Immunological and structural evidence for patterned intussusceptive surface growth in a unicellular organism. A postulated role for submembranous proteins and microtubules

The distinctive flagellar proteome of Euglena gracilis illuminates the complexities of protistan flagella adaptation

The eukaryotic flagellum/cilium is a prominent organelle with conserved structure and diverse functions. Euglena gracilis, a photosynthetic and highly adaptable protist, employs its flagella for both locomotion and environmental sensing. Using proteomics of isolated E. gracilis flagella we identify nearly 1700 protein groups, which challenges previous estimates of the protein complexity of motile eukaryotic flagella. We not only identified several unexpected similarities shared with mammalian flagella, including an entire glycolytic pathway and proteasome, but also document a vast array of flagella-based signal transduction components that coordinate gravitaxis and phototactic motility. By contrast, the pellicle was found to consist of > 900 protein groups, containing additional structural and signalling components. Our data identify significant adaptations within the E. gracilis flagellum, many of which are clearly linked to the highly flexible lifestyle.

Epiplasts: Membrane Skeletons and Epiplastin Proteins in Euglenids, Glaucophytes, Cryptophytes, Ciliates, Dinoflagellates, and Apicomplexans

Animals and amoebae assemble actin/spectrin-based plasma membrane skeletons, forming what is often called the cell cortex, whereas euglenids and alveolates (ciliates, dinoflagellates, and apicomplexans) have been shown to assemble a thin, viscoelastic, actin/spectrin-free membrane skeleton, here called the epiplast. Epiplasts include a class of proteins, here called the epiplastins, with a head/medial/tail domain organization, whose medial domains have been characterized in previous studies by their low-complexity amino acid composition. We have identified two additional features of the medial domains: a strong enrichment of acid/base amino acid dyads and a predicted β-strand/random coil secondary structure. These features have served to identify members in two additional unicellular eukaryotic radiations-the glaucophytes and cryptophytes-as well as additional members in the alveolates and euglenids. We have analyzed the amino acid composition and domain structure of 219 epiplastin sequences and have used quick-freeze deep-etch electron microscopy to visualize the epiplasts of glaucophytes and cryptophytes. We define epiplastins as proteins encoded in organisms that assemble epiplasts, but epiplastin-like proteins, of unknown function, are also encoded in Insecta, Basidiomycetes, and Caulobacter genomes. We discuss the diverse cellular traits that are supported by epiplasts and propose evolutionary scenarios that are consonant with their distribution in extant eukaryotes.IMPORTANCE Membrane skeletons associate with the inner surface of the plasma membrane to provide support for the fragile lipid bilayer and an elastic framework for the cell itself. Several radiations, including animals, organize such skeletons using actin/spectrin proteins, but four major radiations of eukaryotic unicellular organisms, including disease-causing parasites such as Plasmodium, have been known to construct an alternative and essential skeleton (the epiplast) using a class of proteins that we term epiplastins. We have identified epiplastins in two additional radiations and present images of their epiplasts using electron microscopy. We analyze the sequences and secondary structure of 219 epiplastins and present an in-depth overview and analysis of their known and posited roles in cellular organization and parasite infection. An understanding of epiplast assembly may suggest therapeutic approaches to combat infectious agents such as Plasmodium as well as approaches to the engineering of useful viscoelastic biofilms.

The four-transmembrane protein IP39 of Euglena forms strands by a trimeric unit repeat

Euglenoid flagellates have striped surface structures comprising pellicles, which allow the cell shape to vary from rigid to flexible during the characteristic movement of the flagellates. In Euglena gracilis, the pellicular strip membranes are covered with paracrystalline arrays of a major integral membrane protein, IP39, a putative four-membrane-spanning protein with the conserved sequence motif of the PMP-22/EMP/MP20/Claudin superfamily. Here we report the three-dimensional structure of Euglena IP39 determined by electron crystallography. Two-dimensional crystals of IP39 appear to form a striated pattern of antiparallel double-rows in which trimeric IP39 units are longitudinally polymerised, resulting in continuously extending zigzag-shaped lines. Structural analysis revealed an asymmetric molecular arrangement in the trimer, and suggested that at least four different interactions between neighbouring protomers are involved. A combination of such multiple interactions would be important for linear strand formation of membrane proteins in a lipid bilayer.

