Cloning and sequence analysis of genes coding for paramecium secretory granule (trichocyst) proteins. A unique protein fold for a family of polypeptides with different primary structures

Enhancement of microspore embryogenesis induction and plantlet regeneration of sweet pepper (Capsicum annuum L.) using putrescine and ascorbic acid

Production of doubled haploid (DH) plants is an efficient tool in genetic and plant breeding programs; however, sweet pepper (Capsicum annuum L.) is recalcitrant to microspore embryogenesis and DH production. Trying to break the barrier of DH production, three independent experiments were conducted on microspore embryogenesis of sweet pepper. In the first experiment, the effect of cold (4 °C) and heat (32 °C) pretreatments were investigated on microspore embryogenesis of three genotypes of sweet pepper including "Inspiration F1," "Maratus F1," and "Magno F1" cultivars in a factorial design with three replications. Heat shock (32 °C for 7 days), applied to mannitol-starved anthers of "Inspiration F1," showed higher multinuclear microspore percent, number of multicellular structures, total embryos, cotyledonary embryos, and regenerants. In the second experiment, the effect of different concentrations of putrescine (0, 0.5, 1, 2, and 5 mg l^-1) was evaluated on microspore embryogenesis of the three aforementioned cultivars of sweet pepper. The highest mean number of multicellular structures, cotyledonary embryos, and regenerants were achieved by applying 0.5-1 mg l^-1 putrescine during the mannitol starvation and heat shock (32 °C) treatments of isolated microspore culture of "Inspiration F1" cultivar. Significant decrease in microspore embryogenesis efficiency was observed when high levels of putrescine (2 and 5 mg l^-1) were used. Microspore embryogenesis was prevented completely at 5.0 mg l^-1 putrescine. In the third experiment, the effect of different concentrations of ascorbic acid (0, 20, 50, 100, and 200 mg l^-1) was investigated and the results showed that the application of ascorbic acid (20 and 50 mg l^-1) during mannitol starvation and heat shock treatment (32 °C) caused remarkable improvement in the number of produced cotyledonary embryos and their regeneration ability compared to control treatment. However, the application of higher levels of ascorbic acid (100 and 200 mg l^-1) inhibited microspore cell divisions and embryogenesis. In conclusion, the results indicated that both putrescine and ascorbic acid have significant effect on microspore embryogenesis efficiency of sweet pepper when they are used in appropriate concentrations.

Candidates of trichocyst matrix proteins of the dinoflagellate Oxyrrhis marina

Trichocysts are a common cell organelle of ciliates and dinoflagellates. They are composed of trichocyst matrix proteins and have been intensely investigated and characterized in ciliates. Here, for the first time, data have been obtained for trichocyst matrix proteins of a dinoflagellate. A DELTA-BLAST search using 14 available and complete amino acid sequences of mature trichocyst matrix proteins of the ciliate Paramecium tetraurelia resulted in 16 hits for the dinoflagellate Oxyrrhis marina when the E values and bit values to be scored were <10^-4 and >40. They code for proteins with acidic pI values and exceeded the precursors of the trichocyst matrix proteins of the ciliate approximately twofold in length. The values calculated for coverage, identity, and positives ranged from 76 to 100, 21.5 to 28.3, and 44.9 to 53.9%, respectively. Protein conformation predictions indicate coiled-coil domains which are a common feature of mature ciliate trichocyst matrix proteins. As often several EST sequences of O. marina matched with a queried mature trichocyst matrix protein of P. tetraurelia, a multigene family can be assumed for trichocyst proteins in this dinophyte, too. Trichocyst-enriched fractions of O. marina were isolated and subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis. When samples were incubated with loading buffer without a reducing agent, the banding pattern was mainly composed of three regions in the range of >90, 75-60, and 50-35 kDa, with each region consisting of four to five bands. Tryptic in gel digestion of proteins excised from these three gel regions followed by mass spectrometry confirmed that up to 14 of the 16 predicted proteins were present within the trichocyst-enriched fractions. When the samples were reduced with either ß-mercaptoethanol or dithiothreitol, the proteins of the three regions disappeared almost completely and proteins in the range of 27 to 15 kDa became the dominating bands. Up to 12 of the predicted proteins were detected within these bands.

