Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Biogenesis”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 973 records · Page 54Linked to original sources
PubMed2004

Role of calcium and AMP kinase in the regulation of mitochondrial biogenesis and GLUT4 levels in muscle.

Contractile activity induces mitochondrial biogenesis and increases glucose transport capacity in muscle. There has been much research on the mechanisms responsible for these adaptations. The present paper reviews the evidence, which indicates that the decrease in the levels of high-energy phosphates, leading to activation of AMP kinase (AMPK), and the increase in cytosolic Ca(2+), which activates Ca(2+)/calmodulin-dependent protein kinase (CAMK), are signals that initiate these adaptative responses. Although the events downstream of AMPK and CAMK have not been well characterized, these events lead to activation of various transcription factors, including: nuclear respiratory factors (NRF) 1 and 2, which cause increased expression of proteins of the respiratory chain; PPAR-alpha, which up regulates the levels of enzymes of beta oxidation; mitochondrial transcription factor A, which activates expression of the mitochondrial genome; myocyte-enhancing factor 2A, the transcription factor that regulates GLUT4 expression. The well-orchestrated expression of the multitude of proteins involved in these adaptations is mediated by the rapid activation of PPAR gamma co-activator (PGC) 1, a protein that binds to various transcription factors to maximize transcriptional activity. Activating AMPK using 5-aminoimidizole-4-carboxamide-1-beta-D-riboside (AICAR) and increasing cytoplasmic Ca(2+) using caffeine, W7 or ionomycin in L6 myotubes increases the concentration of mitochondrial enzymes and GLUT4 and enhances the binding of NRF-1 and NRF-2 to DNA. AICAR and Ca-releasing agents also increase the levels of PGC-1, mitochondrial transcription factor A and myocyte-enhancing factors 2A and 2D. These results are similar to the responses seen in muscle during the adaptation to endurance exercise and show that L6 myotubes are a suitable model for studying the mechanisms by which exercise causes the adaptive responses in muscle mitochondria and glucose transport.

Adaptation, Physiological↗
PubMed2004

Ribosomal RNA processing and ribosome biogenesis in eukaryotes.

In eukaryotes nearly 500 rRNAs, ribosomal proteins, snoRNAs and trans-acting factors contribute to ribosome biogenesis. After more than 30 years of intense research, the incredible complexities of nucleolar function are revealed but details often remain unclear. Here we review this progress and the many intriguing questions which remain.

Active Transport, Cell Nucleus↗
PubMed2004

Structural aspects of microRNA biogenesis.

One of the biggest surprises at the beginning of the 'post-genome era' was the discovery of numerous genes encoding microRNAs. They were found in genomes of such diverse organisms as Caenorhabditis elegans, Drosophila melanogaster, Arabidopsis thaliana, and Homo sapiens which implies their important role in multicellular life evolution. The number of microRNA genes is estimated to be nearly 1% of that of protein-coding genes. Their products, tiny RNAs, are thought to regulate gene expression during development, organogenesis, and very likely during many other processes, by hybridizing to their target mRNAs. The cellular functions of mRNAs that are regulated by microRNAs are only beginning to be revealed, and details of this regulation mechanism are still poorly understood. In this article we discuss the possible mechanisms of microRNA biogenesis with special emphasis on their structural aspects. We have focused on the factors and effects that may be responsible for the existing length differences between different microRNAs, and for the observed length heterogeneity within some individual microRNA species.

Animals↗
PubMed1986

Microtubules and actin filaments are not critically involved in the biogenesis of epithelial cell surface polarity.

