Annual fish oogenesis : I. Differentiation of the mature oocyte and formation of the primary envelope

The chorion surface in the eggs of the annual fishes Cynolebias melanotaenia and C. ladigesi contains an elaborate, three-dimensional species-specific pattern. Two concentric layers form the chorion. The pattern resides in the outer layer, the secondary envelope. It consists of closely packed tubules about 250 Å in diameter. A coat of electron dense “fuzzy” material increases this to 475 Å. The inner layer, the primary envelope, of uniformly low electron density possesses no obvious substructure. Oogenesis is divided into six stages. The oocyte increases in size from 10–20 μm in Stage 1 to 250 μm in Stage 3, 600 μm in Stage 4, and attains maximal size of 900 μm by Stage 6. Massive inclusions of protein and lipid yolk accumulate during Stages 4 and 5. Zone 1, one of the three zones of the primary envelope, first appears late in Stage 2. During Stage 3, Zone 1 is completed and Zone 2 appears between the oocyte surface and Zone 1. The oocyte cytoplasm increases in complexity. Material similar to Zone 1 (light, fibrillar) and Zone 2 (dark, compact) is present in the RER, Golgi, derivative vesicles, and apical pits. Micropyle formation also commences. The oocyte secretes Zone 3 during Stage 4 as thin filaments which consolidate into a highly ordered, transitional structure composed of tangentially oriented bundles of interwoven filaments. These partially fuse during Stage 5 except for fenestrations through which oocyte and follicle cell microvilli pass. Complete fusion during Stage 6 produces a continuous layer. Follicle cells retain an unspecialized structure from Stages 1 through 4. Secondary envelope material accumulates in the RER of the follicle cells during Stage 5. It is secreted and deposited during Stage 6.     

Retinal-tectal connections in the embryonic chick: evidence for regionally specific cell surface components which mimic the pattern of innervation.

The chorion surface in the eggs of the annual fishes Cynolebias melanotaenia and C. ladigesi contains an elaborate, three-dimensional species-specific pattern. Two concentric layers form the chorion. The pattern resides in the outer layer, the secondary envelope. It consists of closely packed tubules about 250 Å in diameter. A coat of electron dense “fuzzy” material increases this to 475 Å. The inner layer, the primary envelope, of uniformly low electron density possesses no obvious substructure. Oogenesis is divided into six stages. The oocyte increases in size from 10–20 μm in Stage 1 to 250 μm in Stage 3, 600 μm in Stage 4, and attains maximal size of 900 μm by Stage 6. Massive inclusions of protein and lipid yolk accumulate during Stages 4 and 5. Zone 1, one of the three zones of the primary envelope, first appears late in Stage 2. During Stage 3, Zone 1 is completed and Zone 2 appears between the oocyte surface and Zone 1. The oocyte cytoplasm increases in complexity. Material similar to Zone 1 (light, fibrillar) and Zone 2 (dark, compact) is present in the RER, Golgi, derivative vesicles, and apical pits. Micropyle formation also commences. The oocyte secretes Zone 3 during Stage 4 as thin filaments which consolidate into a highly ordered, transitional structure composed of tangentially oriented bundles of interwoven filaments. These partially fuse during Stage 5 except for fenestrations through which oocyte and follicle cell microvilli pass. Complete fusion during Stage 6 produces a continuous layer. Follicle cells retain an unspecialized structure from Stages 1 through 4. Secondary envelope material accumulates in the RER of the follicle cells during Stage 5. It is secreted and deposited during Stage 6.     

Stages in follicle cell/oocyte interface during vitellogenesis in caecilians <Emphasis Type="Italic">Ichthyophis tricolor</Emphasis> and <Emphasis Type="Italic">Gegeneophis ramaswamii</Emphasis>: a transmission electron-microscopic study

