Biosynthesis and intracellular transport of a membrane glycoprotein (plgp57) of the prelysosome compartment

We have characterized the biosynthesis and intracellular transport of a membrane glycoprotein, designated plgp57, which is found predominantly in the prelysosome compartment (PLC) of Madin-Darby bovine kidney cells. In pulse-chase experiments, plgp57 was found to be initially synthesized as a 35 kDa precursor which was modified to yield a diffuse approximately 57 kDa mature form. Digestion with endoglycosidase H (endo H) demonstrated that the 35 kDa precursor contained three endo H-sensitive, high mannose-type oligosaccharides which became modified to endo H-resistant, complex-type sugars on the approximately 57 kDa mature form. Labelling cells in the presence of tunicamycin and treatment of the 35 kDa precursor with endo H revealed that plgp57 has a core protein of 24 kDa to which three N-asparagine-liked oligosaccharides are attached. Other experiments indicated that plgp57 could be differentially glycosylated on a common 24 kDa core protein in different cell types. The half-lives of the glycosylated and non-glycosylated forms of plgp57 were approximately 18 and approximately 13 h, respectively. Glycosylated and non-glycosylated plgp57 exhibited similar steady-state intracellular distributions, indicating that targeting of plgp57 to the PLC does not require carbohydrate address markers. Pulse-labeling of cells followed by organelle fractionation at various chase times revealed a t1/2 = approximately 1 h for the transit of newly synthesized plgp57 to the PLC. Finally, amino terminal sequencing of plgp57 revealed the similarity of this protein to the CD63/ME491 family of membrane glycoproteins.

The vascular endothelial growth factor (VEGF) isoforms: differential deposition into the subepithelial extracellular matrix and bioactivity of extracellular matrix-bound VEGF

Vascular endothelial growth factor (VEGF)mRNA undergoes alternative splicing events that generate four different homodimeric isoforms, VEGF121, VEGF165, VEGF189, or VEGF206. VEGF121 is a nonheparin-binding acidic protein, which is freely diffusible. The longer forms, VEGF189 or VEGF206, are highly basic proteins tightly bound to extracellular heparin-containing proteoglycans. VEGF165 has intermediate properties. To determine the localization of VEGF isoforms, transfected human embryonic kidney CEN4 cells expressing VEGF165, VEGF189, or VEGF206 were stained by immunofluorescence with a specific monoclonal antibody. The staining was found in patches and streaks suggestive of extracellular matrix (ECM). VEGF165 was observed largely in Golgi apparatus-like structures. Immunogold labeling of cells expressing VEGF189 or VEGF206 revealed that the staining was localized to the subepithelial ECM. VEGF associated with the ECM was bioactive, because endothelial cells cultured on ECM derived from cells expressing VEGF189 or VEGF206 were markedly stimulated to proliferate. In addition, ECM-bound VEGF can be released into a soluble and bioactive form by heparin or plasmin. ECM-bound VEGF189 and VEGF206 have molecular masses consistent with the intact polypeptides. The ECM may represent an important source of VEGF and angiogenic potential.

Three-dimensional structural resemblance between leucine aminopeptidase and carboxypeptidase A revealed by graph-theoretical techniques

Using 3-D searching techniques based on algorithms derived from graph theory we have established a striking structural similarity between the structure of bovine carboxypeptidase A and that of the C-terminal domain of bovine leucine aminopeptidase. There is no significant sequence homology between the aminopeptidases and the carboxypeptidases but the strong structural relationship detected in this complex fold suggests that there may be a very remote divergent evolutionary relationship between these two enzyme classes.

Brefeldin A causes a microtubule-mediated fusion of the trans-Golgi network and early endosomes

Brefeldin A (BFA) is a fungal metabolite that causes a redistribution of the stacked cisternae of the Golgi complex into the endoplasmic reticulum by inhibiting anterograde transport. We report that BFA also causes membrane tubules derived from the trans-Golgi network (TGN) to fuse with early endosomes. In the presence of BFA, a mannose-6-phosphate receptor (M6PR)-enriched tubular network rapidly forms from the TGN, not from the prelysosomal compartment, and can be labeled with endocytic tracers after only 5 min of uptake at either 20 degrees C or 37 degrees C, indicating that it is also functionally an early endosome. Formation of the TGN-early endosome network is microtubule dependent and may involve modification of membrane processes affected by microtubule-associated motor activity. Concomitant with the formation of the fused TGN-early endosome network, there is a greater than 5-fold increase in cell surface M6PRs. The data suggest that BFA has revealed a membrane transport cycle between the TGN and early endosomes, perhaps used for the secretion or delivery of molecules to the cell surface.