We have studied the role of microtubules and actin filaments in the biogenesis of epithelial cell surface polarity, using influenza hemagglutinin and vesicular stomatitis G protein as model apical and basolateral proteins in infected Madin-Darby canine kidney cells. Addition of colchicine or nocodazole to confluent monolayers at concentrations sufficient to completely disassemble microtubules did not affect the asymmetric budding of influenza or vesicular stomatitis virus and only slightly reduced the typical asymmetric surface distribution of their envelope proteins, despite extensive cytoplasmic redistribution of the Golgi apparatus. Alteration of microtubular function by taxol or dissociation of actin filaments by cytochalasin D also failed to have a significant effect. Furthermore, neither colchicine nor cytochalasin D pretreatment blocked the ability of subconfluent Madin-Darby canine kidney cells to sustain polarized budding of influenza virus a few hours after attachment to the substrate. Our results indicate that domain-specific microtubule or actin filament "tracks" are not responsible for the vectorial delivery of apically or basolaterally directed transport vesicles. In conjunction with currently available evidence, they are compatible with a model in which receptors in the cytoplasmic aspect of apical or basolateral regions provide vectoriality to the transport of vesicles carrying plasma membrane proteins to their final surface localization.

Actin Cytoskeleton↗
PubMed1986

Biogenesis of the crystalloid endoplasmic reticulum in UT-1 cells: evidence that newly formed endoplasmic reticulum emerges from the nuclear envelope.

The crystalloid endoplasmic reticulum (ER), a specialized smooth ER of the compactin-resistant UT-1 cell, is composed of multiple membrane tubules packed together in a hexagonal pattern. This membrane contains large amounts of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, an integral membrane protein that enzymatically regulates endogenous cholesterol biosynthesis. Using morphological and immunocytochemical techniques, we have traced the sequence of events in the biogenesis of this ER when compactin-withdrawn UT-1 cells, which do not have a crystalloid ER, are incubated in the presence of compactin. After 15 h of incubation in the presence of compactin, many cells had profiles of ER cisternae that were juxtaposed to the nuclear envelope and studded with ribosomes on their outer membrane. Both the outer nuclear membrane and the ER membrane contained HMG CoA reductase; however, there was little or no detectable enzyme in rough ER that was free in the cytoplasm. With longer times of incubation in the presence of compactin, these cells had lamellar stacks of smooth ER next to the nuclear envelope that contained HMG CoA reductase. Coordinate with the appearance of the smooth ER, crystalloid ER appeared in the same cell. Often regions of continuity were found between the membrane of the smooth ER and the membrane of the crystalloid ER tubules. These studies suggest that HMG CoA reductase is synthesized along the outer nuclear membrane and in response to increased enzyme synthesis, a membrane emerges from the outer nuclear membrane as smooth ER cisternae, which then transforms into crystalloid ER tubules.

Acyl Coenzyme A↗
PubMed1987

Biogenesis of the rat hepatocyte plasma membrane in vivo: comparison of the pathways taken by apical and basolateral proteins using subcellular fractionation.

We have used pulse-chase metabolic radiolabeling with L-[35S]methionine in conjunction with subcellular fractionation and specific protein immunoprecipitation techniques to compare the posttranslational transport pathways taken by endogenous domain-specific integral proteins of the rat hepatocyte plasma membrane in vivo. Our results suggest that both apical (HA 4, dipeptidylpeptidase IV, and aminopeptidase N) and basolateral (CE 9 and the asialoglycoprotein receptor [ASGP-R]) proteins reach the hepatocyte plasma membrane with similar kinetics. The mature molecular mass form of each of these proteins reaches its maximum specific radioactivity in a purified hepatocyte plasma membrane fraction after only 45 min of chase. However, at this time, the mature radiolabeled apical proteins are not associated with vesicles derived from the apical domain of the hepatocyte plasma membrane, but instead are associated with vesicles which, by several criteria, appear to be basolateral plasma membrane. These vesicles: (a) fractionate like basolateral plasma membrane in sucrose density gradients and in free-flow electrophoresis; (b) can be separated from the bulk of the likely organellar contaminants, including membranes derived from the late Golgi cisternae, transtubular network, and endosomes; (c) contain the proven basolateral constituents CE 9 and the ASGP-R, as judged by vesicle immunoadsorption using fixed Staphylococcus aureus cells and anti-ASGP-R antibodies; and (d) are oriented with their ectoplasmic surfaces facing outward, based on the results of vesicle immunoadsorption experiments using antibodies specific for the ectoplasmic domain of the ASGP-R. Only at times of chase greater than 45 min do significant amounts of the mature radiolabeled apical proteins arrive at the apical domain, and they do so at different rates. Approximate half-times for arrival are in the range of 90-120 min for aminopeptidase N and dipeptidylpeptidase IV whereas only 15-20% of the mature radiolabeled HA 4 associated with the hepatocyte plasma membrane fraction has become apical even after 150 min of chase. Our results suggest a mechanism for hepatocyte plasma membrane biogenesis in vivo in which all integral plasma membrane proteins are shipped first to the basolateral domain, followed by the specific retrieval and transport of apical proteins to the apical domain at distinct rates.