Regulatory volume decrease in Leishmania mexicana: effect of anti-microtubule drugs

The trypanosomatid cytoskeleton is responsible for the parasite's shape and it is modulated throughout the different stages of the parasite's life cycle. When parasites are exposed to media with reduced osmolarity, they initially swell, but subsequently undergo compensatory shrinking referred to as regulatory volume decrease (RVD). We studied the effects of anti-microtubule (Mt) drugs on the proliferation of Leishmania mexicana promastigotes and their capacity to undergo RVD. All of the drugs tested exerted antiproliferative effects of varying magnitudes [ansamitocin P3 (AP3)> trifluoperazine > taxol > rhizoxin > chlorpromazine]. No direct relationship was found between antiproliferative drug treatment and RVD. Similarly, Mt stability was not affected by drug treatment. Ansamitocin P3, which is effective at nanomolar concentrations, blocked amastigote-promastigote differentiation and was the only drug that impeded RVD, as measured by light dispersion. AP3 induced 2 kinetoplasts (Kt) 1 nucleus cells that had numerous flagella-associated Kts throughout the cell. These results suggest that the dramatic morphological changes induced by AP3 alter the spatial organisation and directionality of the Mts that are necessary for the parasite's hypotonic stress-induced shape change, as well as its recovery.

NOVEL PELLICLE SURFACE PATTERNS ON EUGLENA OBTUSA (EUGLENOPHYTA) FROM THE MARINE BENTHIC ENVIRONMENT: IMPLICATIONS FOR PELLICLE DEVELOPMENT AND EVOLUTION(1)

Euglena obtusa F. Schmitz possesses novel pellicle surface patterns, including the greatest number of strips (120) and the most posterior subwhorls of strip reduction in any euglenid described so far. Although the subwhorls form a mathematically linear pattern of strip reduction, the pattern observed here differs from the linear pattern described for Euglena mutabilis F. Schmitz in that it contains seven linear subwhorls, rather than three, and is developmentally equivalent to three whorls of exponential reduction, rather than two. These properties imply that the seven-subwhorled linear pattern observed in E. obtusa is evolutionarily derived from an ancestral bilinear pattern, rather than from a linear pattern, of strip reduction. Furthermore, analysis of the relative lateral positions of the strips forming the subwhorls in E. obtusa indicates that (1) the identity (relative length, lateral position, and maturity) of each strip in any mother cell specifies that strip's identity in one of the daughter cells following pellicle duplication and cell division, (2) the relative length of any given pellicle strip regulates the length of the nascent strip it will produce during pellicle duplication, and (3) pellicle pores develop within the heels of the most mature pellicle strips. These observations suggest that continued research on pellicle development could eventually establish an ideal system for understanding mechanisms associated with the morphogenesis and evolution of related eukaryotic cells.

Macroevolution of complex cytoskeletal systems in euglenids

Euglenids comprise a group of single-celled eukaryotes with diverse modes of nutrition, including phagotrophy and photosynthesis. The level of morphological diversity present in this group provides an excellent system for demonstrating evolutionary transformations in morphological characters. This diversity also provides compelling evidence for major events in eukaryote evolution, such as the punctuated effects of secondary endosymbiosis and mutations in underlying developmental mechanisms. In this essay, we synthesize evidence for the origin, adaptive significance and diversification of the euglenid cytoskeleton, especially pellicle ultrastructure, pellicle surface patterns, pellicle strip number and the feeding apparatus. We also highlight holes in our knowledge that must be filled before we are able to confidently describe euglenid cell biology and infer the earliest stages in euglenid evolution. Nonetheless, by possessing combinations of characters resulting from adaptive change and morphostasis, euglenids have retained key pieces of evidence necessary for reconstructing the early evolution and diversification of eukaryotic life.

