Cytoskeletal function of cortical microtubules in the ciliateColpoda

Summary The cytoskeletal function of cortical microtubular structures is explored by high-pressure treatment of the ciliated protozoonColpoda cucullus. This ciliate has two regions of form asymmetry which are apparently maintained by microtubules, namely the somatic groove and the right oral lip. Pressure induced changes in cellular morphology and motility were found to be a function of the magnitude of pressure and duration of compression. Cells exposed to 5,000 psi for 25 minutes, 7,500 psi for 12 minutes, and 10,000 psi for 3 minutes are quiescent and acquire a rounded shape. Observation by electron microscopy of cells exposed to 5,000 psi for 25 minutes indicates that the disappearance of the somatic groove and eversion of the oral apparatus are coincident with the disassembly of the microtubular rootlets in the groove and the supraepiplasmic microtubules in the right oral lip. Other changes accompanying the pressure-induced disassemblies include the reduction in numbers of overlapping microtubular ribbons in the cortical ridges and the appearance of cortical granular accumulations. The essential role in form-maintenance played by microtubular components is discussed.Financial support provided by Natural Science and Engineering Research Council Grant A6544 awarded to DHL and Natural Science and Engineering Research Council Grant A2404 awarded to AMZ.     

The Cytology of Sheep Rumen Ciliates. I. Ultrastructure of Epidinium caudatum Crawley1

Epidinium caudatum has an anterior vestibulum containing the adoral zone syncilia (AZS) on an extrusible peristome. The cytopharyngeal structures include a funnel-shaped arrangement of nematodesmata, longitudinal and transversely oriented microtubular ribbons all of which are located in the peristome, a structure which also contains filamentous phagoplasm. The origins of the microtubular ribbons indicate affinities to the rhabdos type of cytopharynx. The peristomal base is continuous with the tubular esophagus, the region connecting the two being ensheathed by a fibrous layer and low density cytoplasm. The esophagus has a microtubular/microfilamentous wall. A distinct cytoproct with associated myonemal structures occurs posteriorly. The skeletal plates consist of a large number of interconnected, variably shaped platelets and may have dual skeletal and storage functions. The endoplasm is more vesicular than the ectoplasm, the two separated by a fibrous boundary layer. The five-layered cortex has an external glycocalyx, a plasma membrane with two subtending membranes, homogeneous, microtubular, and microfilamentous layers. The syncilia of the AZS are mounted in a U-shaped band on the peristome with transversely oriented kinetics consisting of kinetosomes linked by a sub-kinetosomal rod. There is a bifurcated kinetodesma, dense support material forming a lateral spur with associated transverse microtubules, and postciliary, interkinetal, and occasional basal microtubules, nematodesmata, and a subciliary reticulum. A barren, possibly vestigial, somatic infraciliature consists of non-ciliated kinetosomes and a basal striated fiber with associated basal and perpendicular (cortical) microtubules.     

Morphogenesis and Cortical Ultrastructure of Brooklynella hostilis,a Dysteriid Ciliate Ectoparasitic on Marine Fishes*

SYNOPSIS. Morphogenesis, and the cortical structures of Brooklynella hostilis, a cyrtophorine gymnostome ciliate ectoparasitic on marine fishes, were studied from protargol silver-impregnated preparations and with the aid of electron microscopy. The pattern of morphogenesis of Brooklynella is close to that found in less differentiated species of the families Chlamydodontidae (e.g., in the genus Trithigmostoma) and Dysteriidae (e.g., in the genus Hartmanula). The full number of kineties in the opisthe is restored after division from a segment of the left one of the 3 kinetics producing the oral rows. The oral rows consist of a double row of kinetosomes arranged in a zig-zag pattern; only the outer row is ciliated, the inner one being barren. However, the positions of the postciliary and transverse fibers indicate that the oral rows are not homologs of an undulating membrane but are akin to a membranelle. In association with the ventral somatic kinetosomes there are 4 postciliary fibers; a rather aberrant, transversally oriented kinetodesma; 2 microtubular, transverse fibers plus a transverse fibrousspur; and one to several ribbons of subkinetal microtubular fibers. Not directly associated with the kinetosomes are fibrous strands running subpellicularly between the kinetosomes and also deep into the cytoplasm. The cortical structures of Brooklynella are compared with those of some other groups of ciliates of about the same phylogenetic level in which the subkinetal microtubules can also be found– rhynchodine, suctorian, and chonotrich ciliates. The nasse consists of 6–8 nematodesmata not closely associated with the microtubular cytopharyngeal tube. The former have a distinctly developed densely fibrous capitulum containing barren kinetosomes which originally produced the nematodesma during stomatogenesis; the capitulum is connected by a fibrous link to the microtubular shaft. Extending from the oral rows to the capitula are fibrous structures strongly reminiscent of filamentous reticulum in hymenostome and peritrich ciliates. The structure of the posterio-ventral glandular organelle is also described and discussed.     

