The Golgi apparatus of the scaly green flagellate Scherffelia dubia: uncoupling of glycoprotein and polysaccharide synthesis during flagellar regeneration

The flagella of the green alga Scherffelia dubia are covered by scales which consist of acidic polysaccharides and glycoproteins. Experimental deflagellation results in the regeneration of flagella complete with scales. During flagellar regeneration, scales are newly synthesized in the Golgi apparatus, exocytosed and deposited on the growing flagella. Flagellar regeneration is dependent upon protein synthesis and N-glycosylation, as it is blocked by cycloheximide and partially inhibited by tunicamycin. Metabolic labeling with [³⁵S]methionine/cysteine demonstrated that scale-associated proteins were not newly synthesized during flagellar regeneration, suggesting that the proteins deposited on regenerating flagella were drawn from a pool. Quantitative immunoelectron microscopy using a monospecific antibody directed against a scale-associated protein of 126 kDa (SAP126) revealed that the pool of SAP126 was primarily located at the plasma membrane, with minor labeling of the scale reticulum and trans-Golgi cisternae, both before deflagellation and during flagellar regeneration. Since SAP126 was sequestered during flagellar regeneration into secretory vesicles together with newly synthesized scales, it is concluded that the persistent presence of SAP126 in the trans-Golgi cisternae during scale biogenesis requires retrograde transport of the protein from the plasma membrane to the Golgi apparatus. Received: 3 July 1999 / Accepted: 21 August 1999     

Flagellar regeneration in the scaly green flagellateTetraselmis striata (Prasinophyceae): regeneration kinetics and effect of inhibitors

Flagellar regeneration after experimental amputation was studied in synchronized axenic cultures of the scaly green flagellateTetraselmis striata (Prasinophyceae). After removal of flagella by mechanical shearing, 95% of the cells regrow all four flagella (incl. the scaly covering) to nearly full length with a linear velocity of 50 nm/min under standard conditions. Flagellar regeneration is independent of photosynthesis (no effect of DCMU; the same regeneration rate in the light or in the dark), but depends on de novo protein synthesis: cycloheximide at a low concentration (0.35 μM) blocks flagellar regeneration reversibly. No pool of flagellar precursors appears to be present throughout the flagellated phase of the cell cycle. A transient pool of flagellar precursors, sufficient to generate 2.5 μm of flagellar length, however, develops during flagellar regeneration. Tunicamycin (2 μg/ml) inhibits flagellar regeneration only after a second flagellar amputation, when flagella reach only one third the length of the control. Flagellar regeneration inT. striata differs considerably from that ofChlamydomonas reinhardtii and represents an excellent model system for the study of synchronous Golgi apparatus (GA) activation, and transport and exocytosis of GA-derived macromolecules (scales).     

A Role for the Membrane in Regulating Chlamydomonas Flagellar Length

Flagellar assembly requires coordination between the assembly of axonemal proteins and the assembly of the flagellar membrane and membrane proteins. Fully grown steady-state Chlamydomonas flagella release flagellar vesicles from their tips and failure to resupply membrane should affect flagellar length. To study vesicle release, plasma and flagellar membrane surface proteins were vectorially pulse-labeled and flagella and vesicles were analyzed for biotinylated proteins. Based on the quantity of biotinylated proteins in purified vesicles, steady-state flagella appeared to shed a minimum of 16% of their surface membrane per hour, equivalent to a complete flagellar membrane being released every 6 hrs or less. Brefeldin-A destroyed Chlamydomonas Golgi, inhibited the secretory pathway, inhibited flagellar regeneration, and induced full-length flagella to disassemble within 6 hrs, consistent with flagellar disassembly being induced by a failure to resupply membrane. In contrast to membrane lipids, a pool of biotinylatable membrane proteins was identified that was sufficient to resupply flagella as they released vesicles for 6 hrs in the absence of protein synthesis and to support one and nearly two regenerations of flagella following amputation. These studies reveal the importance of the secretory pathway to assemble and maintain full-length flagella.     

