Ultrastructural identification of sulphated glycoconjugates in the Golgi apparatus in human colonic absorptive cells

Summary The subcellular localization of sulphated glycoconjugates was determined at the ultrastructural level by using the high iron diamine (HID) technique for sulphate groups in the absorptive cells of human colonic mucosa. Stained material was observed on the apical plasma membrane, in intracytoplasmic vesicles and in the Golgi complex. In this organelle, the last two or three cisternae of the trans side and the trans-Golgi network (TGN) were labelled, as well as a variable number of coated and noncoated vesicles facing the trans side and surrounding trans-Golgi network. These findings point to the trans side of the Golgi apparatus and trans-Golgi network as the subcompartments functionally involved in the sulphation of glycoconjugates.     

Ultrastructural localization of CMPase, TPPase, and NADPase activity in neurons, satellite cells, and Schwann cells in frog dorsal root ganglia

Sections of bullfrog dorsal root ganglia were analyzed for cytidine monophosphatase (CMPase), thiamine pyrophosphatase (TPPase), and nicotinamide adenine dinucleotide phosphatase (NADPase) activity, and the distributions of these enzymatic activities were compared with those traditionally found in other cell types (e.g., CMPase: Golgi trans-sacculotubular network; TPPase: trans-Golgi saccule(s); NADPase: intermediate Golgi saccules). In the present study, CMPase activity in neurons was localized mainly to the Golgi trans-sacculotubular network and lysosomes, but sometimes also occurred at the ends of the trans and most distal intermediate Golgi saccules. A similar distribution was found in satellite and Schwann cells. TPPase activity in neurons occurred not only in the trans-Golgi saccule but also in the trans-sacculotubular network, lysosomes, and scattered tubular elements. In satellite and Schwann cells, activity was found in both the trans saccule and trans-sacculotubular network, and substantial activity often appeared in the more distal of the intermediate saccules. NADPase activity in neurons was usually absent from the intermediate Golgi saccules and was confined to the trans-sacculotubular network and lysosomes; however, activity was sometimes also found in the intermediate and/or trans-Golgi saccules. In satellite and Schwann cells, activity appeared consistently in both the trans-sacculotubular network and intermediate saccules, as well as in lysosomes. These distributions, especially in the case of TPPase and NADPase, differ substantially from the most frequently reported localizations of the above enzymes, indicating that the Golgi complex may exhibit considerable plasticity of structure and function in different cell types.     

Further studies of the glandular tissue of the Sauromatum guttatum (Araceae) appendix

Electron microscopic studies showed that the trans-Golgi network (trans indicates the polarity of cisternae within the Golgi apparatus; it is opposite to the cis-face that is adjacent to the rough endoplasmic reticulum) was involved in the processing of the osmiophilic material present in the appendix of the inflorescence of Sauromatum guttatum. This material accumulated in the rough endoplasmic reticulum and in special pockets of the plasma membrane prior to heat production. Associations between the endoplasmic reticulum and trans-Golgi network were observed. The Golgi apparatus was composed of 5–6 dictyosomes on one side and one or two somewhat detached cisternae on the other side. Various nonosmiophilic Golgi-derived vesicles were observed: small ones covered with spike-like material, large ones with a smooth surface, and irregularly shaped ones. These electron-translucent vesicles seemed to accumulate in specific localities at the plasma membrane surface in the vicinity of the osmiophilic material; they were not found when the aroma was released. During heat production, the Golgi structures shrank and the activity of the trans-Golgi network seemed to be reduced. At the same time, coated pits were seen at the plasma membrane surface. In some cells, hypertrophic Golgi apparatuses were seen with only 2–3 dictyosomes that contained granulated material in their lumens. Finally, the osmiophilic material was also found in the plasmodesmata.     

Targeting of active sialyltransferase to the plant Golgi apparatus.

