Molecular characterization of two human autoantigens: unique cDNAs encoding 95- and 160-kD proteins of a putative family in the Golgi complex

Vesicles on strings: morphological evidence for processive transport within the Golgi stack

Cis-Golgi cisternae have a higher freeze-fracture particle density than trans-cisternae. Transport vesicles neighboring cis or trans positions of the Golgi stack have a particle concentration comparable to that of the adjacent cisterna and the buds emerging from it. This implies that transport vesicles remain locally within the stack during their lifetime, near their origin, favoring a processive pattern of transport in which vesicle transfers occur preferentially between adjacent cisternae in the stack. A "string theory" is proposed to account for processive transport, in which a carpet of fibrous attachment proteins located at the surface of cisternae (the strings) prevent budded vesicles from diffusing away but still allow them to diffuse laterally, effectively limiting transfers to adjoining cisternae in the stack. Fibrous elements that multivalently connect otherwise free COPI-coated vesicles and uncoated transport vesicles to one or two cisternae simultaneously are discerned readily by electron microscopy. It is suggested that long, coiled coil, motif-rich, Golgi-specific proteins including p115, GM130, and possibly giantin, among others, function as the proposed strings.

Role of NAD+ and ADP-ribosylation in the maintenance of the Golgi structure

We have investigated the role of the ADP- ribosylation induced by brefeldin A (BFA) in the mechanisms controlling the architecture of the Golgi complex. BFA causes the rapid disassembly of this organelle into a network of tubules, prevents the association of coatomer and other proteins to Golgi membranes, and stimulates the ADP-ribosylation of two cytosolic proteins of 38 and 50 kD (GAPDH and BARS-50; De Matteis, M.A., M. DiGirolamo, A. Colanzi, M. Pallas, G. Di Tullio, L.J. McDonald, J. Moss, G. Santini, S. Bannykh, D. Corda, and A. Luini. 1994. Proc. Natl. Acad. Sci. USA. 91:1114-1118; Di Girolamo, M., M.G. Silletta, M.A. De Matteis, A. Braca, A. Colanzi, D. Pawlak, M.M. Rasenick, A. Luini, and D. Corda. 1995. Proc. Natl. Acad. Sci. USA. 92:7065-7069). To study the role of ADP-ribosylation, this reaction was inhibited by depletion of NAD+ (the ADP-ribose donor) or by using selective pharmacological blockers in permeabilized cells. In NAD+-depleted cells and in the presence of dialized cytosol, BFA detached coat proteins from Golgi membranes with normal potency but failed to alter the organelle's structure. Readdition of NAD+ triggered Golgi disassembly by BFA. This effect of NAD+ was mimicked by the use of pre-ADP- ribosylated cytosol. The further addition of extracts enriched in native BARS-50 abolished the ability of ADP-ribosylated cytosol to support the effect of BFA. Pharmacological blockers of the BFA-dependent ADP-ribosylation (Weigert, R., A. Colanzi, A. Mironov, R. Buccione, C. Cericola, M.G. Sciulli, G. Santini, S. Flati, A. Fusella, J. Donaldson, M. DiGirolamo, D. Corda, M.A. De Matteis, and A. Luini. 1997. J. Biol. Chem. 272:14200-14207) prevented Golgi disassembly by BFA in permeabilized cells. These inhibitors became inactive in the presence of pre-ADP-ribosylated cytosol, and their activity was rescued by supplementing the cytosol with a native BARS-50-enriched fraction. These results indicate that ADP-ribosylation plays a role in the Golgi disassembling activity of BFA, and suggest that the ADP-ribosylated substrates are components of the machinery controlling the structure of the Golgi apparatus.

