Re-entering the translocon from the lumenal side of the endoplasmic reticulum. Studies on mutated carboxypeptidase yscY species.
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Misfolded or unassembled secretory proteins are retained in the endoplasmic reticulum (ER) and subsequently degraded by the cytosolic ubiquitin-proteasome system. This requires their retrograde transport from the ER lumen into the cytosol, which is mediated by the Sec61 translocon. It had remained a mystery whether ER-localised soluble proteins are at all capable of re-entering the Sec61 channel de novo or whether a permanent contact of the imported protein with the translocon is a prerequisite for retrograde transport. In this study we analysed two new variants of the mutated yeast carboxypeptidase yscY, CPY*: a carboxy-terminal fusion protein of CPY* and pig liver esterase and a CPY* species carrying an additional glycosylation site at its carboxy-terminus. With these constructs it can be demonstrated that the newly synthesised CPY* chain is not retained in the translocation channel but reaches its ER lumenal side completely. Our data indicate that the Sec61 channel provides the essential pore for protein transport through the ER membrane in either direction; persistent contact with the translocon after import seems not to be required for retrograde transport. (+info)
The protein disulphide-isomerase family: unravelling a string of folds.
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The mammalian protein disulphide-isomerase (PDI) family encompasses several highly divergent proteins that are involved in the processing and maturation of secretory proteins in the endoplasmic reticulum. These proteins are characterized by the presence of one or more domains of roughly 95-110 amino acids related to the cytoplasmic protein thioredoxin. All but the PDI-D subfamily are composed entirely of repeats of such domains, with at least one domain containing and one domain lacking a redox-active -Cys-Xaa-Xaa-Cys- tetrapeptide. In addition to their known roles as redox catalysts and isomerases, the last few years have revealed additional functions of the PDI proteins, including peptide binding, cell adhesion and perhaps chaperone activities. Attention is now turning to the non-redox-active domains of the PDIs, which may play an important role in all of the known activities of these proteins. Thus the presence of both redox-active and -inactive domains within these proteins portends a complexity of functions differentially accommodated by the various family members. (+info)
Ligand recognition and domain structure of Vps10p, a vacuolar protein sorting receptor in Saccharomyces cerevisiae.
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Vp10p is a receptor that sorts several different vacuolar proteins by cycling between a late Golgi compartment and the endosome. The cytoplasmic tail of Vps10p is necessary for the recycling, whereas the lumenal domain is predicted to interact with the soluble ligands. We have studied ligand binding to Vps10p by introducing deletions in the lumenal region. This region contains two domains with homology to each other. Domain 2 binds carboxypeptidase Y (CPY), proteinase A (PrA) and hybrids of these proteases with invertase. Moreover, we show that aminopeptidase Y (APY) is a ligand of Vps10p. The native proteases compete for binding to domain 2. Binding of CPY(156)-invertase or PrA(137)-invertase, on the other hand, do not interfere with binding of CPY to Vps10p. Furthermore, the Q24RPL27 sequence known to be important for vacuolar sorting of CPY, is of little importance in the Vps10p-dependent sorting of CPY-invertase. Apparently, domain 2 contains two different binding sites; one for APY, CPY and PrA, and one for CPY-invertase and PrA-invertase. The latter interaction seems not to be sequence specific, and we suggest that an unfolded structure in these ligands is recognized by Vps10p. (+info)
Effect of prolonged administration of a urinary kinase inhibitor, ebelactone B on the development of deoxycorticosterone acetate-salt hypertension in rats.