Morphology and Ecology of Siroloxophyllum utriculariae (Penard, 1922) N. G., N. Comb. (Ciliophora, Pleurostomatida) and an Improved Classification of Pleurostomatid Ciliates

ABSTRACT. The morphology and infraciliature of Siroloxophyllum utriculariae (Penard, 1922) n. g., n. comb. were studied in live cells, with the scanning and transmission electron microscope, as well as in specimens impregnated with protargol and silver carbonate. The new genus, Siroloxophyllum , belongs to the Loxophyllidae and has a specific combination of characters, viz. an oral bulge surrounding almost the entire cell, three perioral kineties, a single brush kinety, and a single right dorsolateral kinety. The ecology and faunistics of S. utriculariae are reviewed. It is a rare and infrequent predator preferring clean freshwaters. The somatic monokinetid of S. utriculariae has typical haptorid ultrastructure, including two transverse microtubular ribbons. The oral bulge is patterned string-like with riffles containing the transverse microtubular ribbons originating from the oral kinetids. Perioral kineties 1 and 2 consist of dikinetids having one basal body each ciliated; the nonciliated basal body is associated with a nematodesmal and a transverse microtubular ribbon. Perioral kinety 3 consists of ciliated monokinetids having a fine structure similar to the somatic kinetids; they form triads with the dikinetids from perioral kinety 2. The classification of pleurostomatid ciliates is reviewed. Two suborders (Amphileptina, Litonotina) and three families (Amphileptidae, Litonotidae, Loxophyllidae n. fam.) are recognized and defined.     

Polymerization of tubulin in the presence of colchicine or podophyllotoxin. Formation of a ribbon structure induced by guanylyl-5'-methylene diphosphonate

GTP-dependent in vitro polymerization of rat brain microtubular protein is inhibited to 50% by substoichiometric concentrations of the antimitotic drugs colchicine (0.12 mol/mol of tubulin) and podophyllotoxin (0.14 mol/mol of tubulin). Substitution of pp(CH₂)pG² for GTP, however, results in an extensive microtubular protein polymerization at such concentrations. In the presence of pp(CH₂)pG, suprastoichiometric concentrations of podophyllotoxin (19 mol/mol of tubulin) are required to inhibit the polymerization process by 50%. Colchicine is very ineffective since 3 × 10⁵ moles/mole of tubulin are required to give a 50% inhibition. Electron microscopical analysis shows that the polymers formed by microtubular protein in the presence of suprastoichiometric concentrations of drugs are not the normal short microtubules typical of pp(CH₂)pG-driven polymerization, but are ribbons with three or four protofilaments. The colchicine content of the harvested ribbons has been measured directly and found to be approximately 0.8 moles colchicine/mole of tubulin. Treatment of microtubular protein with substoichiometric concentrations of drugs results in an increase in the number of protofilaments forming the ribbons. Many of the ribbons can close into morphologically normal microtubules when microtubular protein is treated with only 0.05 moles of either colchicine or podophyllotoxin per mole of tubulin.     

