Quantitative immunogold localization of protein phosphatase 2B (calcineurin) in Paramecium cells

Calmodulin in Paramecium: Focus on Genomic Data

Calcium (Ca²⁺) is a universal second messenger that plays a key role in cellular signaling. However, Ca²⁺ signals are transduced with the help of Ca²⁺-binding proteins, which serve as sensors, transducers, and elicitors. Among the collection of these Ca²⁺-binding proteins, calmodulin (CaM) emerged as the prototypical model in eukaryotic cells. This is a small protein that binds four Ca²⁺ ions and whose functions are multiple, controlling many essential aspects of cell physiology. CaM is universally distributed in eukaryotes, from multicellular organisms, such as human and land plants, to unicellular microorganisms, such as yeasts and ciliates. Here, we review most of the information gathered on CaM in Paramecium, a group of ciliates. We condense the information here by mentioning that mature Paramecium CaM is a 148 amino acid-long protein codified by a single gene, as in other eukaryotic microorganisms. In these ciliates, the protein is notoriously localized and regulates cilia function and can stimulate the activity of some enzymes. When Paramecium CaM is mutated, cells show flawed locomotion and/or exocytosis. We further widen this and additional information in the text, focusing on genomic data.

Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

During evolution, the cell as a fine-tuned machine had to undergo permanent adjustments to match changes in its environment, while "closed for repair work" was not possible. Evolution from protists (protozoa and unicellular algae) to multicellular organisms may have occurred in basically two lineages, Unikonta and Bikonta, culminating in mammals and angiosperms (flowering plants), respectively. Unicellular models for unikont evolution are myxamoebae (Dictyostelium) and increasingly also choanoflagellates, whereas for bikonts, ciliates are preferred models. Information accumulating from combined molecular database search and experimental verification allows new insights into evolutionary diversification and maintenance of genes/proteins from protozoa on, eventually with orthologs in bacteria. However, proteins have rarely been followed up systematically for maintenance or change of function or intracellular localization, acquirement of new domains, partial deletion (e.g. of subunits), and refunctionalization, etc. These aspects are discussed in this review, envisaging "evolutionary cell biology." Protozoan heritage is found for most important cellular structures and functions up to humans and flowering plants. Examples discussed include refunctionalization of voltage-dependent Ca²⁺ channels in cilia and replacement by other types during evolution. Altogether components serving Ca²⁺ signaling are very flexible throughout evolution, calmodulin being a most conservative example, in contrast to calcineurin whose catalytic subunit is lost in plants, whereas both subunits are maintained up to mammals for complex functions (immune defense and learning). Domain structure of R-type SNAREs differs in mono- and bikonta, as do Ca²⁺ -dependent protein kinases. Unprecedented selective expansion of the subunit a which connects multimeric base piece and head parts (V0, V1) of H⁺ -ATPase/pump may well reflect the intriguing vesicle trafficking system in ciliates, specifically in Paramecium. One of the most flexible proteins is centrin when its intracellular localization and function throughout evolution is traced. There are many more examples documenting evolutionary flexibility of translation products depending on requirements and potential for implantation within the actual cellular context at different levels of evolution. From estimates of gene and protein numbers per organism, it appears that much of the basic inventory of protozoan precursors could be transmitted to highest eukaryotic levels, with some losses and also with important additional "inventions."

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

Calcium regulation in the protozoan model, Paramecium tetraurelia

Early in eukaryotic evolution, the cell has evolved a considerable inventory of proteins engaged in the regulation of intracellular Ca(2+) concentrations, not only to avoid toxic effects but beyond that to exploit the signaling capacity of Ca(2+) by small changes in local concentration. Among protozoa, the ciliate Paramecium may now be one of the best analyzed models. Ciliary activity and exo-/endocytosis are governed by Ca(2+) , the latter by Ca(2+) mobilization from alveolar sacs and a superimposed store-operated Ca(2+) -influx. Paramecium cells possess plasma membrane- and endoplasmic reticulum-resident Ca(2+) -ATPases/pumps (PMCA, SERCA), a variety of Ca(2+) influx channels, including mechanosensitive and voltage-dependent channels in the plasma membrane, furthermore a plethora of Ca(2+) -release channels (CRC) of the inositol 1,4,5-trisphosphate and ryanodine receptor type in different compartments, notably the contractile vacuole complex and the alveolar sacs, as well as in vesicles participating in vesicular trafficking. Additional types of CRC probably also occur but they have not been identified at a molecular level as yet, as is the equivalent of synaptotagmin as a Ca(2+) sensor for exocytosis. Among established targets and sensors of Ca(2+) in Paramecium are calmodulin, calcineurin, as well as Ca(2+) /calmodulin-dependent protein kinases, all with multiple functions. Thus, basic elements of Ca(2+) signaling are available for Paramecium.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Quantitative assessment of specificity in immunoelectron microscopy

