STRUCTURES LINKING THE MYONEMES, ENDOPLASMIC RETICULUM, AND SURFACE MEMBRANES IN THE CONTRACTILE CILIATE VORTICELLA

An electron microscope investigation of the interface between the myonemes of Vorticella convallaria and their associated endoplasmic reticulum (ER) has revealed structures of a complex morphology linking these two organelles. These structures are named "linkage complexes". Each complex contains a spindle-shaped midpiece which lies in a groove of the ER membrane. Microfilaments splay out from the tips of the midpiece and may come in contact with the inner alveolar sac membrane. Three to six raillike structures lie on each side of the midpiece and parallel it. The ER membrane appears to pass through the sides of the rails. In the lumen of the ER these rails are associated with a meshwork of filaments. A cradle of five rods lies within the groove under the midpiece. The ER membrane also passes through these rods which contact the same meshwork. In the scopular region and in the stalk the microfilaments from the midpiece form a bundle which passes into the lumen of modified basal bodies. These basal bodies are connected to the alveolar sac which, in the stalk, passes as a flattened tube along its length. The parts of the dissociated linkage complex are scattered throughout the spasmoneme of the stalk along membranes of the intraspasmonemal tubules. Thus, both stalk and body contractile bundles have linkage complexes that link their associated membrane systems to the microfibrils and, in turn, connect this membrane-microfibrillar interface to the pellicular membranes. The arrangement of the linkage complex suggests an involvement in the control of the transport of calcium ions between ER and microfibrils, and possibly the transfer of a message from the surface membranes to the sites of calcium release to trigger myonemal contraction.     

Motility processes in Acantharia. II. A Ca2+ dependent system of contractile 2–4 nm filaments isolated from demembranated myonemes

Myonemes of the acantharians are contractile ribbon-like organelles. As previously shown, their motility is based on the coiling mechanism of double-twisted 2–4 nm nonactin filaments [14]. Myonemes have been isolated and manipulated in vitro. After demembranation, the contraction takes place when the Ca²⁺ concentration is above 10^?7 M, whereas relaxation occurs below this threshold concentration. The response to Ca²⁺ ions is an on/off mechanism. Both contraction and relaxation can be induced repeatedly without fatigue phenomena. Other divalent cations such as Sr²⁺, Ba²⁺, Ma²⁺, CO²⁺, and La³⁺ can replace Ca²⁺ in inducing contraction of the demembranated myonemes although with less efficiency. Contraction and relaxation are ATP-independent and calmodulin is not involved in this in vitro motility process. The myoneme is a strongly resistant structure which is capable of contracting and relaxing under various extreme conditions which indicates very stable proteins and resistant functions.     

The Fine Structure of Rhynchocystis pilosa (Sporozoa, Eugregarinida)

SYNOPSIS. The trophozoite of Rhynchocystis pilosa obtained from the seminal vesicles of the earthworm Lumbricus terrestris was studied by light and electron microscopy. The trophozoite's cortical organization is particularly interesting because of its unusual evaginations and associated fibrillar structures. The pellicle is formed by 2 concentric membranes elevated into 60–70 alternating primary and secondary ridges extending posteriad. Numerous long ‘hairs’ or cytopilia originate along the primary ridges and each contains a system of fibrils originating from an underlying longitudinal myoneme. Longitudinal rows of pores lie between adjacent pollicular ridges. Three systems of fibrils lie in the cortex of the trophozoite. A longitudinal myoneme consisting of 12–18 fibrils lies below each primary pellicular ridge. Circular myonemes lie below the pellicle in a parallel array along the length of the organism. Each myoneme consists of 4–8 fibrils structurally similar to those of the longitudinal myonemes. Pairs of fine filaments also lie in the inner pellicular membrane along the apex of each ridge. The trophozoite's anterior end is modified as an attachment organelle consisting of 30–35 delicate pellicular folds which originate at the base of an anterior papilla. The folds extend approximately 15 μ posteriad where they become continuous with the primary pellicular ridges. The nucleus lies in the cytoplasm near the posterior level of the attachment organelle and is surrounded by a double membrane perforated by numerous pores. The cytoplasm contains numerous small vesicles which may be found in dense aggregations. These aggregations often occur in proximity to Golgi complexes and certain membrane-bound bodies. Mitochondria are abundant in the cytoplasm as are large, ovoid paraglycogen bodies. Occasionally layers of granular membranes are arranged parallel to the surface of the paraglycogen bodies but also occur thruout the cytoplasm.     