Light and Electron Microscopical Observations on the Stomatogenesis of the Ciliate Coleps amphacanthus Ehrenberg, 1833

Light and electron microscopical observations on the stomatogenesis of Coleps amphacanthus Ehrenberg, 1833, show that this "gymno"-stome ciliate has a well developed oral ciliature made of 19–23 "paroral dikinetids" and three "adoral organelles." These structures were previously known as "circumoral ciliature" and "dorsal brosse," and it was thought that they originated from the distal ends of all the 22–26 somatic kineties. Contrary to this view, only four stomatogenic kineties (K1, Kn, Kn-1, and Kn-2) are involved in stomatogenesis of the opisthe. All paroral dikinetids arise from one single kinetofragment (KF1) to the right of the oral anlage while the adoral organelles originate from the three left kinetofragments (KFn, KFn-1, and KFn-2). In particular, the future paroral dikinetids perform a complex morphogenetic movement that leads to a situation where the postciliary microtubules of the once posterior kinetosome of each oral dikinetid give rise to the cytopharyngeal microtubular ribbons. The postciliary origin of the cytopharyngeal ribbons which could only be detected by an EM study of stomatogenesis shows that the basket of Coleps belongs to the cyrtos-type and not to the rhabdos-type basket, where transverse microtubules accompany the basket-forming nematodesmata. The taxonomic implications of these observations, which may lead to a revision of the systematic position of the genus Coleps , are discussed.     

Oral Monokinetids in the Free-Living Haptorid Ciliate Enchelydium polynucleatum (Ciliophora,Enchelyidae): Ultrastructural Evidence and Phylogenetic Implications1

Special ultrastructural characteristics of the haptorid soil ciliate Enchelydium polynucleatum Foissner, 1984 are the restriction of the parasomal sacs to the area of the “brush” and finger-like projections of the food vacuole membrane into the lumen of the vacuole. The general organization of the infraciliature is similar to that of Spathidium and some buetschliids because the anterior ends of the somatic kineties are condensed and obliquely bent. Enchelydium is similar to haptorids and buetschliids in possessing monokinetid somatic fibrillar structures with the classical fibrillar associates: 1) a short kinetodesmal fiber; 2) two transverse microtubular ribbons; 3) a long postciliary microtubular ribbon; and 4) a system of overlapping subkinetal microtubules, which seems to be absent in the buetschliids. Unlike Spathidium and all other haptorids so far investigated ultrastructurally, serial sections show that there are no oral dikinetids, as in the endocommensal buetschliids and balantidiids. Instead, three to six anterior kinetids in each ciliary row have nematodesmal bundles extending into the cytoplasm and surrounding the cytopharynx. These kinetids lack cilia and all fibrillar associates except enlarged transverse ribbons, which extend anteriorly and inwards to support the cytopharynx. Other similarities between the buetschliids and Enchelydium are the conspicuous rough endoplasmic reticulum and abundant sausage-like vesicles in the oral region. As in other haptorids, Enchelydium has two types of toxicysts and one type of mucocyst. These observations strongly suggest that Enchelydium belongs to the ancestral stock of both the Haptorida and the Archistomatida. The similarities in the somatic and oral infraciliature and ultrastructure of the Haptorida and the Archistomatida suggest that they belong to the same subclass, Haptoria Corliss, 1974.     

The Cytology of Sheep Rumen Ciliates. III. Ultrastructure of the Genus Entodinium (Stein)1

This study examines previously undescribed general and cytopharyngeal features of the genus Entodinium. The cytopharynx contains three types of microtubular ribbons underlying the cytostomal membrane as well as a loose palisade of nematodesmata. A protoesophagus composed of microtubular bundles associated with a fibrous wall lies internally to one side of an extrusible peristome on which the adoral zone of syncilia (AZS) is mounted. Macronuclear structures are very similar to those of other ophryoscolecids. The micronucleus has chromatin bodies forming a compact mass but lacks the thick wall found in other species. A tubular spongiome surrounds the contractile vacuole and the cytoproct is relatively undifferentiated. Cortical structure follows the usual five-layered ophryoscolecid pattern with subcortical barren kinetosomes arranged into indistinct kineties. The infraciliature of the AZS has kinetosomes set upon a subkinetal rod and with associated bifurcated kinetodesmata and transverse microtubules, some of which extend into the cytopharynx. Components newly described for Entodinium are the one to three postciliary microtubules and the interkinetosomal centro-lateral strand, all of which are present in other species of ophryoscolecid ciliates. The infraciliature of the paralabial ciliary tuft shows similar components to that of the main AZS, but lacks the subkinetal rod. The microtubular components of the cytopharynx are discussed in relation to the “alimentary” structures in other ophryoscolecids, and a relationship of these structures to dietary differences is suggested.     

