The formation of a bezoar is a relatively infrequent disorder that affects the gastrointestinal system. Bezoars are mainly classified into four types depending on the material constituting the indigestible mass of the bezoar: phytobezoars, trichobezoars, pharmacobezoars, and lactobezoars. Gastric bezoars often cause ulcerative lesions in the stomach and subsequent bleeding, whereas small intestinal bezoars present with small bowel obstruction and ileus. A number of articles have emphasized the usefulness of Coca-Cola(®) administration for the dissolution of phytobezoars. However, persimmon phytobezoars may be resistant to such dissolution treatment because of their harder consistency compared to other types of phytobezoars. Better understanding of the etiology and epidemiology of each type of bezoar will facilitate prompt diagnosis and management. Here we provide an overview of the prevalence, classification, predisposing factors, and manifestations of bezoars. Diagnosis and management strategies are also discussed, reviewing mainly our own case series. Recent progress in basic research regarding persimmon phytobezoars is also briefly reviewed.
Expression of nuclear-encoded plastid proteins and import of those proteins into plastids are indispensable for plastid biogenesis. One possible cellular mechanism that coordinates these two essential processes is retrograde signaling from plastids to the nucleus. However, the molecular details of how this signaling occurs remain elusive. Using the plastid protein import2 mutant of Arabidopsis (Arabidopsis thaliana), which lacks the atToc159 protein import receptor, we demonstrate that the expression of photosynthesis-related nuclear genes is tightly coordinated with their import into plastids. Down-regulation of photosynthesis-related nuclear genes is also observed in mutants lacking other components of the plastid protein import apparatus. Genetic studies indicate that the coordination of plastid protein import and nuclear gene expression is independent of proposed plastid signaling pathways such as the accumulation of Mg-protoporphyrin IX and the activity of ABA INSENSITIVE4 (ABI4). Instead, it may involve GUN1 and the transcription factor AtGLK. The expression level of AtGLK1 is tightly correlated with the expression of photosynthesis-related nuclear genes in mutants defective in plastid protein import. Furthermore, the activity of GUN1 appears to down-regulate the expression of AtGLK1 when plastids are dysfunctional. Based on these data, we suggest that defects in plastid protein import generate a signal that represses photosynthesis-related nuclear genes through repression of AtGLK1 expression but not through activation of ABI4.Plastids are a diverse group of organelles that perform essential metabolic and signaling functions within all plant cells. It is generally believed that plastids originated from a unicellular photosynthetic bacterium that was taken up by a eukaryotic host cell (Dyall et al., 2004). During evolution, most of the genes encoded by the bacterial ancestor have been transferred to the host nuclear genome; for example, the plastid genome of Arabidopsis (Arabidopsis thaliana) encodes fewer than 100 open reading frames (Martin et al., 1998). Consequently, plastid biogenesis is dependent on the import of nuclear-encoded plastid proteins (Keegstra and Cline, 1999;Soll and Schleiff, 2004;Kessler and Schnell, 2006;Inaba and Schnell, 2008;Jarvis, 2008), the genes for which must be expressed at an appropriate level. For example, many of the photosynthesis-related nuclear genes that are required for chloroplast biogenesis are induced via photoreceptors, such as phytochrome, in response to light quality and quantity (Terzaghi and Cashmore, 1995), so that the photosynthesis-related proteins will be available for import into the developing chloroplasts. Other types of plastids, according to their specific metabolic functions, need other sets of nuclear-encoded proteins. Therefore, the expression of specific sets of nuclear genes and the import of their translation products are indispensable for plastid differentiation.After they have been imported into plastids, nuclear-encoded plastid proteins combi...
The translocon at the inner envelope membrane of chloroplasts (Tic) plays a central role in plastid biogenesis by coordinating the sorting of nucleus-encoded preproteins across the inner membrane and coordinating the interactions of preproteins with the processing and folding machineries of the stroma. Despite these activities, the precise roles of known Tic proteins in translocation, sorting, and preprotein maturation have not been defined. In this report, we examine the in vivo function of a major Tic component, Tic110. We demonstrate that Arabidopsis thaliana Tic110 (atTic110) is essential for plastid biogenesis and plant viability. The downregulation of atTic110 expression results in the reduced accumulation of a wide variety of plastid proteins. The expression of dominant negative mutants of atTic110 disrupts assembly of Tic complexes and the translocation of preproteins across the inner envelope membrane. Together, these data suggest that Tic110 plays a general role in the import of nuclear-encoded preproteins as a common component of Tic complexes.
The translocon of the inner envelope membrane of chloroplasts (Tic) mediates the late events in the translocation of nucleus-encoded preproteins into chloroplasts. Tic110 is a major integral membrane component of active Tic complexes and has been proposed to function as a docking site for translocation-associated stromal factors and as a component of the protein-conducting channel. To investigate the various proposed functions of Tic110, we have investigated the structure, topology, and activities of a 97.5-kDa fragment of Arabidopsis Tic110 (atTic110) lacking only the amino-terminal transmembrane segments. The protein was expressed both in Escherichia coli and Arabidopsis as a stable, soluble protein with a high ␣-helical content. Binding studies demonstrate that a region of the atTic110-soluble domain selectively associates with chloroplast preproteins at the late stages of membrane translocation. These data support the hypothesis that the bulk of Tic110 extends into the chloroplast stroma and suggest that the domain forms a docking site for preproteins as they emerge from the Tic translocon.Chloroplast biogenesis is dependent upon the import of ϳ3000 different nucleus-encoded proteins (1). The majority of these proteins are synthesized as preproteins carrying an amino-terminal transit peptide that serves as the essential signal for targeting to the organelle. The transit peptide is recognized by receptor components of the translocon at the outer envelope membrane of chloroplasts (Toc), and a GTP-regulated switch initiates translocation through the protein-conducting channel of the Toc complex. At this stage, the Toc complex associates with the translocon at the inner envelope membrane (Tic), and this Toc-Tic supercomplex mediates the direct transport of the preproteins from the cytoplasm into the chloroplast stroma (2).Although many mechanistic details remain to be defined, the activities of the Toc components have been extensively investigated. Two membrane-associated GTPases, Toc159 and Toc34/33, mediate transit peptide recognition and regulate the initiation of translocation (3-7). The Toc GTPases form a complex with Toc75, an integral membrane protein that, along with Toc159, constitutes a major component of the protein-conducting channel (1, 8 -10).In contrast to the Toc complex, the activities and functions of the Tic components are less well defined. The biochemical analysis of Tic function has been complicated by the fact that assembly of functional Tic complexes is dynamic and occurs in response to preprotein translocation (11). Therefore, the isolation of a stable Tic complex has thus far been elusive. Tic110 was the first Tic component identified and represents a major component of active Tic complexes (12, 13). It is an integral inner envelope membrane protein, and structural predictions suggest that it consists of two predicted transmembrane helices at its extreme amino terminus and a 97.5-kDa carboxyl-terminal region that is largely hydrophilic. Tic110 transiently associates with at least five o...
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