A STUDY OF CELL WALL AND FLAGELLA FORMATION DURING CELL DIVISION IN THE SCALY GREEN ALGA,SCHERFFELIA DUBIA (CHLOROPHYTA)1

The thecate green flagellate Scherffelia dubia (Perty) Pascher divides within the parental cell wall into two progeny cells. It sheds all four flagella before cell division, and the maturing progeny cells regenerate new walls and flagella. By synchronizing cell division, we observed mitosis, cytokinesis, cell maturation, flagella extension, and cell wall formation via differential interference contrast microscopy of live cells and serial thin‐section EM. Synthesis of thecal and flagellar scales is spatially and temporally strictly separated. Flagellar scales are collected in a pool during late interphase. Before prophase, Golgi stacks divide, flagella are shed, the parental theca separates from the plasma membrane, and flagellar scales are deposited on the plasma membrane near the flagellar bases. At prophase, Golgi bodies start to synthesize thecal scales, continuing into interphase after cytokinesis. During cytokinesis, vesicles containing thecal scales coalesce near the cell posterior, forming a cleavage furrow that is initially oriented slightly diagonal to the longitudinal cell axis but later becomes transverse. After the progeny nuclei have moved into opposite directions, resulting in a “head to tail” orientation of the progeny cells, theca biogenesis is completed and flagellar scale synthesis resumes. Progeny cells emerge through a hole near the posterior end of the parental theca with four flagella of about 8 μm long. The precise timing of flagellar and thecal scale synthesis appears to be an evolutionary adaptation in a scaly green flagellate for the thecal condition, necessary for the evolution of the phycoplast and thus multicellularity in the Chlorophyta.     

Flagellar Development During the Cell Cycle in Chlamydomonas reinhardtii

Flagellar development during the asexual synchronous cell cycle of Chlamydomonas reinhardtii (11.32 aM) was studied by light microscopy. Cell walls of sporangia of different developmental status were dissolved using gamete lysin (g-lysin) enabling direct observation of flagellar development. Flagellar growth in progeny cells exhibits a linear kinetic with a growth rate of 28 nm/min at 30°C leading to a flagellar length of 7–7.5 μm in 4–4.5 h. After this time the flagellar growth rate drops to 2.8 nm/min (as in interphase). Both flagella of a single cell and all flagella within a sporangium grow out at the same time and with the same rate. Cycloheximide (10 μg/ml) completely blocks flagellar development. If cycloheximide is removed flagellar growth resumes at the normal rate with no lag-phase. Flagellar development during the cell cycle in C. reinhardtii differs considerably from the well-studied model system of flagellar regeneration following amputation in the same species.     

Flagellar regeneration and associated scale deposition inPyramimonas gelidicola (Prasinophyceae,Chlorophyta)

Summary During regeneration of mechanically amputated flagella, flagellar scales and the subtending membrane accumulate in a villiform scale reservoir in which the scales interact to form patterns on the villi reminiscent of the arrangement they later assume on the flagellum. The reservoir membrane is continuous with the plasmalemma, and the scales, attached directly and indirectly to the membrane, leave the reservoir and migrate toward the developing flagella where they assemble into highly ordered layers. It is proposed that scale-scale interactions induce a process of auto-assembly initiating the complex arrangement of scale tiers on the flagellum and cell body.     

Golgi apparatus activity and membrane flow during scale biogenesis in the green flagellateScherffelia dubia (Prasinophyceae). II: Cell wall secretion and assembly

Summary Secretion of the cell wall (theca) in the scaly green flagellateScherffelia dubia (Prasinophyceae) has been examined by electron microscopy during cytokinesis. The bi-laminate wall forms by the extracellular amalgamation of two layers of scales produced in the Golgi apparatus (GA). Each mature GA cisterna contains ca. 12,000 scales of two distinct varieties arranged in two layers on the cisternal membrane. GA cisternae undergo turnover and one scale containing cisterna matures from thetransface of each dictyosome every 3–4 minutes. Cisternae then fuse with the plasma membrane at the anterior end of the cell releasing the scales onto the cell surface. The two layers of wall scales integrate on the cell surface in a time-dependent self-assembly process. The first scales deposited commence assembly at the cell posterior and the wall develops anteriorly by edge growth. The daughter cell wall is composed of ca. 1.2 million scales deposited in about 3 hours. Calculations of net membrane flow strongly indicate extensive endocytosis during wall deposition.     