Molecular characterization of GCP170, a 170-kDa protein associated with the cytoplasmic face of the Golgi membrane

We have isolated a cDNA clone encoding a protein (designated GCP170) of 1530 amino acid residues with a calculated molecular mass of 170 kDa that is localized to the Golgi complex. Hydropathy analysis shows that GCP170 contains no NH2-terminal signal sequence nor a hydrophobic domain sufficient for participating in membrane localization. It is also predicted that GCP170 has characteristic secondary structures including an extremely long alpha-helical domain that likely forms a coiled-coil between non-coil domains at the NH2 and COOH termini, suggesting that the protein is organized as a globular head, a stalk, and a tail. Immunocytochemical observations revealed that GCP170 was localized to the Golgi complex and the cytoplasm, consistent with biochemical data indicating that the protein exits as a membrane-associated form and a soluble form. GCP170 was dissociated from the Golgi membrane in response to brefeldin A as rapidly as a coat protein complex of non-clathrin-coated vesicles (beta-COP, a subunit of coatomer), but did not co-localize with beta-COP on the Golgi membrane when examined by immunoelectron microscopy. The protein was detected as phosphorylated and unphosphorylated forms, of which the unphosphorylated form was more tightly associated with the Golgi membrane. When cells were extracted with 1% Triton X-100 under microtubule-stabilizing conditions, GCP170 remained in the cells in association with the Golgi complex. These results indicate that GCP170 is a peripheral membrane protein with a long coiled-coil domain that may be involved in the structural organization or stabilization of the Golgi complex.

SSX: a multigene family with several members transcribed in normal testis and human cancer

Analysis of t(X;18) translocation in synovial sarcoma had previously led to the definition of the SSX2 gene, the fusion partner on chromosome X. Subsequent screening of testicular cDNA libraries identified 2 highly homologous genes, SSX1 and SSX3. Among these 3 genes, SSX2 has been found to be identical to HOM-MEL-40, which codes for an immunogenic tumor antigen expressed in various human cancers. SSX2 thus belongs to the family of cancer/testis (CT) antigens, i.e., immunogenic protein antigens with characteristic mRNA expression in normal testis and in cancer. To define additional CT antigens, we have immuno-screened a testicular cDNA expression library with an allogeneic serum from a melanoma patient, and both SSX2 and SSX3 were isolated. Further studies using testicular cDNA and SSX probes defined 2 new members of this gene family, SSX4 and SSX5, while a shorter cDNA variant of SSX4 was also identified. All 5 members of the SSX family shared strong sequence homology, with nucleotide homology ranging from 88 to 95% and amino acid homology ranging from 77 to 91%. Genomic cloning of a prototype SSX gene (SSX2) showed that its coding region is encoded by 6 exons, and the shortened form of SSX4 cDNA represents an alternatively spliced product lacking the 5th exon. Analysis of SSX mRNA expression by gene-specific RT-PCR confirmed that all 5 SSX genes are expressed in testis. In addition, analysis of a panel of 12 melanoma cell lines showed strong mRNA expression of either SSX1 (3/12), SSX2 (3/12), SSX4 (1/12), or SSX5 (1/12), indicating variable activation of the genes in malignant cells.

Molecular cloning of a novel 97-kd Golgi complex autoantigen associated with Sjögren's syndrome

OBJECTIVE

To identify a Golgi complex autoantigen bound by Sjögren's syndrome (SS) autoantibodies.

METHODS

Serum from a patient with secondary SS and anti-Golgi antibodies was used as a probe to isolate a complementary DNA (cDNA) insert from a HeLa cDNA library.

RESULTS

A 3.7-kb cDNA encoding a 56-kd recombinant protein was immunoprecipitated by the human anti-Golgi serum and immune rabbit serum. Western blot analysis showed that the immune rabbit sera recognized a protein of 97 kd (golgin-97), suggesting that the isolated clone contained a partial cDNA. The 5' upstream sequence was obtained by rapid amplification of the cDNA ends. The complete cDNA contained 4,860 basepairs, encoding a protein with a calculated Mr of 88 kd. Antibodies to golgin-97 were found in 12 (20%) of 60 sera known to have anti-Golgi autoantibodies, and the majority of these sera (8 of 12, or 75%) were from patients who had secondary SS.

CONCLUSION

Golgin-97 is a unique Golgi complex antigen that appears to be a target of SS autoantibodies.

Autoantibodies to fibrillarin in systemic sclerosis (scleroderma). An immunogenetic, serologic, and clinical analysis

OBJECTIVE

To determine the frequency, clinical associations, and any major histocompatibility complex correlations of antifibrillarin antibodies in patients with systemic sclerosis (SSc).

