Two proteins isolated from sea urchin sperm flagella: structural components common to the stable microtubules of axonemes and centrioles

Tetrahymena RIB72A and RIB72B are microtubule inner proteins in the ciliary doublet microtubules

Doublet and triplet microtubules are essential and highly stable core structures of centrioles, basal bodies, cilia, and flagella. In contrast to dynamic cytoplasmic micro­tubules, their luminal surface is coated with regularly arranged microtubule inner proteins (MIPs). However, the protein composition and biological function(s) of MIPs remain poorly understood. Using genetic, biochemical, and imaging techniques, we identified Tetrahymena RIB72A and RIB72B proteins as ciliary MIPs. Fluorescence imaging of tagged RIB72A and RIB72B showed that both proteins colocalize to Tetrahymena cilia and basal bodies but assemble independently. Cryoelectron tomography of RIB72A and/or RIB72B knockout strains revealed major structural defects in the ciliary A-tubule involving MIP1, MIP4, and MIP6 structures. The defects of individual mutants were complementary in the double mutant. All mutants had reduced swimming speed and ciliary beat frequencies, and high-speed video imaging revealed abnormal highly curved cilia during power stroke. Our results show that RIB72A and RIB72B are crucial for the structural assembly of ciliary A-tubule MIPs and are important for proper ciliary motility.

Human FAM154A (SAXO1) is a microtubule-stabilizing protein specific to cilia and related structures

Cilia and flagella are microtubule-based organelles present at the surface of most cells, ranging from protozoa to vertebrates, in which these structures are implicated in processes from morphogenesis to cell motility. In vertebrate neurons, microtubule-associated MAP6 proteins stabilize cold-resistant microtubules through their Mn and Mc modules, and play a role in synaptic plasticity. Although centrioles, cilia and flagella have cold-stable microtubules, MAP6 proteins have not been identified in these organelles, suggesting that additional proteins support this role in these structures. Here, we characterize human FAM154A (hereafter referred to as hSAXO1) as the first human member of a widely conserved family of MAP6-related proteins specific to centrioles and cilium microtubules. Our data demonstrate that hSAXO1 binds specifically to centriole and cilium microtubules. We identify, in vivo and in vitro, hSAXO1 Mn modules as responsible for microtubule binding and stabilization as well as being necessary for ciliary localization. Finally, overexpression and knockdown studies show that hSAXO1 modulates axoneme length. Taken together, our findings suggest a fine regulation of hSAXO1 localization and important roles in cilium biogenesis and function.

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.

The mouse transcription factor-like 5 gene encodes a protein localized in the manchette and centriole of the elongating spermatid

Spermiogenesis is the final phase of spermatogenesis. During this process, haploid round spermatids differentiate into spermatozoa, with dramatic morphological changes, including elongation and condensation of the nuclei, and formation of the flagella. Meig1 is one of many genes involved in the regulation of this process. Male mice deficient in MEIG1 are sterile with a severe defect in spermiogenesis, associated with dramatic disruption of the spermatid manchette and failure of flagellogenesis. A yeast two-hybrid screen using full-length MEIG1 as bait identified transcription factor-like 5 protein (TCFL5) as a putative interacting proteins. Interestingly, this protein was also identified as a potential binding partner of SPAG16, another protein essential for spermatogenesis, and also a binding partner of MEIG1. The interaction between TCFL5 and MEIG1 was confirmed in cultured cells over-expressing the two proteins. The mouse Tcfl5 transcript is present only in the testis, and its expression is significantly increased during spermiogenesis. However, little is known about TCFL5 protein and its role in male germ cells. A rabbit polyclonal antibody was generated against the C-terminal region of TCFL5. Mouse TCFL5 protein was expressed in the testis but not in mature spermatozoa. During the first wave of spermatogenesis, TCFL5 expression was dramatically increased at day 30 after birth. In the testis and a mixture of dispersed testicular cells, the protein co-localized with α-tubulin, a manchette marker in early elongating spermatids. The protein also localized in the centrioles of late elongating spermatids. No obvious differences in TCFL5 epitope abundance and localization were observed between wild type and the Meig1-deficient mice. These findings suggest that TCFL5 may play a role upstream of MEIG1 action, and based on putative binding partners and localization is likely to be involved in spermiogenesis and formation of the sperm flagella.