Glycosyltransferases in the Golgi apparatus synthesize cell wall polysaccharides and elaborate the complex glycans of glycoproteins. To investigate the targeting of this type of enzyme to plant Golgi compartments, we generated transgenic Arabidopsis plants expressing alpha-2,6-sialyltransferase, a glycosyltransferase of the mammalian trans-Golgi cisternae and the trans-Golgi network. Biochemical analysis as well as immunolight and immunoelectron microscopy of these plants indicate that the protein is targeted specifically to the Golgi apparatus. Moreover, the protein is predominantly localized to the cisternae and membranes of the trans side of the organelle. When supplied with the appropriate substrates, the enzyme has significant alpha-2,6-sialyltransferase activity. These results indicate a conservation of glycosyltransferase targeting mechanisms between plant and mammalian cells and also demonstrate that glycosyltransferases can be subcompartmentalized to specific cisternae of the plant Golgi apparatus.     

Targeting of the GRIP domain to the trans-Golgi network is conserved from protists to animals

The GRIP domain, found in a family of coiled-coil peripheral membrane Golgi proteins, is a specific targeting sequence for the trans-Golgi network of animal cells. In this study we show that a coiled-coil protein with a GRIP domain occurs in the primitive eukaryote, Trypanosoma brucei, and that reporter proteins containing this domain can be used as a marker for the poorly characterized trans Golgi/trans-Golgi network of trypanosomatid parasites. The T. brucei GRIP domain, when fused to the carboxyl terminus of the green fluorescent protein (GFP-TbGRIP), was efficiently localized to the Golgi apparatus of transfected COS cells. Overexpression of GFP-TbGRIP in COS cells displaced the endogenous GRIP protein, GCC1p, from the Golgi apparatus indicating that the trypanosomatid and mammalian GRIP sequences interact with similar membrane determinants. GFP fusion proteins containing either the T. brucei GRIP domain or the human p230 GRIP (p230GRIP) domain were also expressed in the trypanosomatid parasite, Leishmania mexicana, and localized by fluorescence and immuno-electron microscopy to the trans face of the single Golgi apparatus and a short tubule that extended from the Golgi apparatus. Binding of GFP-p230GRIP to Golgi membranes in L. mexicana was abrogated by mutation of a critical tyrosine residue in the p230 GRIP domain. The levels of GFP-GRIP fusion proteins were dramatically reduced in stationary-phase L. mexicana promastigotes, suggesting that specific Golgi trafficking steps may be down-regulated as the promastigotes cease dividing. This study provides a protein marker for the trans-Golgi network of trypanosomatid parasites and suggests that the GRIP domain binds to a membrane component that has been highly conserved in eukaryotic evolution.     

Immunocytochemical localization of beta 1,4 galactosyltransferase in epithelial cells from bovine tissues using monoclonal antibodies.

Post-embedding immunocytochemistry was employed to investigate the distribution of UDP-galactose:N-acetylglucosamine galactosyltransferase (beta 1,4-GT) in epithelial cells from various bovine organs. Several well characterized monoclonal antibodies previously demonstrated to recognize distinct polypeptide epitopes within the primary structure of beta 1,4-GT were applied to thin sections from tissues embedded in Lowicryl K4M, followed by the protein A-gold technique. Immunoreactivity was observed in the Golgi apparatus of epithelial cells from intestine, thymus and trachea. No immunoreactivity was observed in other intracellular structures, including rough endoplasmic reticulum, nuclear envelope and goblet cell mucus droplets. Within the Golgi apparatus, the staining was restricted to several cisternae in the trans region, with most portions of the trans-Golgi network appearing unlabelled. However, in thymic epithelial-reticular cells trans-Golgi network portions resembling classical GERL elements were stained by the antibodies. Thus, although immunoreactivity was subcompartmentalized within the Golgi apparatus in all epithelial cell types examined, the extent of staining within the trans-Golgi network was variable. Immunoreactivity was not detected at the plasma membrane (ecto-galactosyl-transferase), except in the case of a subpopulation of tracheal cells that resemble brush cells. These results suggest that in the epithelial cells examined, the subcompartmental distribution of beta 1,4-GT within the Golgi apparatus is maintained across different types of epithelial cell organization. Moreover, no evidence for a general epithelial cell ecto-galactosyltransferase could be discerned with these reagents.     