A model for the morphogenesis of strip reduction patterns in phototrophic euglenids: evidence for heterochrony in pellicle evolution

We propose a general developmental model that explains the evolutionary origin, diversification, and inheritance of pellicle strip patterns in phototrophic euglenids. Dividing cells of Euglena gracilis, E. viridis, and Phacus similis were observed with scanning electron microscopy in order to study the morphogenesis of posterior whorls of strip reduction. We found evidence that constant whorl numbers are maintained through cell division because of organized strip growth before and during cytokinesis. Alternating nascent strips form a new whorl of strip reduction at each of the anterior and posterior ends of daughter cells. Strips that terminated to form posterior whorls in the mother cell change in length during the development of daughter cells. In the mother cells of E. gracilis, the strips forming whorls I and II grow to become whorls II and III, respectively, in the daughter cells; the strips forming whorl III in the mother cell lengthen and meet with other strips already present at the posterior tip of daughter cells. This process of whorl morphogenesis during asexual reproduction is consistent with known variation in pellicle strip patterns and suggests that heterochrony played a major role in the ultrastructural evolution of phototrophic euglenids.

The molecular biology of Euglena gracilis. XV. Recovery from centrifugation-induced stratification

The contents of Euglena gracilis cells can be separated in vivo by ultracentrifugation. Within the unbroken cell, each set of components forms a distinct layer according to their respective densities. The degree of segregation increases with both the g-force and the time of centrifugation, up to a maximum at 100,000 x g for 1 h, when six distinct strata can be observed. When returned to normal growth conditions, essentially all the cells return to the normal state and growth pattern. Greater g-forces or longer exposures do not alter the observable strata, but the ability of the cells to recover is diminished. Smaller g-forces result in less separation of cellular contents and all cells recover, even after 18 h of exposure. Euglena cells stratified at 100,000 x g for 1 h were returned to normal growth conditions; recovery was followed microscopically and by the rate of utilization of oxygen as well as that of the single carbon source. The cells recovered their normal state within 1 to 2 h, which is only a tenth of the normal doubling time. The mechanism for this recovery involves a natural process of change in cell shape caused by contraction and relaxation of the pellicle, a cell surface structure.

Did trypanosomatid parasites have photosynthetic ancestors?

Some molecular phylogenies of plastid-like genes suggest that chloroplasts (the structures responsible for photosynthesis in plants and algae) might have been secondarily lost in trypanosomatid parasites. Chloroplasts are present in some euglenids, which are closely related to trypanosomatids, and it has been argued that chloroplasts arose early in the diversification of the lineage Euglenozoa, to which trypanosomatids and euglenids belong (plastids-early hypothesis). This article reviews how euglenid ultrastructural systems are functionally integrated and phylogenetically correlated. I argue that chloroplast acquisition profoundly altered the structure of certain euglenids, and that the complete absence of these modifications in other euglenozoans is most consistent with their never having had a chloroplast. Ultrastructural evidence suggests that chloroplasts arose relatively recently within a specific subgroup of euglenids and that trypanosomatids are not secondarily non-photosynthetic (plastids-recent hypothesis).

Trends in the evolution of the euglenid pellicle

Trends in the evolution of the euglenid pellicle were described using phylogenetic methods on 18S rDNA, morphological, and combined data from 25 mostly phototrophic taxa. The tree topology from a total-evidence analysis formed a template for a synthetic tree that took into account conflicting results derived from the partitioned datasets. Pellicle character states that can only be observed with the assistance of transmission and scanning electron microscopy were phylogenetically mapped onto the synthetic tree to test a set of previously established homology statements (inferences made independently from a cladogram). The results permitted us to more confidently infer the ancestral-derived polarities of character state transformations and provided a framework for understanding the key cytoskeletal innovations associated with the evolution of phototrophic euglenids. We specifically addressed the character evolution of (1) the maximum number of pellicle strips around the cell periphery; (2) the patterns of terminating strips near the cell posterior end; (3) the substructural morphology of pellicle strips; (4) the morphology of the cell posterior tip; and (5) patterns of pellicle pores on the cell surface.

