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PubMed1997

Parallel dermal subcompartments for modeling chemical absorption.

Understanding the absorption of chemicals through the skin is of importance to many fields of study. Biologically-based models can be used to simulate the absorption process and predict the rate of absorption and the amount of the chemical in various parts of the body and skin. When these models consist of physiological and biochemical parameters that can be measured, they can be extremely useful. When a model is appropriately validated, the results can be extrapolated across species to predict the effect of human exposure. In this paper we develop two new physiologically-based pharmacokinetic (PBPK) models which predict the concentration of Dibromomethane in the blood of rats after dermal vapor exposure. These two new models expand a previously developed homogeneous skin model by adding parallel skin subcompartments to represent skin appendages and layered subcompartments to represent the distinct layers of the skin. The predictions of these new models match the experimental data better than the original homogeneous model, as well as being more physiologically descriptive. Sensitivity analysis showed us which parameters were the most sensitive to change and thus revealing the parameters we should be most concerned with measuring. After being properly validated, these models could be a great improvement over previous models in the ability to extrapolate results for different species, doses, and durations.

Animals↗
PubMed2006

Luteinizing hormone receptor ectodomain splice variant misroutes the full-length receptor into a subcompartment of the endoplasmic reticulum.

The luteinizing hormone receptor (LHR) is a G protein-coupled receptor that is expressed in multiple RNA messenger forms. The common rat ectodomain splice variant is expressed concomitantly with the full-length LHR in tissues and is a truncated transcript corresponding to the partial ectodomain with a unique C-terminal end. Here we demonstrate that the variant alters the behavior of the full-length receptor by misrouting it away from the normal secretory pathway in human embryonic kidney 293 cells. The variant was expressed as two soluble forms of M(r) 52,000 and M(r) 54,000, but although the protein contains a cleavable signal sequence, no secretion to the medium was observed. Only a very small fraction of the protein was able to gain hormone-binding ability, suggesting that it is retained in the endoplasmic reticulum (ER) by its quality control due to misfolding. This was supported by the finding that the variant was found to interact with calnexin and calreticulin and accumulated together with these ER chaperones in a specialized juxtanuclear subcompartment of the ER. Only proteasomal blockade with lactacystin led to accumulation of the variant in the cytosol. Importantly, coexpression of the variant with the full-length LHR resulted in reduction in the number of receptors that were capable of hormone binding and were expressed at the cell surface and in targeting of immature receptors to the juxtanuclear ER subcompartment. Thus, the variant mediated misrouting of the newly synthesized full-length LHRs may provide a way to regulate the number of cell surface receptors.

Acetylcysteine↗
PubMed1992

Ca2+ stores in Purkinje neurons: endoplasmic reticulum subcompartments demonstrated by the heterogeneous distribution of the InsP3 receptor, Ca(2+)-ATPase, and calsequestrin.

The nature of second messenger-responsive intracellular Ca2+ stores in neurons remains open for discussion. Here, we demonstrate the existence in Purkinje cells (PCs) of endoplastic reticulum (ER) subcompartments characterized by an uneven distribution of three proteins involved in Ca2+ storage and release: the inositol 1,4,5-trisphosphate (InsP3) receptor, Ca(2+)-ATPase, and calsequestrin. Ca(2+)-ATPase and the InsP3 receptor have a widespread, although not identical, distribution throughout the ER. Calsequestrin is localized throughout the smooth ER and is particularly concentrated in pleiomorphic vesicles with a moderately electron-dense core, which appear to represent a subcompartment of the smooth ER. In double-labeling experiments many of these vesicles were unlabeled by InsP3 receptor antibodies. These results suggest a key role of the ER as an intracellular Ca2+ store and demonstrate a possible structural basis for distinct intracellular Ca2+ pools regulated by different second messengers.

Animals↗
PubMed1988

Organelle-specific antibodies: production of antibodies to Golgi subcompartments.

We have devised a strategy for producing Golgi subcompartment-specific antibodies that involves utilizing Triton X-114-phase separated membrane proteins derived from rat pancreatic Golgi subfractions as immunogens. When we tested the approach by immunizing rabbits with membrane proteins derived from heavy Golgi subfractions that are known to be enriched in cis Golgi elements, we succeeded in generating an antibody that recognized a 58 kD protein that was restricted in its distribution to cis Golgi cisternae in several cell types. Thus we have demonstrated the feasibility of the approach we devised for generation of Golgi subcompartment-specific antibodies, and we have also succeeded in identifying a heretofore unknown cis Golgi marker protein.

