Induction of cellular immunity by immunization with novel hybrid peptides complexed to heat shock protein 70.

Heat shock proteins 70 (hsp70) derived from tissues and cells can elicit cytotoxic T lymphocyte (CTL) responses against peptides bound to hsp70. However, peptides can markedly differ in their affinity for hsp, and this potentially limits the repertoire of peptides available to induce CTL by the hsp immunization. Hybrid peptides consisting of a high-affinity ligand for the peptide-binding site of hsp70 joined to T cell epitopes by a glycine-serine-glycine linker were constructed. Immunization with hybrid peptides complexed to mouse hsp70 effectively primed specific CTL responses in mice and were more potent than T cell peptide epitopes alone with hsp70. In vivo immunization with hsp70 and hybrid peptides led to rejection of tumors expressing antigen with greater efficacy than immunization with peptide epitope plus hsp70. Induction of CTL responses occurred independently of CD4(+) T cells, suggesting that immunization directly primed antigen-presenting cells to elicit CD8(+) cytotoxic T cell responses without T cell help. Both peptide/hsp70 complexes and mouse hsp70 alone were able to induce cultures of mouse bone marrow-derived dendritic cells (DC) to release cytokines, including DC from endotoxin-resistant C57BL/10Sc mice. Thus, hsp70/hybrid peptide complexes can activate DC for cytokine release, providing a potential adjuvant effect that could bypass T cell help.

Evidence for recycling of the resident medial/trans Golgi enzyme, N-acetylglucosaminyltransferase I, in ldlD cells.

ldlD cells, which lack the UDP-Gal/UDP-GalNAc 4-epimerase, were stably transfected with a Myc-tagged version of N-acetylglucosaminyltransferase I (Myc-Glc-NAc-T I). In the absence of GalNAc and Gal, newly synthesized GlcNAc-T I did not acquire O-linked oligosaccharides but was catalytically active and was transported to the Golgi region as defined using both immunofluorescence and immunoelectron microscopy. After addition of cycloheximide to prevent further synthesis, GalNAc and Gal were added, and the unglycosylated GlcNAc-T I was found to acquire mature, O-linked oligosaccharides with a half-time of about 150 min. The addition of these sugars was sensitive to N-ethylmaleimide and okadaic acid, both inhibitors of vesicle-mediated traffic. Together, these results suggest that Myc-Glc-NAc-T I undergoes retrograde transport to the early part of the Golgi apparatus where the first O-linked sugar, GalNAc, is added followed by anterograde transport back to the Golgi stack, where addition of Gal and sialic acid occurs.

An integral membrane component of coatomer-coated transport vesicles defines a family of proteins involved in budding.

We have isolated a major integral membrane protein from Golgi-derived coatomer-coated vesicles. This 24-kDa protein, p24, defines a family of integral membrane proteins with homologs present in yeast and humans. In addition to sequence similarity, all p24 family members contain a motif with the characteristic heptad repeats found in coiled coils. When the yeast p24 isoform, yp24A, is knocked out in a strain defective for vesicle fusion, a dramatic reduction in the accumulation of transport vesicles is observed. Together, these results indicate a role for this protein family in the budding of coatamer-coated and other species of coated vesicles.

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.

Kin recognition. A model for the retention of Golgi enzymes.

The surprising result that the spanning domain causes retention of proteins in the Golgi stack poses the question as to the actual mechanism. Here we present a simple model that might have general applicability.

Role of oligosaccharides in the processing and function of human transferrin receptors. Effect of the loss of the three N-glycosyl oligosaccharides individually or together.

When the coding sequence for human transferrin receptors was expressed in a Chinese hamster ovary cell line lacking endogenous transferrin receptors, 86-kDa molecules containing three N-glycosidically linked oligosaccharides were synthesized. These rapidly dimerized to form 172-kDa molecules which increased in size to 190 kDa. After site-directed mutagenesis of all three N-glycosylation sites, 80-kDa receptors were synthesized and only a few dimers were formed. 84-kDa monomers were synthesized in the absence of the oligosaccharide attached to Asn727 or Asn317. Dimerization and maturation through the Golgi body of the Asn727 mutant receptors were much slower than the wild type whereas the Asn317 mutant receptors behaved more similarly to the wild type. Lack of the oligosaccharide at Asn251 gave rise to 73-kDa monomers because of proteolytic processing (Hoe, M. H., and Hunt, R. C. (1992) J. Biol. Chem. 267, 4916-4923), but a second mutation at a potential cleavage site allowed the formation of 84-kDa receptors. These also dimerized at a similar rate to wild type receptors. The three-site mutant receptors were degraded in the endoplasmic reticulum but all three 84-kDa single site mutant receptor species migrated to the cell surface. However, receptors lacking the oligosaccharide at Asn727 bound and internalized little transferrin as a result of reduced affinity.

Overlapping distribution of two glycosyltransferases in the Golgi apparatus of HeLa cells.

Thin, frozen sections of a HeLa cell line were double labeled with specific antibodies to localize the trans-Golgi enzyme, beta 1,4 galactosyltransferase (GalT) and the medial enzyme, N-acetylglucosaminyltransferase I (NAGT I). The latter was detected by generating a HeLa cell line stably expressing a myc-tagged version of the endogenous protein. GalT was found in the trans-cisterna and trans-Golgi network but, contrary to expectation, NAGT I was found both in the medial- and trans-cisternae, overlapping the distribution of GalT. About one third of the NAGT I and half of the GalT were found in the shared, trans-cisterna. These data show that the differences between cisternae are determined not by different sets of enzymes but by different mixtures.

Loss of one asparagine-linked oligosaccharide from human transferrin receptors results in specific cleavage and association with the endoplasmic reticulum.

The coding sequence for the human transferrin receptor gene has been mutated in order to abolish the attachment of the oligosaccharide closest to the transmembrane sequence. Expression of the mutant receptor in Chinese hamster ovary cells resulted, after analysis under nonreducing conditions, in an 85- and a 73-kDa receptor species. After reduction, the 85-kDa receptors were mostly converted to 73 kDa, although with short labeling periods some 85-kDa receptor remained suggesting that the mutated gene was capable of coding for the entire polypeptide which was then proteolytically processed. This was supported by in vitro translation of mRNA from either wild type or mutant cells which in both cases yielded an 80-kDa protein, the full size of the nonglycosylated protein. The solubility characteristics of the mutant receptor suggest that it contains the COOH-terminal extracellular domain but not the transmembrane sequence. This is supported by the endoglycosidase H sensitivity of the 73-kDa protein which is compatible with the retention of two of the original three high mannose oligosaccharides. The receptor that lacks one oligosaccharide is unable to form intermonomer disulfide bridges or to migrate to the cell surface and is located in the endoplasmic reticulum where it is further degraded after a lag period of about 30 min.