Comparative morphology of the euglenid pellicle. II. Diversity of strip substructure

The morphological diversity associated with the strip substructure of the euglenid pellicle was examined, and after identifying characters and states, we outlined hypotheses about their evolution. We have attempted to standardize terms necessary for analytical comparisons of strips by providing a glossary and comparing published synonyms. Most of the substructural diversity found in euglenids is demonstrated with 13 representative taxa. Strips are generally composed of two subcomponents: frames and projections. Frames support the basic shape of strips and many can be described as either S-shaped, plateau-shaped, M-shaped, or A-shaped. Projections branch laterally from the frames, are usually periodic, and can be described as thread-like structures, an indented plate, tooth-like structures, and plate-like structures. The ancestral state included strips that were few in number, flat, and fused. The strips became S-shaped and disjoined in the lineage leading to most euglenid taxa. These strips became secondarily flattened and fused in one lineage. In some lineages of phototrophs, the strips became increasingly robust. Two strips of different morphology formed the repeating pellicular unit or doublet in four taxa. These doublets evolved convergently at least three times and may provide insights into developmental patterns of the cytoskeleton.

Ultrastructure of the pellicle of Euglena gracilis

Deep-etching technique was used to investigate the organization of the pellicle complex of Euglena gracilis. The interpretation of the images was further supported by SEM and TEM investigations. Our results mainly validate data obtained by previous freeze-fracture studies on the E and P faces of the outer cortical membrane. At the level of the ridges, the outer E fracture face is highly organized in a regular striated pattern, whereas the P inner face shows a particulate structure. However, our images reveal that this particulate organization of the P face is not limited to the ridges, but it is displayed also by the grooves. Moreover, this face shows two distinct layers, a particulate layer facing the cytoplasm and a striated layer facing the E face; these layers represent different true fracture levels of the same P face.

Distribution of F-actin, alpha-actinin, tropomyosin, tubulin and organelles in Euglena gracilis by immunofluorescence microscopy

Euglena gracilis, a unicellular flagellated alga, can display numerous shape changes. These changes are most probably caused by a pellicle and an internal cytoskeleton. In this paper we studied the distribution of the cytoskeletal proteins actin, alpha-actinin, tropomyosin and tubulin in dark-adapted Euglena, using immunofluorescence microscopy. We found that F-actin, alpha-actinin, tropomyosin and tubulin have a distribution that is coincident in the plasma membrane and, in addition, alpha-actinin and tropomyosin are seen in small patches in the cytoplasm, and tubulin in the flagella. We have also studied the distribution of the endoplasmic reticulum, nucleus, and Golgi apparatus of these cells, using fluorescent probes. Both the endoplasmic reticulum and the Golgi apparatus have a meshwork pattern distributed throughout the cytoplasm, and the nucleus has a chromatin evenly distributed in the nucleoplasm.

Cortical structure and function in euglenoids with reference to trypanosomes, ciliates, and dinoflagellates

The membrane skeletal complex (cortex) of euglenoids generates and maintains cell form. In this review we summarize structural, biochemical, physiological, and molecular studies on the euglenoid membrane skeleton, focusing specifically on four principal components: the plasma membrane, a submembrane layer (epiplasm), cisternae of the endoplasmic reticulum, and microtubules. The data from euglenoids are compared with findings from representative organisms of three other protist groups: the trypanosomes, ciliates, and dinoflagellates. Although there are significant differences in cell form and phylogenetic affinities among these groups, there are also many similarities in the organization and possibly the function of their cortical components. For example, an epiplasmic (membrane skeletal) layer is widely used for adding strength and rigidity to the cell surface. The ER/alveolus/amphiesmal vesicle may function in calcium storage and regulation, and in mediating assembly of surface plates. GPI-linked variable surface antigens are characteristic of both ciliates and the unrelated trypanosomatids. Microtubules are ubiquitous, and cortices in trypanosomes may relay exclusively on microtubules and microtubule-associated proteins for maintaining cell form. Also, in agreement with previous suggestions, there is an apparent preservation of many cortical structures during cell duplication. In three of the four groups there is convincing evidence that part or all of the parental cortex persists during cytokinesis, thereby producing mosaics or chimeras consisting of both inherited and newly synthesized cortical components.