Scale biogenesis in the green alga,Mesostigma viride

Summary The unicellular green algal flagellate,Mesostigma viride, is characterized by an extracellular matrix of multiple layers of scales. These scales are processed within the Golgi apparatus (GA). The GA consists of 11–13 closely stacked cisternae. The cis cisternae are highly fenestrated and grow via vesicles from adjacent transition ER. Medial-trans cisternae are plate-like with swollen peripheries. The calcified basket scales are produced in the peripheries of GA cisternae, usually first observable in the medial zone of the cisternal stack. Cisternal membrane closely conforms to the precise architecture of the developing scale. Antimonate labeling reveals that a population of smooth cytoplasmic vacuoles situated near the GA contains a store of calcium, perhaps used for scale processing. Vesicles carry calcium from these vacuoles to the cisternal loci where basket scale ontogenesis is occurring. The smaller scale types are produced within the central areas of the GA. A discussion concerning membrane flow through the GA is provided.     

A new 'paralyzed' flagella mutant, OC-10, in Chlamydomonas reinhardtii (Volvocales, Chlorophyceae) that can be reactivated with adenosine triphosphate

A new ‘paralyzed’ mutant. OC–10, was isolated in Chlamydomonas reinhardtii Dangeard. OC-10 cannot swim and generally shows very little flagellar movement. However, when OC-10 was demembranated, axonemal motility was reactivated in the presence of adenosine triphosphate (ATP) or adenosine diphosphate (ADP). The beating form of the reactivated axonemes was almost the same as that of the wild-type axonemes. Flagellar regeneration of OC-10 was slower than that of the wild-type. Electron microscopic examination showed no abnormality in OC-10 flagella, but SDS/PAGE revealed that mobility of a flagellar membrane protein was changed and a few bands disappeared in OC-10 flagella, When the mutant was crossed to wild-type to form temporary dikaryon cells with 4 flagella, OC-10 flagella did not regain motility. Tetrad analysis of crosses between OC–10 and wild-type demonstrated a 1:1 segregation on the basis of flagellar motility. From these results, we suppose that OC-10 may be limited in ATP availability inside the flagella, or altered in flagellar membrane proteins important for motility.     

Concentration of amylase along its secretory pathway in the pancreatic acinar cell as revealed by high resolution immunocytochemistry

Summary The modified protein A-gold immunocytochemical technique was applied to the localization of amylase in rat pancreatic acinar cells. Due to the good ultrastructural preservation of the cellular organelles obtained on glutaraldehyde-fixed, osmium tetroxide-postfixed tissue, the labelling was detected with high resolution over the cisternae of the rough endoplasmic reticulum (RER), the Golgi apparatus, the condensing vacuoles, the immature pre-zymogen granules, and the mature zymogen granules. Over the Golgi area, the labelling was present over the transitional elements of the endoplasmic reticulum, some of the smooth vesicular structures at thecis- andtrans-faces and all the different Golgi cisternae. The acid phosphatase-positive rigidtrans-cisternae as well as the coated vesicles were either negative or weakly labelled. Quantitative evaluations of the degree of labelling demonstrated an increasing intensity which progresses from the RER, through the Golgi, to the zymogen granules and have identified the sites where protein concentration occurs. The results obtained have thus demonstrated that amylase is processed through the conventional RER-Golgi-granule secretory pathway in the pancreatic acinar cells. In addition a concomitance has been found between some sites where protein concentration occurs: thetrans-most Golgi cisternae, the condensing vacuoles, the pre- and the mature zymogen granules, and the presence of actin at the level of the limiting membranes of these same organelles as reported previously (Bendayan, 1983). This suggests that beside their possible role in transport and release of secretory products, contractile proteins may also be involved in the process of protein concentration.     