Isolation and characterisation of the trichocysts of the dinophyte Prorocentrum micans

Trichocyst-enriched fractions were isolated from the marine dinophyte Prorocentrum micans. Transmission electron microscopy revealed that most of the trichocysts were discharged and had elongated to long filaments. Some trichocysts were still condensed. Fragments of discharged trichocysts measured up to 20 μm in length and 260 nm in width, those still condensed measured up to 1 μm in width and 16 μm in length. A distinct banding pattern with a transversal periodicity of approximately 16-18 nm and a periodic longitudinal striation of 3-4 nm could be measured along the trichocyst filaments. At higher magnifications, a fragile, alveolated, net-like organisation became obvious which resembled the one shown for the trichocysts of ciliates. When trichocyst-enriched fractions were treated with sodium dodecyl sulfate and centrifuged subsequently, no trichocysts were registered any longer in the sodium dodecyl sulfate-insoluble fraction by electron microscopy. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of trichocyst-enriched fractions and of the SDS-soluble fractions revealed a protein banding pattern which was dominated by polypeptides of 50-30, 12.5, and approximately 8.5 kDa. The polypeptide banding pattern deviated significantly from those registered for ejectisomes of cryptophytes and of the prasinophyte Pyramimonas grossii, for the Reb polypeptides which constitute the R-bodies of Caedibacter taeniospiralis, and also from the banding pattern of trichocysts of Paramecium. An antiserum directed against trichocysts of Paramecium did not cross-react with the polypeptides present in the trichocyst-enriched fraction of Prorocentrum micans.

An aspartyl cathepsin, CTH3, is essential for proprotein processing during secretory granule maturation in Tetrahymena thermophila

In animal cells, the assembly of dense cores in secretory granules is controlled by proteolytic processing of proproteins. The same phenomenon occurs in the ciliate Tetrahymena thermophila, but the proteases involved appear to be highly unrelated, suggesting that similar regulatory mechanisms have different molecular origins.

In Tetrahymena thermophila, peptides secreted via dense-core granules, called mucocysts, are generated by proprotein processing. We used expression profiling to identify candidate processing enzymes, which localized as cyan fluorescent protein fusions to mucocysts. Of note, the aspartyl cathepsin Cth3p plays a key role in mucocyst-based secretion, since knockdown of this gene blocked proteolytic maturation of the entire set of mucocyst proproteins and dramatically reduced mucocyst accumulation. The activity of Cth3p was eliminated by mutation of two predicted active-site mutations, and overexpression of the wild-type gene, but not the catalytic-site mutant, partially rescued a Mendelian mutant defective in mucocyst proprotein processing. Our results provide the first direct evidence for the role of proprotein processing in this system. Of interest, both localization and the CTH3 disruption phenotype suggest that the enzyme provides non–mucocyst-related functions. Phylogenetic analysis of the T. thermophila cathepsins, combined with prior work on the role of sortilin receptors in mucocyst biogenesis, suggests that repurposing of lysosomal enzymes was an important step in the evolution of secretory granules in ciliates.

Further investigations on the polypeptides and reconstitution of prasinophycean ejectisomes

Ejectisome fragments were isolated from the prasinophyte Pyramimonas grossii and subjected to different treatments, i.e. Percoll density gradient centrifugation, incubation at pH 2.5 or at pH 10.8, or incubation in 6M guanidine hydrochloride. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that Percoll density gradient centrifugation did not improve the purity of the ejectisome fragment-enriched fractions. The ejectisome fragments withstood pH 2.5 and pH 10.8 treatment, and no loosely bound polypeptides became detached. The disintegration of ejectisome fragments was achieved in 6M guanidine hydrochloride, and reassembly into filamentous, ejectisome-like structures occurred after dialysis against distilled water. Fractions enriched either in ejectisome fragments or in reconstituted ejectisome-like structures were dominated by three polypeptides with relative molecular weights of approximately 12.5-19kDa and two additional polypeptides of 23 and 26kDa. A polyclonal antiserum directed against an ejectisome fragment-enriched fraction weakly cross-reacted with these polypeptides, and no significant immuno-labelling of ejectisome fragments was registered. A positive immuno-label was achieved using immunoglobulin (IgG) fractions which were gained by selectively incubating nitrocellulose stripes of these polypeptides with the antiserum.