Membrane skeletal proteins and their integral membrane protein anchors are targets for tyrosine and threonine kinases in Euglena

Proteins of the membrane skeleton of Euglena gracilis were extensively phosphorylated in vivo and in vitro after incubation with [32P]-orthophosphate or gamma-[32P] ATP. Endogenous protein threonine/serine activity phosphorylated the major membrane skeletal proteins (articulins) and the putative integral membrane protein (IP39) anchor for articulins. The latter was also the major target for endogenous protein tyrosine kinase activity. A cytoplasmic domain of IP39 was specifically phosphorylated, and removal of this domain with papain eliminated the radiolabeled phosphoamino acids and eliminated or radically shifted the PI of the multiple isoforms of IP39. In gel kinase assays IP39 autophosphorylated and a 25 kDa protein which does not autophosphorylate was identified as a threonine/serine (casein) kinase. Plasma membranes from the membrane skeletal protein complex contained threonine/serine (casein) kinase activity, and cross-linking experiments suggested that IP39 was the likely source for this membrane activity. pH optima, cation requirements and heparin sensitivity of the detergent solubilized membrane activity were determined. Together these results suggest that protein kinases may be important modulators of protein assembly and function of the membrane skeleton of these protistan cells.

Identification of the epiplasmins, a new set of cortical proteins of the membrane cytoskeleton in Paramecium

In most ciliates, the epiplasm, a superficial cytoskeletal layer of variable thickness, both surrounds basal bodies and interacts tightly with adjacent membrane networks; it constitutes the predominant structure in Paramecium cell ghosts. Previous indirect data suggested several cortical proteins as potential constituents of the epiplasm. New sharp monoclonal antibodies presented in this paper, positive both on immunotransfers and in immunocytochemical tests carried out on permeabilized cells and ultrathin sections, definitively identify the epiplasmins: a set of about twenty protein bands ranging from 45 to 33 kDa and making up the bulk of the epiplasmic layer. The complete epiplasmin pattern characterized from gradient-purified cortex is also present in unfractionated whole cells, confirming that the pattern is not generated artifactually. Comparative one-step extractions, performed either in 1 M KI or in 4 M urea, solubilize the epiplasmins as a whole, indicating that all of them share very similar biochemical properties. Two-dimensional electrophoresis shows the great complexity of this epiplasmin group. Epiplasmin solubilization properties are discussed with respect to other models of membrane-cytoskeleton interaction developed among protists and metazoans and also to intermediate filaments, specially lamins. Immunofluorescent labelling combined with confocal microscopy permits a more detailed study of epiplasm formation at the level of the fission furrow, with new insights into two successive steps of epiplasm growth. A first series of interspecific reactions has been carried out with one of the anti- epiplasmin antibodies, yielding results which are discussed in an evolutionary framework.

Properties and topography of the major integral plasma membrane protein of a unicellular organism

The cellular distribution, membrane orientation, and biochemical properties of the two major NaOH-insoluble (integral) plasma membrane proteins of Euglena are detailed. We present evidence which suggests that these two polypeptides (Mr 68 and 39 kD) are dimer and monomer of the same protein: (a) Antibodies directed against either the 68- or the 39-kD polypeptide bind to both 68- and 39-kD bands in Western blots. (b) Trypsin digests of the 68- and 39-kD polypeptides yield similar peptide fragments. (c) The 68- and 39-kD polypeptides interconvert during successive electrophoresis runs in the presence of SDS and beta-mercaptoethanol. (d) The 39-kD band is the only major integral membrane protein evident after isoelectric focusing in acrylamide gels. The apparent shift from 68 to 39 kD in focusing gels has been duplicated in denaturing SDS gels by adding ampholyte solutions directly to the protein samples. The membrane orientation of the 39-kD protein and its 68-kD dimer has been assessed by radioiodination in situ using intact cells or purified plasma membranes. Putative monomers and dimers are labeled only when the cytoplasmic side of the membrane is exposed. These results together with trypsin digestion data suggest that the 39-kD protein and its dimer have an asymmetric membrane orientation with a substantial cytoplasmic domain but with no detectable extracellular region. Immunolabeling of sectioned cells indicates that the plasma membrane is the only cellular membrane with significant amounts of 39-kD protein. No major 68- or 39-kD polypeptide bands are evident in SDS acrylamide gels or immunoblots of electrophoresed whole flagella or preparations enriched in flagellar membrane vesicles, nor is there a detectable shift in any flagellar polypeptide in the presence of ampholyte solutions. These findings are considered with respect to the well-known internal crystalline organization of the euglenoid plasma membrane and to the potential for these proteins to serve as anchors for membrane skeletal proteins.

