Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.4.99.7 (sialyltransferase)
 1,534 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The cation-independent mannose 6-phosphate receptor (MPRCI) functions in the packaging of both newly made and extracellular lysosomal enzymes into lysosomes. The subcellular location of MPRCI reflects these two functions; receptor is found in the Golgi complex, in endosomes, and on the cell surface. To learn about the intracellular pathway followed by surface receptor and to study the relationship between the receptor pools, we examined the entry of the surface MPRCI into Golgi compartments that contain sialyltransferase. Sialic acid was removed from surface-labeled K562 cultured human erythroleukemia cells by neuraminidase treatment. When the cells were returned to culture at 37 degrees C, surface MPRCI was resialylated by the cells with a half-time of 1-2 h. Resialylation was inhibited by reduced temperature, a treatment that allows surface molecules to reach endosomes but blocks further transport. These results indicate that surface MPRCI is transported to the sialyltransferase compartment in the Golgi complex. After culture at 37 degrees C, a small fraction (10-20%) of the resialylated receptor was found on the cell surface. Because a similar fraction of the total receptor pool is found on the cell surface, it is likely that cell surface MPRCI mixes with the cellular pool after resialylation. These data also support the idea that extracellular and newly made lysosomal enzymes are transported to lysosomes through a common compartment.
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PMID:Transport of surface mannose 6-phosphate receptor to the Golgi complex in cultured human cells. 254 Feb

We have developed a method for the isolation of the subcellular organelles from bovine liver which are enriched in the cation-independent mannose 6-phosphate receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor (CD-MPR). The purification scheme consists of sedimentation of a postnuclear supernatant fraction on a sucrose gradient followed by immunoisolation using specific anti-peptide antibodies conjugated to magnetic polystyrene beads. Antibodies that recognize the cytoplasmic domain of either the CI-MPR or the CD-MPR routinely give membrane preparations that are approximately 50-fold enriched in each of the respective receptors, as determined by quantitative Western blotting. The immunoisolated membranes are also enriched in the other MPR, as well as in the asialoglycoprotein receptor. They contain significantly lower levels of enzyme activities representative of the plasma membrane (5' nucleotidase) or the Golgi complex (galactosyltransferase and sialyltransferase). There is little or no enrichment for either the lysosomal enzymes beta-hexosaminidase and tartrate-resistant acid phosphatase, or the mitochondrial enzyme succinate-tetrazolium reductase. These data, together with electron microscopy of the immunoisolated material, suggest that the bulk of MPR-containing membranes we have isolated from bovine liver correspond to endosomes. Analysis by SDS-PAGE indicates that several proteins, including two with apparent molecular weights of 170 K and 400 K, are significantly enriched in the purified fractions and may represent potential markers for MPR-containing endosomes.
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PMID:Isolation and characterization of membranes from bovine liver which are highly enriched in mannose 6-phosphate receptors. 254 3

Insulin regulates glucose transport in muscle and adipose tissue by triggering the translocation of a facilitative glucose transporter, GLUT4, from an intracellular compartment to the cell surface. It has previously been suggested that GLUT4 is segregated between endosomes, the trans-Golgi network (TGN), and a postendosomal storage compartment. The aim of the present study was to isolate the GLUT4 storage compartment in order to determine the relationship of this compartment to other organelles, its components, and its presence in different cell types. A crude intracellular membrane fraction was prepared from 3T3-L1 adipocytes and subjected to iodixanol equilibrium sedimentation analysis. Two distinct GLUT4-containing vesicle peaks were resolved by this procedure. The lighter of the two peaks (peak 2) was comprised of two overlapping peaks: peak 2b contained recycling endosomal markers such as the transferrin receptor (TfR), cellubrevin, and Rab4, and peak 2a was enriched in TGN markers (syntaxin 6, the cation-dependent mannose 6-phosphate receptor, sortilin, and sialyltransferase). Peak 1 contained a significant proportion of GLUT4 with a smaller but significant amount of cellubrevin and relatively little TfR. In agreement with these data, internalized transferrin (Tf) accumulated in peak 2 but not peak 1. There was a quantitatively greater loss of GLUT4 from peak 1 than from peak 2 in response to insulin stimulation. These data, combined with the observation that GLUT4 became more sensitive to ablation with Tf-horseradish peroxidase following insulin treatment, suggest that the vesicles enriched in peak 1 are highly insulin responsive. Iodixanol gradient analysis of membranes isolated from other cell types indicated that a substantial proportion of GLUT4 was targeted to peak 1 in skeletal muscle, whereas in CHO cells most of the GLUT4 was targeted to peak 2. These results indicate that in insulin-sensitive cells GLUT4 is targeted to a subpopulation of vesicles that appear, based on their protein composition, to be a derivative of the endosome. We suggest that the biogenesis of this compartment may mediate withdrawal of GLUT4 from the recycling system and provide the basis for the marked insulin responsiveness of GLUT4 that is unique to muscle and adipocytes.
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PMID:Characterization of insulin-responsive GLUT4 storage vesicles isolated from 3T3-L1 adipocytes. 1059 43

Previously, we demonstrated that beta 1,4galactosyltransferase (gal-T1) reversibly segregates from alpha 2,6sialyltransferase (ST6Gal) to swollen vesicles after monensin treatment of the cells. To further explore this phenomenon, we investigated the response to monensin of various Golgi proteins. Within 30 min of monensin treatment, gal-T1 moved from the Golgi apparatus, as defined by localization of giantin, to swollen vesicles whereas ST6Gal, alpha 2,3(N)sialyltransferase, mannosidase II, and N-acetylgalactosaminyltransferase 2 remained associated with the Golgi apparatus. Stably transfected CHO cells exhibited a similar phenomenon of monensin-induced displacement of recombinant gal-T1 to swollen vesicles while recombinant ST6Gal remained colocalized with endogenously expressed giantin. Gal-T1 and the cation-insensitive mannose 6-phosphate receptor colocalized in swollen vesicles as observed at both light and electron microscopic levels. When monensin was replaced by chloroquine, gal-T1 remained arrested in swollen vesicles. Brefeldin A treatment known to cause relocation of Golgi-associated gal-T1 to the endoplasmic reticulum had no effect on gal-T1 trapped in swollen vesicles. This evidence suggests that monensin blocks gal-T1 trafficking in post-Golgi structures and argues against swelling of gal-T1-containing trans Golgi cisternae as previously assumed.
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PMID:Colocalization of beta 1,4galactosyltransferase with mannose 6-phosphate receptor in monensin-induced TGN-derived structures. 1144 50