Intracellular transport of a variant surface glycoprotein in Trypanosoma brucei

Trypanosome variant surface glycoproteins (VSGs) have a novel glycan-phosphatidylinositol membrane anchor, which is cleavable by a phosphatidylinositol-specific phospholipase C. A similar structure serves to anchor some membrane proteins in mammalian cells. Using kinetic and ultrastructural approaches, we have addressed the question of whether this structure directs the protein to the cell surface by a different pathway from the classical one described in other cell types for plasma membrane and secreted glycoproteins. By immunogold labeling on thin cryosections we were able to show that, intracellularly, VSG is associated with the rough endoplasmic reticulum, all Golgi cisternae, and tubulovesicular elements and flattened cisternae, which form a network in the area adjacent to the trans side of the Golgi apparatus. Our data suggest that, although the glycan-phosphatidylinositol anchor is added in the endoplasmic reticulum, VSG is nevertheless subsequently transported along the classical intracellular route for glycoproteins, and is delivered to the flagellar pocket, where it is integrated into the surface coat. Treatment of trypanosomes with 1 microM monensin had no effect on VSG transport, although dilation of the trans-Golgi stacks and lysosomes occurred immediately. Incubation of trypanosomes at 20 degrees C, a treatment that arrests intracellular transport from the trans-Golgi region to the cell surface in mammalian cells, caused the accumulation of VSG molecules in structures of the trans-Golgi network, and retarded the incorporation of newly synthesized VSG into the surface coat.     

Transport into and out of the Golgi complex studied by transfecting cells with cDNAs encoding horseradish peroxidase

We have developed a novel technique with which to investigate the morphological basis of exocytotic traffic. We have used expression of HRP from cDNA in a variety of cells in combination with peroxidase cytochemistry to outline traffic into and out of the Golgi apparatus at the electron microscopic level with very high sensitivity. A secretory form of the peroxidase (ssHRP) is active from the beginning of the secretory pathway and the activity is efficiently cleared from cells. Investigation of the morphological elements involved in the itinerary of soluble ER proteins using ssHRP tagged with the ER retention motif (ssHRPKDEL) shows that it progresses through the Golgi stack no further than the cis-most element. Traffic between the RER and the Golgi stack as outlined by ssHRPKDEL occurs via vesicular carriers as well as by tubular elements. ssHRP has also been used to investigate the trans side of the Golgi complex, where incubation at reduced temperatures outlines the trans-Golgi network with HRP reaction product. Tracing the endosomal compartment with transferrin receptor in double-labeling experiments with ssHRP fails to show any overlap between these two compartments.     

Tubules of the trans Golgi apparatus visualized by immunoelectron microscopy

 Tubules constitute an integral part of the Golgi apparatus and have been shown to form a complex and dynamic network at its trans side. We have studied in detail structural features of the trans Golgi network and its relationship with the cisternal stack in thin sections of Lowicryl K4M embedded human absorptive enterocytes by immunolectron microscopy. Immunoreactive sites for α1,3 N-acetylgalactosaminyltransferase and blood group A substance were detectable troughout the cisternal stack and the entire trans Golgi network. Furthermore, the entire trans Golgi network was reactive for CMPase activity. Evidence for two kinds of tubules at the trans side of the Golgi apparatus was found: tubules that laterally connect adjacent and distant cisternal stacks, and others extending from central and lateral portions of trans cisternae to form the complex and extensive trans Golgi network. Trans cisternae showed often the peeling-off phenomenon and were continuous with the trans Golgi network. Both, trans cisternae and tubules of the trans Golgi network exhibited regionally buds and vesicles with a lace-like, non clathrin coat, previously reported by others in NRK cells, which contained glycoproteins with terminal N-acetylgalactosamine residues. These buds and vesicle are therefore involved in constitutive exocytosis. Accepted: 12 January 1998     