Animals↗
PubMed1988

Biogenesis of thylakoid membranes is controlled by light intensity in the conditional chlorophyll b-deficient CD3 mutant of wheat.

Biogenesis of thylakoid membranes in the conditional chlorophyll b-deficient CD3 mutant of wheat is dramatically altered by relatively small differences in the light intensity under which seedlings are grown. When the CD3 mutant is grown at 400 microE/m2 S (high light, about one-fifth full sunlight) plants are deficient in chlorophyll b (chlorophyll a/b ratio greater than 6.0) and lack or contain greatly reduced amounts of the chlorophyll a/b-binding complexes CPII/CPII (mobile or peripheral LHCII), CP29, CP24 and LHCI, as shown by mildly denaturing 'green gel' electrophoresis, by fully denaturing SDS-PAGE, and by Western blot analysis. High light CD3 chloroplasts display an unusual morphology characterized by large, sheet-like stromal thylakoids formed into parallel unstacked arrays and a limited number of small grana stacks displaced toward the edges of the arrays. Changes in the supramolecular organization of CD3 thylakoids, seen with freeze-fracture electron microscopy, include a reduction in the size of EFs particles, which correspond to photosystem II centers with variable amounts of attached LHCII, and a redistribution of EF particles from the stacked to the unstacked regions. When CD3 seedlings are grown at 150 microE/m2 S (low light) there is a substantial reversal of all of these effects. Thus, chlorophyll b and the chlorophyll a/b-binding proteins accumulate to near wild-type levels (chlorophyll a/b ratio = 3.5-4.5) and thylakoid morphology is more nearly wild type in appearance. Growth of the CD3 mutant in the presence of chloramphenicol stimulates the accumulation of chlorophyll b and its binding proteins (Duysen, M. E., T. P. Freeman, N. D. Williams, and L. L. Huckle. 1985. Plant Physiol. 78:531-536). We show that this partial rescue of the CD3 high light phenotype is accompanied by large changes in thylakoid structure. The CD3 mutant, which defines a new class of chlorophyll b-deficient phenotype, is discussed in the more general context of chlorophyll b deficiency.

Chlorophyll↗
PubMed1989

Steady-state distribution and biogenesis of endogenous Madin-Darby canine kidney glycoproteins: evidence for intracellular sorting and polarized cell surface delivery.