Ultrastructure of the Somatic and Buccal Cortex of the Tetrahymenine Hymenostome Glaucoma chattoni

SYNOPSIS The membranes, epiplasm, and fiber systems are described in the somatic cortex of Glaucoma chattoni strain HZ-1. Kinetodesmal fibers, postciliary and transverse microtubular ribbons, basal microtubules, transverse fibers and transverse accessory material are associated with kinetosomes. Longitudinal microtubular ribbons and mitochondria occur interkinetally. In the buccal cortex, the membranes, epiplasm and fibers of the 3 membranelles, the paroral kinety, the ribbed wall, and the cytostome are described. Comparisons between G. chattoni and other ciliates reveal ultrastructural differences of possible systematic significance. In the somatic cortex of this and other tetrahymenines. Iongitudinal microtubular ribbons and basal microtubules occur concurrently. In the buccal cortex, alveoli are absent in tetrahymenine membranelles. A table is presented of the fiber systems associated with single somatic kinetosomes of various ciliates whose cortical ultrastructure has been studied to date.     

FOOD VACUOLE MEMBRANE GROWTH WITH MICROTUBULE-ASSOCIATED MEMBRANE TRANSPORT IN PARAMECIUM

Evidence from a morphological study of the oral apparatus of Paramecium caudatum using electron microscope techniques have shown the existence of an elaborate structural system which is apparently designed to recycle digestive-vacuole membrane. Disk-shaped vesicles are filtered out of the cytoplasm by a group of microtubular ribbons. The vesicles, after being transported to the cytostome-cytopharynx region in association with these ribbons, accumulate next to the cytopharynx before they become fused with the cytopharyngeal membrane. This fusion allows the nascent food vacuole to grow and increase its membrane surface area. The morphology of this cytostome-cytopharynx region is described in detail and illustrated with a three-dimensional drawing of a portion of this region and a clay sculpture of the oral apparatus of Paramecium. Evidence from the literature for the transformation of food vacuole membrane into disk-shaped vesicles both from condensing food vacuoles in the endoplasm and from egested food vacuoles at the cytoproct is presented. This transformation would complete a system of digestive vacuole membrane recycling.     

The Ultrastructure of Chaenea teres and an Analysis of the Phylogeny of the Haptorid Ciliates

Chaenea teres has typical haptorid ultrastructure. The somatic monokinetid has two transverse microtubular ribbons, an overlapping postciliary microtubular ribbon, and a laterally directed kinetodesmal fiber. The evered cytopharynx forms a dome at the apical end of the cell. The base of the dome is surrounded by oral dikinetids. The left, anterior kinetosome of the oral pair is not ciliated and has a transverse microtubular ribbon, a nematodesmata and a single postciliary microtubule. The right, posterior kinetosome is ciliated and has only postciliary microtubules. The kinetosomes at the anterior ends of the somatic kinetics are close together and their transverse microtubules and nematodesmata contribute to the support of the cytopharynx. The transverse microtubules of these oralized somatic kinetosomes, together with those from the oral dikinetids, line the cytopharynx. Accessory or bulge microtubules arise perpendicular to the transverse microtubules. A dorsal brush of three kineties of clavate cilia is found on the cell surface just posterior to the oral region. Mucocysts and a single type of toxicyst are present. The toxicysts are confined to the oral region. There are multiple ovoid macronuclei that stain weakly. Micronuclei were not observed. Cladistic analysis indicates the Chaenea may be most closely related to Fuscheria and Acropisthium. The cladistic analysis also suggests that existing taxonomies of the subclass Haptoria need to be revised. We propose some modifications to Foissner & Foissner's classification that include transferring Helicoprorodon, Actinobolina, the buetschiliids, and the balantidiids to the order Haptorida and recognizing the close relationship between pleurostomes and spathidiids.     