Biosynthesis of lysosomal enzymes in cells of the End3 complementation group conditionally defective in endosomal acidification

Various aspects of lysosome biogenesis have been studied in Chinese hamster ovary (CHO) cells of the End3 complementation group (designated G.7.1 cells), which display a temperature-sensitive defect in the acidification of endosomes, but not lysosomes. In G.7.1 and normal wild-type cells grown at the permissive temperature (34 degrees C), the lysosomal enzymes alpha-glucosidase and cathepsin D were synthesized as high-molecular-weight precursors that subsequently underwent intracellular proteolytic processing to yield lower molecular weight mature forms. The mature forms of the enzymes were retained in cells, and small amounts of each precursor were secreted. However, in G.7.1 cells grown at the restrictive temperature (41 degrees C), there was a massive and inappropriate oversecretion of lysosomal enzyme precursors, which resulted in very little of the mature forms being processed and retained by the cells. This mistargeting of lysosomal enzymes was not due to an absence of phosphorylated oligosaccharides on the enzymes, nor to a defect in mannose 6-phosphate (Man6P) receptors. However, it was found that whereas G.7.1 cells had the same number of cell surface Man6P receptors at 34 degrees C and 41 degrees C, the rate of accumulation and degradation of Man6P-containing ligands was about two to three times more rapid in cells maintained at the permissive temperature. There did not appear to be any gross changes in Golgi function as the oligosaccharides of alpha-glucosidase and the Man6P receptor were processed in a similar fashion at both 34 degrees C and 41 degrees C. In addition to these studies, electron microscopic observations revealed that at 41 degrees C, G.7.1 cells accumulated inclusion-type bodies reminiscent of those found in I-cell disease fibroblasts. Thus, the biochemical and electron microscopic results on G.7.1 cells provide further evidence that acidified endosomes are important for the biogenesis of lysosomes.

Identification of a membrane glycoprotein found primarily in the prelysosomal endosome compartment

Cells contain an intracellular compartment that serves as both the "prelysosomal" delivery site for newly synthesized lysosomal enzymes by the mannose 6-phosphate (Man6P) receptor and as a station along the endocytic pathway to lysosomes. We have obtained mAbs to a approximately 57-kD membrane glycoprotein, (called here plgp57), found predominantly in this prelysosomal endosome compartment. This conclusion is supported by the following results: (a) plgp57 was primarily found in a population of late endosomes that were located just distal to the 20 degrees C block site in the endocytic pathway to lysosomes (approximately 83% of the prelysosomes were positive for plgp57 but less than 5% of the early endosomes had detectable amounts of this marker); (b) plgp57 and the cation-independent (CI) Man6P receptor were located in many of the same intracellular vesicles; (c) plgp57 was found in the membranes of an acidic compartment; (d) immunoelectron microscopy showed that plgp57 was located in characteristic multilamellar- and multivesicular-type vacuoles believed to be prelysosomal endosomes; and (e) cell fractionation studies demonstrated that plgp57 was predominantly found in low density organelles which comigrated with late endosomes and CI Man6P receptors, and only approximately 10-15% of the antigen was found in high density fractions containing the majority of secondary lysosomes. These results indicate that plgp57 is a novel marker for a unique prelysosomal endosome compartment that is the site of confluence of the endocytic and biosynthetic pathways to lysosomes.

A large pedunculated left ventricular thrombus with recurrent systemic thromboembolism in a young man

We present the case of a young man who has suffered from recurrent systemic thromboembolism since he developed an acute interior wall myocardial infarction at the age of 27. A large elongated, pendunculated left ventricular thrombus was found by two dimensional echocardiography and was successfully removed by open heart surgery.