Tetrahymena thermophila: a divergent perspective on membrane traffic

Tetrahymena thermophila, a member of the Ciliates, represents a class of organisms distantly related from commonly used model organisms in cell biology, and thus offers an opportunity to explore potentially novel mechanisms and their evolution. Ciliates, like all eukaryotes, possess a complex network of organelles that facilitate both macromolecular uptake and secretion. The underlying endocytic and exocytic pathways are key mediators of a cell's interaction with its environment, and may therefore show niche-specific adaptations. Our laboratory has taken a variety of approaches to identify key molecular determinants for membrane trafficking pathways in Tetrahymena. Studies of Rab GTPases, dynamins, and sortilin-family receptors substantiate the widespread conservation of some features but also uncover surprising roles for lineage-restricted innovation.

Trichocyst ribbons of a cryptomonads are constituted of homologs of R-body proteins produced by the intracellular parasitic bacterium of Paramecium

Trichocysts are ejectile organelles found in cryptomonads, dinoflagellates, and peniculine ciliates. The fine structure of trichocysts differs considerably among lineages, and their evolutionary relationships are unclear. The biochemical makeup of the trichocyst constituents has been studied in the ciliate Paramecium, but there have been no investigations of cryptomonads and dinoflagellates. Furthermore, morphological similarity between the contents of cryptomonad trichocysts and the R-bodies of the endosymbiotic bacteria of Paramecium has been reported. In this study, we identified the proteins of the trichocyst constituents in a red cryptomonad, Pyrenomonas helgolandii, and found their closest relationships to be with rebB that comprises the R-bodies of Caedibacter taeniospiralis (gammaproteobacteria), which is an endosymbiont of Paramecium. In addition, the biochemical makeups of the trichocysts are entirely different between cryptomonads and peniculine ciliates, and therefore, cryptomonad trichocysts have an evolutionary origin independent from the peniculine ciliate trichocysts.

Isolation, purification and some ultrastructural details of discharged ejectisomes of cryptophytes

The first successful isolation of discharged ejectisomes from pigmented cryptophytes is reported. Discharged ejectisomes from a Chroomonas and two Cryptomonas species were characterized by transmission electron microscopy using negative staining and freeze-etching. Tubular-shaped fragments of variable lengths and diameters were obtained which showed a paracrystalline lattice. Particle periodicities of 4.1 nm along the longitudinal axis and 3.1 nm in the transverse direction were measured in negative-stained fragments. The dimensions measured from freeze-etched ejectisome fragments were about 0.5-1 nm larger. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed a protein banding pattern, dominated by polypeptides of 40-44, 23-25 and 16-18 kDa. The results are discussed in the context of what is currently known about extrusomes of protists.

Gene expression in a paleopolyploid: a transcriptome resource for the ciliate Paramecium tetraurelia

Background

The genome of Paramecium tetraurelia, a unicellular model that belongs to the ciliate phylum, has been shaped by at least 3 successive whole genome duplications (WGD). These dramatic events, which have also been documented in plants, animals and fungi, are resolved over evolutionary time by the loss of one duplicate for the majority of genes. Thanks to a low rate of large scale genome rearrangement in Paramecium, an unprecedented large number of gene duplicates of different ages have been identified, making this organism an outstanding model to investigate the evolutionary consequences of polyploidization. The most recent WGD, with 51% of pre-duplication genes still in 2 copies, provides a snapshot of a phase of rapid gene loss that is not accessible in more ancient polyploids such as yeast.

Results

We designed a custom oligonucleotide microarray platform for P. tetraurelia genome-wide expression profiling and used the platform to measure gene expression during 1) the sexual cycle of autogamy, 2) growth of new cilia in response to deciliation and 3) biogenesis of secretory granules after massive exocytosis. Genes that are differentially expressed during these time course experiments have expression patterns consistent with a very low rate of subfunctionalization (partition of ancestral functions between duplicated genes) in particular since the most recent polyploidization event.