Cloning and Expression of a cDNA Encoding a Vorticella convallaria Spasmin: an EF-Hand Calcium-Binding Protein

The stalked, ciliated protozoan Vorticella convallaria possesses a highly contractile cytoskeleton consisting of spasmonemes and myonemes. The major component of these contractile organelles is the calcium-binding protein(s) called spasmin. Cloning and characterization of spasmin would help elucidate this contractile system. Therefore, enriched spasmoneme protein preparations from these contractile stalks were used to produce a monoclonal antibody to spasmin. A monoclonal antibody, 1F5, was obtained that immunolocalized specifically to the spasmonemes and the myonemes and recognized a 20-kD calcium-binding protein in spasmoneme protein preparations. A putative spasmin cDNA was obtained from a V. convallaria cDNA library and the derived amino acid sequence of this cDNA revealed an acidic, 20-kD protein with calcium-binding helix-loop-helix domains. The physical properties of the putative spasmin were assessed by characterization of a recombinantly-produced spasmin protein. The recombinant spasmin protein was shown to bind calcium using calcium gel-shift assays and was recognized by the anti-spasmin antibody. Therefore, a V. convallaria spasmin was cloned and shown to be a member of the EF-hand superfamily of calcium-binding proteins.     

Motility processes in Acantharia (protozoa) III. Calcium regulation of the contraction - relaxation cycles of in vivo myonemes

The myonemes in the marine pelagic protozoa Acantharia are contractile organelles involved in buoyancy regulation. It was previously shown that they can perform three kinds of movement: rapid contraction, slow undulation and slow relaxation. They consist of a periodically striated bundle of 2–4 nm nonactin filaments that are twisted in pairs and shortened by a coiling mechanism. After permeabilization or demembranation, contraction and relaxation can still be performed by varying Ca²⁺ concentration and ATP is not needed. In the present paper, we have studied the role of Ca²⁺ and inhibitors of energy production in intact cells. Our data suggest that; (i) the in vivo rapid contraction subsequent to mechanical or electrical stimulation is triggered by Ca²⁺ influx across the cell membrane; (ii) the slow contraction that takes place during the undulating movement depends on Ca²⁺ release provided by internal calcium stores; (iii) the rapid contraction as well as the progressive shortening that occurs during the slow undulating movement are caused by Ca²⁺ binding to the myoneme filaments; (iv) ATP is not directly involved in the saturation by Ca²⁺ of Ca²⁺ sensitive sites located along the myoneme microstrands; (v) regulation of the movements of Ca²⁺ within the cytoplasm depends mainly upon the alternative pathway of ATP production; (vi) calmodulin is presumably involved in this regulation. A tentative cytophysiologic interpretation of the mechanism of contractility is proposed.     

Scanning Electron Microscopy of the Cortex of the Ciliate Stentor coeruleus. A View from the Inside*

SYNOPSIS. A microdissection procedure was developed which permits the viewing of the inside surface of the cortex of Stentor coeruleus with scanning electron microscopy. Parallel bands of myonemes cover the entire inner surface of the cortex. The myonemes of the stalk region are ribbon-shaped and lack cross connections. The myonemes of the anterior cortex are flattened against the surface and are interconnected by an extensive system of cross branches. The inner surface of the frontal field is covered with a regularly cross-branched myoneme system which follows the curved pattern of frontal field kinety. The observed branching patterns and shapes of the myonemes support the hypothesis that they cause contraction of the cell. The membranellar root system was examined. Each membranellar root makes a 90° counterclockwise twist along its vertical axis (viewed from the inside) as it descends into the cell. The outer edge of each root fuses with the inner edge of the adjacent one, forming a continuous fiber sheet linking the roots together.     

Ciliate Ultrastructure: Some Problems in Cell Biology*

SYNOPSIS. Protozoology, along with other cell sciences, has profited immeasurably during the last 25 years from the application of electron microscopy and other new technics to studies of cell structure and function. Protozoa were among the first objects examined with the electron microscope in the 1940's and an extensive literature in protozoan ultrastructure has accumulated since then. Some examples of recent investigations of ciliate cortical ultrastructure are selected to illustrate the importance of protozoan studies to the development of fundamental concepts of cell biology. Ciliates are choice subjects for analysis of ciliary structure and motility and of myoneme contractility. Membrane-limited alveoli contribute to the structure of the characteristic ciliate pellicle and provide evidence of how multiple pellicular membranes may affect cell-surface activities. The striated fibrils and microtubules associated with ciliate kinetosomes resemble those of other cell types but are more highly organized; wherever they occur, these structures appear to be related to the development and maintenance of polarity and asymmetry of cells and organelles. Their stability and the precision of their arrangement in ciliates make them peculiarly suitable for the study of the properties and behavior of such fibrous organelles and also for investigation of the morphogenetic role of the kinetosome.     