Lectin-gold cytochemistry of the Golgi apparatus in rabbit luteal cells, with special emphasis on the formation of a lysosomal-type membrane

In rabbit luteal cells embedded in glycolmethacrylate and stained with PTA at low pH highly glycosylated membrane patches can be observed after vesiculation of the trans-Golgi network. As these membranes could be prelysosomal, their sialic acid content was investigated by post-embedding labeling with Limax flavus agglutinin (LFA)/fetuin-Au. Additional labeling of the Golgi apparatus was performed with Wheat germ agglutinin (WGA)/ovomucoid Au, Ricinus communis agglutininI (RCAI)/Au and Helix pomatia agglutinin (HPA)/Au. The sections were then counterstained with PTA at low pH, which allows a clear distinction between the elements of the trans-Golgi network (G2-G1) and the saccules of the stack (g). With WGA, LFA and RCAI the trans-Golgi network was observed to be clearly more reactive than the stack. After vesiculation most intense labeling was found over the highly glycosylated vacuolar membranes derived from the G2-element. The limiting membrane of lysosomes, the MvB's and the plasma membrane also reacted strongly. Colloidal gold particles were also found over the membranes of the vacuoles derived from G1. The Golgi stack showed a lower reactivity and label for all three lectins could be found over three to four saccules of the stack (g3-g4). The matrix of the lysosomes was slightly labeled. Labeling with HPA was absent from the trans saccules and was consistently found in the cis and cis-most (g4-g5) saccules of the stack. Some cytoplasmic vesicles near the cell border were also labeled. With our procedure the Golgi apparatus can easily be detected and it is apparent that in rabbit luteal cells the highest lectin reactivity is found in the trans-Golgi network.(ABSTRACT TRUNCATED AT 250 WORDS)     

Immunolocalization of blood group A gene specified alpha 1,3N-acetylgalactosaminyltransferase and blood group A substance in the trans-tubular network of the Golgi apparatus and mucus of intestinal goblet cells

The subcellular distribution of blood group A gene specified alpha 1,3N-acetylgalactosaminyltransferase and its product was studied in human intestinal goblet cells by immunoelectron microscopy. The O-glycosylation step yielding blood group A-active glycoconjugates occurred in the trans region of the Golgi apparatus as indicated by the presence of immunolabel for both antigens. In the Golgi apparatus, immunoreactive alpha 1,3N-acetylgalactosaminyltransferase was detectable in trans cisternae and in the trans-tubular network which was found to be continuous with the cisternal stack and exhibited acid phosphatase activity. This demonstrates that in intestinal goblet cells (i) the trans-tubular network does not constitute a compartment distinct from trans cisternae, and (ii) structures corresponding to GERL are structurally and functionally part of the Golgi apparatus. In addition to immunolabel for transferase at the inner surface of the cisternal membranes, luminally located immunolabel indicating the presence of free, not membrane-associated transferase became first detectable in the trans-tubular network and early forming mucus droplets contained therein. Further, the content of mature mucus droplets as well as the extracellular mucus layer were labeled. Absence of immunolabel in blood group 0 subjects lacking the blood group A gene specified transferase and the apparent non-reactivity of the antibodies with carbohydrate epitopes indicates that free alpha 1,3N-acetylgalactosaminyltransferase is present in mucus droplets and becomes secreted by intestinal goblet cells.     

Isolation of a fraction enriched in the trans-Golgi network from baby hamster kidney cells

Incubation of cultured cells at 20 degrees C blocks the transport of newly synthesized plasma membrane proteins, and the proteins accumulate intracellularly in a terminally glycosylated form. When baby hamster kidney cells are infected with the ts O45 mutant of vesicular stomatitis virus, and incubated at 20 degrees C, the terminally glycosylated spike glycoprotein G of the virus accumulates in the membranes of a tubular network localized on the trans side of the Golgi cisternae, the trans-Golgi network (TGN). We have used the G protein of ts O45 as a marker for the TGN and isolated a TGN fraction using a combination of conventional cell fractionation techniques and immunoisolation. The TGN was separated from the bulk of the endoplasmic reticulum, mitochondria, lysosomes, plasma membrane, and endosomes, while the activity of trans-Golgi marker galactosyltransferase copurified with the G protein. Using G protein as the TGN marker we have determined that the TGN was enriched 25-fold in the final fraction relative to the total homogenate. Several polypeptides (Mr 75,000, 87,000, 92,000, and 120,000) copurified with the G protein in the isolated TGN fraction and most likely represent resident markers of the compartment.     