METHODS

Antifibrillarin antibodies were determined by indirect immunofluorescence, immunoblotting, and immunoprecipitation, and HLA class II alleles by DNA oligotyping, in a large cohort of SSc patients.

RESULTS

Antifibrillarin was found in 8% of 335 SSc sera and was significantly more common in blacks (16%) than whites (5%), in males (33%) than females (14%), and in patients with cardiac, renal, or gut involvement. The HLA class II haplotype DRB1*1302, DQB1*0604 was found significantly more frequently in SSc patients with antifibrillarin compared with race-matched normal controls and 260 SSc patients without antifibrillarin. In addition, 1 or more of the HLA-DQB1 alleles *0604, *0301, *0602, and/or *0302 was found in all antifibrillarin-positive patients, and 62% of the antifibrillarin-positive patients had 2 of these HLA-DQB1 alleles, a highly significant difference from both race-matched normal controls and antifibrillarin-negative SSc patients.

CONCLUSION

Antifibrillarin, although an infrequent nucleolar autoantibody, is a marker for severe SSc, especially in blacks and males, and is strongly associated with a unique HLA haplotype, as well as with combinations of certain HLA-DQB1 alleles.

Molecular characterization of trans-Golgi p230. A human peripheral membrane protein encoded by a gene on chromosome 6p12-22 contains extensive coiled-coil alpha-helical domains and a granin motif

Using autoantibodies from a Sjögren's syndrome patient, we have previously identified a 230-kDa peripheral membrane protein associated with the cytosolic face of the trans-Golgi (Kooy, J., Toh, B. H., Pettitt, J. M., Erlich, R. and Gleeson, P. A. (1992) J. Biol. Chem. 267, 20255-20263). Here we report the molecular cloning and sequence analysis of human p230 and the localization of its gene to chromosome 6p12 22. Partial cDNA clones, isolated from a HeLa cell cDNA library using autoantibodies, were used to obtain additional cDNAs, which together span 7695 base pairs (bp). The p230 mRNA is approximately 7.7 kilobases. Two alternatively spliced mRNAs for p230 were detected. These differed by 21- and 63-bp insertions in the 3'-sequence, resulting in differences in amino acid sequence at the carboxyl terminus. The predicted 261-kDa protein is highly hydrophilic with 17-20% homology with many proteins containing coiled-coil domains. Apart from two proline-rich regions (amino acids 1-117 and 239-270), p230 contains a very high frequency of heptad repeats, characteristic of alpha-helices that form dimeric coiled-coil structures. p230 also includes the sequence ESLALEELEL (amino acids 538-546), a motif found in the granin family of acidic proteins present in secretory granules of neuroendocrine cells. This is the first report of a cytosolic Golgi protein containing a granin motif. The structural characteristics of p230 indicate that it may play a role in vesicular transport from the trans-Golgi.

Molecular characterization of Golgin-245, a novel Golgi complex protein containing a granin signature

The serum from a Sjögren's syndrome patient with anti-Golgi antibodies was used as a probe to isolate a 4.6-kilobase pair cDNA insert from a HeLa cDNA library. Expression of the cDNA in Escherichia coli and the in vitro translation products of the cDNA yielded a recombinant protein that migrated in SDS-polyacrylamide gel electrophoresis at 180 kDa. This protein was immuno-precipitated by the human anti-Golgi serum and by immune rabbit serum but not by normal human serum or preimmune rabbit serum. Western blot analysis showed that the prototype human and immune rabbit sera recognized a 245-kDa protein, suggesting that the isolated clone contained a partial cDNA. The 5'-upstream sequence obtained by the rapid amplification of cDNA ends methodology using human placental cDNA and the combined HeLa cDNA contained 6965 base pairs and combined HeLa cDNA contained 6965 base pairs and encoded a protein of 245 kDa and, like other Golgi autoantigens described earlier, is highly rich in coiled-coils. The deduced amino acid sequence included the decapeptide ESLALEELEL, which was identified as one of two signature sequences previously reported in a family of peptide hormones and neuropeptides known as "granins". This is the first report of a Golgi complex autoantigen that bears structural similarities to the granin family of proteins.