Animals↗
PubMed1998

Association of spectrin with a subcompartment of the endoplasmic reticulum in honeybee photoreceptor cells.

The endoplasmic reticulum (ER) in honeybee photoreceptors is organized into structurally distinct subregions. The most prominent of these, the submicrovillar network of ER cisternae, is tightly associated with actin filaments. Electron microscopic techniques have demonstrated that the ER-associated actin filaments are regularly spaced at 60-80 nm and cross-bridged by filamentous structures. A polyclonal antibody against Drosophila alpha-spectrin has been used to examine the distribution of spectrin in the photoreceptors. On Western blots of bee retina, the antibody identifies a 260-kDa protein that exhibits biochemical and immunological properties characteristic of alpha-spectrin. Immunofluorescence microscopy has shown that alpha-spectrin codistributes with the submicrovillar ER but not with other ER subdomains. After cytochalasin-B-induced depolymerization of the ER-associated F-actin system, alpha-spectrin remains colocalized with the ER, indicating that alpha-spectrin is bound to the ER membrane. The F-actin/spectrin system associated with the submicrovillar ER may stabilize the shape of this ER subcompartment and may play a role in maintaining functional ER subregions.

Actin Cytoskeleton↗
PubMed2000

Detection of cell cycle subcompartments by flow cytometric estimation of DNA-RNA content in combination with dual-color immunofluorescence.

BACKGROUND: Correlated flow cytometric measurements of phenotype and DNA-RNA content offer detailed information on cell cycle status of subpopulations in heterogeneous cell preparations in response to stimulation. We have developed a method for flow cytometric analysis of DNA-RNA content that has been optimized for simultaneous measurement of dual-color immunofluorescence. METHODS: Nucleic acid staining was performed at low pH in the presence of saponin. DNA was stained with 7-aminoactinomycin D (7-AAD) and RNA with pyronin Y(G) (PY); both dyes were used at low concentrations, and 7-AAD was exchanged with nonfluorescent actinomycin D after DNA staining to minimize fluorochrome-fluorochrome interactions. For cell surface antigen staining, allophycocyanin was combined with pH-independent Alexa488 instead of fluorescein-isothiocyanate (FITC) because FITC is pH sensitive. RESULTS: This method identified cell cycle subcompartments in CEM cells comparable to published results on cell lines using other dyes and staining methods. Measurement of DNA-RNA content in CD8 lymphocyte subsets of human peripheral blood mononuclear cells costimulated with CD3/CD28.2 showed that, after 48 h of stimulation, 80% of CD8(+) T cells were in the proliferative state, whereas 86% of CD8(+) non-T cells remained in G(0). CONCLUSIONS: This technique permits the clear identification of cellular subpopulations by phenotype and assessment of their cell cycle status.

Antigens, CD↗
PubMed1984

Kinetics of partitioning and binding of digoxin and its analogues in the subcompartments of blood.

The kinetics of digoxin and analogues (metabolites) in the subcompartments of blood, erythrocytes and plasma, were first order and concentration independent. The rate and extent of red blood cell partitioning of the tested compounds were, according to first-order kinetics, identical in the presence and absence of structurally related compounds and dependent on lipophilicity and temperature; red blood cell penetration was a reversible process. Erythrocyte partitioning of digoxin and its analogues was postulated to be by passive diffusion. There was significant intracellular binding of the compounds, and hemoglobin was the major ligand. The kinetics of red blood cell partitioning of the individual compounds were fitted according to a closed two- or three-compartment modeL. The latter indicated the existence of two kinetically separable compartments within the red blood cells. Model-independent mean transit times of red blood cell-plasma water partitioning of the compounds depended largely on the lipophilicity and were a thousand times greater for the least lipophilic analogues than for the most lipophilic derivatives. Red blood cell partitioning of digoxin and its metabolites under in vivo and in vitro conditions, were equivalent. Plasma protein binding of the tested compounds was concentration independent, unaltered in the presence of the analogues, and temperature independent between 24 degrees C and 37 degrees C. Binding to albumin in buffer solution was significantly larger than to albumin in plasma for all the compounds. Binding of digoxin and its analogues to albumin in buffer solution increased with increasing lipophilicity of the compounds.