Spasmin and a Putative Spasmin Binding Protein(s) Isolated from Solubilized Spasmonemes1

ABSTRACT The amino acid composition and hydrophobicity scale (hydropathy) of calcium-binding proteins contained in the contractile spasmoneme of Carchesium polypinum was compared with other calcium-binding proteins from eukaryotes. Spasmins which may hind at most 4 calcium ions simultaneously and initiate spasmoneme contraction cooperatively belong to a super family of proteins including; centrin/caltractin and calmodulin. Based on chemical modification of tryptophan and methionine, these residues are involved in contraction but the spasmin proteins contain little or none of these amino acids. Based on this evidence, it is suggested that another, non-calcium binding protein(s) is involved in spasmoneme contraction.     

Electron microscope investigation of the evolutionary stages of the trophozoite of Didymophyes gigantea (Sporozoa, Gregarinida). III. The fine structure of the epicyte with emphasis on the contractile elements (author's transl)

The fine structure of the epicyte of D. gigantea was investigated. The motility of the gregarine and the contractile elements are described. Four essential types of movements can be observed in this gregarine: (1) rolling up and pendular movements, (2) locomotion by gliding forward, (3) cytoplasmic streaming (Fig. 1), (4) peristaltic contractions (Fig. 2) which seem to be accompanied by the contraction of annular myonemes (Fig. 2). The epicyte is formed by the folding of the parasitic cell wall which is made from three membranes (Figs. 3 and 4). At the top of each fold one can see apical struts between the outer and middle membrane and apical filaments under the inner membrane (Fig. 3). In addition, the epicytic folds are covered by a cell coat which is made from tubular structures (Fig. 5). At the base of the epicytic folds can be observed the basal lamina (Fig. 3) composed of very fine fibrillar material with an average thickness of 2.5 nm (Fig. 6). These fibrils are oriented in the longitudinal axis of the gregarine. Beneath the epicytic fold in the ectoplasm are found the annular myonemes with a width of up to 0.5 micrometers (Fig. 7). They are composed of many fine fibrils with an average thickness of 5 nm. In young trophozoites, the myonemes also contain microtubuli (Fig. 8). Between the epicytic folds, the cell wall is interrupted by three different types of vesicles: the vesicles with an electrondense content (Fig. 9), the three-membranous vesicles (Fig. 10), and the hose-shaped vesicles (Fig. 11). Glycerol-extraction of the parasites was performed in order to define the contractile structures. After extraction the annular myonemes are difficult to recognize (Fig. 13). When ATP is added, the gregarine does not contract but the myonemes reappear after 3 to 4 min (Fig. 14). Differences can also be observed in the myoneme structure using electron microscopy: After extraction, the myonemes are composed of a very limp fibrillar network (Fig. 15) which becomes very dense after the action of ATP (Fig. 16). Glycerol extraction does not disturb either the apical struts and apical filaments or the fibrils of the basal lamina (Figs. 15--17). In addition, cytoplasmic fibrillar structures appear after glycerol extraction (Figs. 15 and 16). The experimental and electron microscope results indicate that the motility of the gregarine depends upon four different systems: (1) the ectoplasmic annular myonemes, (2) the apical structures in the undulating epicytic folds, (3) the cytoplasmic fibrils, and (4) the basal lamina.     

Use of an ionophore to maintain repeated caffeine contractures in holothurian muscle

When treated with caffeine (10 mM) isolated longitudinal retractor muscles of $\text{Isostichopus badionotus}$ show a contracture which diminishes progressively with repeated doses. Brief treatment with X-537A (10^?5 M) restores contractility for the duration of the exposure. However, after longer treatment with X-537A, restored contractility persists even after X-537A has been washed off. The persistent contractility may indicate that X-537A has partitioned into muscle cell membranes, thus speeding the time course of refilling of calcium stores.     