The membrane skeleton of a unicellular organism consists of bridged, articulating strips

In this paper we show that a membrane skeleton associated with the plasma membrane of the unicellular organism Euglena consists of approximately 40 individual S-shaped strips that overlap along their lateral margins. The region of strip overlap is occupied by a set of microtubule-associated bridges and microtubule-independent bridges. Both cell form and plasma membrane organization are dependent on the integrity of this membrane skeleton. Removal of the membrane skeleton with a low-molar base results in loss of membrane form and randomization of the paracrystalline membrane interior characteristic of untreated cells. Conversely, removal of the plasma membrane and residual cytoplasm with lithium 3,5-diiodosalicylate/Nonidet P-40 yields cell ghosts that retain the form of the original cell but consist only of the membrane skeleton. Two major polypeptides of 86 and 80 KD persist in the skeleton and two other major proteins of 68 and 39 kD are associated with the plasma membrane fraction. None of these components appears to be the same as the major polypeptides (spectrins, band 3) of the erythrocyte ghost, the other cell system in which a well-defined peripheral membrane skeleton has been identified. We suggest that the articulating strips of euglenoids are not only the basic unit of cell and surface form, but that they are also positioned to mediate or accommodate surface movements by sliding, and to permit surface replication by intussusception.

Endogenous glycosyltransferases glucosylate lipids in flagella of Euglena

Flagella, intact deflagellated cells and isolated cell surfaces of the unicell , Euglena were separately assayed for glycosyltransferase activity by incubating these fractions with uridine diphosphate-[3H]glucose and isolating radiolabeled products. Most of the label was incorporated into lipophilic products, soluble in chloroform/methanol, which could be separated via thin layer chromatography or LH-60 chromatography into four distinct classes. The most polar of these products was extracted from flagella and purified by column chromatography for use as an in vitro substrate to identify flagella-associated glycosyltransferases. After flagella were treated with the detergent CHAPS , a soluble fraction was removed that was capable of glycosylation in solution. The glycosyltransferase(s) responsible for this activity were further enriched on sucrose or fructose gradients and ultimately identified on acrylamide gels through the combined use of nondenaturing gels, dial-[3H]uridine diphosphate binding, and fluorography. The enzyme had an apparent monomer molecular weight of 32,000 and consisted of four or fewer subunits. The occurrence of endogenous glycosyltransferase(s) in flagella suggests that modifications and/or assembly of the flagella surface can take place in situ in this organism.

Disassembly and reconstitution of a membrane-microtubule complex

The cell membrane of the unicellular algae Distigma proteus is associated with arrays of parallel microtubules. Fragments of the membrane-microtubule complex have been isolated and partially purified. The microtubules were stable in vitro at room temperature as well as at 0 degree C, but were specifically and rapidly disassembled by Ca2+. After removal of all endogenous microtubules, the membrane-microtubule complex could be reassembled from brain microtubule protein and denuded Distigma membrane fragments. The readded microtubules bound in a fixed orientation, and only to those regions of membrane that are normally associated with microtubules in vivo.

Three-dimensional structure of a membrane-microtubule complex

The unicellular algae Distigma proteus contain a group of aligned microtubules associated with their cell membrane. The association is maintained in isolated membrane fragments. The membrane-microtubule complex also includes a crystalline array of membrane particles. The major peptide component of this array was identified by labeling whole cells with radioiodine. The entire complex of membrane, particles, and microtubules is sufficiently well ordered to permit reconstruction from electron micrographs by Fourier techniques. A three-dimensional model of the membrane array at a nominal resolution of 2.5 nm has been calculated. Some similarities were apparent between lattice spacings in the membrane array and in microtubules. Analysis of these lattice correlations suggests a way in which the array of membrane particles may serve as scaffolding for microtubule attachment.