Clathrin-coated vesicular transport of secretory proteins during the formation of ACTH-containing secretory granules in AtT20 cells

We have studied by electron microscopy and immunocytochemistry the formation of secretory granules containing adrenocorticotropic hormone (ACTH) in murine pituitary cells of the AtT20 line. The first compartment in which condensed secretory protein appears is a complex reticular network at the extreme trans side of the Golgi stacks beyond the TPPase-positive cisternae. Condensed secretory protein accumulates in dilated regions of this trans Golgi network. Examination of en face and serial sections revealed that "condensing vacuoles" are in fact dilations of the trans Golgi network and not detached vacuoles. Only after presumptive secretory granules have reached an advanced stage of morphological maturation do they detach from the trans Golgi network. Frequently both the dilations of the trans Golgi network containing condensing secretory protein and the detached immature granules in the peri-Golgi region have surface coats which were identified as clathrin by immunocytochemistry. Moreover both are the site of budding (or fusion) of coated vesicles, some of which contain condensed secretory protein. The mature granules below the plasma membrane do not, however, have surface coats. Immunoperoxidase labeling with an antiserum specific for ACTH and its precursor polypeptide confirmed that many of the coated vesicles associated with the trans Golgi network contain ACTH. The involvement of the trans Golgi network and coated vesicles in the formation of secretory granules is discussed.     

Ultrastructural distribution of NADPase within the Golgi apparatus and lysosomes of mammalian cells

Cytochemical studies with over 40 different mammalian cell types have indicated that NADPase activity is associated with the Golgi apparatus and/or lysosomes of all cells. In the majority of cases, NADPase is restricted to saccular elements comprising the medial region of the Golgi stack and an occasional lysosome. There is often weak NADPase activity in other Golgi compartments such as the trans Golgi saccules and/or elements of the trans Golgi network. In some cells, however, strong NADPase activity is found within these latter compartments, either exclusively in trans Golgi saccules or elements of the trans Golgi network, or in combination with medial Golgi saccules and each other including (1) medial Golgi saccules + trans Golgi saccules, (2) medial Golgi saccules + trans Golgi saccules + trans Golgi network, or (3) trans Golgi saccules + trans Golgi network. In some rare cases, no NADPase activity is detectable in either Golgi saccules or elements of the trans Golgi network, but it is observed in an occasional lysosome or throughout the lysosomal system of these cells. It is unclear at present if these variations in the distribution of NADPase across the Golgi apparatus, and between the Golgi apparatus and lysosomal system, are due to differences in targeting mechanisms or to the existence of "bottlenecks" in the natural flow of NADPase along the biosynthetic pathway toward lysosomes. While no clear pattern in the association of strong NADPase activity with lysosomes was apparent relative to the ultrastructural distribution of NADPase activity in Golgi saccules or elements of the trans Golgi network, the results of this investigation suggested that cells having NADPase localized predominantly toward the trans aspect of the Golgi apparatus (in trans Golgi saccules or elements of the trans Golgi network or both) have few NADPase-positive lysosomes. The only exception is hepatocytes which were classified as predominantly trans but had noticeable NADPase activity within medial Golgi saccules and elements of the trans Golgi network as well, and highly reactive lysosomes. Other cells showing highly reactive lysosomes including (1) Kupffer cells of liver and those forming the proximal convoluted tubules of the kidney, both of which also had strong NADPase activity within medial and trans Golgi saccules and elements of the trans Golgi network, (2) Leydig cells of the testis and interstitial cells of the ovary, which also showed strong NADPase activity within medial Golgi saccules, and (3) macrophages from lung, spleen and testis, and Sertoli cells from the testis all of which showed no Golgi associated NADPase activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vesicle-associated Membrane Protein 4 is Implicated in Trans-Golgi Network Vesicle Trafficking