We used domain-selective biotinylation/125I-streptavidin blotting (Sargiacomo, M., M. P. Lisanti, L. Graeve, A. Le Bivic, and E. Rodriguez-Boulan. 1989 J. Membr. Biol. 107:277-286), in combination with lectin precipitation, to analyze the apical and basolateral glycoprotein composition of Madin-Darby canine kidney (MDCK) cells and to explore the role of glycosylation in the targeting of membrane glycoproteins. All six lectins used recognized both apical and basolateral glycoproteins, indicating that none of the sugar moieties detected were characteristic of the particular epithelial cell surface. Pulse-chase experiments coupled with domain-selective glycoprotein recovery were designed to detect the initial appearance of newly synthesized glycoproteins at the apical or basolateral cell surface. After a short pulse with a radioactive precursor, glycoproteins reaching each surface were biotinylated, extracted, and recovered via precipitation with immobilized streptavidin. Several basolateral glycoproteins (including two sulfated proteins) and at least two apical glycoproteins (one of them the major sulfated protein of MDCK cells) appeared at the corresponding surface after 20-40 min of chase, but were not detected in the opposite surface, suggesting that they were sorted intracellularly and vectorially delivered to their target membrane. Several "peripheral" apical proteins were detected at maximal levels on the apical surface immediately after the 15-min pulse, suggesting a very fast intracellular transit. Finally, domain-selective labeling of surface carbohydrates with biotin hydrazide (after periodate oxidation) revealed strikingly different integral and peripheral glycoprotein patterns, resembling the Con A pattern, after labeling with sulfo-N-hydroxy-succinimido-biotin. The approaches described here should be useful in characterizing the steady-state distribution and biogenesis of endogenous cell surface components in a variety of epithelial cell lines.

Animals↗
PubMed1990

Biogenesis of transverse tubules: immunocytochemical localization of a transverse tubular protein (TS28) and a sarcolemmal protein (SL50) in rabbit skeletal muscle developing in situ.

To study the biogenesis of transverse tubules, the temporal appearance and distribution of TS28 (a specific marker of transverse tubules absent from the sarcolemma in adult skeletal muscle; 28,000 Mr) and SL50 (specifically associated with the sarcolemma and absent from the region of the transverse tubules in adult rabbit skeletal muscle) (Jorgensen, A.O., W. Arnold, A. C.-Y. Shen, S. Yuan, M. Gaver, and K.P. Campbell. 1990. J. Cell Biol. 110:1173-1185) were determined in rabbit skeletal muscle developing in situ (day 17 of gestation to day 15 newborn) by indirect immunofluorescence labeling. The results presented show that the temporal appearance and subcellular distribution of TS28 is distinct from that of SL50 at the developmental stages examined. TS28 was first detected in some, but not all, multinucleated myotubes on day 17 of gestation. At this stage of development, SL50 and the Ca2(+)-ATPase of the sarcoplasmic reticulum were already present in all myotubes. TS28 first appeared in discrete foci mostly confined to the cell periphery of the myotubes. At subsequent stages of development (days 19-24 of gestation), TS28 was also found in shoft finger-like structures extending obliquely and transversely from the cell periphery towards the center of the myotubes. 1-2 d after birth, TS28 was observed in an anastomosing network composed of transversely oriented chickenwire-like networks extending throughout the cytoplasm and interconnected by longitudinally oriented fiber-like structures. As development proceeded, the transversely oriented network became increasingly dominant. By day 10 of postnatal development, the longitudinally oriented component of the tubular network was not regularly observed. At none of the developmental stages examined was TS28 observed to be uniformly distributed at the cell periphery. SL50, like TS28, first appeared in discrete foci at the cell periphery. However, shortly after its first appearance it appeared to be distributed along the entire cell periphery. Although the intensity of SL50 labeling increased with development, it remained confined to the sarcolemma and was absent from the interior regions of the myofibers, where transverse tubules were present at all subsequent developmental stages examined. Immunoblotting of cell extracts from skeletal muscle tissue at various stages of development showed that SL50 was first detected on day 24 of gestation, while TS28 was not detected until days 1-2 after birth. Comparison of these results with previous ultrastructural studies of the formation of transverse tubules supports the idea that the temporal appearance and subcellular distribution of TS28 correspond very closely to that of the distribution of forming transverse tubules in rabbit skeletal muscle developing in situ.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1991

Participation of a novel 88-kD protein in the biogenesis of murine class I histocompatibility molecules.