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The effect of prolonged administration of a carboxypeptidase Y-like kininase inhibitor, ebelactone B (EB) (2-ethyl-3, 11-dihydroxy-4, 6, 8, 10, 12-pentamethyl-9-oxo-6-tetradecenoic 1, 3-lactone), on the development of deoxycorticosterone acetate (DOCA)-salt hypertension was tested. The systolic blood pressure (SBP) of non-treated 6-week-old Sprague-Dawley strain rats was gradually increased by DOCA-salt treatment from 137+/-2 mmHg (n=11) to 195+/-7 mmHg at 10 weeks of age. With daily oral administration of lisinopril (5 mg kg(-1), twice a day), which is an inhibitor of angiotensin converting enzyme, a major kininase in plasma, the development of hypertension was not suppressed. By contrast, administration of EB (5 mg kg(-1), twice a day), completely inhibited the development of hypertension (SBP: 146+/-1 mmHg, n=5, 10 weeks old). The reduced SBP at 10 weeks of age was equal to the SBP before any treatment (142+/-1 mmHg, n=5). Direct determination of mean blood pressure (MBP) in conscious, unrestrained rats confirmed that MBP elevation was completely inhibited by EB. Continuous subcutaneous infusion (5 mg kg(-1) day(-1)) of HOE140, a bradykinin B2 receptor antagonist, restored the elevation of SBP, which was suppressed by EB. The weights of left ventricle of DOCA-salt treated rats 10-weeks-old (0.36+/-0.02 g 100 g body weight(-1), n=11) was significantly reduced by EB (0.27+/-0.01, n=5), as were the sodium levels in serum, cerebrospinal fluid and erythrocyte. These findings suggested that EB is effective in preventing salt-related hypertension presumably by eliminating sodium retention. (+info)
Stable expression of protective protein/cathepsin A-green fluorescent protein fusion genes in a fibroblastic cell line from a galactosialidosis patient. Model system for revealing the intracellular transport of normal and mutated lysosomal enzymes.
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Fibroblastic cell lines derived from a galactosialidosis patient, stably expressing the chimaeric green fluorescent protein variant (EGFP) gene fused to the wild-type and mutant human lysosomal protective protein/cathepsin A (PPCA) cDNA, were first established as a model system for revealing the sorting and processing of lysosomal enzymes and for investigating the molecular bases of their deficiencies. In the cell line expressing the wild-type PPCA-EGFP chimaera gene (EGFP-PPwild), an 81 kDa form (27 kDa EGFP fused to the C-terminus of the 54 kDa PPCA precursor) was produced, then processed into the mature 32/20 kDa two-chain form free of the EGFP domain. The intracellular cathepsin A, alpha-N-acetylneuraminidase and beta-galactosidase activities, which are deficient in the parent fibroblastic cells, could also be significantly restored in the cells. In contrast with the uniform and strong fluorescence throughout the cytoplasm and nucleus in the mock-cell line expressing only EGFP cDNA, weak reticular and punctate fluorescence was distributed throughout the EGFP-PPwild cell line. Bafilomycin A1, a potent inhibitor of vacuolar ATPase and intracellular acidification, induced the distribution of Golgi-like perinuclear fluorescence throughout the living and fixed cells, in which only the 81 kDa product was detected. After removal of the agent, time-dependent transport of the chimaeric protein from the Golgi apparatus to the prelysosomal structure in living cells was monitored with a confocal laser scanning microscope system. Leupeptin caused the distribution of lysosome-like granular fluorescence throughout the cytoplasm in the fixed cells, although it was hardly observed in living cells. The latter agent also dose-dependently induced an increase in the intracellular amount of the 81 kDa product containing the EGFP domain and inhibited the restoration of cathepsin A activity in the EGFP-PPwild cells after the removal of bafilomycin A1. In parallel, both the mature two-chain form and PPCA function disappeared. These results suggested that the chimaera gene product was transported to acidic compartments (endosomes/lysosomes), where proteolytic processing of the PPCA precursor/zymogen, quenching of the fluorescence, and random degradation of the EGFP portion occurred. A cell line stably expressing a chimaeric gene with a mutant PPCA cDNA containing an A1184-->G (Y395C) mutation, commonly detected in Japanese severe early-infantile type of galactosialidosis patients, showed an endoplasmic reticulum (ER)-like reticular fluorescence pattern. The PPCA-immunoreactive gene product was hardly detected in this cell line. The mutant chimaeric product was suggested to be degraded rapidly in the ER before transport to post-ER compartments. A cell line expressing the chimaeric gene with a T746-->A (Y249N) PPCA mutation exhibited both ER-like reticular and granular fluorescence on the reticular structure that was stronger than that in the EGFP-PPwild cells. Some of them contained large fluorescent inclusion-body-like structures. The ineffectiveness of transport inhibitors in the distribution changes in the two mutant chimaeric proteins suggested that they were not delivered to acidic compartments. Therefore this expression system can possibly be applied to the direct analysis of the sorting defects of mutant gene products in living cells and will be useful for the molecular investigation of lysosomal diseases, including galactosialidosis. (+info)
Contribution of the carbohydrate moiety to conformational stability of the carboxypeptidase Y high pressure study.