In immunoelectron microscopy (immuno-EM) on ultrathin sections, gold particles are used for localization of molecular components of cells. These particles are countable, and quantitative methods have been established to estimate and evaluate the density and distribution of "raw" gold particle counts from a single uncontrolled labeling experiment. However, these raw counts are composed of two distinct elements: particles that are specific (specific labeling) and particles that are not (nonspecific labeling) for the target component. So far, approaches for assessment of specific labeling and for correction of raw gold particle counts to reveal specific labeling densities and distributions have not attracted much attention. Here, we discuss experimental strategies for determining specificity in immuno-EM, and we present methods for quantitative assessment of (1) the probability that an observed gold particle is specific for the target, (2) the density of specific labeling, and (3) the distribution of specific labeling over a series of compartments. These methods should be of general utility for researchers investigating the distribution of cellular components using on-section immunogold labeling.

Protein phosphatase 2B (PP2B, calcineurin) in Paramecium: partial characterization reveals that two members of the unusually large catalytic subunit family have distinct roles in calcium-dependent processes

We characterized the calcineurin (CaN) gene family, including the subunits CaNA and CaNB, based upon sequence information obtained from the Paramecium genome project. Paramecium tetraurelia has seven subfamilies of the catalytic CaNA subunit and one subfamily of the regulatory CaNB subunit, with each subfamily having two members of considerable identity on the amino acid level (>or=55% between subfamilies, >or=94% within CaNA subfamilies, and full identity in the CaNB subfamily). Within CaNA subfamily members, the catalytic domain and the CaNB binding region are highly conserved and molecular modeling revealed a three-dimensional structure almost identical to a human ortholog. At 14 members, the size of the CaNA family is unprecedented, and we hypothesized that the different CaNA subfamily members were not strictly redundant and that at least some fulfill different roles in the cell. This was tested by selecting two phylogenetically distinct members of this large family for posttranscriptional silencing by RNA interference. The two targets resulted in differing effects in exocytosis, calcium dynamics, and backward swimming behavior that supported our hypothesis that the large, highly conserved CaNA family members are not strictly redundant and that at least two members have evolved diverse but overlapping functions. In sum, the occurrence of CaN in Paramecium spp., although disputed in the past, has been established on a molecular level. Its role in exocytosis and ciliary beat regulation in a protozoan, as well as in more complex organisms, suggests that these roles for CaN were acquired early in the evolution of this protein family.

Cell surface protein detection with immunogold labelling in ESEM: optimisation of the method and semi-quantitative analysis

In this work we used a combination of immunogold labelling (IGL) and environmental scanning electron microscopy (ESEM) to detect the presence of a protein on the cell surface. To achieve this purpose we chose as experimental system 3T3 Swiss Albino Mouse Fibroblasts and galectin-3. This protein, whose sub-cellular distribution is still under discussion, is involved in a large number of cell physiological and pathological processes. IGL technique has been utilised by many authors in combination with SEM and TEM to obtain the identification/localisation of receptors and antigens, both in cells and tissues. ESEM represents an important tool in biomedical research, since it does not require any severe processing of the sample, lowering the risk of generating artefacts and interfere with IGL procedure. The absence of metal coating could yield further advantages for our purpose as the labelling detection is based on the atomic number difference between Nanogold spheres and the biological material. Using the gaseous secondary electron detector (GSED) compositional contrast is easily revealed by the backscattered electrons component of the signal. In spite of this fact, only few published papers present a combination of ESEM and IGL. Hereby we present our method, optimised to improve the intensity and the specificity of the labelling signal, in order to obtain a semi-quantitative evaluation of the labelling signal.

Molecular aspects of rapid, reversible, Ca2+-dependent de-phosphorylation of pp63/parafusin during stimulated exo-endocytosis in Paramecium cells