Chemical cross-linking and gel permeation chromatography were used to examine early events in the biogenesis of class I histocompatibility molecules. We show that newly synthesized class I heavy chains associate rapidly and quantitatively with an 88-kD protein in three murine tumor cell lines. This protein (p88) does not appear to possess Asn-linked glycans and it is not the abundant ER protein, GRP94. The class I-p88 complex exists transiently (t1/2 = 20-45 min depending on the specific class I heavy chain) and several lines of evidence suggest that p88 dissociates from the complex while still in the ER. Dissociation is not triggered upon binding of beta 2-microglobulin to the heavy chain (t1/2 = 2-5 min). However, the rate of dissociation does correlate with the characteristic rate of ER to Golgi transport for the particular class I molecule studied. Consequently, dissociation of p88 may be rate limiting for ER to Golgi transport. Class I molecules bind antigenic peptides, apparently in the ER, for subsequent presentation to cytotoxic T lymphocytes at the cell surface. p88 could promote peptide binding or it may retain class I molecules in the ER during formation of the ternary complex of heavy chain, beta 2-microglobulin, and peptide.

Animals↗
PubMed1993

Caveolin forms a hetero-oligomeric protein complex that interacts with an apical GPI-linked protein: implications for the biogenesis of caveolae.

Glycosyl-phosphatidylinositol (GPI)-linked proteins are transported to the apical surface of epithelial cells where they undergo cholesterol-dependent clustering in membrane micro-invaginations, termed caveolae or plasmalemmal vesicles. However, the sorting machinery responsible for this caveolar-clustering mechanism remains unknown. Using transfected MDCK cells as a model system, we have identified a complex of cell surface molecules (80, 50, 40, 22-24, and 14 kD) that interact in a pH- and cholesterol-dependent fashion with an apical recombinant GPI-linked protein. A major component of this hetero-oligomeric protein complex is caveolin, a type II transmembrane protein. As this hetero-oligomeric caveolin complex is detectable almost immediately after caveolin synthesis, our results suggest that caveolae may assemble intracellularly during transport to the cell surface. As such, our studies have implications for understanding both the intracellular biogenesis of caveolae and their subsequent interactions with GPI-linked proteins in epithelia and other cell types.

Animals↗
PubMed1994

A crucial role of the mitochondrial protein import receptor MOM19 for the biogenesis of mitochondria.

The novel genetic method of "sheltered RIP" (repeat induced point mutation) was used to generate a Neurospora crassa mutant in which MOM19, a component of the protein import machinery of the mitochondrial outer membrane, can be depleted. Deficiency in MOM19 resulted in a severe growth defect, but the cells remained viable. The number of mitochondrial profiles was not grossly changed, but mutant mitochondria were highly deficient in cristae membranes, cytochromes, and protein synthesis activity. Protein import into isolated mutant mitochondria was decreased by factors of 6 to 30 for most proteins from all suborganellar compartments. Proteins like the ADP/ATP carrier, MOM19, and cytochrome c, whose import into wild-type mitochondria occurs independently of MOM19 became imported normally showing that the reduced import activities are solely caused by a lack of MOM19. Depletion of MOM19 reveals a close functional relationship between MOM19 and MOM22, since loss of MOM19 led to decreased levels of MOM22 and reduced protein import through MOM22. Furthermore, MOM72 does not function as a general backup receptor for MOM19 suggesting that these two proteins have distinct precursor specificities. These findings demonstrate that the import receptor MOM19 fulfills an important role in the biogenesis of mitochondria and that it is essential for the formation of mitochondria competent in respiration and phosphorylation.

Cytochromes↗
PubMed1997

Functional dissection of COP-I subunits in the biogenesis of multivesicular endosomes.