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The process of pressure-induced denaturation of carboxypeptidase Y and the role of the carbohydrate moiety in its response to pressure and low temperature were investigated by measuring in situ the catalytic activity and, the intrinsic and 8-anilino-1-naphthalene sulfonic acid binding fluorescences. Pressure-induced denaturation of carboxypeptidase Y is a process involving at least three transitions. Low pressures (below 150 MPa) induced slight conformational changes characterized by a slight decrease in the center of the spectral mass of intrinsic fluorescence, whereas no changes in 8-anilino-1-naphthalene sulfonic acid binding fluorescence were observed and 80% of the catalytic activity remained. Higher pressure (150-500 MPa) induced further conformational changes, characterized by a large decrease in the center of the spectral mass of intrinsic fluorescence, a large increase in the 8-anilino-1-naphthalene sulfonic acid binding fluorescence and the loss of all catalytic activity. Thus, this intermediate exhibited characteristics of molten globule-like state. A further increase, in pressure (above 550 MPa) induced transition from this first molten globule-like state to a second molten globule-like state. This two-stage denaturation process can be explained by assuming the existence of two independent structural domains in the carboxypeptidase molecule. A similar three-transition process was found for unglycosylated carboxypeptidase Y, but, the first two transitions clearly occurred at lower pressures than those for glycosylated carboxypeptidase Y. These findings indicate that the carbohydrate moiety protects carboxypeptidase Y against pressure-induced denaturation. The origin of the protective effects is discussed based on the known crystallographic structure of CPY. (+info)
Crystal structure of human gastric lipase and model of lysosomal acid lipase, two lipolytic enzymes of medical interest.
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Fat digestion in humans requires not only the classical pancreatic lipase but also gastric lipase, which is stable and active despite the highly acidic stomach environment. We report here the structure of recombinant human gastric lipase at 3.0-A resolution, the first structure to be described within the mammalian acid lipase family. This globular enzyme (379 residues) consists of a core domain belonging to the alpha/beta hydrolase-fold family and a "cap" domain, which is analogous to that present in serine carboxypeptidases. It possesses a classical catalytic triad (Ser-153, His-353, Asp-324) and an oxyanion hole (NH groups of Gln-154 and Leu-67). Four N-glycosylation sites were identified on the electron density maps. The catalytic serine is deeply buried under a segment consisting of 30 residues, which can be defined as a lid and belonging to the cap domain. The displacement of the lid is necessary for the substrates to have access to Ser-153. A phosphonate inhibitor was positioned in the active site that clearly suggests the location of the hydrophobic substrate binding site. The lysosomal acid lipase was modeled by homology, and possible explanations for some previously reported mutations leading to the cholesterol ester storage disease are given based on the present model. (+info)
The phosphatidylinositol 3-phosphate binding protein Vac1p interacts with a Rab GTPase and a Sec1p homologue to facilitate vesicle-mediated vacuolar protein sorting.
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Activated GTP-bound Rab proteins are thought to interact with effectors to elicit vesicle targeting and fusion events. Vesicle-associated v-SNARE and target membrane t-SNARE proteins are also involved in vesicular transport. Little is known about the functional relationship between Rabs and SNARE protein complexes. We have constructed an activated allele of VPS21, a yeast Rab protein involved in vacuolar protein sorting, and demonstrated an allele-specific interaction between Vps21p and Vac1p. Vac1p was found to bind the Sec1p homologue Vps45p. Although no association between Vps21p and Vps45p was seen, a genetic interaction between VPS21 and VPS45 was observed. Vac1p contains a zinc-binding FYVE finger that may bind phosphatidylinositol 3-phosphate [PtdIns(3)P]. In other FYVE domain proteins, this motif and PtdIns(3)P are necessary for membrane association. Vac1 proteins with mutant FYVE fingers still associated with membranes but showed vacuolar protein sorting defects and reduced interactions with Vps45p and activated Vps21p. Vac1p membrane association was not dependent on PtdIns(3)P, Pep12p, Vps21p, Vps45p, or the PtdIns 3-kinase, Vps34p. Vac1p FYVE finger mutant missorting phenotypes were suppressed by a defective allele of VPS34. These data indicate that PtdIns(3)P may perform a regulatory role, possibly involved in mediating Vac1p protein-protein interactions. We propose that activated-Vps21p interacts with its effector, Vac1p, which interacts with Vps45p to regulate the Golgi to endosome SNARE complex. (+info)