Characterization of a cis-Golgi matrix protein, GM130

Antisera raised to a detergent- and salt-resistant matrix fraction from rat liver Golgi stacks were used to screen an expression library from rat liver cDNA. A full-length clone was obtained encoding a protein of 130 kD (termed GM130), the COOH-terminal domain of which was highly homologous to a Golgi human auto-antigen, golgin-95 (Fritzler et al., 1993). Biochemical data showed that GM130 is a peripheral cytoplasmic protein that is tightly bound to Golgi membranes and part of a larger oligomeric complex. Predictions from the protein sequence suggest that GM130 is an extended rod-like protein with coiled-coil domains. Immunofluorescence microscopy showed partial overlap with medial- and trans-Golgi markers but almost complete overlap with the cis-Golgi network (CGN) marker, syntaxin5. Immunoelectron microscopy confirmed this location showing that most of the GM130 was located in the CGN and in one or two cisternae on the cis-side of the Golgi stack. GM130 was not re-distributed to the ER in the presence of brefeldin A but maintained its overlap with syntaxin5 and a partial overlap with the ER-Golgi intermediate compartment marker, p53. Together these results suggest that GM130 is part of a cis-Golgi matrix and has a role in maintaining cis-Golgi structure.

A normal rabbit serum containing Golgi-specific autoantibodies 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.

MG-160, a membrane sialoglycoprotein of the medial cisternae of the rat Golgi apparatus, binds basic fibroblast growth factor and exhibits a high level of sequence identity to a chicken fibroblast growth factor receptor

We report the primary structure of MG-160, a 160 kDa membrane sialoglycoprotein residing in the medial cisternae of the Golgi apparatus of rat neurons, pheochromocytoma (PC-12), and several other cells. The cDNA encodes a polypeptide of 1,171 amino acids with an M(r) of 133,403. An intralumenal cleavable signal peptide is followed by a Pro-Gln-rich segment and 16 contiguous, approx. 60-residue-long, regularly spaced cysteine-rich segments showing sequence identities ranging from 15 to 35%. The lumenal domain is followed by a single membrane spanning domain and a short carboxy-terminal cytoplasmic tail. The protein contains 5 potential NXT glycosylation sites. The sequence of MG-160 shows no homologies with enzymes and other membrane proteins of the Golgi apparatus. MG-160 displays a so far unique feature for a membrane protein of the Golgi apparatus: namely, an upstream, open reading frame (uORF), encoding 58 amino acids, located in front of the major open reading frame (ORF). Most vertebrate mRNAs containing uORF or AUG codons in front of the major ORF encode growth factors and cell surface receptors (Geballe and Morris 1994). In that regard a 90% identity between the primary structure of MG-160 and a receptor for acidic and basic fibroblast growth factors (CFR), isolated from chicken embryos (Burrus et. al., 1992), may be relevant. Immunoreactivity for MG-160 has been detected in the Golgi apparatus of neural and other cells of 2-day-old chicken embryos and adult chicken; furthermore, recombinant human basic fibroblast growth factor (bFGF) binds MG-160 purified from rat brain. MG-160 shows no sequence similarity with members of the family of fibroblast growth factor receptors (FGFR) involved in signal transduction. These findings are consistent with the hypothesis that MG-160 is involved in the traffic and processing of endogenous or autocrine FGFs. This is the first example of an intrinsic membrane protein of the Golgi apparatus which binds a growth factor and may be involved in its regulation.