Functional organization of battery cell complexes in tentacles of Hydra attenuata

Ultrastructural and light microscopic observations on the organization of thick and thin regions of hydra's tentacles, made on serial sections and on whole fixed, plastic-embedded tentacles, reveal the existence of two levels of anatomical order in the tentacle ectoderm: (1) The battery-cell complex (BCC), composed of a single epitheliomuscular cell (EMC) and its content of enclosed nematocytes and neurons; and (2) the battery cell complex ring (BCC ring), an arrangement of 4 or more BCCs into larger units organized as rings around the circumference of the tentacle. All EMCs of the distal tentacle appear to contain batteries of nematocytes, and are, therefore, called “battery cells.” Apart from battery cell complexes and migrating nematocytes, there are no other cell types in the tentacle ectoderm. Battery cells are composed of three distinct regions: the cell body, peripheral attenuated extensions and myonemes. Thick tentacle bands are composed of cell bodies, whereas thin bands are made up of attenuated extensions. Myonemes contribute to both thick and thin regions. It was confirmed that each battery cell has several myonemes, which appear to interdigitate with myonemes of other more proximal and distal battery cells, but not with battery cells of the same BCC ring. Nematocytes have several basal processes. Some processes insert between myonemes and contact the mesoglea; other processes insert into cuplike extensions of myonemes, and are connected to myonemal cups by desmosomal junctions. These observations are discussed in relation to mechanical and electrical aspects of tentacular contraction and bending.     

THE CONTRACTILE PROCESS IN THE CILIATE, STENTOR COERULEUS : I. The Role of Microtubules and Filaments

The structural basis for the function of microtubules and filaments in cell body contractility in the ciliate Stentor coeruleus was investigated. Cells in the extended state were obtained for ultrastructural analysis by treatment before fixation with a solution containing 10 mM EGTA, 50–80 mM Tris, 3 mM MgSO₄, 7.5 mM NH₄Cl, 10 mM phosphate buffer (pH 7.1). The response of Stentor to changes in the divalent cation concentrations in this solution suggests that Ca⁺² and Mg⁺² are physiologically important in the regulation of ciliate contractility. The generation of motive force for changes in cell length in Stentor resides in two distinct longitudinal cortical fiber systems, the km fibers and myonemes. Cyclic changes in cell length are associated with (a) the relative sliding of parallel, overlapping microtubule ribbons in the km fibers, and (b) a distinct alteration in the structure of the contractile filaments constituting the myonemes. The microtubule and filament systems are distinguished functionally as antagonistic contractile elements. The development of motive force for cell extension is accomplished by active microtubule-to-microtubule sliding generated by specific intertubule bridges. Evidence is presented which suggests that active shortening of contractile filaments, reflected in a reversible structural transformation of dense 4-nm filaments to tubular 10–12-nm filaments, provides the basis for rapid cell contraction.     

Perforated pit membranes in imperforate tracheary elements of some angiosperms

BACKGROUND AND AIMS: The structure of pit membranes in angiosperms has not been fully examined and our understanding about the structure is incomplete. Therefore, this study aims to illustrate the micromorphology of pit membranes in fibres and tracheids of woody species from various families. METHODS: Specimens from ten species from ten genera and eight families were prepared using two techniques and examined by field-emission scanning electron microscopy. KEY RESULTS: Interfibre pit membranes with an average diameter of <4 microm were frequently perforated or appeared to be very porous. In contrast, pit membranes in imperforate tracheary elements with distinctly bordered pits and an average diameter of >or=4 microm were homogeneous and densely packed with microfibrils. These differences were observed consistently not only among species but also within a single species in which different types of imperforate tracheary elements were present. CONCLUSIONS: This study demonstrates that the structure of interfibre pit membranes differs among cell types and the differences are closely associated with the specialization of the fibre cells. It is suggested that perforated pit membranes between specialized fibres contribute to the dehydration of the fibre cells at or soon after maturation.     