The flagellar developmental cycle in algae: flagellar transformation inCyanophora paradoxa (Glaucocystophyceae)

Summary Flagellar development during cell division was studied inCyanophora paradoxa using agarose-embedded cells, Nomarski optics and electronic flash photography. The cells bear two heterodynamic and differently oriented (anterior and posterior) flagella. Prior to cell division, cells produce two new anterior flagella while the parental anterior flagellum transforms into a posterior flagellum. The parental posterior flagellum remains a posterior flagellum throughout this and subsequent cell divisions. The development of a single flagellum thus extends through at least two cell cycles and flagellar heterogeneity is achieved by semiconservative distribution of the flagella during cell division. Based on these principles a universal numbering system for basal bodies and flagella of eukaryotic cells is proposed.     

Flagellar transformation in the heterokontEpipyxis pulchra (Chrysophyceae): Direct observations using image enhanced light microscopy

Summary Cells ofEpipyxis pulchra possess two heteromorphic flagella that differ markedly in function, particularly during motility and prey capture. Flagellar heterogeneity is achieved during the course of at least three cell cycles. Prior to cell division, cells produce two new long, hairy flagella while the parental long flagellum is transformed into a new short, smooth flagellum. The parental short flagellum remains a short flagellum for this and subsequent cell division cycles. Although flagellar transformation requires only two cell cycles, developmental differences exist between daughter cells and the maturation of a flagellum/basal body requires at least three cycles.     

Ultrastructural analysis of flagellar development in plurilocular sporangia of Ectocarpus siliculosus (Phaeophyceae)

Flagellar development in the plurilocular zoidangia of sporophytes of the brown alga Ectocarpus siliculosus was analyzed in detail using transmission electron microscopy and electron tomography. A series of cell divisions in the plurilocular zoidangia produced the spore-mother cells. In these cells, the centrioles differentiated into flagellar basal bodies with basal plates at their distal ends and attached to the plasma membrane. The plasma membrane formed a depression (flagellar pocket) into where the flagella elongated and in which variously sized vesicles and cytoplasmic fragments accumulated. The anterior and posterior flagella started elongating simultaneously, and the vesicles and cytoplasmic fragments in the flagellar pocket fused to the flagellar membranes. The two flagella (anterior and posterior) could be clearly distinguished from each other at the initial stage of their development by differences in length, diameter and the appendage flagellar rootlets. Flagella continued to elongate in the flagellar pocket and maintained their mutually parallel arrangement as the flagellar pocket gradually changed position. In mature zoids, the basal part of the posterior flagellum (paraflagellar body) characteristically became swollen and faced the eyespot region. Electron dense materials accumulated between the axoneme and the flagellar membrane, and crystallized materials could also be observed in the swollen region. Before liberation of the zoospores from the plurilocular zoidangia, mastigoneme attachment was restricted to the distal region of the anterior flagellum. Structures just below the flagellar membrane that connected to the mastigonemes were clearly visible by electron tomography.     

Structure,composition, and biogenesis of prasinophyte cell coverings

Summary The cell body and flagellar surfaces of certain green flagellates are covered by non-mineralized scales. Scale structure has been widely used in the systematics of this group of algae commonly known as the Prasinophyceae. The special importance of the flagellar hairs as a taxonomic marker is discussed. We summarize current knowledge about the structure and chemical composition of these scales with emphasis on thecate flagellates. Scales consist mainly of acidic polysaccharides involving unusual 2-keto sugar acids. Glycoproteins as minor components are mainly involved in mediating scale subunit and scale-membrane interactions and species specific glycosylation patterns exist. In thecate prasinophytes the elaboration of 3-deoxy-manno-2-octulosonic acid and galacturonic acid side chains presumably favours a complex of thecal scales with calcium ions and thus extracellular coalescence of the scales to a rigid cell wall. Scales are formed within the Golgi apparatus (GA) and especially in thecate prasinophytes scale formation (i.e., during flagellar regeneration) represents an excellent model system for GA function. Movement of developing scales through the GA requires cisternal progression. Biogenesis of scales involves mainly polysaccharide synthesis, whereas about 50% of the scale-associated glycoproteins are added from a pre-existing pool. Possible functions of prasinophyte scales are briefly discussed.Abbreviations Dha 3-deoxy-lyxo-2-heptulosaric acid - DSA Datura stramonium agglutinin - ER endoplasmic reticulum - GA Golgi apparatus - GNA Galanthus nivalis agglutinin - Kdo 3-deoxy-manno-2-octulosonic acid - MeKdo 3-deoxy-5-O-methyl-manno-2-octulosonic acid - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis     