Ca2+ signalling governs stimulated exocytosis and exocytosis-coupled endocytosis also in Paramecium cells. Upon stimulation, the < or =10(3) dense-core exocytotic organelles (trichocysts) can be synchronously (80 ms) released, followed by endocytotic membrane resealing (350 ms) and retrieval. Paramecium is the most synchronous dense-core exocytotic system known, allowing to dissect rapidly reversible Ca2+-dependent phenomena. This holds for the reversible de-/re-phosphorylation cycle of a 63 kD phosphoprotein, pp63/parafusin (pf), which we have cloned, immuno-localised, and characterised as phosphoglucomutase, the enzyme funneling glucose into the glycolytic pathway. It was isolated ex vivo, followed by MALDI analysis, while X-ray structure analysis was performed after heterologous expression. We found multiple phosphorylation of superficial Ser/Thr residues. Although present also in exo(-) mutants, pp63/pf is selectively de-phosphorylated only in exo(+) strains during synchronous exocytosis (80 ms) and re-phosphorylated within approximately 20 s, i.e., the time required to re-establish [Ca2+] homeostasis. We have isolated relevant protein phosphatases and kinases and probed their activity on pp63/pf in vitro. We consider Ca2+/calmodulin-activated PP2B (calcineurin, whose subunits have been cloned) relevant for de-phosphorylation. Re-phosphorylation can be achieved by two protein kinases that also have been cloned. One is activated by cGMP (PKG) which in turn is formed by Ca2+-activated guanylate cyclase. Another kinase, casein kinase 2, is inhibited by Ca2+ and, hence, activated with some delay in parallel to decreasing [Ca2+] after exocytosis. In total, several Ca2+-sensitive cycles cooperate whose protein components have been localised to the cell cortex. Regulation of the phosphorylation degree of pp63/pf may affect structure binding on a microscale and/or its enzymatic activity. All this may serve fueling substrate into glycolysis with increased ATP re-formation (compromised in exo(-) mutants) and NADH formation, with effects on Ca2+ signalling including mobilisation from cortical stores (alveolar sacs) and overall effects on ATP and Ca2+ dynamics during synchronous exo- and endocytosis.

Dephosphorylation of inner arm 1 is associated with ciliary reversals in Tetrahymena thermophila

In many organisms, depolarizing stimuli cause an increase in intraciliary Ca2+, which results in reversal of ciliary beat direction and backward swimming. The mechanism by which an increase in intraciliary Ca2+ causes ciliary reversal is not known. Here we show that Tetrahymena cells treated with okadaic acid or cantharidin to inhibit protein phosphatases do not swim backwards in response to depolarizing stimuli. We also show that both okadaic acid and cantharidin inhibit backward swimming in reactivated, extracted cell models treated with Ca2+. In contrast, treatment of whole cells or extracted cell models with protein kinase inhibitors has no effect on backward swimming. These results suggest that a component of the axonemal machinery is dephosphorylated during ciliary reversal. The phosphorylation state of inner arm dynein 1 (I1) was determined before and after cells were exposed to depolarizing conditions that induce ciliary reversal. An I1 intermediate chain is phosphorylated in forward swimming cells but is dephosphorylated in cells treated with a depolarizing stimulus. Our results suggest that dephosphorylation of Tetrahymena inner arm dynein 1 may be an essential part of the mechanism of ciliary reversal in response to increased intraciliary Ca2+.

Molecular identification of a calcium-inhibited catalytic subunit of casein kinase type 2 from Paramecium tetraurelia

We have previously described the occurrence in Paramecium of a casein kinase (CK) activity (EC 2.7.1.37) with some unusual properties, including inhibition by Ca(2+) (R. Kissmehl, T. Treptau, K. Hauser, and H. Plattner, FEBS Lett. 402:227-235, 1995). We now have cloned four genes, PtCK2alpha1 to PtCK2alpha4, all of which encode the catalytic alpha subunit of type 2 CK (CK2) with calculated molecular masses ranging from 38.9 to 39.4 kDa and pI values ranging from 8.8 to 9.0. They can be classified into two groups, which differ from each other by 28% on the nucleotide level and by 18% on the derived amino acid level. One of them, PtCK2alpha3, has been expressed in Escherichia coli and characterized in vitro. As we also have observed with the isolated CK, the recombinant protein preferentially phosphorylates casein but also phosphorylates some Paramecium-specific substrates, including the exocytosis-sensitive phosphoprotein pp63/parafusin. Characteristically, Ca(2+) inhibits the phosphorylation at elevated concentrations occurring during stimulation of a cell. Reconstitution with a recombinant form of the regulatory subunit from Xenopus laevis, XlCK2beta, confirms Ca(2+) sensitivity also under conditions of autophosphorylation. This is unusual for CK2 but correlates with the presence of two EF-hand calcium-binding motifs, one of which is located in the N-terminal segment essential for constitutive activity, as well as with an aberrant composition of normally basic domains recognizing acidic substrate domains. Immunogold localization reveals a considerable enrichment in the outermost cell cortex layers, excluding cilia. We discuss a potential role of this Ca(2+)-inhibited PtCK2alpha species in a late step of signal transduction.

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.

