Localization of Sed5, a putative vesicle targeting molecule, to the cis-Golgi network involves both its transmembrane and cytoplasmic domains

The yeast Sed5 protein, which is required for vesicular transport between ER and Golgi complex, is a membrane protein of the syntaxin family. These proteins are thought to provide the specific targets that are recognized by transport vesicles. We have investigated the mechanism by which Sed5 protein is itself localized. Expression of epitope-tagged versions of the yeast, Drosophila and rat Sed5 homologues in COS cells results in a perinuclear distribution; immuno-EM reveals that the majority of the protein is in a tubulo-vesicular compartment on the cis side of the Golgi apparatus. A similar distribution was obtained with a chimeric molecule consisting of a plasma membrane syntaxin with the Drosophila Sed5 transmembrane domain. This indicates that the membrane-spanning domain contains targeting information, as is the case with resident Golgi enzymes. However, alterations to the transmembrane domain of Drosophila Sed5 itself did not result in its mistargeting, implying that an additional targeting mechanism exists which involves only the cytoplasmic part of the protein. This was confirmed by modifying the transmembrane domain of the yeast Sed5 protein: substitution with the corresponding region from the Sso1 protein (a plasma membrane syntaxin homologue) did not affect yeast Sed5 function in vivo.

The TGN38 glycoprotein contains two non-overlapping signals that mediate localization to the trans-Golgi network

The membrane-spanning and cytoplasmic domains of CD4 and CD8 were replaced by those of TGN38. After transient expression in HeLa cells, the location of the hybrid proteins was determined using immunofluorescence and quantitative immuno-electron microscopy, FACS analysis and metabolic labeling. The membrane-spanning domain was found to contain a signal that localized hybrid proteins to the TGN. This was in addition to the signal previously identified in the cytoplasmic domain (Bos, K., C. Wraight, and K. Stanley. 1993. EMBO (Eur. Mol. Biol. Organ) J. 12:2219-2228. Humphrey, J. S., P. J. Peters, L. C. Yuan, and J. S. Bonifacino. 1993. J. Cell Biol. 120:1123-1135. Wong, S. H., and W. Hong. 1993. J. Biol. Chem. 268:22853-22862). The different properties of these two signals suggest that each operates by a different mechanism.

Kin recognition between medial Golgi enzymes in HeLa cells

The medial Golgi enzymes, N-acetylglucosaminyltransferase I (NAGT I) and mannosidase II (Mann II), and the trans Golgi enzyme, beta-1,4-galactosyltransferase (GalT) were each retained in the endoplasmic reticulum (ER) by grafting on the cytoplasmic tail of the p33 invariant chain. Transient and stable expression of p33/NAGT I in HeLa cells caused relocation of endogenous Mann II to the ER and transient expression of p33/Mann II had a similar effect on endogenous NAGT I. Neither of these endogenous medial enzymes were affected by transient expression of p33/GalT. These data provide strong evidence for kin recognition between medial Golgi enzymes and suggest a role for them in the organization of the Golgi stack.

The differential degradation of two cytosolic proteins as a tool to monitor autophagy in hepatocytes by immunocytochemistry

The major pathway for cytosolic constituents to enter lysosomes is by autophagy. We used two cytosolic proteins, CuZn superoxide dismutase (SOD) and carbonic anhydrase III (CAIII), as autophagic markers in male rat hepatocytes. We took advantage of the differential presence of the two proteins in autophagic vacuoles because of the high resistance of SOD to lysosomal degradation as compared with CAIII. This allows us to determine the sequence of autophagic vacuole formation. We have double immunogold-labeled SOD and CAIII in cryosections of fasted rat liver and calculated the ratios of SOD over CAIII labeling densities (SOD/CAIII) in autophagic vacuoles (AV), as compared with the cytoplasm. Different classes of AV were defined according to their SOD/CAIII, their morphology, and their additional immunolabeling for the lysosomal markers lgp120 and cathepsin D. Of all AV, 15% exhibited a cytosol-like SOD/CAIII, indicating that degradation had not yet begun. Most of these initial AV (AVi) showed two enveloping membranes. The formation of AVi was prevented by 3-methyladenine, a potent inhibitor of autophagy. Of all AV, 85% showed a SOD/CAIII that exceeded the cytosolic ratio. These single membrane-bound vacuoles were called degradative AV (AVd). Labeling for lysosomal markers allowed the characterization of AV that shared features with both AVi and AVd. These AVi/d had a cytosol-like SOD/CAIII and a double membrane, but showed some labeling for lysosomal markers. Probably these AVi/d represent the recipient compartment for lysosomal components. AVd were positive for cathepsin D and lgp120. We discerned two AVd subclasses. Early AVd with cytosol-like SOD labeling density while CAIII labeling density was consistently lower than in the cytosol. Their size was similar to AVi and AVi/d. Late AVd contained higher SOD concentrations and were mostly larger. Our findings suggest that AV acquire lysosomal constituents by fusion with small nonautophagic structures and that after subsequent elimination of the inner membrane of AVi, degradation starts resulting in the formation of early AVd and late AVd.