Albumins↗
PubMed2001

Calcium transients in subcompartments of the leech Retzius neuron as induced by single action potentials.

Regional Ca(2+) influx into neurons plays an essential role for fast signal processing, yet it is little understood. We have investigated intracellular Ca(2+) transients induced by a single action potential (AP) in Retzius neurons in situ of isolated ganglia of the leech Hirudo medicinalis using confocal laser scanning microscopy in the cell body, in different axonal branches, and in dendrites. In the cell body, a single AP induced a Ca(2+) transient in submembrane regions, while in central regions no fluorescence change was detected. Burst activity evoked a much larger Ca(2+) influx, which elicited Ca(2+) signals in central somatic regions, including the cell nucleus. A single AP induced a Ca(2+) transient in distal branches of the axon and in dendrites that was significantly larger than in the proximal axon and in the cell body (p <.05), and the recovery of the Ca(2+) transient was significantly faster in axonal branches than in dendrites (p <.01). The AP-induced Ca(2+) transient was inhibited by Co(2+) (2 mM). The P/Q-type Ca(2+) channel blocker omega-agatoxin TK (500 nM) and the L-type Ca(2+) channel blocker nifedipine (20 microM) had no effect on the Ca(2+) transient, whereas the L-type Ca(2+) channel blocker methoxyverapamil (D600, 0.5-1 mM) irreversibly reduced the Ca(2+) transient by 37% in axons and by 42% in dendrites. Depletion of intracellular Ca(2+) stores following inhibition of endoplasmic Ca(2+)-ATPases by cyclopiazonic acid (10 microM) decreased the AP-induced Ca(2+) transient in the dendrites by 21% (p <.01), but not in axons, and increased the Ca(2+) recovery time constant (tau) in the axonal branches by 129% (p <.01), but not in dendrites. The results indicate that an AP evokes a voltage-gated Ca(2+) influx into all subcompartments of the Retzius neuron, where it produces a Ca(2+) signal of different size and/or kinetics. This may contribute to the modulation of electrical excitation and propagation of APs, and to different modes of synaptic and nonsynaptic processes.

Action Potentials↗
PubMed1992

Transport of proteins into the various subcompartments of mitochondria.

The import of proteins into mitochondria is an intricate process comprised of multiple steps. The first step involves the sorting of cytosolically synthesized precursor proteins to the mitochondrial surface. There precursor proteins are recognized by specific receptors which deliver them to the general import site present in the outer membrane. The second stage of import involves a series of complex intraorganelle sorting events which results in the delivery of the proteins to one of the four possible submitochondrial destinations, namely the outer and inner membranes, the matrix and intermembrane space. Here in this review, we discuss the current knowledge on these intramitochondrial sorting events. We especially focus on targeting of proteins to the intermembrane space. Sorting to the intermembrane space represents a particularly interesting situation, as at least three separate targeting pathways to this subcompartment are known to exist.

Biological Transport↗
PubMed1998

Muscle spindles are concentrated in the superior vocalis subcompartment of the human thyroarytenoid muscle.

It is hypothesized that different parts of the thyroarytenoid muscle (TA) are functionally specialized. Specifically, the TA is divided into a lateral muscularis compartment and a medial vocalis compartment. This study examined the distribution of muscle spindles throughout the human TA as an indicator of these functional differences. Histological cross-sections from the anterior, middle, and posterior regions of five human membranous vocal folds were examined for the number and location of muscle spindles. There was an average of 6.1 muscle spindles in sections from each region with no significant variation between the different regions (p < .05). However, in sections from all three regions, the muscle spindles were always found to be concentrated in the superior medial quadrant of the TA (mean 85.9%, p < .01). The inferior medial, superior lateral, and inferior lateral quadrants of the TA contained 11.96%, 2.17%, and 0%, respectively, of the total muscle spindles. Within the superior medial quadrant, most of the muscle spindles were localized in the most superficial part of the muscle. The results of this study demonstrate that the majority of TA muscle spindles are concentrated in its superior medial quadrant, an area we have termed the superior vocalis subcompartment (SC). This finding suggests that the superior vocalis SC is functionally distinct from the remainder of the TA. It is hypothesized that tension in the superior vocalis SC can be controlled independently from the remainder of the TA, and this capability is used to effect the biomechanics of vocal fold vibration during phonation.

Humans↗
PubMed2003

The mitochondria-associated endoplasmic-reticulum subcompartment (MAM fraction) of rat liver contains highly active sphingolipid-specific glycosyltransferases.