We describe the ultrastructural organization of the vitellogenic follicle stages in two caecilian species. Monthly samples of slices of ovary of Ichthyophis tricolor and Gegeneophis ramaswamii from the Western Ghats of India were subjected to transmission electron-microscopic analysis, with special attention to the follicle cell/oocyte interface. In order to maintain uniformity of the stages among the amphibians, all the stages in the caecilian follicles were assigned to stages I–VI, the vitellogenic and post-vitellogenic follicles being assigned to stages III–VI. Stage III commences with the appearance of precursors of vitelline envelope material in the perivitelline space. Stages IV and V have been assigned appropriate substages. During the transition of stage III to stage VI oocytes, a sequential change occurs in the manifestations of follicle cells, perivitelline space, vitelline envelope and oocyte cortex. The vitelline envelope becomes a tough coat through the tunnels of which the macrovilli pass to interdigitate between the microvilli. The oocyte surface forms pinocytic vesicles that develop into coated pits and, later, coated vesicles. Contributions of the oocyte cortex to the vitelline envelope and of the follicle cells to yolk material via synthesis within them are indicated. The follicle cell/oocyte interface of vitellogenic follicles of these two caecilians resembles that in anurans and urodeles, with certain features being unique to caecilians. Thus, this paper throws light on the possible relationships of caecilians to anurans and urodeles with special reference to ovarian follicles. This research was supported by funds from the Kerala State Council for Science, Technology and Environment (KSCSTE), through the SARD facility, and by the FIST scheme of Department of Science and Technology, Government of India, New Delhi, to the Department of Zoology, University of Kerala, Thiruvananthapuram, and to the Department of Animal Science, Bharathidasan University, Thiruchirapalli (SR/FST/LSI-233/2002).     

Cytoskeleton mediating transport between the ER system and the Golgi apparatus in the green alga Scenedesmus acutus

In the green alga Scenedesmus acutus, Golgi bodies are located near the nucleus and supplied with transition vesicles that bud from the outer nuclear envelope membrane. Using this alga, we have shown previously that thiamine pyrophosphatase (TPPase), a marker enzyme of Golgi bodies, migrates in vesicles from the Golgi bodies to the ER via the nuclear envelope in the presence of BFA (Noguchi et al., Protoplasma 201, 202-212, 1998). In this study we demonstrate that both cytochalasin B and oryzalin (microtubule-disrupting agent) inhibit the BFA-induced migration of TPPase from Golgi bodies to the nuclear envelope. However, only actin filaments--not microtubules--can be detected between the nuclear envelope and the Golgi bodies in both BFA-treated and untreated cells. These observations suggest that actin filaments mediate the BFA-induced retrograde transport of vesicles. This mechanism differs from that found in mammalian cells, in which microtubules mediate BFA-induced retrograde transport by the elongation of membrane tubules from the Golgi cisternae. We also discuss the non-participation of the cytoskeleton in anterograde transport from the nuclear envelope to the Golgi bodies.     

Cytochemical characterization of the endomembranous system during oocyte secondary growth in Serrasalmus spilopleura (Teleostei, Characiformes, Characidae)

The endomembranous system of Serrasalmus spilopleura oocyte secondary growth was analysed using structural and ultrastructural cytochemical techniques. In vitellogenic oocytes, the endoplasmic reticulum components, the nuclear envelope intermembranous space, some Golgi dictiossomes, lysosomes, yolk granules, regions of the egg envelope and sites of the follicle cells react to acid phosphatase detection (AcPase). The cortical alveoli, some heterogeneous cytoplasmic structures, regions of the egg envelope, and sites of the follicle cells are strongly contrasted by osmium tetroxide and zinc iodide impregnation (ZIO). The endoplasmic reticulum components, some vesicles, and sites of the follicle cells also react to osmium tetroxide and potassium iodide impregnation (KI). The biosynthetic pathway of lysosomal proteins, such as acid phosphatase, required for vitellogenesis, involves the endoplasmic reticulum, Golgi complex, vesicles with inactive hydrolytic enzymes, and, finally, lysosomes. In S. spilopleura oocytes at secondary growth, the endomembranous system takes part in the production of the enzymes needed for vitellogenesis, and in the metabolism of yolk exogenous components (AcPase detection). The endomembranous system compartments also show reduction capacity (KI reaction) and are involved in the metabolism of proteins rich in SH‐groups (ZIO reaction).     

The ultrastructure of the anal vesicle of the parasitoid,Microplitis croceipes (Hymenoptera:Braconidae)

The fine structural characteristics of epithelial cells of the anal vesicle in the hymenopteran parasitoid, Microplitis croceipes (Cresson), are similar to those of transport cells. Apical and basal infoldings, an abundance of mitochondria, ribosomes, rough endoplasmic reticulum, Golgi complexes and pinocytotic vesicles all indicate a transport function for these epithelial cells. The medial portions of both Malpighian tubules located within the anal vesicle also were examined and on the basis of morphology appear to be active. These observations support earlier physiological data which indicate that the anal vesicle functions in absorption of nutrients and excretion.     