In the present paper, we show that transport from early to late endosomes is inhibited at the restrictive temperature in a mutant CHO cell line (ldlF) with a ts-defect in epsilon coatomer protein (epsilonCOP), although internalization and recycling continue. Early endosomes then appear like clusters of thin tubules devoid of the typical multivesicular regions, which are normally destined to become vesicular intermediates during transport to late endosomes. We also find that the in vitro formation of these vesicles from BHK donor endosomes is inhibited in cytosol prepared from ldlF cells incubated at the restrictive temperature. Although epsilonCOP is rapidly degraded in ldlF cells at the restrictive temperature, cellular amounts of the other COP-I subunits are not affected. Despite the absence of epsilonCOP, we find that a subcomplex of beta, beta', and zetaCOP is still recruited onto BHK endosomes in vitro, and this binding exhibits the characteristic properties of endosomal COPs with respect to stimulation by GTPgammaS and sensitivity to the endosomal pH. Previous studies showed that gamma and deltaCOP are not found on endosomes. However, alphaCOP, which is normally present on endosomes, is no longer recruited when epsilonCOP is missing. In contrast, all COP subunits, except obviously epsilonCOP itself, still bind BHK biosynthetic membranes in a pH-independent manner in vitro. Our observations thus indicate that the biogenesis of multivesicular endosomes is coupled to early endosome organization and depends on COP-I proteins. Our data also show that membrane association and function of endosomal COPs can be dissected: whereas beta, beta', and zetaCOP retain the capacity to bind endosomal membranes, COP function in transport appears to depend on the presence of alpha and/or epsilonCOP.

Animals↗
PubMed1998

Peroxisome biogenesis: involvement of ARF and coatomer.

Peroxisomal membrane protein (Pmp)26p (RnPex11p), a major constituent of induced rat liver peroxisomal membrane, was found to contain a COOH-terminal, cytoplasmically exposed consensus dilysine motif with the potential to bind coatomer. Biochemical as well as immunocytochemical evidence is presented showing that peroxisomes incubated with preparations of bovine brain or rat liver cytosol recruit ADP-ribosylation factor (ARF) and coatomer in a strictly guanosine 5'-O-(3-thiotriphosphate)-dependent manner. Consistent with this observation, ldlF cells expressing a temperature-sensitive mutant version of the epsilon-subunit of coatomer exhibit elongated tubular peroxisomes possibly due to impaired vesiculation at the nonpermissive temperature. Since overexpression of Pex11p in Chinese hamster ovary wild-type cells causes proliferation of peroxisomes, these data suggest that Pex11p plays an important role in peroxisome biogenesis by supporting ARF- and coatomer-dependent vesiculation of the organelles.

ADP-Ribosylation Factors↗
PubMed2000

The unfolded protein response regulates multiple aspects of secretory and membrane protein biogenesis and endoplasmic reticulum quality control.

The unfolded protein response (UPR) is an intracellular signaling pathway that relays signals from the lumen of the ER to activate target genes in the nucleus. We devised a genetic screen in the yeast Saccharomyces cerevisiae to isolate mutants that are dependent on activation of the pathway for viability. Using this strategy, we isolated mutants affecting various aspects of ER function, including protein translocation, folding, glycosylation, glycosylphosphatidylinositol modification, and ER-associated protein degradation (ERAD). Extending results gleaned from the genetic studies, we demonstrate that the UPR regulates trafficking of proteins at the translocon to balance the needs of biosynthesis and ERAD. The approach also revealed connections of the UPR to other regulatory pathways. In particular, we identified SON1/RPN4, a recently described transcriptional regulator for genes encoding subunits of the proteasome. Our genetic strategy, therefore, offers a powerful means to provide insight into the physiology of the UPR and to identify novel genes with roles in many aspects of secretory and membrane protein biogenesis.

Cloning, Molecular↗
PubMed2000

Dissection of autophagosome biogenesis into distinct nucleation and expansion steps.