Although most glycosphingolipids (GSLs) are thought to be located in the outer leaflet of the plasma membrane, recent evidence indicates that GSLs and their precursor, ceramide, are also associated with intracellular organelles and, particularly, mitochondria. GSL biosynthesis starts with the formation of ceramide in the endoplasmic reticulum (ER), which is transported by controversial mechanisms to the Golgi apparatus, where stepwise addition of monosaccharides on to ceramides takes place. We now report the presence of GSL-biosynthetic enzymes in a subcompartment of the ER previously characterized and termed 'mitochondria-associated membrane' (MAM). MAM is a membrane bridge between the ER and mitochondria that is involved in the biosynthesis and trafficking of phospholipids between the two organelles. Using exogenous acceptors coated on silica gel, we demonstrate the presence of ceramide glucosyltransferase (Cer-Glc-T), glucosylceramide galactosyltransferase and sialyltransferase (SAT) activities in the MAM. Estimation of the marker-enzyme activities showed that glycosyltransferase activities could not be ascribed to cross-contamination of MAM by Golgi membranes. Cer-Glc-T was found to have a marked preference for ceramide bearing phytosphingosine as sphingoid base. SAT activities in MAM led to the synthesis of G(M3) ganglioside and small amounts of G(D3). G(M1) was also synthesized along with G(M3) upon incubation of the fraction with exogenous unlabelled G(M3), underlying the presence of other sphingolipid-specific glycosyltransferases in MAM. On the basis of our results, we propose MAM as a privileged compartment in providing GSLs for mitochondria.

Animals↗
PubMed1980

Subcompartments of the G1 phase of cell cycle detected by flow cytometry.

Cellular DNA and RNA were simultaneously quantitated in individual cells of cycling and quiescent populations by flow cytometry. Based on differences in RNA content, two distinct subcompartments, G1A and G1B, were recognized in the G1 phase of exponentially growing cells. After mitosis, cells reside in the low-RNA, G1A compartment from which they cannot enter the S phase directly. An increase in RNA above a critical level is required for G1 cells to be able to initiate DNA replication; G1 cells with RNA values above this level are classified as G1B. Cell transition from G1A to G1B and from G1B to S was analyzed in a stathmokinetic experiment on L1210 cells with a doubling time of 10 hr. The half-time of cell residence in the indeterminate state of G1 was found to be 1 hr, and the duration of the deterministic portion of G1 phase was 2 hr. The indeterminate state, although not identical with G1A, is most likely located in G1A. Cell quiescence induced by serum deprivation (3T3 cells) or by addition of n-butyrate (L1210) results in cell arrest at a state which, judged by RNA content, is similar to that of G1A of exponentially growing cells. The exit from this state, however, is much slower after stimulation of these blocked cells than the transition of G1A to G1B in cells growing exponentially. Thymidine or hydroxyurea arrest cells in G1B or perhaps at the border between G1B and S. Prolonged incubation of 3T3 cells to confluence results in a marked loss of cellular RNA below the level of the G1A state. This deep quiescent state (G1Q) is distinctly different from the G1A state of cycling cells or cells blocked in G1A by serum deprivation.

Acridine Orange↗
PubMed2002

GRIP domain-mediated targeting of two new coiled-coil proteins, GCC88 and GCC185, to subcompartments of the trans-Golgi network.

The GRIP domain is a targeting sequence found in a family of coiled-coil peripheral Golgi proteins. Previously we demonstrated that the GRIP domain of p230/golgin245 is specifically recruited to tubulovesicular structures of the trans-Golgi network (TGN). Here we have characterized two novel Golgi proteins with functional GRIP domains, designated GCC88 and GCC185. GCC88 cDNA encodes a protein of 88 kDa, and GCC185 cDNA encodes a protein of 185 kDa. Both molecules are brefeldin A-sensitive peripheral membrane proteins and are predicted to have extensive coiled-coil regions with the GRIP domain at the C terminus. By immunofluorescence and immunoelectron microscopy GCC88 and GCC185, and the GRIP protein golgin97, are all localized to the TGN of HeLa cells. Overexpression of full-length GCC88 leads to the formation of large electron dense structures that extend from the trans-Golgi. These de novo structures contain GCC88 and co-stain for the TGN markers syntaxin 6 and TGN38 but not for alpha2,6-sialyltransferase, beta-COP, or cis-Golgi GM130. The formation of these abnormal structures requires the N-terminal domain of GCC88. TGN38, which recycles between the TGN and plasma membrane, was transported into and out of the GCC88 decorated structures. These data introduce two new GRIP domain proteins and implicate a role for GCC88 in the organization of a specific TGN subcompartment involved with membrane transport.