Conclusions

A public transcriptome resource is now available for Paramecium tetraurelia. The resource has been integrated into the ParameciumDB model organism database, providing searchable access to the data. The microarray platform, freely available through NimbleGen Systems, provides a robust, cost-effective approach for genome-wide expression profiling in P. tetraurelia. The expression data support previous studies showing that at short evolutionary times after a whole genome duplication, gene dosage balance constraints and not functional change are the major determinants of gene retention.

Independent transport and sorting of functionally distinct protein families in Tetrahymena thermophila dense core secretory granules

Dense core granules (DCGs) in Tetrahymena thermophila contain two protein classes. Proteins in the first class, called granule lattice (Grl), coassemble to form a crystalline lattice within the granule lumen. Lattice expansion acts as a propulsive mechanism during DCG release, and Grl proteins are essential for efficient exocytosis. The second protein class, defined by a C-terminal beta/gamma-crystallin domain, is poorly understood. Here, we have analyzed the function and sorting of Grt1p (granule tip), which was previously identified as an abundant protein in this family. Cells lacking all copies of GRT1, together with the closely related GRT2, accumulate wild-type levels of docked DCGs. Unlike cells disrupted in any of the major GRL genes, DeltaGRT1 DeltaGRT2 cells show no defect in secretion, indicating that neither exocytic fusion nor core expansion depends on GRT1. These results suggest that Grl protein sorting to DCGs is independent of Grt proteins. Consistent with this, the granule core lattice in DeltaGRT1 DeltaGRT2 cells appears identical to that in wild-type cells by electron microscopy, and the only biochemical component visibly absent is Grt1p itself. Moreover, gel filtration showed that Grl and Grt proteins in cell homogenates exist in nonoverlapping complexes, and affinity-isolated Grt1p complexes do not contain Grl proteins. These data demonstrate that two major classes of proteins in Tetrahymena DCGs are likely to be independently transported during DCG biosynthesis and play distinct roles in granule function. The role of Grt1p may primarily be postexocytic; consistent with this idea, DCG contents from DeltaGRT1 DeltaGRT2 cells appear less adhesive than those from the wild type.

Novel types of Ca2+ release channels participate in the secretory cycle of Paramecium cells

A database search of the Paramecium genome reveals 34 genes related to Ca(2+)-release channels of the inositol-1,4,5-trisphosphate (IP(3)) or ryanodine receptor type (IP(3)R, RyR). Phylogenetic analyses show that these Ca(2+) release channels (CRCs) can be subdivided into six groups (Paramecium tetraurelia CRC-I to CRC-VI), each one with features in part reminiscent of IP(3)Rs and RyRs. We characterize here the P. tetraurelia CRC-IV-1 gene family, whose relationship to IP(3)Rs and RyRs is restricted to their C-terminal channel domain. CRC-IV-1 channels localize to cortical Ca(2+) stores (alveolar sacs) and also to the endoplasmic reticulum. This is in contrast to a recently described true IP(3) channel, a group II member (P. tetraurelia IP(3)R(N)-1), found associated with the contractile vacuole system. Silencing of either one of these CRCs results in reduced exocytosis of dense core vesicles (trichocysts), although for different reasons. Knockdown of P. tetraurelia IP(3)R(N) affects trichocyst biogenesis, while CRC-IV-1 channels are involved in signal transduction since silenced cells show an impaired release of Ca(2+) from cortical stores in response to exocytotic stimuli. Our discovery of a range of CRCs in Paramecium indicates that protozoans already have evolved multiple ways for the use of Ca(2+) as signaling molecule.

Genetic, genomic, and functional analysis of the granule lattice proteins in Tetrahymena secretory granules

In some cells, the polypeptides stored in dense core secretory granules condense as ordered arrays. In ciliates such as Tetrahymena thermophila, the resulting crystals function as projectiles, expanding upon exocytosis. Isolation of granule contents previously defined five Granule lattice (Grl) proteins as abundant core constituents, whereas a functional screen identified a sixth family member. We have now expanded this screen to identify the nonredundant components required for projectile assembly. The results, further supported by gene disruption experiments, indicate that six Grl proteins define the core structure. Both in vivo and in vitro data indicate that core assembly begins in the endoplasmic reticulum with formation of specific hetero-oligomeric Grl proprotein complexes. Four additional GRL-like genes were found in the T. thermophila genome. Grl2p and Grl6p are targeted to granules, but the transcripts are present at low levels and neither is essential for core assembly. The DeltaGRL6 cells nonetheless showed a subtle change in granule morphology and a marked reduction in granule accumulation. Epistasis analysis suggests this results from accelerated loss of DeltaGRL6 granules, rather than from decreased synthesis. Our results not only provide insight into the organization of Grl-based granule cores but also imply that the functions of Grl proteins extend beyond core assembly.