Five New Species of Lagenophrys (Ciliophora,Peritricha, Lagenophryidae) from the United States with Observations on Their Developmental Stages1

Five species of the loricate genus Lagenophrys were found on freshwater hosts and are described for the first time. Lagenophrys dennisi n. sp., L. incompta n. sp., and L. oregonensis n. sp. are ectosymbionts of astacid crayfish. Lagenophrys foxi n. sp and L. missouriensis n. sp. are ectosymbionts of gammarid amphipods. All five species appear to occur only m North America. Protargol preparations of the five species reveal that the peristomial myoneme is much broader and more extensive in telotrochs and metamorphosing individuals than in adults. Darkly staining bands appearing to be somatic myonemes were also seen underneath the surface of the body and in the center of the body of telotrochs and metamorphosing individuals. The telotroch of Lagenophrys is so different from the adult that it constitutes a true larval form rather than a simple dispersal stage. Structural parallels between the telotroch of Lagenophrys and mobiline peritrichs suggest the hypothesis that mobilines evolved from the telotroch of a sessiline pentrich which had first evolved into a true larval form.     

Acute inhibition of PMCA4, but not global ablation,reduces blood pressure and arterial contractility via a nNOS‐dependent mechanism

Cardiovascular disease is the world's leading cause of morbidity and mortality, with high blood pressure (BP) contributing to increased severity and number of adverse outcomes. Plasma membrane calcium ATPase 4 (PMCA4) has been previously shown to modulate systemic BP. However, published data are conflicting, with both overexpression and inhibition of PMCA4 in vivo shown to increase arterial contractility. Hence, our objective was to determine the role of PMCA4 in the regulation of BP and to further understand how PMCA4 functionally regulates BP using a novel specific inhibitor to PMCA4, aurintricarboxylic acid (ATA). Our approach assessed conscious BP and contractility of resistance arteries from PMCA4 global knockout (PMCA4KO) mice compared to wild‐type animals. Global ablation of PMCA4 had no significant effect on BP, arterial structure or isolated arterial contractility. ATA treatment significantly reduced BP and arterial contractility in wild‐type mice but had no significant effect in PMCA4KO mice. The effect of ATAin vivo and ex vivo was abolished by the neuronal nitric oxide synthase (nNOS) inhibitor Vinyl‐l ‐NIO. Thus, this highlights differences in the effects of PMCA4 ablation and acute inhibition on the vasculature. Importantly, for doses here used, we show the vascular effects of ATA to be specific for PMCA4 and that ATA may be a further experimental tool for elucidating the role of PMCA4.     

Hormonal control of locust oviducts

The oviducal muscles of the locust, Locusta migratoria, contract in a spontaneous and rhythmic fashion when isolated from the central nervous system. Hemolymph of ovipositing females, when added to isolated locust oviducts, altered the spontaneous contractility of the oviduct. This response was not evident after addition of hemolymph from a nonovipositing female and was still present after addition of the α-aminergic receptor antagonist, phentolamine. Oviducts in which mature eggs were present responded to homogenates of the corpus cardiacum by increasing both the frequency and amplitude of muscular contraction, whereas oviducts devoid of eggs showed no response. Extracts of ventral nerve cord also increased the spontaneous activity of the oviduct musculature. Although the muscles of the oviduct responded to homogenates of the brain, this response differed in two ways from the response due to corpus cardiacum homogenates. First, oviducts devoid of mature eggs responded to brain homogenates; and second, the response caused by the brain homogenates could be eliminated by the addition of 1 μM phenoxybenzamine. The significance of these results is discussed.     

Amiodarone-Induced Changes in Intracellular Ca2+ Homeostasis in the Atrial Myocardium of Patients with Chronic Coronary Insufficiency

Changes in intracellular Ca²⁺ homeostasis induced by 1 M Amiodarone solution were studied with muscle strips isolated from the right atrial auricles of patients with ischemic heart disease (IHD). The intracellular Ca²⁺ homeostasis was evaluated through the changes in contractility in the muscle strips caused by a 60-s discontinuation of their stimulation by electrical pulses. It was found that, in 30% of the IHD patients, the atrial myocardium displayed an abnormal contractility. Myocardium samples with normal contractility displayed two types of inotropic response to the resumption of electrical stimulation. The first type was associated with a 30% decrease in the parameters of the contraction–relaxation cycle, whereas, for the second type, this decrease was stronger (by 70% or more). An incubation of the muscle samples with the Amiodarone solution suppressed the abnormal contractility, did not affect the muscles with the first type of response, and significantly (p < 0.05) increased the inotropic response in the muscles with the second type of reaction to a short interruption in their electrical stimulation. As Amiodarone itself induces no direct positive inotropic effects, it was supposed that, in some cases of IHD, the presence of Amiodarone in the atrial myocardium is associated with an increased ability of the sarcoplasmic reticulum to retain calcium ions accumulated. This phenomenon may contribute to the suppression of abnormal activity of the atrial myocardium and strengthen the direct antiarrhythmic effect of Amiodarone.     