A MAP6-related protein is present in protozoa and is involved in flagellum motility

In vertebrates the microtubule-associated proteins MAP6 and MAP6d1 stabilize cold-resistant microtubules. Cilia and flagella have cold-stable microtubules but MAP6 proteins have not been identified in these organelles. Here, we describe TbSAXO as the first MAP6-related protein to be identified in a protozoan, Trypanosoma brucei. Using a heterologous expression system, we show that TbSAXO is a microtubule stabilizing protein. Furthermore we identify the domains of the protein responsible for microtubule binding and stabilizing and show that they share homologies with the microtubule-stabilizing Mn domains of the MAP6 proteins. We demonstrate, in the flagellated parasite, that TbSAXO is an axonemal protein that plays a role in flagellum motility. Lastly we provide evidence that TbSAXO belongs to a group of MAP6-related proteins (SAXO proteins) present only in ciliated or flagellated organisms ranging from protozoa to mammals. We discuss the potential roles of the SAXO proteins in cilia and flagella function.

The proteomic response of the mussel congeners Mytilus galloprovincialis and M. trossulus to acute heat stress: implications for thermal tolerance limits and metabolic costs of thermal stress

The Mediterranean blue mussel, Mytilus galloprovincialis, an invasive species in California, has displaced the more heat-sensitive native congener, Mytilus trossulus, from its former southern range, possibly due to climate change. By comparing the response of their proteomes to acute heat stress we sought to identify responses common to both species as well as differences that account for greater heat tolerance in the invasive. Mussels were acclimated to 13°C for four weeks and exposed to acute heat stress (24°C, 28°C and 32°C) for 1 h and returned to 13°C to recover for 24 h. Using two-dimensional gel electrophoresis and tandem mass spectrometry we identified 47 and 61 distinct proteins that changed abundance in M. galloprovincialis and M. trossulus, respectively. The onset temperatures of greater abundance of some members of the heat shock protein (Hsp) 70 and small Hsp families were lower in M. trossulus. The abundance of proteasome subunits was lower in M. galloprovincialis but greater in M. trossulus in response to heat. Levels of several NADH-metabolizing proteins, possibly linked to the generation of reactive oxygen species (ROS), were lower at 32°C in the cold-adapted M. trossulus whereas proteins generating NADPH, important in ROS defense, were higher in both species. The abundance of oxidative stress proteins was lower at 32°C in M. trossulus only, indicating that its ability to combat heat-induced oxidative stress is limited to lower temperatures. Levels of NAD-dependent deacetylase (sirtuin 5), which are correlated with lifespan, were lower in M. trossulus in response to heat stress. In summary, the expression patterns of proteins involved in molecular chaperoning, proteolysis, energy metabolism, oxidative damage, cytoskeleton and deacetylation revealed a common loci of heat stress in both mussels but also showed a lower sensitivity to high-temperature damage in the warm-adapted M. galloprovincialis, which is consistent with its expanding range in warmer waters.

From zero to many: control of centriole number in development and disease

Centrioles are essential for the formation of microtubule-derived structures, including cilia, flagella and centrosomes. These structures are involved in a variety of functions, from cell motility to division. In most dividing animal cells, centriole formation is coupled to the chromosome cycle. However, this is not the case in certain specialized divisions, such as meiosis, and in some differentiating cells. For example, oocytes loose their centrioles upon differentiation, whereas multiciliated epithelial cells make several of those structures after they exit the cell cycle. Aberrations of centriole number are seen in many cancer cells. Recent studies began to shed light on the molecular control of centriole number, its variations in development, and how centriole number changes in human disease. Here we review the recent developments in this field.

Centrioles: active players or passengers during mitosis?

Centrioles are cylinders made of nine microtubule (MT) triplets present in many eukaryotes. Early studies, where centrosomes were seen at the poles of the mitotic spindle led to their coining as "the organ for cell division". However, a variety of subsequent observational and functional studies showed that centrosomes might not always be essential for mitosis. Here we review the arguments in this debate. We describe the centriole structure and its distribution in the eukaryotic tree of life and clarify its role in the organization of the centrosome and cilia, with an historical perspective. An important aspect of the debate addressed in this review is how centrioles are inherited and the role of the spindle in this process. In particular, germline inheritance of centrosomes, such as their de novo formation in parthenogenetic species, poses many interesting questions. We finish by discussing the most likely functions of centrioles and laying out new research avenues.