Molecular genetic analyses of a 376-kilodalton Golgi complex membrane protein (giantin)

Molecular genetic analyses of a 376-kDa Golgi complex (GC) membrane protein (giantin) are described. The immunoglobulin G fraction of a human serum containing antibodies against GC antigens as revealed by indirect immunofluorescence microscopy with Hep-2 cells was used to screen a HeLa cDNA expression library, yielding four overlapping cross-hybridizing clones. Additional cDNA clones were retrieved from a lambda gt11 human thyroid cDNA library or generated by reverse transcriptase-mediated PCR from HeLa cell mRNA. Alignment of the clones resulted in a consensus cDNA of 10,300 bp encoding a protein of 376 kDa. The corresponding mRNA with a size of about 10 kb was detected by Northern (RNA) blotting of HeLa, Hep-G2, and Jurkat cell RNA. Sequence analyses of the protein revealed an extraordinarily high content of heptad repeats with the probability of forming coiled coils similar to the proteins of the myosin family. Five overlapping recombinant proteins covering the entire sequence were synthesized and used for antibody production in rabbits and for affinity purification of human and rabbit antibodies. Indirect immunofluorescence experiments also done with brefeldin A-treated Hep-2 and Pt K1 cells revealed an identical GC staining of both the affinity-purified human and rabbit antibodies. Double labeling experiments with antibodies against the GC marker mannosidase II as well as immunoelectron microscopic studies confirmed the localization of the protein within the GC. A corresponding endogenous large-molecular-mass protein of about 390 kDa was found in [35S]methionine-labeled Hep-2 cell lysates as well as in GC-enriched subcellular fractions from rat liver. The protein as well as the recently described proteins golgin-95 and golgin-160 (M. J. Fritzler, J. C. Hamel, R. L. Ochs, and E. K. L. Chan, J. Exp. Med. 178:49-62, 1993) may belong to a new group of Golgi proteins with a high content of heptad repeats which may exert functions in scaffold formation or vesicle transport. As far as can be concluded from immunological and personally communicated partial cDNA sequence data, the protein seems to be identical with a 400-kDa Golgi protein (giantin) recently described (A. D. Linstedt and H. P. Hauri, Mol. Biol. Cell 4:679-693, 1993). Therefore, we agreed to adopt the name giantin.

Molecular genetic analyses of a 376-kilodalton Golgi complex membrane protein (giantin)

Molecular genetic analyses of a 376-kDa Golgi complex (GC) membrane protein (giantin) are described. The immunoglobulin G fraction of a human serum containing antibodies against GC antigens as revealed by indirect immunofluorescence microscopy with Hep-2 cells was used to screen a HeLa cDNA expression library, yielding four overlapping cross-hybridizing clones. Additional cDNA clones were retrieved from a lambda gt11 human thyroid cDNA library or generated by reverse transcriptase-mediated PCR from HeLa cell mRNA. Alignment of the clones resulted in a consensus cDNA of 10,300 bp encoding a protein of 376 kDa. The corresponding mRNA with a size of about 10 kb was detected by Northern (RNA) blotting of HeLa, Hep-G2, and Jurkat cell RNA. Sequence analyses of the protein revealed an extraordinarily high content of heptad repeats with the probability of forming coiled coils similar to the proteins of the myosin family. Five overlapping recombinant proteins covering the entire sequence were synthesized and used for antibody production in rabbits and for affinity purification of human and rabbit antibodies. Indirect immunofluorescence experiments also done with brefeldin A-treated Hep-2 and Pt K1 cells revealed an identical GC staining of both the affinity-purified human and rabbit antibodies. Double labeling experiments with antibodies against the GC marker mannosidase II as well as immunoelectron microscopic studies confirmed the localization of the protein within the GC. A corresponding endogenous large-molecular-mass protein of about 390 kDa was found in [35S]methionine-labeled Hep-2 cell lysates as well as in GC-enriched subcellular fractions from rat liver. The protein as well as the recently described proteins golgin-95 and golgin-160 (M. J. Fritzler, J. C. Hamel, R. L. Ochs, and E. K. L. Chan, J. Exp. Med. 178:49-62, 1993) may belong to a new group of Golgi proteins with a high content of heptad repeats which may exert functions in scaffold formation or vesicle transport. As far as can be concluded from immunological and personally communicated partial cDNA sequence data, the protein seems to be identical with a 400-kDa Golgi protein (giantin) recently described (A. D. Linstedt and H. P. Hauri, Mol. Biol. Cell 4:679-693, 1993). Therefore, we agreed to adopt the name giantin.