The Saccharomyces cerevisiae TRG1 gene is essential for growth and encodes a lumenal endoplasmic reticulum glycoprotein involved in the maturation of vacuolar carboxypeptidase

A Complex of Htm1 and the Oxidoreductase Pdi1 Accelerates Degradation of Misfolded Glycoproteins

A quality control system in the endoplasmic reticulum (ER) efficiently discriminates polypeptides that are in the process of productive folding from conformers that are trapped in an aberrant state. Only the latter are transported into the cytoplasm and degraded in a process termed ER-associated protein degradation (ERAD). In the ER, an enzymatic cascade generates a specific N-glycan structure of seven mannosyl and two N-acetylglucosamine residues (Man7GlcNAc2) on misfolded glycoproteins to facilitate their disposal. We show that a complex encompassing the yeast lectin-like protein Htm1 and the oxidoreductase Pdi1 converts Man8GlcNAc2 on glycoproteins into the Man7GlcNAc2 signal. In vitro the Htm1-Pdi1 complex processes both unfolded and native proteins albeit with a preference for the former. In vivo, elevated expression of HTM1 causes glycan trimming on misfolded and folded proteins, but only degradation of the non-native species is accelerated. Thus, modification with a Man7GlcNAc2 structure does not inevitably commit a protein for ER-associated protein degradation. The function of Htm1 in ERAD relies on its association with Pdi1, which appears to regulate the access to substrates. Our data support a model in which the balanced activities of Pdi1 and Htm1 are crucial determinants for the efficient removal of misfolded secretory glycoproteins.

A second protein disulfide isomerase plays a protective role against nitrosative and nutritional stresses in Schizosaccharomyces pombe

In the present work, a second gene encoding protein disulfide isomerase (PDI2) was cloned and characterized from Schizosaccharomyces pombe, and its regulation was studied. The structural gene encoding PDI2 was amplified from the genomic DNA using PCR, and ligated into the E. coli-yeast shuttle vector pRS316 to generate the recombinant plasmid pYPDI2. The determined DNA sequence carries 2,578 bp and is able to encode a protein of 726 amino acid sequence with CGAC at the putative active site. The fission yeast cells harboring pYPDI2 contained 1.62- and 2.73-fold higher PDI activity than the control yeast cells in exponential and stationary phases, respectively, indicating that the cloned gene is in vivo functioning. The PDI2 mRNA levels in both vector control and pYPDI2-containing yeast cells were found to be significantly higher in the stationary phase than in the exponential phase, suggesting that expression of the PDI2 gene is under stationary control. The yeast cells harboring pYPDI2 showed enhanced survival on minimal media plates containing nitric oxide (NO)-generating sodium nitroprusside (SNP) and no nitrogen. The synthesis of β-galactosidase from the PDI2-lacZ fusion gene was markedly enhanced in the Pap1-positive KP1 cells by SNP and nitrogen starvation. However, the enhancement in the synthesis of β-galactosidase from the PDI2-lacZ fusion gene by SNP and nitrogen starvation appeared to be relatively reduced in the Pap1-negative TP108-3C cells than in the Pap1-positive KP1 cells. The PDI2 mRNA level was elevated by SNP and nitrogen starvation in the Pap1-positive cells but not in the Pap1-negative cells. In brief, the S. pombe PDI2 plays a protective role against nitrosative and nutritional stresses, and is positively regulated by NO and nitrogen starvation in a Pap1-dependent manner.

Cloning, characterization and regulation of a protein disulfide isomerase from the fission yeast Schizosaccharomyces pombe

To elucidate the physiological roles and regulation of a protein disulfide isomerase (PDI) from the fission yeast Schizosaccharomyces pombe, the full-length PDI gene was ligated into the shuttle vector pRS316, resulting in pPDI10. The determined DNA sequence carries 1,636 bp and encodes the putative 359 amino acid sequence of PDI with a molecular mass of 39,490 Da. In the amino acid sequence, the S. pombe PDI appears to be very homologous to A. thaliana PDI. The S. pombe cells harboring pPDI10 showed increased PDI activity and accelerated growth, suggesting that the cloned PDI gene is functioning and involved in the yeast growth. The 460 bp upstream region of the PDI gene was fused into promoterless beta-galactosidase gene of the shuttle vector YEp367R to generate pYUPDI10. The synthesis of beta-galactosidase from the PDI-lacZ fusion gene was enhanced by oxidative stress, such as superoxide anion and hydrogen peroxide. It was also induced by some non-fermentable and fermentable carbon sources. Nitrogen starvation was able to enhance the synthesis of beta-galactosidase from the PDI-lacZ fusion gene. The enhancement by oxidative stress and fermentable carbon sources did not depend on the presence of Pap1. The PDI mRNA levels were increased in both Pap1-positive and Pap1-negative cells treated with glycerol. Taken together, the S. pombe PDI gene is involved in cellular growth and response to nutritional and oxidative stress.