Biosynthetic protein transport through the early secretory pathway

 Newly synthesized proteins destined for delivery to the cell surface are inserted cotranslationally into the endoplasmic reticulum (ER) and, after their correct folding, are transported out of the ER. During their transport to the cell surface, cargo proteins pass through the various cisternae of the Golgi apparatus and, in the trans-most cisternae of the stack, are sorted into constitutive secretory vesicles that fuse with the plasma membrane. Simultaneously with anterograde protein transport, retrograde protein transport occurs within the Golgi complex as well as from the Golgi back to the ER. Vesicular transport within the early secretory pathway is mediated by two types of non-clathrin coated vesicles: COPI- and COPII-coated vesicles. The formation of these carrier vesicles depends on the recruitment of cytosolic coat proteins that are thought to act as a mechanical device to shape a flattened donor membrane into a spherical vesicle. A general molecular machinery that mediates targeting and fusion of carrier vesicles has been identified as well. Beside a general overview of the various coat structures known today, we will discuss issues specifically related to the biogenesis of COPI-coated vesicles: (1) a possible role of phospholipase D in the formation of COPI-coated vesicles; (2) a functional role of a novel family of transmembrane proteins, the p24 family, in the initiation of COPI assembly; and (3) the direction COPI-coated vesicles may take within the early secretory pathway. Moreover, we will consider two alternative mechanisms of protein transport through the Golgi stack: vesicular transport versus cisternal maturation. Accepted: 24 October 1997     

Vesicle production and fusion during lobe formation inMicrasterias visualized by high-pressure freeze fixation

Summary Two different types of Golgi vesicles involved in wall formation can be visualized during lobe growth inMicrasterias when using high-pressure freeze fixation followed by freeze substitution. One type that corresponds to the dark vesicles (DV) described in literature seems to arise by a developmental process occurring at the Golgi bodies with the single vesicles being forwarded from one cisterna to the next. The other vesicle type appears to be produced at thetrans Golgi network without any visible precursors at the Golgi cisternae. A third type of vesicle, produced by median andtrans cisternae, contains slime; these are considerably larger than those previously mentioned and they do not participate in wall formation. The distribution of the two types of cell wall vesicles at the cell periphery and their fusion with the plasma membrane are shown for the first time, since chemical fixation is too slow to preserve a sufficient number of vesicles in the cortical cytoplasm. The results indicate that fusions of both types of vesicles with the plasma membrane are possible all over the entire surface of the growing half cell. However, the DVs are much more concentrated at the growing lobes, where they form queues several vesicles deep behind zones on the plasma membrane thought to be specific fusion sites. The structural observations reveal that the regions of enhanced vesicle fusion correspond in general to the sites of calcium accumulation determined in earlier studies. By virtue of the absence of the DVs in the region of cell wall indentations the second type of wall forming vesicle appears prominent; they too fuse with the plasma membrane and discharge their contents to the wall.     

The golgi-mediated scale production in the marine haptophycean alga Ochrosphaera neapolitana Schussnig

Abstract

Some ultrastructural features of cells of the marine haptophycean alga, Ochrosphaera neapolitana Schussnig in the palmelloid stage were examined. Chloroplasts which are contained in a compartment isolated from the cytoplasm by ER profiles and nuclear envelope, display trilamellated thylakoids running along the major axis. The stalked pyrenoid with inner bilamellated thylakoids, protrudes in a large membrane-bounded vacuole. Other structures, as the haptonematic and flagellar bases, autophagic vacuoles and mitochondria, are typical of the chrysophycean and haptophycean genera so far investigated.