Amino Acid Sequence↗
PubMed1990

The inositol 1,4,5,-trisphosphate receptor in cerebellar Purkinje cells: quantitative immunogold labeling reveals concentration in an ER subcompartment.

The Ca2+ mobilization effect of inositol 1,4,5-trisphosphate, the second messenger generated via receptor-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, is mediated by binding to intracellular receptors, which are expressed in high concentration in cerebellar Purkinje cells. Partially conflicting previous reports localized the receptor to various subcellular structures: elements of ER, both rough and smooth-surfaced, the nuclear envelope, and even the plasma membrane. We have now reinvestigated the problem quantitatively by using cryosections of rat cerebellar tissue immunolabeled with polyclonal monospecific antibodies against the inositol 1,4,5-trisphosphate receptor. By immunofluorescence the receptor was detected only in Purkinje cells, whereas the other cells of the cerebellar cortex remained negative. In immunogold-decorated ultrathin cryosections of the Purkinje cell body, the receptor was concentrated in cisternal stacks (piles of up to 12 parallel cisternae separated by regularly spaced bridges, located both in the deep cytoplasm and beneath the plasma membrane; average density, greater than 5 particles/micron of membrane profile); in cisternal singlets and doublets adjacent to the plasma membrane (average density, approximately 2.5 particles/micron); and in other apparently smooth-surfaced vesicular and tubular profiles. Additional smooth-surfaced elements were unlabeled. Perinuclear and rough-surfaced ER cisternae were labeled much less by themselves (approximately 0.5 particles/micron, two- to threefold the background), but were often in direct membrane continuity with heavily labeled, smooth-surfaced tubules and cisternal stacks. Finally, mitochondria, Golgi cisternae, multivesicular bodies, and the plasma membrane were unlabeled. In dendrites, approximately half of the nonmitochondrial, membrane-bound structures (cisternae, tubules, and vesicles), as well as small cisternal stacks, were labeled. Dendritic spines always contained immunolabeled cisternae and vesicles. The dendritic plasma membrane, of both shaft and spines, was consistently unlabeled. These results identify a large, smooth-surfaced ER subcompartment that appears equipped to play a key role in the control of Ca2+ homeostasis: in particular, in the generation of [Ca2+]i transients triggered by activation of specific receptors, such as the quisqualate-preferring trans(+/-)-1-amino-1,3-cyclopentamedicarboxylic acid glutamatergic receptors, which are largely expressed by Purkinje cells.

Animals↗
PubMed1993

Proteolytic cleavage of haptoglobin occurs in a subcompartment of the endoplasmic reticulum: evidence from membrane fusion in vitro.

The primary translation product of haptoglobin mRNA is a 45-kD polypeptide which is proteolytically cleaved shortly after its synthesis. Previous studies have indicated that the cleavage of this proform of haptoglobin occurs in the ER. In an attempt to characterize the cleaving enzyme, we found that upon incubation of microsomes from rat hepatocytes pulse labeled with [35S]methionine, little cleavage of labeled prohaptoglobin occurred. In contrast, when cells whose cytoplasmic proteins had been released by saponin treatment were incubated, 30-40% of the prohaptoglobin was cleaved. The addition of GTP caused a twofold stimulation, which was abolished by the nonhydrolyzable analog GTP gamma S. With a homogenate of the cells, the addition of GTP resulted in a fourfold stimulation of the degree of cleavage--from 15 to 60%. Differential centrifugation revealed that most of the cleaving activity resided in membranes sedimenting similarly to mitochondria and to a small fraction of the ER. These rapidly sedimenting membranes were therefore prepared from a rat liver homogenate. Upon treatment with high salt, light membranes were released which, when incubated with microsomes of pulse-labeled hepatocytes in the presence of detergent (and in the absence of GTP), induced specific cleavage of prohaptoglobin. The cleaving enzyme had an alkaline pH optimum indicating that it was not of lysosomal origin. These results suggest that cleavage of prohaptoglobin occurs in a subcompartment of the ER. Apparently, the connection between this compartment and the bulk of the ER is broken upon saponin treatment or homogenization but can be reestablished through a process requiring GTP hydrolysis.