Molecular architecture of the centriole proteome: the conserved WD40 domain protein POC1 is required for centriole duplication and length control

Centrioles are intriguing cylindrical organelles composed of triplet microtubules. Proteomic data suggest that a large number of proteins besides tubulin are necessary for the formation and maintenance of a centriole's complex structure. Expansion of the preexisting centriole proteome from the green alga Chlamydomonas reinhardtii revealed additional human disease genes, emphasizing the significance of centrioles in normal human tissue homeostasis. We found that two classes of ciliary disease genes were highly represented among the basal body proteome: cystic kidney disease (especially nephronophthisis) syndromes, including Meckel/Joubert-like and oral-facial-digital syndrome, caused by mutations in CEP290, MKS1, OFD1, and AHI1/Jouberin proteins and cone-rod dystrophy syndrome genes, including UNC-119/HRG4, NPHP4, and RPGR1. We further characterized proteome of the centriole (POC) 1, a highly abundant WD40 domain-containing centriole protein. We found that POC1 is recruited to nascent procentrioles and localizes in a highly asymmetrical pattern in mature centrioles corresponding to sites of basal-body fiber attachment. Knockdown of POC1 in human cells caused a reduction in centriole duplication, whereas overexpression caused the appearance of elongated centriole-like structures. Together, these data suggest that POC1 is involved in early steps of centriole duplication as well as in the later steps of centriole length control.

Tektin 2 is required for central spindle microtubule organization and the completion of cytokinesis

During anaphase, the nonkinetochore microtubules in the spindle midzone become compacted into the central spindle, a structure which is required to both initiate and complete cytokinesis. We show that Tektin 2 (Tek2) associates with the spindle poles throughout mitosis, organizes the spindle midzone microtubules during anaphase, and assembles into the midbody matrix surrounding the compacted midzone microtubules during cytokinesis. Tek2 small interfering RNA (siRNA) disrupts central spindle organization and proper localization of MKLP1, PRC1, and Aurora B to the midzone and prevents the formation of a midbody matrix. Video microscopy revealed that loss of Tek2 results in binucleate cell formation by aberrant fusion of daughter cells after cytokinesis. Although a myosin II inhibitor, blebbistatin, prevents actin-myosin contractility, the microtubules of the central spindle are compacted. Strikingly, Tek2 siRNA abolishes this actin-myosin-independent midzone microtubule compaction. Thus, Tek2-dependent organization of the central spindle during anaphase is essential for proper midbody formation and the segregation of daughter cells after cytokinesis.

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.

Structural insights into microtubule doublet interactions in axonemes

Coordinated sliding of microtubule doublets, driven by dynein motors, produces periodic beating of eukaryotic cilia and flagella. Recent structural studies of the axoneme, which forms the core of cilia and flagella, have used cryo-electron tomography to reveal new details of the interactions between some of the multitude of proteins that form the axoneme and regulate its movement. Connections between the several types of dyneins, in particular, suggest ways in which their action might be coordinated. Study of the molecular architecture of isolated doublets has provided a structural basis for understanding mechanical properties related to the bending of the axoneme, and has also offered insight into the potential role of doublets in the mechanism of dynein activity regulation.

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.

Axonemal protofilament ribbons, DM10 domains, and the link to juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is a common neurological disorder that results in short uncontrolled muscle contractions and sometimes more severe seizures. Genetic studies have suggested that JME may be caused by mutations in EFHC1. The Efhc1 protein consists of three DM10 domains and a C-terminal region containing a potential Ca2+ -binding motif. In Chlamydomonas, a protein (Rib72) of almost identical domain structure is a component of the protofilament ribbons within the doublet microtubules of the flagellar axoneme. Here I discuss recent work that supports assignment of human Efhc1 as a ciliary component and the resulting implications for the mechanism of disease causation.

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.

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.