Vesicle transport along microtubular ribbons and isolation of cytoplasmic dynein from Paramecium

Cytoplasmic microtubule-based motility in Paramecium was investigated using video-enhanced contrast microscopy, the quick-freeze, deep-etch technique, and biochemical isolations. Three distinct vesicle populations were found to be transported unidirectionally along the cytopharyngeal microtubular ribbons. This minus-end-directed movement exhibited unique in vivo features in that the vesicle transport was nonsaltatory, rapid, and predominantly along one side of the microtubular ribbons. To identify candidate motor proteins which may participate in vesicle transport, we prepared cytosolic extracts of Paramecium and used bovine brain microtubules as an affinity matrix. These preparations were found to contain a microtubule-stimulated ATPase which supported microtubule gliding in vitro. This protein was verified as a cytoplasmic dynein based upon its relative molecular mass, sedimentation coefficient of 16S, susceptibility to vanadate photocleavage, elevated CTPase/ATPase ratio, and its typical two-headed dynein morphology. This dynein was directly compared with the axonemal dyneins from Paramecium and found to differ by five criteria: morphology, sedimentation coefficient, CTPase/ATPase ratio, vanadate cleavage patterns, and polypeptide composition. The cytoplasmic dynein is therefore not an axonemal dynein precursor, but rather it represents a candidate for supporting the microtubule-based vesicle transport which proceeds along the microtubular ribbons.     

Ultrastructural aspects of the somatic and buccal infraciliature ofColeps amphacanthus Ehrenberg 1833

Summary Somatic and buccal infraciliature ofColeps amphacanthus Ehrenberg 1833 were studied by light and electron microscopy. The somatic kineties are composed of monokinetids and 2 dikinetids at the anterior end of each kinety. The monokinetids are associated with postciliary microtubules at triplet 9, a kinetodesmal fiber at triplet 5 and 7 and nearly radially arranged transverse microtubules at triplet 4. The associated fibrillar systems of the posterior kinetosome of the dikinetids are like those of the monokinetids. The anterior kinetosome is associated with transverse microtubules at triplet 4 and one or few postciliary microtubules at triplet 9. The anterior kinetosome bears only a short cilium.The oral ciliature is composed of a kinety of nearly circumorally arranged paroral dikinetids and 3 adoral organelles at the ventral left side of the oral opening. Nematodesmata arising from the oral ciliature form the major component of the cytopharyngeal apparatus which is lined by microtubular ribbons of postciliary origin. The buccal cavity is surrounded by oral papillae which often contain toxicysts.     

Ultrastructure of the Oral Apparatus of Mesodinium rubrum from Korea

Mesodinium rubrum Lohmann is a photosynthetic marine ciliate that has functional chloroplasts of cryptophyte origin. Little is known about the oral ultrastructure of M. rubrum compared with several reports on the sequestration of nuclei and plastids from prey organisms, such as Geminigera cryophila and Teleaulax species. Here, we describe the fine structure of the oral apparatus of a M. rubrum strain from Gomso Bay, Korea. The cytopharynx was cone‐shaped and supported by 20–22 ribbons of triplet microtubules. At the anterior end of the cytopharynx, an annulus anchored small cylinders composed of 11 microtubules. The small cylinders were spaced at regular intervals, each reinforced by one set of the triplet microtubules. At the opening of the cytostome, larger 14‐membered microtubular cylinders were set adjacent to the small, 11‐membered microtubular cylinders, each pair surrounded by separate membranes, however, only the large cylinders extended into the oral tentacles. There were 20–22 oral tentacles each having one to five extrusomes at its tip. At the anterior end of the oral apparatus, microtubular bands supporting the cytostome curved posteriad, extending beneath the cell cortex to the kinetosomes of the somatic cirri. The microtubular bands were connected by striated fibers and originated from kinetosomes anchored by fibers. Each cirrus consisted of eight cilia associated with 16 kinetosomes. The ultrastructure of M. rubrum from Korea provides information useful for taxonomic characterization of the genus Mesodinium and relevant to developing a better understanding of the acquisition of foreign organelles through phagocytosis by M. rubrum.     