Adenosine Triphosphate↗
PubMed1983

Lectin-binding sites as markers of Golgi subcompartments: proximal-to-distal maturation of oligosaccharides.

We investigated the subcellular sites of glycoprotein oligosaccharide maturation by using lectin conjugates to stain lightly-fixed, saponin-permeabilized myeloma cells. At the electron microscopic level, concanavalin A-peroxidase stains the cisternal space of the nuclear envelope, the rough endoplasmic reticulum, and cisternae along the proximal face of the Golgi stack. Conversely, wheat germ agglutinin-peroxidase stains cisternae along the distal face of the Golgi stack, associated vesicles, and the cell surface. These observations confirm the existence of two qualitatively distinct Golgi subcompartments, show that the lectin conjugates can be employed as relatively proximal or distal Golgi markers under conditions of excellent ultrastructural preservation, suggest that the asymmetric distribution of qualitatively distinct oligosaccharides is a property of underlying cellular components and not simply of the principal secretory product, and suggest that the oligosaccharide structure recognized by wheat germ agglutinin is attained during transport from the proximal toward the distal face of the Golgi stack.

Animals↗
PubMed1996

HLA-DM and MHC class II molecules co-distribute with peptidase-containing lysosomal subcompartments.

MHC class II molecules associate with peptides in the endocytic pathway. Different endosomal locations for peptide loading of class II molecules, varying from early endosomes (EE) to lysosomes, have been assigned on the basis of subcellular fractionation experiments. We have determined the intracellular location of HLA-DM, a molecule that supports peptide loading of class II molecules, by separating vesicles from the melanoma cell line Mel JuSo on the basis of buoying density and surface charge. In both fractionations, HLA-DM co-fractionated with a lysosomal compartment containing beta-hexosaminidase (beta-hex) activity and not with endosomes. Further analysis showed that HLA-DM mainly co-fractionated with a sub-lysosomal structure characterized by a relative low density and containing both pro- and mature cathepsin D and MHC class II molecules. Fluid phase markers first enter this compartment before entering high-density lysosomes that contain exclusively mature cathepsin D, some HLA-DM and no detectable MC class II molecules. Finally we determined the intracellular location of neutral and acidic peptidases. Whereas neutral peptidase activity was detected in the endoplasmic reticulum and/or plasma membrane fractions, acidic peptidase activity exclusively migrated at the position of HLA-DM containing lysosomal vesicles. Our results show that class II molecules co-migrate with HLA-DM, pro- and mature cathepsin D, beta-hex and acidic peptidase activity. HLA-DM, cathepsin d and class II molecules were not observed at the position of EE. Our data suggest that HLA-DM-mediated peptide loading of class II molecules occurs in a lysosomal subcompartment.

Cathepsin D↗
PubMed2006

Plastoglobules are lipoprotein subcompartments of the chloroplast that are permanently coupled to thylakoid membranes and contain biosynthetic enzymes.

Plastoglobules are lipoprotein particles inside chloroplasts. Their numbers have been shown to increase during the upregulation of plastid lipid metabolism in response to oxidative stress and during senescence. In this study, we used state-of-the-art high-pressure freezing/freeze-substitution methods combined with electron tomography as well as freeze-etch electron microscopy to characterize the structure and spatial relationship of plastoglobules to thylakoid membranes in developing, mature, and senescing chloroplasts. We demonstrate that plastoglobules are attached to thylakoids through a half-lipid bilayer that surrounds the globule contents and is continuous with the stroma-side leaflet of the thylakoid membrane. During oxidative stress and senescence, plastoglobules form linkage groups that are attached to each other and remain continuous with the thylakoid membrane by extensions of the half-lipid bilayer. Using three-dimensional tomography combined with immunolabeling techniques, we show that the plastoglobules contain the enzyme tocopherol cyclase (VTE1) and that this enzyme extends across the surface monolayer into the interior of the plastoglobules. These findings demonstrate that plastoglobules function as both lipid biosynthesis and storage subcompartments of thylakoid membranes. The permanent structural coupling between plastoglobules and thylakoid membranes suggests that the lipid molecules contained in the plastoglobule cores (carotenoids, plastoquinone, and tocopherol [vitamin E]) are in a dynamic equilibrium with those located in the thylakoid membranes.

Arabidopsis↗