Copper,zinc superoxide dismutase is primarily a cytosolic protein in human cells

The intracellular localization of human copper,zinc superoxide dismutase (Cu,Zn-SOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) was evaluated by using EM immunocytochemistry and both isolated human cell lines and human tissues. Eight monoclonal antibodies raised against either native or recombinant human Cu,Zn-SOD and two polyclonal antibodies raised against either native or recombinant human Cu,Zn-SOD were used. Fixation with 2% paraformaldehyde/0.2% glutaraldehyde was found necessary to preserve normal distribution of the protein. Monoclonal antibodies were less effective than polyclonal antibodies in recognizing the antigen after adequate fixation of tissue. Cu,Zn-SOD was found widely distributed in the cell cytosol and in the cell nucleus, consistent with it being a soluble cytosolic protein. Mitochondria and secretory compartments did not label for this protein. In human cells, peroxisomes showed a labeling density slightly less than that of cytoplasm.

Nonselective utilization of the endomannosidase pathway for processing glycoproteins by human hepatoma (HepG2) cells

Endo-alpha-D-mannosidase, a Golgi-situated processing enzyme, provides a glucosidase-independent pathway for the formation of complex N-linked oligosaccharides of glycoproteins (Moore, S. E. H., and Spiro, R. G. (1990) J. Biol. Chem. 265, 13104-13112). The present report demonstrates that at least five distinct glycoproteins secreted by HepG2 cells (alpha 1-antitrypsin, transferrin, alpha 1-acid glycoprotein, alpha 1-antichymotrypsin, and alpha-fetoprotein) as well as cell surface components can effectively utilize this alternate processing route. During a castanospermine (CST)-imposed glucosidase blockade, these glycoproteins apparently were produced with their usual complement of complex carbohydrate units, and upon addition of the mannosidase I inhibitor, 1-deoxymannojirimycin (DMJ), to prevent further processing of deglucosylated N-linked oligosaccharides, Man6-8GlcNAc, but not Man9GlcNAc, were identified; the Man8GlcNAc component occurred as the characteristic isomer generated by endomannosidase cleavage. Although the endomannosidase-mediated deglucosylation pathway appeared to be nonselective, a differential inhibitory effect on the secretion of the various glycoproteins was noted in the presence of CST which was directly related to the number of their N-linked oligosaccharides, ranging from minimal in alpha-fetoprotein to substantial (approximately 65%) in alpha 1-acid glycoprotein. Addition of DMJ to CST-incubated cells did not further decrease secretion of the glycoproteins, although processing was now arrested at the polymannose stage, and a portion of the oligosaccharides were still in the glucosylated form. These latter findings indicate that complex carbohydrate units are not required for secretion of these glycoproteins and that any effect which glucose residues exert on their intracellular transit would be related to movement from the endoplasmic reticulum to the Golgi compartment.

Fractal organisation in biological macromolecular lattices

Macromolecular solutions of proteic, glycoproteic or polysaccharidic nature in the presence of salt gave rise (when slowly dried) to dendritic-like fractal patterns. A fractal dimension D = 1.79 +/- 0.018 was obtained for dendritic patterns of a sonicated ovomucin gel (40% ovomucin-60% ovalbumin) in the presence of 0.1 M NaCl. The calculated D is similar to that described in percolation clusters. The appearance of fractal patterns was dependent upon the protein:salt ratio with an optimum in the range 0.75-1.25. Patterns disappeared at either lower or higher ratios. We conclude that the salt percolates through the macromolecular lattice and precipitates in fractal clusters during the drying process. Dendritic-like fractal patterns with similar D and morphologies were obtained with solutions of fetuin, ovalbumin, albumin or starch suggesting that fractal patterning is a general property of biological polymers. That cellular polymers would also aggregate in a fractal way was implied from the analysis of cellular cytoskeleton and microtrabecular lattices.