OBSERVATIONS ON PREMEIOCYTES OF WHEAT

Electron microscopic observations of wheat premeiocytes in premeiotic mitosis (PMM) and premeiotic interphase (PMI) are reported. In archesporial cells of columns at last PMM there is a significant increase in activity of endoplasmic reticulum. Characteristic states of chromatin of premeiocyte nuclei in the stages of PMI are illustrated. Changes in the primary walls of PMCs at Stage 1, PMI, precede the formation of callose in Stage 2. In general, callose deposition throughout the PMC column follows the pattern described elsewhere in Sorghum and other Gramineae. The behavior of the plasmalemma during callose synthesis resembles that described elsewhere in Cucurbita. A role for the cell endomembrane system in callose synthesis is suggested. Blebs arising from the inner membrane of the nuclear envelope, described elsewhere in endosperm cells of wheat and by us in wheat PMCs in this report, give rise to vesicles which carry nuclear material into the cell cytoplasm. Differences in this transport mechanism as it appears in the two tissue systems are described.     

Ultrastructural observations on possible sites of steroid biosynthesis in the ovarian follicular epithelium of two species of cichlid fish,Cichlasoma nigrofasciatum and Haplochromis multicolor

Summary The follicular epithelial layers of the developing ovary of two cichlid species were examined by electron microscopy for evidence of steroid secretion. As each oocyte grew, its follicular cell layers increased in height, eventually becoming somewhat columnar; no development could be detected in follicle cells of non-activated oocytes. Isolated cells close to capillaries in the thecal layer developed large amounts of smooth membrane indicative of steroidogenesis, appearing similar at maturity to testicular Leydig cells. In Cichlasoma nigrofasciatum the mitochondria of differentiated thecal elements contained microtubule-like inclusions. It is suggested that these cells may produce estrogens during vitellogenesis.In developing granulosa cells, active synthesis of granular endoplasmic reticulum occurred. This membrane appeared to arise from the nuclear envelope, and in the pre-ovulatory stage was always intermediate between smooth and granular forms, being only partly associated with ribosomes. Evidence for steroid biosynthesis in the granulosa at this time was therefore equivocal. Evidence was found of transfer of micropinocytotic vesicles from the granulosa cells into the ooplasm.The fate of the post-ovulatory follicle was investigated in Cichlasoma. Thecal elements remained separate from granulosa and unchanged in ultrastructure for up to ten days. The granulosa cells proliferated and differentiated within a few hours after ovulation into a cell type containing much smooth reticulum, characteristic of steroidogenesis. However, after approximately three days numerous signs of degenerative processes became visible. The significance of the observed ultrastructural changes in relation to endocrine function is discussed.     

Ultrastructural analysis of transitional endoplasmic reticulum and pre-Golgi intermediates: a highway for cars and trucks

Cargo selection and export from the endoplasmic reticulum occurs at specialized sites in cells. Export complexes consist of transitional elements of the endoplasmic reticulum and pre-Golgi intermediates. It is generally assumed that 60 to 80 nm initially COPII-coated vesicles derived from the transitional endoplasmic reticulum are the main carriers for transport of cargo to the Golgi apparatus. We have analyzed on serial ultrathin sections the transitional endoplasmic reticulum and pre-Golgi intermediates of beta cells of islets of Langerhans in mouse pancreas. In addition to Golgi-associated complexes, others were observed in the periphery of the cells or close to the nuclear envelope. Upon three-dimensional reconstruction, non-coated ribosome-free tubules with an average diameter of 115 nm (range 60–195 nm) and a length of up to 500 nm were detected in the pre-Golgi intermediates in addition to small vesiculo-tubular elements. Furthermore, evidence was found that the large tubular elements may directly arise from transitional elements of the endoplasmic reticulum. In a given cell, pre-Golgi intermediates were found to be composed solely of small vesiculo-tubular elements or additionally of tubules or solely of tubules. Immunogold labeling for proinsulin indicated that the large tubular elements contained cargo and thus appear to take part in ER-to-Golgi transport.Presented at the 43rd Symposium of the Society for Histochemistry, Vienna, Austria, 27–29 September 2001     