The vacuolar proton-ATPase plays a major role in several membrane-bounded organelles inParamecium

The vacuolar proton-ATPase (V-ATPase) is a multisubunit enzyme complex that is able to transfer protons over membranes against an electrochemical potential under ATP hydrolysis. The enzyme consists of two subcomplexes: V0, which is membrane embedded; and V1, which is cytosolic. V0 was also reported to be involved in fusion of vacuoles in yeast. We identified six genes encoding c-subunits (proteolipids) of V0 and two genes encoding F-subunits of V1 and studied the role of the V-ATPase in trafficking in Paramecium. Green fluorescent protein (GFP) fusion proteins allowed a clear subcellular localization of c- and F-subunits in the contractile vacuole complex of the osmoregulatory system and in food vacuoles. Several other organelles were also detected, in particular dense core secretory granules (trichocysts). The functional significance of the V-ATPase in Paramecium was investigated by RNA interference (RNAi), using a recently developed feeding method. A novel strategy was used to block the expression of all six c- or both F-subunits simultaneously. The V-ATPase was found to be crucial for osmoregulation, the phagocytotic pathway and the biogenesis of dense core secretory granules. No evidence was found supporting participation of V0 in membrane fusion.

Proprotein processing within secretory dense core granules of Tetrahymena thermophila

In the ciliate Tetrahymena thermophila, the polypeptides stored in secretory dense core granules (DCGs) are generated by proteolytic processing of precursors, and the mature products assemble as a crystal. Previous observations suggested that this maturation involves precise cleavage at distinct motifs by a small number of enzymes. To test these inferences, we analyzed the determinants for site-specific processing of pro-Grl1p (Granule lattice protein 1) by complete gene replacement with modified alleles. Contrary to the predictions of previous models, none of the component amino acids in a putative processing motif was necessary for targeted cleavage. Indeed, cleavage at a range of alternative positions near the native site was consistent with normal DCG assembly. Furthermore, substitution of other classes of processing site motifs did not perturb DCG structure or function. These results suggest that processing can be catalyzed by multiple proteases, for which substrate accessibility may be the prime determinant of site specificity. Consistent with this, inhibition of either subtilisin or cathepsin family proteases resulted in delayed processing of pro-Grl1p.

Molecular Aspects of Membrane Trafficking in Paramecium

Results achieved in the molecular biology of Paramecium have shed new light on its elaborate membrane trafficking system. Paramecium disposes not only of the standard routes (endoplasmic reticulum --> Golgi --> lysosomes or secretory vesicles; endo- and phagosomes --> lysosomes/digesting vacuoles), but also of some unique features, e.g. and elaborate phagocytic route with the cytoproct and membrane recycling to the cytopharynx, as well as the osmoregulatory system with multiple membrane fusion sites. Exocytosis sites for trichocysts (dense-core secretory vesicles), parasomal sacs (coated pits), and terminal cisternae (early endosomes) display additional regularly arranged predetermined fusion/fission sites, which now can be discussed on a molecular basis. Considering the regular, repetitive arrangements of membrane components, availability of mutants for complementation studies, sensitivity to gene silencing, and so on, Paramecium continues to be a valuable model system for analyzing membrane interactions. This review intends to set a new baseline for ongoing work along these lines.

