dyskinesia and molecular genetics

Primary ciliary dyskinesia (PCD) is a phenotypically and genetically heterogeneous condition in which three genetic mutations have already been identified. The primary defect is in the ultrastructure or function of cilia, highly complex organelles that are structurally related to the flagella of sperm and protozoa. The clinical features of PCD include recurrent sinopulmonary infections, subfertility and laterality defects; the latter due to ciliary dysfunction at the embryological node. Completion of the human genome sequence has accelerated the identification and characterisation of disease genes, and the current molecular strategy in PCD includes candidate gene analysis, positional cloning, model organism analysis and proteomic analysis. The identification of these genes will provide new insights into the molecular mechanisms involved in the assembly and function of cilia and the pathway that determines left-right axis in man. This may also allow the development of new methods for diagnosis, prevention and treatment of PCD.