The Ultrastructure of Zosterodasys agamalievi (Ciliophora: Synhymeniida)

ABSTRACT. The cell surface of the synhymeniid ciliate, Zosterodasys agamalievi , consists of shallow kinetal grooves separated by low cortical ridges. Numerous electron-opaque bodies are located in the cortical ridges, inside the kinetal grooves, and are distributed in parallel rows between adjacent kineties. Well-developed alveoli are present beneath the cell surface membrane. Zosterodasys agamalievi has a single micronucleus and a homomerous macronucleus. The infraciliature of the somatic monokinetid consists of an anteriorly-directed kinetodesmal fiber, a well-developed divergent postciliary microtubular ribbon, radially-oriented transverse microtubules, and a short striated rootlet, which extends anteriorly from the base of the kinetosome into the cell. Zosterodasys agamalievi has a perioral band of paired cilia, the synhymenium, that winds obliquely across the ventral surface of the body, just posterior to the cytostome. The infraciliature of the anterior kinetosome of the synhymenium consists of two postciliary microtubules; a well-developed, divergent post-ciliary ribbon of microtubules and a short kinetodesmal fiber are associated with the posterior kinetosome. The cytopharynx is supported by 14-16 nematodesmata which are capped distally by a capitulum. The cytopharynx is bound proximally by a fibrous sheath and is lined by radially-arranged microtubular ribbons. No obvious oral ciliature is present.     

THE ORGANIZATION AND EVOLUTION OF MICROTUBULAR ORGANELLES IN CILIATED PROTOZOA

(1) Ciliated protozoa are viewed as unicellular organisms structured in a hierarchy of organizational levels that include the macromolecular, suborganellar, unit organellar, organellar complex, and organellar system. (2) The ciliate cortex is divided into two major functional regions, the somatic region and the oral region. The fundamental component of the cortex is an organellar complex, the kinetid, whose organizing centre is the kinetosome with which are associated three fibrillar associates diagnostic of ciliates. These three fibrillar associates are the periodically striated kinetodesmal fibril and two microtubular ribbons, the transverse and postciliary ribbons. (3) Somatic and oral kinetids are found to be of three major types: monokinetids are composed of one kinetosome and its fibrillar associates; dikinetids are composed of two kinetosomes and their fibrillar associates; polykinetids are composed of more than two kinetosomes and their fibrillar associptes. (4) The mechanisms underlying kinetid function and development remain largely unexplored. Research into the molecular biology and ultrastructure, especially of mutant forms, should provide basic insights in the near future. (5) The conservation of kinetid structure across major phyla of organisms suggests that this subcellular structure should be useful in phylogenetic analysis despite the concepts of ‘chemical identity’ and ‘organic design’. (6) The evolutionary rate of change of oral features is greater than that of somatic features, probably due to developmental and ecological factors. Nevertheless both cortical regions are constrained by the phenomenon of structural conservatism; that is, the conservation of structure through time is inversely related to the level of biological organization. (7) Eight major groupings of ciliate species are recognized, based on ultrastruc-tural features of the cortex. Several examples of differences between these eight groups and the groups presently recognized are discussed.     