New class of cargo protein in Tetrahymena thermophila dense core secretory granules

Regulated exocytosis of dense core secretory granules releases biologically active proteins in a stimulus-dependent fashion. The packaging of the cargo within newly forming granules involves a transition: soluble polypeptides condense to form water-insoluble aggregates that constitute the granule cores. Following exocytosis, the cores generally disassemble to diffuse in the cell environment. The ciliates Tetrahymena thermophila and Paramecium tetraurelia have been advanced as genetically manipulatable systems for studying exocytosis via dense core granules. However, all of the known granule proteins in these organisms condense to form the architectural units of lattices that are insoluble both before and after exocytosis. Using an approach designed to detect new granule proteins, we have now identified Igr1p (induced during granule regeneration). By structural criteria, it is unrelated to the previously characterized lattice-forming proteins. It is distinct in that it is capable of dissociating from the insoluble lattice following secretion and therefore represents the first diffusible protein identified in ciliate granules.

My favorite cell--Paramecium

A Paramecium cell has a stereotypically patterned surface, with regularly arranged cilia, dense-core secretory vesicles and subplasmalemmal calcium stores. Less strikingly, there is also a patterning of molecules; for instance, some ion channels are restricted to certain regions of the cell surface. This design may explain very effective and selective responses, such as that to Ca(2+) upon stimulation. It enables the cell to respond to a Ca(2+) signal precisely secretion (exocytosis) or by changing its ciliary activity. These responses depend on the location and/or type of signal, even though these two target structures co-exist side-by-side, and normally only limited overlap occurs between the different functions. Furthermore, the patterning of exocytotic sites and the possibility of synchronous exocytosis induction in the sub-second time range have considerably facilitated analyses, and thus led to new concepts of exocytotic membrane fusion. It has been possible to dissect complicated events like overlapping Ca(2+) fluxes produced from external sources and from internal stores. Since molecular genetic approaches have become available for Paramecium, many different gene products have been identified only some of which are known from "higher" eukaryotes. Although a variety of basic cellular functions are briefly addressed to demonstrate the uniqueness of this unicellular organism, this article focuses on exocytosis regulation.

An antisense approach to phenotype-based gene cloning in Tetrahymena

We report a pioneering approach using Tetrahymena thermophila that permits rapid identification of genes based on their null or hypomorphic phenotypes. This technique involves cell transformation with a library of plasmids that encode 26S ribosomal subunits containing short insertions. The insertions correspond to antisense sequences for a large number of genes. The majority of cells each acquires a single antisense sequence, which silences a single genomic locus. Because the insertion site within the ribosomal sequence is known, the silenced gene is easily amplified. We demonstrate that this approach can be used to identify genes required for dense core granule exocytosis.

Growth and form of secretory granules involves stepwise assembly but not differential sorting of a family of secretory proteins in Paramecium

Paramecium trichocysts are voluminous secretory vesicles consisting of a spindle-shaped body surmounted by a tip that serves to anchor them at exocytotic sites in the plasma membrane. This constrained shape is conferred by the proteins stored in the vesicles, which form an insoluble three-dimensional crystalline array. The constituent polypeptides (Trichocyst Matrix Proteins, TMPs), which assemble during trichocyst biogenesis, are produced by proteolytic processing of soluble proproteins encoded by a large multigene family. In order to investigate the functional significance of the TMP multigene family, which assures the synthesis of a mixture of related polypeptides, we have designed synthetic genes for heterologous expression of three different mature polypeptides, which were used to obtain sequence-specific rabbit antisera. We used these antisera to carry out immunolocalization experiments with wild-type trichocysts at different stages of development and found that the trichocyst matrix consists of two concentric layers containing different TMPs, and that the assembly of each layer corresponds to a distinct phase of trichocyst growth. Examination of mutant trichocysts created by targeted gene silencing of different TMP genes showed that the layer containing the products of the silenced genes is specifically affected, as are all subsequently assembled parts of the structure, consistent with an ordered assembly pathway. This stepwise assembly is not controlled by differential sorting of the TMPs, as single and double label experiments provided evidence that the different TMPs are delivered together to post-Golgi vesicles and developing trichocysts. We present a model for trichocyst biogenesis in which TMP assembly is controlled by protein processing.