Regulation of Cell Shape in Euglena gracilis: I. Involvement of the Biological Clock, Respiration, Photosynthesis, and Cytoskeleton

The alga Euglena gracilis Z. changes its shape two times per day when grown under the synchronizing effect of a daily light-dark cycle. At the beginning of the light period when photosynthetic capacity is low, the population of cells is largely spherical in shape. The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. The population becomes spherical by the end of the 24-hour period when the cycle reinitiates. These changes are also observed under constant dim light conditions (up to 72 hours) and are therefore controlled by the biological clock and represent a circadian rhythm in cell shape. In constant dim light, the cell division rhythm is either arrested or slowed considerably, while the cell shape rhythm continues.The involvement of respiratory and photosynthetic pathways in the cell shape changes was investigated with energy pathway inhibitors. Antimycin A and NaN(3) both inhibited the round to long and long to round shape changes, indicating that the respiratory pathways are involved. DCMU and atrazine inhibited the round to long shape change but did not affect the long to round transition, indicating that light-induced electron flow is necessary only for the round to long shape change.The influence of the cell shape changes on the photosynthetic reactions was investigated by altering cell shape with the cytoskeletal inhibitors cytochalasin and colchicine. Both inhibitors blocked the round to long and long to round shape changes. Cytochalasin B was found to have minimal cytotoxic effects on the photosynthetic reactions, but colchicine significantly inhibited light-induced electron flow and the in vivo expression of the photosynthetic rhythm.

Flagellar surface antigens in Euglena: immunological evidence for an external glycoprotein pool and its transfer to the regenerating flagellum

Antibodies raised against the Sarkosyl-insoluble, major flagellar glycoprotein fraction, mastigonemes, were used to determine the source of flagellar surface glycoproteins and to define the general properties of flagellar surface assembly in Euglena. After suitable absorption, mastigoneme antiserum reacts with several specific mastigoneme glycoproteins but does not bind either to the other major flagellar glycoprotein, xyloglycorien, or to other Sarkosyl-soluble flagellar components. When Fab' fragments of this mastigoneme-specific antiserum were used in combination with a biotin-avidin secondary label, antigen was localized not only on the flagellum as previously described but also in the contiguous reservoir region. If deflagellated cells are reservoir pulse-labeled with Fab' antibody, this antibody appears subsequently on the newly regenerated flagellum. This chased antibody is uniformly distributed throughout the length of the flagellum and shows no preferred growth zone after visualization with either fluorescein or ferritin-conjugated secondary label. From these and tunicamycin inhibition experiments it is concluded that (a) a surface pool of at least some flagellar surface antigens is present in the reservoir membrane adjacent to the flagellum and that (b) the reservoir antigen pool is transferred to the flagellar surface during regeneration.

Synthesis and mobilization of flagellar glycoproteins during regeneration in Euglena

Flagellar glycoprotein synthesis and mobilization of flagellar glycoprotein pools have been followed during flagellar regeneration in Euglena. The glycosylation inhibitor tunicamycin has little effect on either regeneration kinetics or the complement of flagellar peptides as seen in SDS acrylamide gels, but tunicamycin totally inhibits incorporation of exogenously supplied [14C]xylose into flagellar glycoproteins. Moreover, deflagellated cells pulsed with tunicamycin for 0 min or more, regenerated for 180 min, and then redeflagellated are completely or partially inhibited from undergoing a second regeneration even when tunicamycin is no longer present. These facts are interpreted as indicating that Euglena retains sufficient glycoprotein pool for one complete flagellar assembly. Some of this pool is present on the cell surface since [125I]-labeled surface peptides can be chased into the regenerating flagellum. Glycosylation may also be taking place in the flagellum directly because [14C]xylose has been found in three flagellar fractions: glycoprotein and two others, which are lipophilic and have properties similar to those described for lipid-carrier glycoprotein intermediates in other systems. Pulse-chase experiments also suggest a precursor-product relationship between the presumptive lipid carriers and flagellar glycoproteins. From these results a model is postulated in which Euglena is visualized as retaining sufficient pool of glycoprotein for one complete flagellar regeneration, but the pool is normally supplemented by active xylosylation in situ during regeneration.