The rate-limiting enzyme in phosphatidylcholine synthesis regulates proliferation of the nucleoplasmic reticulum

The nucleus contains a network of tubular invaginations of the nuclear envelope (NE), termed the nucleoplasmic reticulum (NR), implicated in transport, gene expression, and calcium homeostasis. Here, we show that proliferation of the NR, measured by the frequency of NE invaginations and tubules, is regulated by CTP:phosphocholine cytidylyltransferase-alpha (CCTalpha), the nuclear and rate-limiting enzyme in the CDP-choline pathway for phosphatidylcholine (PtdCho) synthesis. In Chinese hamster ovary (CHO)-K1 cells, fatty acids triggered activation and translocation of CCTalpha onto intranuclear tubules characteristic of the NR. This was accompanied by a twofold increase in NR tubules quantified by immunostaining for lamin A/C or the NE. CHO MT58 cells expressing a temperature-sensitive CCTalpha allele displayed reduced PtdCho synthesis and CCTalpha expression and minimal proliferation of the NR in response to oleate compared with CHO MT58 cells stably expressing CCTalpha. Expression of CCTalpha mutants in CHO58 cells revealed that both enzyme activity and membrane binding promoted NR proliferation. In support of a direct role for membrane binding in NR tubule formation, recombinant CCTalpha caused the deformation of liposomes into tubules in vitro. This demonstrates that a key nuclear enzyme in PtdCho synthesis coordinates lipid synthesis and membrane deformation to promote formation of a dynamic nuclear-cytoplasmic interface.     

Development,structure, and occurrence of secretory trichomes ofPharbitis

Summary Secretory trichomes develop from epidermal cells on the leaf primordia and stem ofPharbitis nil. Following an initial growth phase, trichomes begin active secretion of a protein-carbohydrate mucilage. This mucilage covers the shoot apex and developing leaves ofPharbitis.The secretory cells possess cellular organelles in forms usually associated with actively secreting cells: many mitochondria, an elaborate network of rough endoplasmic reticulum (RER), many free ribosomes, and numerous dictyosomes. The role of the dictyosomes is twofold: 1. dictyosome vesicles bud coated vesicles which transport materials from the cell and, 2. dictyosome vesicles coalesce, forming large storage vesicles. The storage vesicles are surrounded by, and often in contact with, poculiform RER. The RER forms an interconnected network throughout the cytoplasm, extending from the nuclear envelope to the plasmalemma. Distended profiles of RER are frequently in direct contact with the plasmalemma. Thus, inPharbitis secretory trichomes, it is the coated vesicles and RER which are active in secretion export. These findings imply a secretory pathway which deviates from the usual pattern in glandular cells.Predoctoral fellow of National Science Foundation during part of the investigation.     

Translocation of UDP-N-acetylglucosamine into vesicles derived from rat liver rough endoplasmic reticulum and Golgi apparatus

A mixture of UDP-N-acetylglucosamine labeled with different radioisotopes in the uridine and glucosamine was used to show that the intact sugar nucleotide was translocated across the membrane of vesicles derived from rat liver rough endoplasmic reticulum (RER) and Golgi apparatus. Translocation was dependent on temperature, saturable at high concentrations of sugar nucleotide, and inhibited by treatment of vesicles with proteases, suggesting protein carrier mediated transport. Translocation of UDP-GlcNAc by RER-derived vesicles appeared to be specific since these vesicles were unable to translocate UDP-galactose, in contrast to those derived from the Golgi apparatus. Preliminary results suggest that the mechanism of UDP-GlcNAc translocation into RER-derived vesicles is via a coupled exchange with lumenal nucleoside monophosphate. This is similar to the recently postulated mechanism for translocation of sugar nucleotides into vesicles derived from the Golgi apparatus.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphoniidae) II. Vitellogenesis, follicle cell structure and egg shell formation