Regulation of secretory protein gene expression in paramecium role of the cortical exocytotic sites

In cells that possess a regulated secretory pathway, exocytosis can lead to transcriptional activation of genes encoding products stored in secretory granules as well as genes required for granule biogenesis. With the objective of understanding this response, we have examined the expression of Paramecium secretory protein genes in different physiological and genetic contexts. The genes belong to the trichocyst matrix protein (TMP) multigene family, encoding polypeptides that form the crystalline matrix of the secretory granules, known as trichocysts. Approximately 1000 trichocysts per cell are docked at pre-formed cortical exocytotic sites. Their rapid and synchronous exocytosis can be triggered by vital secretagogues such as aminoethyldextran without harming the cells. Using this exocytotic trigger, we found that the transcription of TMP genes undergoes rapid, transient and co-ordinate 10-fold activation in response to massive exocytosis, leading to a 2.5-fold increase in the pool of TMP mRNA. Experiments with exocytosis-deficient mutants show that the secretagogue-induced increase in intracellular free calcium implicated in stimulus/secretion coupling is not sufficient to activate TMP gene expression. We present evidence that the state of occupation of the cortical exocytotic sites can affect TMP gene expression and suggest that these sites play a role in gene activation in response to exocytosis.

Molecular genetics of regulated secretion in paramecium

Paramecium is a unicell in which cellular processes are amenable to genetic dissection. Regulated secretion, which designates a secretory pathway where secretory products are first stored in intracellular granules and then released by exocytotic membrane fusion upon external trigger, is an important function in Paramecium, involved in defensive response through the release of organelles called trichocysts. In this review, we focus on recent advances in the molecular genetics of two major aspects of the regulated pathway in Paramecium, the biogenesis of the secretory organelles and their exocytosis.

An indexed genomic library for Paramecium complementation cloning

Recent pioneering work opened the way for cloning genes in Paramecium by functional complementation of mutants. We present here the construction and pilot utilization of a new indexed library of Paramecium macronuclear DNA. The library is made of 61,440 clones containing inserts mostly between 6 and 12 kilobases. It has already allowed the complementation cloning of four new genes, and this library has proven to be very useful for rapid hybridization cloning.

Sorting and storage during secretory granule biogenesis: looking backward and looking forward

Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

An exocytotic mutant of Paramecium caudatum: membrane fusion without secretory contents release

This is a detailed characterization of a secretory mutant incapable of releasing secretory contents despite normal exocytotic membrane fusion performance. Trichocyst non-discharge strain tnd1 of Paramecium caudatum and its wildtype (wt) both show a transient cortical [Ca2+]i increase and exocytotic membrane fusion in response to the polyamine secretagogue, aminoethyldextran (AED), or to caffeine. tnd1 cells frequently display spontaneous Ca2+ signals parallelled by spontaneous exocytotic membrane fusion. This remains undetected, unless the trichocyst matrix is shown to be freely accessible to the inert, non-membrane permeable fluorochrome, F2FITC, from the outside. In these tnd1 cells, spontaneous and AED- or caffeine-induced membrane fusion, always without contents expulsion by decondensation (i.e. several-fold stretching), is ascertained by electron microscopy. Exocytotic openings, with condensed trichocysts retained, may persist for hours without impairing cells. Trichocyst decondensation normally requires micromolar [Ca2+]e, but an increase to 10 mM has no effect on tnd1 trichocyst expansion in vivo or in vitro (when isolated and exposed to ionophore A23187 + Ca2+). Paracrystalline packing of the major secretory components (trichynins) does occur, despite incomplete proteolytic precursor processing (according to SDS-PAGE). However, 45Ca(2+)-binding by secretory components is considerably reduced--the likely cause of the non-discharge phenotype. Our findings imply significant untriggered membrane fusion in a system normally following the triggered pathway and clear separation of exocytotic membrane fusion from any later Ca(2+)-dependent steps of the secretory cycle.

Homology-dependent gene silencing in Paramecium

Microinjection at high copy number of plasmids containing only the coding region of a gene into the Paramecium somatic macronucleus led to a marked reduction in the expression of the corresponding endogenous gene(s). The silencing effect, which is stably maintained throughout vegetative growth, has been observed for all Paramecium genes examined so far: a single-copy gene (ND7), as well as members of multigene families (centrin genes and trichocyst matrix protein genes) in which all closely related paralogous genes appeared to be affected. This phenomenon may be related to posttranscriptional gene silencing in transgenic plants and quelling in Neurospora and allows the efficient creation of specific mutant phenotypes thus providing a potentially powerful tool to study gene function in Paramecium. For the two multigene families that encode proteins that coassemble to build up complex subcellular structures the analysis presented herein provides the first experimental evidence that the members of these gene families are not functionally redundant.