Ca2+-binding proteins of cilia and infraciliary lattice ofParamecium tetraurelia: their phosphorylation by purified endogenous Ca2+-dependent protein kinases

We purified two small, acidic calcium-binding proteins(ParameciumCa2+-binding proteins, PCBP-25α and PCBP-25β) from Paramecium tetraurelia by Ca2+-dependent chromatography on phenyl-Sepharose and by anion-exchange chromatography. The proteins were immunologically distinct. Monoclonal antibodies against PCBP-25β did not react with PCBP-25α, and antibodies against centrin from Chlamydomonas reacted with PCBP-25α but not with PCBP-25β. Like the centrins described previously, both PCBPs were associated with the infraciliary lattice (ICL), a fibrillar cytoskeletal element in Paramecium. Both were also present in isolated cilia, from which they could be released (with dynein) by a high-salt wash, and both PCBPs cosedimented with dynein in a sucrose gradient. PCBP-25β was especially prominent in cilia and in the deciliation supernatant, a soluble fraction released during the process of deciliation. The results of immunoreactivity and localization experiments suggest that PCBP-25α is a Paramecium centrin and that PCBP-25β is a distinct Ca2+-binding protein that confers Ca2+ sensitivity on some component of the cilium, ciliary basal body or ICL.

We characterized these proteins and Paramecium calmodulin as substrates for two Ca2+-dependent protein kinases purified from Paramecium. PCBP-25α and calmodulin were in vitro substrates for one of the two Ca2+-dependent protein kinases (CaPK-2), but only PCBP-25α was phosphorylated by CaPK-1. These results raise the possibility that the biological activities of PCBP-25α and calmodulin are regulated by phosphorylation.

Glycosylphosphatidyl inositol-anchored proteins and fyn kinase assemble in noncaveolar plasma membrane microdomains defined by reggie-1 and -2

Using confocal laser scanning and double immunogold electron microscopy, we demonstrate that reggie-1 and -2 are colocalized in < or =0.1-microm plasma membrane microdomains of neurons and astrocytes. In astrocytes, reggie-1 and -2 do not occur in caveolae but clearly outside these structures. Microscopy and coimmunoprecipitation show that reggie-1 and -2 are associated with fyn kinase and with the glycosylphosphatidyl inositol-anchored proteins Thy-1 and F3 that, when activated by antibody cross-linking, selectively copatch with reggie. Jurkat cells, after cross-linking of Thy-1 or GM1 (with the use of cholera toxin), exhibit substantial colocalization of reggie-1 and -2 with Thy-1, GM1, the T-cell receptor complex and fyn. This, and the accumulation of reggie proteins in detergent-resistant membrane fractions containing F3, Thy-1, and fyn imparts to reggie-1 and -2 properties of raft-associated proteins. It also suggests that reggie-1 and -2 participate in the formation of signal transduction centers. In addition, we find reggie-1 and -2 in endolysosomes. In Jurkat cells, reggie-1 and -2 together with fyn and Thy-1 increase in endolysosomes concurrent with a decrease at the plasma membrane. Thus, reggie-1 and -2 define raft-related microdomain signaling centers in neurons and T cells, and the protein complex involved in signaling becomes subject to degradation.

Green fluorescent protein-tagged sarco(endo)plasmic reticulum Ca2+-ATPase overexpression in Paramecium cells: isoforms, subcellular localization, biogenesis of cortical calcium stores and functional aspects

We have followed the time-dependent transfection of Paramecium cells with a vector containing the gene of green fluorescent protein (GFP) attached to the C-terminus of the PtSERCA1 gene. The outlines of alveolar sacs (ASs) are labelled, as is the endoplasmic reticulum (ER) throughout the cell. When GFP fluorescence is compared with previous anti-PtSERCA1 antibody labelling, the much wider distribution of GFP (ER+ASs) indicates that only a small amount of SERCA molecules is normally retained in the ER. A second isoform, PtSERCA2, also occurs and its C-terminal GFP-tagging results in the same distribution pattern. However, when GFP is inserted in the major cytoplasmic loop, PtSERCA1 and two fusion proteins are mostly retained in the ER, probably because of the presence of the overt C-terminal KKXX ER-retention signal and/or masking of a signal for transfer into ASs. On the overall cell surface, new SERCA molecules seem to be permanently delivered from the ER to ASs by vesicle transport, whereas in the fission zone of dividing cells ASs may form anew. In cells overexpressing PtSERCA1 (with C-terminal GFP) in ASs, [Ca2+]i regulation during exocytosis is not significantly different from controls, probably because their Ca2+ pump has to mediate only slow reuptake.