The supramolecular organization of ovomucin. Biophysical and morphological studies

Ovomucin participates in the ovomucin-gel-forming properties because of its shape and its ability to interact in a specific spatial organization. Purified from chicken egg-white by exclusion chromatography with Sephacryl S-300 and Sepharose CL-2B and analysed by light-scattering, it exhibited an Mr of about 40 x 10(6). This large Mr can be explained by the aggregation of polymers that can be degraded into 3 x 10(6)-Mr fragments by reduction with dithiothreitol. The values for hydrodynamic parameters such as Mr, radius of gyration, hydrodynamic radius, mass per unit length and combinations of them suggested that ovomucin is a linear and highly flexible molecule conferring upon it a random-coil-like structure in 0.2 M-NaCl solution. Analysis of the ovomucin molecules by electron microscopy revealed its linear character but also indicated a lower Mr than that obtained in the light-scattering experiments. By temperature-induced non-specific aggregation of an ovomucin solution containing other globular egg proteins, an attempt was made to find out what conditions are required for gel formation and to examine the quality of aggregation that is obtained under these conditions. Results show that the viscosity of the solution did not increase after heat treatment. Apparently, in the ovomucin gel, specific spatial organization of the ovomucin molecules is required for hydrogel formation.

Interactions involved in ovomucin gel-forming properties: a rheological-biochemical approach

Different kinds of interactions involved in the properties of ovomucin gel formation from hen egg white were studied by combining physical and biochemical methods. A decrease in viscosity of the ovomucin gel was observed when it was subjected to chymotrypsin or sonication treatment. The viscosity decrease correlated with a change from non-Newtonian to Newtonian properties of the ovomucin gel. By treatment of the gel with either 5 M guanidinium HCl, 6 M urea, or 5% sodium dodecyl sulfate a change from non-Newtonian to Newtonian properties was also obtained. Although high ionic strength or sialic acid liberation from the ovomucin gel by neuraminidase treatment provoked a decrease in viscosity, it was not followed by a change in non-Newtonian properties. The results obtained suggest that different noncovalent interactions might be involved in gel formation. Electrostatic interactions (partially destroyed by sialic acid removal or 2 M NaCl) and hydrophobic interactions might be responsible for protein-mucin and mucin-mucin interactions. Other bonds susceptible to chymotrypsin treatment and sonication would be involved in the interaction between mucin subunits.

Immobilized respiratory chain activities from Escherichia coli utilized to measure D- and L-lactate, succinate, L-malate, 3-glycerophosphate, pyruvate, or NAD(P)H

The respiratory chain (membranous, multienzymatic system) from Escherichia coli, was coimmobilized with gelatin and insolubilized in film form by tanning with glutaraldehyde. The film was fixed onto an oxygen sensor. The enzyme electrode can be used for measuring NAD(P)H, D- and L-lactate, succinate, L-malate, 3-glycerophosphate, or pyruvate. The range of metabolites concentrations was from 1 to 50 mM. It was possible to discriminate between the different metabolites (if mixed): By inducing during bacterial growth the specific flavoproteins necessary for L-lactate, succinate, L-malate, and 3-glycerophosphate respirations. The constitutive activities are unaltered on glucose or glycerol, namely D-lactate, NAD(P)H, and pyruvate respiration. When intact bacteria were immobilized (with or without induction), D- and L-lactate, succinate, 3-glycerophosphate, and L-malate respiration were measured, no activities of pyruvate and NAD(P)H respiration were obtained. For these last activities, French press breakage (see section on Membrane Preparations) of bacteria prior to immobilization was necessary. Products of reactions can be used as enzyme inhibitors: Pyruvate inhibits D- and L-lactate; fumarate inhibits succinate, and oxaloacetate inhibits L-malate respirations. Heat denaturation of the bacteria at 55 degrees C for 1 h maintains full activity of succinate and pyruvate respiration. On the other hand, no activity of D- and L-lactate, L-malate, or NAD(P)H respiration was measurable. These enzyme electrodes have many applications in basic and applied research.