Characterization and localization of a flagellar-specific membrane glycoprotein in Euglena

Purified flagella from Euglena yield a unique high molecular weight glycoprotein when treated with low concentrations of nonionic detergents. This glycoprotein termed "xyloglycorien" cannot be extracted from other regions of the cell, although a minor component that coextracts with xyloglycorien does have a counterpart in deflagellated cell bodies. Xyloglycorien is tentatively identified with a flagellar surface fuzzy layer that appears in negatively stained membrane vesicles of untreated flagella but not in similar vesicles after Nonidet P-40 extraction. The localization of xyloglycorien is further confirmed to be membrane associated by reciprocal extraction experiments using 12.5 mM lithium diiodosalicylate (LIS), which does not appreciably extract xyloglycorien, visibly solubilize membranes, or remove the fuzzy layer. Rabbit antibodies directed against the two major flagellar glycoproteins (xyloglycorien and mastigonemes) to some extent cross react, which may in part be caused by the large percentage of xylose found by thin-layer chromatography (TLC) analysis to be characteristic of both antigens. However, adsorption of anti-xyloglycorien sera with intact mastigonemes produced antibodies responding only to xyloglycorien, and vice versa, indicating the nonidentity of the two antigens. Antibodies or fragments of these antibodies used in immunofluorescence assays demonstrated that xyloglycorien is confined to the flagellum and possibly the adjacent reservoir and gullet. Binding could not be detected on the cell surface. The sum of these experiments suggests that, in addition to mastigonemes, at least one major membrane glycoprotein in Euglena is restricted to the flagellar domain and is not inserted into the contiguous cell surface region.

Formation and positioning of surface-related structures in protozoa

Images

Surface organization and composition of Euglena. II. Flagellar mastigonemes

The surface of the Euglena flagellum is coated with about 30,000 fine filaments of two distinct types. The longer of these nontubular mastigonemes (about 3 micron) appear to be attached to the paraflagellar rod whereas the shorter nontubular mastigonemes (about 1.5 micron) are the centrifugally arranged portions of a larger complex, which consists of an attached unit parallel to and outside of the flagellar membrane. Units are arranged laternally in near registration and longitudinally overlap by one-half of a unit length. Rows of mastigoneme units are firmly attached to the axoneme microtubules or to the paraflagellar rod as evidenced by their persistence after removal of the flagellar membrane with neutral detergents. SDS-acrylamide gels of whole flagella revealed about 30 polypeptides, of which two gave strong positive staining with the periodic acid-Schiff (PAS) procedure. At least one of these two bands (glycoproteins) has been equated with the surface mastigonemes by parallel analysis of isolated and purified mastigonemes, particularly after phenol extraction. The faster moving glycoprotein has been selectively removed from whole flagella and from the mastigoneme fraction with low concentrations of neutral detergents at neutral or high pH. The larger glycoprotein was found to be polydisperse when electrophoresed through 1% agarose/SDS gels. Thin-layer chromatography of hydrolysates of whole flagella or of isolated mastigonemes has indicated that the major carbohydrate moiety is the pentose sugar, xylose, with possibly a small amount of glucose and an unknown minor component.

Biochemical approaches to problems of cellular patterning

The expression of intracellular patterning is perhaps nowhere more impressive than in the arrangements of structural elements associated with the cell surface in protozoa. The view is proposed that biochemical studies of protozoan plasma membranes and associated surface structures represent important contributions of potential significance for the understanding of the perpetuation and expression of positional information at the intracellular level. Some recent work dealing with the isolation, identification, and metabolism of pellicular proteins in Tetrahymena is presented and discussed. Some integral membrane proteins have been identified by iodination and polyacrylamide gel electrophoresis. Labeling studies suggest heterogeneous turnover rates within the group of presently identified membrane proteins. High molecular weight proteins with some similarity to spectrin have been isolated from Tetrahymena epiplasm. It is suggested that the ciliate epiplasm is one example of membrane-associated, actomyosin-like systems found in a variety of cell types. The epiplasm may play a role in the positioning of surface-associated structures and in the control of cell shape.