The Golgi apparatus is represented by a single dictyosome composed of stacked cisternae fonctioning in a way that they form organic scales which constitute the main part of the cell covering. The scales, build up of microfibrils disposed parallel each to other, lie in cisternal lumina of the dictyosomal maturing face; scaly cisternae are numerous in the peripheral cytoplasm and are observed merging in the plasma membrane and discharging the content outside the protoplast.

Dictyosomal activity is evidenced morphologically by massive vesicle production. Three kinds of membrane-bounded vesicles were identified in the present material: i) inner-granulated vesicles, arising from the maturation face; ii) coated vesicles, scattered in the cytoplasm or at the periphery of the golgi body, and iii) dense-cored vesicles, present in the proximity of the maturation face. The possible functional relationships related to scale production and assembly outside the protoplast, and between the nucleus and dictyosome are discussed.     

Isolation,purification, and characterization of flagellar scales from the green flagellateTetraselmis striata (Prasinophyceae)

Summary Flagellar scales from the green flagellateTetraselmis striata (Prasinophyceae) were isolated, purified by isopycnic cesium chloride-gradient and zonal sucrose gradient centrifugation and their structure and biochemical composition investigated. Three types of flagellar scales were purified to more than 90% purity, a fourth type up to 75% purity. In addition to the previously known types of flagellar scales (pentagonal scales, rod-shaped scales, hair-scales), a novel scale type (i.e., the knotted scales) was discovered. New information about the asymmetric structure of the rod-shaped scales is presented and consequently they are renamed man scales. Flagellar scales consist mainly of carbohydrate (50–70%), significant amounts of protein (11% of dry weight) were found only in pentagonal scales. The main sugars (90%) of the pentagonal and man scales are the unusual 2-keto-sugar acids 3-deoxy-5-O-methyl-2-octulosonic acid (5 OMeKDO), 3-deoxy-2-heptulosaric acid (DHA), and 3-deoxy-2-octulosonic acid (KDO), the knotted scales contain as major sugars galactose and arabinose in addition to KDO and 5 OMeKDO but lack DHA. 13 major polypeptides were identified in flagellar scales by one-dimensional SDS-PAGE, 11 of these are of high molecular mass (>116 kDa). While the majority of polypeptides was found associated with pentagonal scales, at least 4 polypeptides were tentatively assigned to the hair-scales and knotted scales.Abbreviations CSF crude scale fraction - PS pentagonal scales - MS man scales - HS hair-scales - KS knotted scales - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - DHA 3-deoxylyxo-2-heptulosaric acid - 5 OMeKDO 3-deoxy-5-O-methyl-manno-2-octulosonic acid - KDO 3-deoxy-manno-2-octulosonic acid - GA Golgi apparatus     

Effect of divalent cations on flagellar scales in the green flagellateTetraselmis cordiformis

Summary The structure and topography of flagellar scales (underlayer scales, rodshaped scales, hair-scales) in the green flagellateTetraselmis cordiformis has been studied in detail and the effect of divalent cations and fixation conditions on scale structure and topography was followed quantitatively. Hair-scales occur in two rows on opposite sides of a flagellum and are linked to the flagellar membrane and to two axonemal doublets by B-tubule-flagellar membrane connectives. Underlayer scales form about 24 longitudinal rows along the flagellum and occur in two distinctive shapes (pentagonal and square). The square shaped underlayer scales are related in position to the attachment sites of the hair-scales. Rod-shaped scales occur in about 20 longitudinal rows along the flagellum and are characteristically positioned as double scales. Calcium in the culture medium is necessary to retain rod-shaped scales on the flagellum, absence of calcium or chelation with EGTA or pyrophosphate leads to disappearance of rod-shaped scales from the flagellum. Other divalent cations can only partially substitute for calcium. It is suggested that calcium provides the linkage between underlayer scales and rod-shaped scales inTetraselmis. Flagellar scales inTetraselmis apparently fall into two categories: a) hair-scales (not affected by fixation or absence of divalent cations, firmly bound to axonemal microtubules via the flagellar membrane), b) underlayer scales and rod-shaped scales (affected by fixation and absence of divalent cations, kept on the flagellum mainly by electrostatic forces). The function of flagellar scales inTetraselmis is discussed.