Proteolytic processing and Ca2+-binding activity of dense-core vesicle polypeptides in Tetrahymena

Formation and discharge of dense-core secretory vesicles depend on controlled rearrangement of the core proteins during their assembly and dispersal. The ciliate Tetrahymena thermophila offers a simple system in which the mechanisms may be studied. Here we show that most of the core consists of a set of polypeptides derived proteolytically from five precursors. These share little overall amino acid identity but are nonetheless predicted to have structural similarity. In addition, sites of proteolytic processing are notably conserved and suggest that specific endoproteases as well as carboxypeptidase are involved in core maturation. In vitro binding studies and sequence analysis suggest that the polypeptides bind calcium in vivo. Core assembly and postexocytic dispersal are compartment-specific events. Two likely regulatory factors are proteolytic processing and exposure to calcium. We asked whether these might directly influence the conformations of core proteins. Results using an in vitro chymotrypsin accessibility assay suggest that these factors can induce sequential structural rearrangements. Such progressive changes in polypeptide folding may underlie the mechanisms of assembly and of rapid postexocytic release. The parallels between dense-core vesicles in different systems suggest that similar mechanisms are widespread in this class of organelles.

Characterization of multigene families in the micronuclear genome of Paramecium tetraurelia reveals a germline specific sequence in an intron of a centrin gene

In Paramecium, as in other ciliates, the transcriptionally active macronucleus is derived from the germline micronucleus by programmed DNA rearrangements, which include the precise excision of thousands of germline-specific sequences (internal eliminated sequences, IESs). We report the characterization of micronuclear versions of genes encoding Paramecium secretory granule proteins (trichocyst matrix proteins, TMPs) and Paramecium centrins. TMP and centrin multigene families, previously studied in the macronuclear genome, consist of genes that are co-expressed to provide mixtures of related polypeptides that co-assemble to form respectively the crystalline trichocyst matrix and the infraciliary lattice, a contractile cytoskeletal network. We present evidence that TMP and centrin genes identified in the macronucleus are also present in the micronucleus, ruling out the possibility that these novel multigene families are generated by somatic rearrangements during macronuclear development. No IESs were found in TMP genes, however, four IESs in or near germline centrin genes were characterized. The only intragenic IES is 75 bp in size, interrupts a 29 bp intron and is absent from at least one other closely related centrin gene. This is the first report of an IES in an intron in Paramecium.

Granule lattice protein 1 (Grl1p), an acidic, calcium-binding protein in Tetrahymena thermophila dense-core secretory granules, influences granule size, shape, content organization, and release but not protein sorting or condensation

The electron-dense cores of regulated secretory granules in the ciliate Tetrahymena thermophila are crystal lattices composed of multiple proteins. Granule synthesis involves a series of steps beginning with protein sorting, followed by the condensation and precise geometric assembly of the granule cargo. These steps may to various degrees be determined by the cargo proteins themselves. A prominent group of granule proteins, in ciliates as well as in vertebrate neuronal and endocrine cells, are acidic, heat-stable, and bind calcium. We focused on a protein with these characteristics named granule lattice protein 1 (Grl1p), which represents 16% of total granule contents, and we have now cloned the corresponding gene. Mutants in which the macronuclear copies of GRL1 have been disrupted continue to synthesize dense-core granules but are nonetheless defective in regulated protein secretion. To understand the nature of this defect, we characterized mutant and wild-type granules. In the absence of Grl1p, the sorting of the remaining granule proteins appears normal, and they condense to form a well-defined core. However, the condensed cores do not demonstrate a visible crystalline lattice, and are notably different from wild type in size and shape. The cellular secretion defect arises from failure of the aberrant granule cores to undergo rapid expansion and extrusion after exocytic fusion of the granule and plasma membranes. The results suggest that sorting, condensation, and precise granule assembly are distinct in their requirements for Grl1p.