Mitochondrial apoptosis induced by BH3‐only molecules in the exclusive presence of endoplasmic reticular Bak

Bak and Bax are critical apoptotic mediators that naturally localize to both mitochondria and the endoplasmic reticulum (ER). Although it is generally accepted that mitochondrial expression of Bak or Bax suffices for apoptosis initiated by BH3‐only homologues, it is currently unclear whether their reticular counterparts may have a similar potential. In this study, we show that cells exclusively expressing Bak in endoplasmic membranes undergo cytochrome c mobilization and mitochondrial apoptosis in response to BimEL and Puma, even when these BH3‐only molecules are also targeted to the ER. Surprisingly, calcium was necessary but not sufficient to drive the pathway, despite normal ER calcium levels. We provide evidence that calcium functions coordinately with the ER‐stress surveillance machinery IRE1α/TRAF2 to transmit apoptotic signals from the reticulum to mitochondria. These results indicate that BH3‐only mediators can rely on reticular Bak to activate an ER‐to‐mitochondria signalling route able to induce cytochrome c release and apoptosis independently of the canonical Bak,Bax‐dependent mitochondrial gateway, thus revealing a new layer of complexity in apoptotic regulation.     

Isolation and Physiological Analysis of Mouse Cardiomyocytes

Cardiomyocytes, the workhorse cell of the heart, contain exquisitely organized cytoskeletal and contractile elements that generate the contractile force used to pump blood. Individual cardiomyocytes were first isolated over 40 years ago in order to better study the physiology and structure of heart muscle. Techniques have rapidly improved to include enzymatic digestion via coronary perfusion. More recently, analyzing the contractility and calcium flux of isolated myocytes has provided a vital tool in the cellular and sub-cellular analysis of heart failure. Echocardiography and EKGs provide information about the heart at an organ level only. Cardiomyocyte cell culture systems exist, but cells lack physiologically essential structures such as organized sarcomeres and t-tubules required for myocyte function within the heart. In the protocol presented here, cardiomyocytes are isolated via Langendorff perfusion. The heart is removed from the mouse, mounted via the aorta to a cannula, perfused with digestion enzymes, and cells are introduced to increasing calcium concentrations. Edge and sarcomere detection software is used to analyze contractility, and a calcium binding fluorescent dye is used to visualize calcium transients of electrically paced cardiomyocytes; increasing understanding of the role cellular changes play in heart dysfunction. Traditionally used to test drug effects on cardiomyocytes, we employ this system to compare myocytes from WT mice and mice with a mutation that causes dilated cardiomyopathy. This protocol is unique in its comparison of live cells from mice with known heart function and known genetics. Many experimental conditions are reliably compared, including genetic or environmental manipulation, infection, drug treatment, and more. Beyond physiologic data, isolated cardiomyocytes are easily fixed and stained for cytoskeletal elements. Isolating cardiomyocytes via perfusion is an extremely versatile method, useful in studying cellular changes that accompany or lead to heart failure in a variety of experimental conditions.     

TFG clusters COPII‐coated transport carriers and promotes early secretory pathway organization

In mammalian cells, cargo‐laden secretory vesicles leave the endoplasmic reticulum (ER) en route to ER‐Golgi intermediate compartments (ERGIC) in a manner dependent on the COPII coat complex. We report here that COPII‐coated transport carriers traverse a submicron, TFG (Trk‐fused gene)‐enriched zone at the ER/ERGIC interface. The architecture of TFG complexes as determined by three‐dimensional electron microscopy reveals the formation of flexible, octameric cup‐like structures, which are able to self‐associate to generate larger polymers in vitro. In cells, loss of TFG function dramatically slows protein export from the ER and results in the accumulation of COPII‐coated carriers throughout the cytoplasm. Additionally, the tight association between ER and ERGIC membranes is lost in the absence of TFG. We propose that TFG functions at the ER/ERGIC interface to locally concentrate COPII‐coated transport carriers and link exit sites on the ER to ERGIC membranes. Our findings provide a new mechanism by which COPII‐coated carriers are retained near their site of formation to facilitate rapid fusion with neighboring ERGIC membranes upon uncoating, thereby promoting interorganellar cargo transport.