Genomic structure and expression analysis of the mouse testis-specific ribbon protein (Trib) gene

During our analyses of genes required for the development and function of the mouse gonads, we identified a novel testis-specific mRNA, transcribed from a gene that we have named testis-specific ribbon protein (Trib). In the mouse, Trib is located on chromosome 15, overlapping with and transcribed in the opposite orientation of the meiosis specific gene Smc1beta. The deduced amino acid sequence of testis ribbon (TRIB) protein is highly conserved between human, mouse, and rat and contains the ribbon motifs found in the largely uncharacterized microtubule ribbon protein ribbon43a (RIB43A). We show by Northern blot analyses and reverse transcription-polymerase chain reaction (RT-PCR) that Trib mRNA is specifically expressed in the adult testis. In situ hybridization indicates that Trib is expressed solely in germ cells during the leptotene-pachytene stages of spermatogenesis. The high level of evolutionary conservation and the cellular and temporal expression suggest that Trib may be required for mouse spermatogenesis and male fertility. Here, we describe the genomic structure and expression profile of mouse Trib and compare its homology with other ribbon proteins.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Basal body/centriole assembly and continuity

The long-standing interest in centrioles and basal bodies stems from the evolutionary conservation of their structural design and from their dual mode of assembly (templated versus de novo), revealed by electron microscopic studies nearly four decades ago and unique for a subcellular organelle. Molecular dissection of the assembly pathway during the past few years has recently progressed, essentially through direct and reverse genetic approaches. These studies revealed essential roles for centrins and the gamma-, delta-, epsilon - and eta-tubulins in assembly or as specific signals for centriole duplication. Identification of further components of basal bodies and centrioles might help to unravel the two assembly pathways and their regulation.

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.

The Rib43a protein is associated with forming the specialized protofilament ribbons of flagellar microtubules in Chlamydomonas

Ciliary and flagellar microtubules contain a specialized set of three protofilaments, termed ribbons, that are composed of tubulin and several associated proteins. Previous studies of sea urchin sperm flagella identified three of the ribbon proteins as tektins, which form coiled-coil filaments in doublet microtubules and which are associated with basal bodies and centrioles. To study the function of tektins and other ribbon proteins in the assembly of flagella and basal bodies, we have begun an analysis of ribbons from the unicellular biflagellate, Chlamydomonas reinhardtii, and report here the molecular characterization of the ribbon protein rib43a. Using antibodies against rib43a to screen an expression library, we recovered a full-length cDNA clone that encodes a 42,657-Da polypeptide. On Northern blots, the rib43a cDNA hybridized to a 1. 7-kb transcript, which was up-regulated upon deflagellation, consistent with a role for rib43a in flagellar assembly. The cDNA was used to isolate RIB43a, an approximately 4.6-kb genomic clone containing the complete rib43a coding region, and restriction fragment length polymorphism analysis placed the RIB43a gene on linkage group III. Sequence analysis of the RIB43a gene indicates that the substantially coiled-coil rib43a protein shares a high degree of sequence identity with clones from Trypanosoma cruzi and Homo sapiens (genomic, normal fetal kidney, and endometrial and germ cell tumors) but little sequence similarity to other proteins including tektins. Affinity-purified antibodies against native and bacterially expressed rib43a stained both flagella and basal bodies by immunofluorescence microscopy and stained isolated flagellar ribbons by immuno-electron microscopy. The structure of rib43a and its association with the specialized protofilament ribbons and with basal bodies is relevant to the proposed role of ribbons in forming and stabilizing doublet and triplet microtubules and in organizing their three-dimensional structure.

Molecular cloning of haploid germ cell-specific tektin cDNA and analysis of the protein in mouse testis

Tektins are a class of proteins that form filamentous polymers in the walls of ciliary and flagellar microtubules. We report here the molecular cloning of a new member of the tektin family, tektin-t, identified from a mouse haploid germ cell-specific cDNA library. Tektin-t mRNA encodes a protein of 430 deduced amino acids possessing RSNVELCRD, the conserved sequence of tektin family proteins. Western blotting showed a single band having a molecular weight of 86 kDa in the mouse testis. Immunohistochemistry of the testis showed that tektin-t is localized in the flagella of elongating spermatids from developmental step 15 to maturity.