The trans-Golgi network (TGN) plays a pivotal role in directing proteins in the secretory pathway to the appropriate cellular destination. VAMP4, a recently discovered member of the vesicle-associated membrane protein (VAMP) family of trafficking proteins, has been suggested to play a role in mediating TGN trafficking. To better understand the function of VAMP4, we examined its precise subcellular distribution. Indirect immunofluorescence and electron microscopy revealed that the majority of VAMP4 localized to tubular and vesicular membranes of the TGN, which were in part coated with clathrin. In these compartments, VAMP4 was found to colocalize with the putative TGN-trafficking protein syntaxin 6. Additional labeling was also present on clathrin-coated and noncoated vesicles, on endosomes and the medial and trans side of the Golgi complex, as well as on immature secretory granules in PC12 cells. Immunoprecipitation of VAMP4 from rat brain detergent extracts revealed that VAMP4 exists in a complex containing syntaxin 6. Converging lines of evidence implicate a role for VAMP4 in TGN-to-endosome transport.     

Detection of sialic acid residues in the axonal reticulum of rat superior cervical ganglion cells by lectin-gold cytochemistry

Highly glycosylated compounds have been demonstrated in the axonal reticulum elements of the superior cervical ganglion cells of the rat, and this is considered to suggest a connection of the reticulum with the trans Golgi side. In the present study, the axonal reticulum and the Golgi elements were further characterized by post-embedding methods of lectin-gold cytochemistry to determine their carbohydrate residues and to see, more specifically, if sialic acid residues could be detected in the axonal reticulum elements. Therefore, the affinity of neuronal cell structures for Limax flavus agglutinin (LFA), wheat germ agglutinin (WGA), and Ricinus communis agglutinin I (RCA-I) was tested in ultra-thin sections of glycolmethacrylate-embedded material, counterstained with phosphotungstic acid (PTA) at low pH. The trans Golgi network, the Golgi-associated axonal reticulum, the reticulum within axons, the large dense-cored vesicles, and the plasma membranes were reactive for all three lectins used. We conclude that the axonal reticulum elements carry sialic acid residues, relating them to the trans Golgi network. The present results support the concept that the axonal reticulum is an extension of the trans network of the Golgi apparatus specialized for neurosecretion.     

The Golgi apparatus ofPhytophthora cinnamomi makes three types of secretory or storage vesicles concurrently

Summary The formation of three types of vesicles in the oomycetePhytophthora cinnamomi was investigated using ultrastructural and immunocytochemical techniques. All three vesicles are synthesised at the same time; one type serves a storage role; the others undergo regulated secretion. A monoclonal antibody Lpv-1 that is specific for glycoproteins contained in the storage vesicles labelled the endoplasmic reticulum (ER), elements in the transition region between ER and Golgi stack, and cis, medial and trans Golgi cisternae. Cpa2, a monoclonal antibody specific for glycoproteins contained within secretory dorsal vesicles labelled the transition region, cis cisternae and a trans-Golgi network. Vesicles possessing a structure characteristic of mature secretory ventral vesicles were observed in close association with the trans face of Golgi stacks. The results suggest that all three vesicles are formed by the Golgi apparatus. Double immunogold labelling with Lpv-1 and Cpa-2 showed that these two sets of glycoproteins occurred within the same Golgi cisternae, indicating that both products pass through and are sorted concurrently within a single Golgi stack.     