Rapamycin, an antifungal macrolide antibiotic, mimics starvation conditions in Saccharomyces cerevisiae through activation of a general G(0) program that includes widespread effects on translation and transcription. Macroautophagy, a catabolic membrane trafficking phenomenon, is a prominent part of this response. Two views of the induction of autophagy may be considered. In one, up-regulation of proteins involved in autophagy causes its induction, implying that autophagy is the result of a signal transduction mechanism leading from Tor to the transcriptional and translational machinery. An alternative hypothesis postulates the existence of a dedicated signal transduction mechanism that induces autophagy directly. We tested these possibilities by assaying the effects of cycloheximide and specific mutations on the induction of autophagy. We find that induction of autophagy takes place in the absence of de novo protein synthesis, including that of specific autophagy-related proteins that are up-regulated in response to rapamycin. We also find that dephosphorylation of Apg13p, a signal transduction event that correlates with the onset of autophagy, is also independent of new protein synthesis. Finally, our data indicate that autophagosomes that form in the absence of protein synthesis are significantly smaller than normal, indicating a role for de novo protein synthesis in the regulation of autophagosome expansion. Our results define the existence of a signal transduction-dependent nucleation step and a separate autophagosome expansion step that together coordinate autophagosome biogenesis.

Active Transport, Cell Nucleus↗
PubMed2001

Biogenesis of porin of the outer mitochondrial membrane involves an import pathway via receptors and the general import pore of the TOM complex.

Porin, also termed the voltage-dependent anion channel, is the most abundant protein of the mitochondrial outer membrane. The process of import and assembly of the protein is known to be dependent on the surface receptor Tom20, but the requirement for other mitochondrial proteins remains controversial. We have used mitochondria from Neurospora crassa and Saccharomyces cerevisiae to analyze the import pathway of porin. Import of porin into isolated mitochondria in which the outer membrane has been opened is inhibited despite similar levels of Tom20 as in intact mitochondria. A matrix-destined precursor and the porin precursor compete for the same translocation sites in both normal mitochondria and mitochondria whose surface receptors have been removed, suggesting that both precursors utilize the general import pore. Using an assay established to monitor the assembly of in vitro-imported porin into preexisting porin complexes we have shown that besides Tom20, the biogenesis of porin depends on the central receptor Tom22, as well as Tom5 and Tom7 of the general import pore complex (translocase of the outer mitochondrial membrane [TOM] core complex). The characterization of two new mutant alleles of the essential pore protein Tom40 demonstrates that the import of porin also requires a functional Tom40. Moreover, the porin precursor can be cross-linked to Tom20, Tom22, and Tom40 on its import pathway. We conclude that import of porin does not proceed through the action of Tom20 alone, but requires an intact outer membrane and involves at least four more subunits of the TOM machinery, including the general import pore.

Amino Acid Sequence↗
PubMed2001

A link between the synthesis of nucleoporins and the biogenesis of the nuclear envelope.

The nuclear pore complex (NPC) is a multicomponent structure containing a subset of proteins that bind nuclear transport factors or karyopherins and mediate their movement across the nuclear envelope. By altering the expression of a single nucleoporin gene, NUP53, we showed that the overproduction of Nup53p altered nuclear transport and had a profound effect on the structure of the nuclear membrane. Strikingly, conventional and immunoelectron microscopy analysis revealed that excess Nup53p entered the nucleus and associated with the nuclear membrane. Here, Nup53p induced the formation of intranuclear, tubular membranes that later formed flattened, double membrane lamellae structurally similar to the nuclear envelope. Like the nuclear envelope, the intranuclear double membrane lamellae enclosed a defined cisterna that was interrupted by pores but, unlike the nuclear envelope pores, they lacked NPCs. Consistent with this observation, we detected only two NPC proteins, the pore membrane proteins Pom152p and Ndc1p, in association with these membrane structures. Thus, these pores likely represent an intermediate in NPC assembly. We also demonstrated that the targeting of excess Nup53p to the NPC and its specific association with intranuclear membranes were dependent on the karyopherin Kap121p and the nucleoporin Nup170p. At the nuclear envelope, the abilities of Nup53p to associate with the membrane and drive membrane proliferation were dependent on a COOH-terminal segment containing a potential amphipathic alpha-helix. The implications of these results with regards to the biogenesis of the nuclear envelope are discussed.

Active Transport, Cell Nucleus↗