By the end of previtellogenesis, the oocytes of Glossiphonia heteroclita gradually protrude into the ovary cavity. As a result they lose contact with the ovary cord (which begins to degenerate) and float freely within the hemocoelomic fluid. The oocyte's ooplasm is rich in numerous well-developed Golgi complexes showing high secretory activity, normal and transforming mitochondria, cisternae of rER and vast amounts of ribosomes. The transforming mitochondria become small lipid droplets as vitellogenesis progresses. The oolemma forms microvilli, numerous coated pits and vesicles occur at the base of the microvilli, and the first yolk spheres appear in the peripheral ooplasm. A mixed mechanism of vitellogenesis is suggested. The eggs are covered by a thin vitelline envelope with microvilli projecting through it. The envelope is formed by the oocyte. The vitelline envelope is produced by exocytosis of vesicles containing two kinds of material, one of which is electron-dense and seems not to participate in envelope formation. The cortical ooplasm of fully grown oocytes contains many cytoskeletal elements (F-actin) and numerous membrane-bound vesicles filled with stratified content. Those vesicles probably are cortical granules. The follicle cells surrounding growing oocytes have the following features: (1) they do not lie on a basal lamina; (2) their plasma membrane folds deeply, forming invaginations which eventually seem to form channels throughout their cytoplasm; (3) the plasma membrane facing the ovary lumen is lined with a layer of dense material; and (4) the plasma membrane facing the oocyte forms thin projections which intermingle with the oocyte microvilli. In late oogenesis, the follicle cells detach from the oocytes and degenerate in the ovary lumen.     

Ultrastructural observations on oogenesis in Drosophila

The ultrastructure of the follicle cells and oocyte periplasm is described during the stages of oogenesis immediately prior to, during, and immediately subsequent to, vitellogenesis. A number of features have not been described previously in Drosophila. Some yolk appears prior to pinocytosis of blood proteins. However, most of the protein yolk forms while the periplasm is filled with micropinocytotic invaginations and tubules derived from the oolemma. These tubules retain the internal layer of material characteristic of coated vesicles and are found to fuse with yolk spheres. No accumulation of electron-dense material in the endoplasmic reticulum or Golgi of the oocyte is found. Both trypan blue and ferritin are accumulated by the oocyte. The follicle cells have an elaborate endoplasmic reticulum during the period of maximum yolk accumulation. Adjacent cells are joined at their base by a zonula adhaerens, forming a band around the cells, and by plaques of gap junctions. Gap junctions are also present between nurse cells and follicle cells. During chorion formation, septate junctions also appear between follicle cells, adjacent to the zonula adhaerens.     

Cystine-glutamate transport interactions in rat renal cortical tubules,brushborder vesicles,and cultured renal tubule cells

Glutamate had no significant effect on the uptake of 0.025 mM cystine by isolated rat renal cortical tubules and brushborder membrane vesicles in contrast to lysine which significantly inhibits cystine transport. Glutamate, however, markedly inhibited cystine uptake by rat renal tubule cells grown in a serum-free, hormonally defined media for 5 days. Lysine also inhibited cystine transport in these cultured renal tubule cells.     

Replication of dengue viruses in mosquito cell cultures--a model from ultrastructural observations.

Mosquito cell cultures infected with human sera from dengue-1 and dengue-2 outbreaks, started in Rio de Janerio by 1986 and 1990 respectively, were examined by electron microscopy at different times post the infection of cell cultures. More information was obtained about cell penetration of virus particles in the presence or not of antibodies, their pathway inside the cells, replication mode and exist. Infectiveness of the virus at those different stages can only be attributed to the particles appearing inside the trans-Golgi vesicles; most of all newly formed virus particles remain inside the RER-derived cell vesicles or inside lysosomes, even during cell lysis. Groups of larger particles, 65-75 nm in diameter at dengue-2 infections, persist during cell passage. The large amounts of smooth membrane structures, as vesicles or tubules inside the RER, are attributed to a cell response to viral infection.     

Potassium pyroantimonate and osmium-zinc iodide reactivity in the tubular epithelium of the opisthonephric kidney of the sea lamprey,Petromyzon marinus L

Pyroantimonate precipitate indicates that the epithelium of the proximal tubule is the only segment of the tubular nephron of the fresh water lamprey where large accumlations of cations are distributed. Unusually large amounts of reaction product are located within the lateral intercellular spaces and within vesicles closely associated with the plasma membrane at the lateral and basal surfaces. This technique suggests the continuity of these vesicles with the plasma membrane and alludes to the possibility of an endomembranous system of vesicles and the intercellular spaces as vehicles for ion transport. Lateral intercellular spaces of proximal tubules of lower vertebrates may play a different role in kidney function that their counterparts in higher vertebrates. Osmium-zinc iodide has a specificity for certain cells within the proximal, intermediate, and distal segments, but no structural differences are noted when these cells are compared to unstained cells. Smooth endoplasmic reticulum remains unstained in the distal segment but the stain has a strong affinity for elements of the Golgi apparatus, lysosomes, and the nuclear envelope of all cell types. This technique does not suggest a structural or functional similarity between cells of the distal segment and the chloride cells of the gills of teleosts.     