An Arabidopsis GRIP domain protein locates to the trans-Golgi and binds the small GTPase ARL1

GRIP domain proteins are a class of golgins that have been described in yeast and animals. They locate to the trans-Golgi network and are thought to play a role in endosome-to-Golgi trafficking. The Arabidopsis GRIP domain protein, AtGRIP, fused to the green fluorescent protein (GFP), locates to Golgi stacks but does not exactly co-locate with the Golgi marker sialyl transferase (ST)-mRFP, nor with the t-SNAREs Memb11, SYP31 and BS14a. We conclude that the location of AtGRIP is further to the trans side of the stack than STtmd-mRFP. The 185-aa C-terminus of AtGRIP containing the GRIP domain targeted GFP to the Golgi, although a proportion of the fusion protein was still found in the cytosol. Mutation of a conserved tyrosine (Y717) to alanine in the GRIP domain disrupted Golgi localization. ARL1 is a small GTPase required for Golgi targeting of GRIP domain proteins in other systems. An Arabidopsis ARL1 homologue was isolated and shown to target to Golgi stacks. The GDP-restricted mutant of ARL1, AtARL1-T31N, was observed to locate partially to the cytosol, whereas the GTP-restricted mutant AtARL1-Q71L labelled the Golgi and a population of small structures. Increasing the levels of AtARL1 in epidermal cells increased the proportion of GRIP-GFP fusion protein on Golgi stacks. We show, moreover, that AtARL1 interacted with the GRIP domain in a GTP-dependent manner in vitro in affinity chromatography and in the yeast two-hybrid system. This indicates that AtGRIP and AtARL1 interact directly. We conclude that the pathway involving ARL1 and GRIP domain golgins is conserved in plants.     

A normal rabbit serum containing Golgi-specific autoatibodies identifies a novel 74-kDa trans-Golgi resident protein

A normal rabbit serum has been identified which contains Golgi-specific autoantibodies. In indirect immunofluorescence experiments the serum was found to stain the juxtanuclear Golgi complex in a variety of cell lines, including human skin fibroblasts, rat osteoblasts, rat myoblasts (L6), baby hamster kidney epithelial cells, and human embryonic kidney cells (293). Thus, the antigen(s) recognized by this serum seems to be well conserved and universally expressed in various mammalian cell types. Immunoelectron microscopy revealed that the epitope resides in the luminal side of the Golgi membranes, and that the antigen is concentrated in the trans-face of the Golgi stacks. In agreement with these results, brefeldin A treatment did not release the antigen from the membranes, but caused its redistribution partly into the endoplasmic reticulum but also into the juxtanuclear area, similarly as with other proteins known to be present in the trans-Golgi cisternae or trans-Golgi network. Our immunoprecipitation studies in human skin fibroblasts demonstrated that the serum recognizes specifically only a single protein with a molecular size of 74 kDa. This protein also cosedimented with a known trans-Golgi-specific marker protein, galactosyltransferase, after fractionation of subcellular organelles by Nycodenz gradient centrifugation. The widespread and polarized expression of this 74-kDa trans-Golgi resident protein suggests that it is required for the late Golgi functions in different mammalian cell types.     

Identify Golgi protein types with modified Mahalanobis discriminant algorithm and pseudo amino acid composition

The Golgi apparatus is an important eukaryotic organelle. Successful prediction of Golgi protein types can provide valuable information for elucidating protein functions involved in various biological processes. In this work, a method is proposed by combining a special mode of pseudo amino acid composition (increment of diversity) with the modified Mahalanobis discriminant for predicting Golgi protein types. The benchmark dataset used to train the predictor thus formed contains 95 Golgi proteins in which none of proteins included has ≥40% pairwise sequence identity to any other. The accuracy obtained by the jackknife test was 74.7%, with the ROC curve of 0.772 in identifying cis-Golgi proteins and trans-Golgi proteins. Subsequently, the method was extended to discriminate cis-Golgi network proteins from cis-Golgi network membrane proteins and trans-Golgi network proteins from trans-Golgi network membrane proteins, respectively. The accuracies thus obtained were 76.1% and 83.7%, respectively. These results indicate that our method may become a useful tool in the relevant areas. As a user-friendly web-server, the predictor is freely accessible at http://immunet.cn/SubGolgi/.