Fine Structure of Pseudokeronopsis carnea (Ciliophora,Hypotrichida)1

ABSTRACT Two kinds of pigment structures, pigment vacuoles and pigmentocysts, cause the orange-red color of Pseudokeronopsis carnea (Cohn, 1866). The pigment vacuoles are undischargeable and two to five layers of them form a characteristic ectoplasmic zone. The pigmentocysts mainly surround the infraciliature and show a unique channel which is probably used for extrusion. Previous data on the fine structure of subpellicular granules and extrusomes of hypotrich ciliates are summarized. Their obviously diverse organization argues for a great value of these structures in species identification. The basic structural features of the infraciliature and the cytoplasmic organelles of P. carnea are similar to those found in other hypotrichs; however, a special kind of linear microtubular array borders the longer sides of the cirral bases and the margins of the adoral membranelles and those of the membranes in the right buccal area. To the left of the endoral membrane, these microtubular arrays result in a highly ordered structure reminiscent of oral ribs. This peculiar arrangement of microtubules in cirri and paramembranelles has also been found in the related form, Thigmokeronopsis jahodai, probably indicating a homogeneity of the fine structure of urostylid hypotrichs. In P. carnea, the basal bodies of the paroral membrane are proximally connected like a polykinetid. Its cilia are unlinked, whereas those of the endoral membrane are fused by microfibrillar material. The terms diplostichomonad and polystichomonad only refer to quantitative aspects and omit the evident, high diversity of microtubular and microfibrillar associates occurring in the membranes in the right buccal area. These terms need to be redefined on the basis of more material that is better described.     

Electron microscope studies of pH effects on assembly of tubulin free of associated proteins. Delineation of substructure by tannic acid staining

Bovine brain tubulin, purified by phosphocellulose chromatography (PC), was assembled in the presence of 10% dimethyl sulfoxide (DMSO), and the reaction was monitored turbidimetrically. Samples were fixed in glutaraldehyde-tannic acid after completion of polymerization, as indicated by no further change in absorbance, and then sectioned and studied electron microscopy, with special attention being given to the arrangement of protofilaments in the walls of formed elements. Samples of PC-tubulin were polymerized in buffer having various pH values from 6.0 to 7.7. At the lower pH values, only branched and flattened ribbons of protofilaments are formed. At intermediate values, the ribbons are unbranched, narrower, and more curved in cross section; complete microtubules are also seen. At the higher pH values, the predominate formed elements are complete microtubules. Most of the complete microtubules examined in this study had 14 wall protofilaments. The effect of pH on tubulin assembly was shown not to be an effect of DMSO. The dimers of associated protofilaments in ribbons and microtubules are conceptually viewed as having trapezoidal profiles in cross section, and, as additional dimers are added, the "C"-shaped ribbon closes to form a tube. The tilt angle of the lateral surfaces of the "trapezoidal" dimers will determine the number of wall protofilaments in the microtubules. At low pH, it is theorized that the trapezoidal profile of the dimer is shifted to a more rectangular configuration such that flat ribbons are formed by the lateral association of dimers. Also, variously shaped ribbon structures are formed at intermediate pH values, including "S"- and "W"-shaped structures, and elements shaped like a figure "6," all representing ribbons viewed in cross section. By visualizing the trapezoidal dimer in three-dimensions, and by arbitrarily indexing its six binding surfaces, it is possible to discuss interdimer binding in terms of preferred and possible binding interactions.     

Ultrastructural alterations in Tetrahymena pyriformis induced by growth on saturated phospholipids at 40.1 °C

Structural changes in Tetrahymena pyriformis, strain WH-14, induced by growth on saturated phospholipids at 40.1 °C, were studied by electron microscopy. Alterations in the ultrastructural organization of the cell membrane and surface regions were common. These alterations were characterized in the displacement of kinetosomes, the spatial disorientation and disorganization of cortical ridges and grooves, and the spatial disorientation of longitudinal and transverse microtubular ribbons. Irregular surface protrusions and multiple invaginations of alveolar membranes were among the most common features encountered. Disorganization of longitudinal microtubular ribbons was also a frequent encounter. The integrity of the ultrastructure of cell surface membranes and of the internal organization and ultrastructure of the kinetosomes, however, appeared to be unaltered. Other alterations included those of a number of cytoplasmic organelles (e.g. mitochondria and endoplasmic reticulum), which showed characteristic changes in structural patterns.