Impermeability of hagfish cerebral capillaries to horseradish peroxidase

Summary Brain capillaries and their permeability to intravenously injected horseradish peroxidase, HRP, (MW: 40,000) were examined electron-microscopically in an attempt to find a structural explanation for the poorly developed blood-brain barrier in the hagfish, Myxine glutinosa. In particular, it was the aim of this study to examine the role of the numerous endothelial vesicles and tubules in the transport of this tracer between blood and brain. Many of the vesicles and tubules were found to be in continuity with the luminal or abluminal surfaces, but tubules generating channels through the endothelial cells were never observed. The cleft between adjacent endothelial cells was obliterated by punctate junctions. HRP, which was allowed to circulate for up to 35 min, was not found in the basal lamina or in the surrounding brain parenchyma. Few of the luminal vesicles and tubules were marked by the tracer. In the intercellular cleft HRP was stopped by the junctions. It is concluded that the hagfish like other vertebrates has a blood-brain barrier to HRP, and the numerous vesicles and tubules occurring in hagfish brain endothelium are not involved in the transendothelial transport of this macromolecule.     

The nuclear reticulum in placental cells ofLilium regale is a part of the endomembrane system

Summary Placental cells line the ovarian transmitting tract inLilium regale and produce exudates for secretion. Sections through the highly lobed nuclei of these cells reveal the presence of membrane profiles which form vesicles with varying dimensions in cross section. Computer reconstruction of the nucleus reveals that the vesicle profiles form a complex reticulum of tubular cisternae, which spans the whole nucleus, enclosing a maze of continuous lumen space. Connections between the vesicles and the inner nuclear envelope are visible at various points along the nuclear envelope. This complex network of tubules which constitutes the reticulum arises from the inner nuclear membrane. The nuclear reticulum dramatically increases the inner-envelope surface area, comprising 82% of the total membrane perimeter of inner nuclear envelope and nuclear reticulum. The inner nuclear envelope invaginates into the nucleus forming the nuclear reticulum and the outer nuclear envelope evaginates into the endoplasmic reticulum (ER), indicating that there is a continuity between the lumens of the nuclear reticulum and the ER. The nuclear reticulum is labelled with zinc iodide-osmium tetroxide, a staining pattern identical to that seen in the ER. Positive reaction to the zinc iodide-osmium tetroxide indicates that the nuclear reticulum is a site for Ca²⁺ deposition. The nuclear reticulum forms an extension of the endomembrane system which reaches deep into the nucleoplasm. The lumenal continuity of this system means that there is a channel for communication from the cytoplasm into the nucleoplasm, and that this channel sequesters calcium.Abbreviations ER endoplasmic reticulum - TEM transmission electron microscope - ZIO zinc iodide-osmium tetroxide     

Crystallization and Preliminary X-Ray Diffraction Analysis of the 190-Å-Long Coiled-Coil Dimerization Domain of the Actin-Bundling Protein Cortexillin I fromDictyostelium discoideum

We have crystallized the ≈190-Å-long parallel two-stranded coiled-coil oligomerization domain of the actin-bundling protein cortexillin I fromDictyostelium discoideum. The orthorhombic crystals belong to the space group C222₁with unit cell dimensions ofa= 71.3 Å,b= 127.8 Å, andc= 91.6 Å. As both native and selenomethionine-substituted protein crystals diffract to 3.0 and 2.85 Å resolution, respectively, using synchrotron radiation, they are suitable for the first high-resolution structural analysis of a two-stranded coiled coil comprising more than six heptad repeats. Moreover, because the polypeptide chain fragment contains a recently identified two-heptad-repeat long sequence that is indispensable for the assembly of the cortexillin I coiled-coil oligomerization domain, its high-resolution structure should enable us to extend our knowledge on the molecular mechanisms underlaying coiled-coil formation and to establish the precise manner in which the two “trigger” sequences interact with one another in the dimer.