Interactions among Yeast Protein-Disulfide Isomerase Proteins and Endoplasmic Reticulum Chaperone Proteins Influence Their Activities

We previously reported that the reductive activities of yeast protein-disulfide isomerase (PDI) family proteins did not completely explain their contribution to the viability of Saccharomyces cerevisiae (Kimura, T., Hosoda, Y., Kitamura, Y., Nakamura, H., Horibe, T., and Kikuchi, M. (2004) Biochem. Biophys. Res. Commun. 320, 359-365). In this study, we examined oxidative refolding activities and found that Mpd1p, Mpd2, and Eug1p exhibit activities of 13.8, 16.0, and 2.16%, respectively, compared with Pdi1p and that activity for Eps1p is undetectable. In analyses of interactions between yeast PDI proteins and endoplasmic reticulum molecular chaperones, we found that Mpd1p alone does not have chaperone activity but that it interacts with and inhibits the chaperone activity of Cne1p, a homologue of mammalian calnexin, and that Cne1p increases the reductive activity of Mpd1p. These results suggest that the interface between Mpd1p and Cne1p is near the peptide-binding site of Cne1p. In addition, Eps1p interacts with Pdi1p, Eug1p, Mpd1p, and Kar2p with dissociation constants (KD) in the range of 10(-7) to 10(-6). Interestingly, co-chaperone activities were completely suppressed in Eps1p-Pdi1p and Eps1p-Mpd1p complexes, although only Eps1p and Pdi1p have chaperone activity. The in vivo consequences of these results are discussed.

Gentamicin binds to the lectin site of calreticulin and inhibits its chaperone activity

Recently, it became clear that aminoglycoside antibiotics affect protein-protein interactions involving protein disulfide isomerase as well as protein synthesis in the endoplasmic reticulum. In this study, we used affinity column chromatography to screen gentamicin-binding proteins in microsomes derived from bovine kidney in order to learn about the possible mechanisms of gentamicin-associated nephrotoxicity. One of the gentamicin-binding proteins was identified as calreticulin (CRT) by N-terminal amino acid sequence analysis. Interestingly, gentamicin inhibited the chaperone and oxidative refolding activities of CRT when N-glycosylated substrates such as alpha1-antitrypsin and alpha-mannosidase were used as substrates, but it did not inhibit the chaperone activity of CRT when unglycosylated citrate synthase was used. Moreover, CRT suppressed the aggregation of deglycosylated and denatured alpha-mannosidase, but gentamicin did not inhibit its chaperone activity. Experiments with domain mutants suggest that the lectin site of CRT is the main target for gentamicin binding and that binding of gentamicin to this site inhibits the chaperone activity of CRT.

Functional differences between human and yeast protein disulfide isomerase family proteins

Previously, it has been reported that a mammalian protein disulfide isomerase (PDI), when expressed on a single copy number plasmid, can rescue growth of a PDI1-disrupted yeast. However, here, for the first time we demonstrated by tetrad analysis that human PDI (hPDI) is unable to replace yeast PDI (yPDI) when hPDI cDNA is integrated into the yeast chromosome. This observation indicates that hPDI is not functionally equivalent to yPDI. Estimation of the actual copy number of the plasmid, as well as comparison of isomerase and chaperone activities between human and yeast PDI homologues, indicates that one copy of hPDI cDNA is not sufficient to rescue the PDI1-disrupted strain. Notably, the isomerase activities of yPDI family proteins, Mpd1p, Mpd2p, and Eug1p, were extremely low, although yPDI itself exhibited twice as much isomerase activity as hPDI in vitro. Moreover, with the exception of Mpd1p, all hPDI and yPDI family proteins had chaperone activity, this being particularly strong in the case of yPDI and Mpd2p. These observations indicate that the growth of Saccharomyces cerevisiae is completely dependent on the isomerase activity of yPDI.

Replacement of domain b of human protein disulfide isomerase-related protein with domain b' of human protein disulfide isomerase dramatically increases its chaperone activity

We have reported that human protein disulfide isomerase-related protein (hPDIR) has isomerase and chaperone activities that are lower than those of the human protein disulfide isomerase (hPDI), and that the b domain of hPDIR is critical for its chaperone activity [J. Biol. Chem. 279 (2004) 4604]. To investigate the basis of the differences between hPDI and hPDIR, and to determine the functions of each hPDIR domain in detail, we constructed several hPDIR domain mutants. Interestingly, when the b domain of hPDIR was replaced with the b' domain of hPDI, a dramatic increase in chaperone activity that was close to that of hPDI itself was observed. However, this mutant showed decreased oxidative refolding of alpha1-antitrypsin. The replacement of the b domain of hPDIR with the c domain of hPDI also increased its chaperone activity. These observations suggest that putative peptide-binding sites of hPDI determine both its chaperone activity and its substrate specificity.

Different Contributions of the Three CXXC Motifs of Human Protein-disulfide Isomerase-related Protein to Isomerase Activity and Oxidative Refolding

Human protein-disulfide isomerase (hPDI)-related protein (hPDIR), which we previously cloned from a human placental cDNA library (Hayano, T., and Kikuchi, M. (1995) FEBS Lett. 372, 210-214), and its mutants were expressed in the Escherichia coli pET system and purified by sequential nickel affinity resin chromatography. Three thioredoxin motifs (CXXC) of purified hPDIR were found to contribute to its isomerase activity with a rank order of CGHC > CPHC > CSMC, although both the isomerase and chaperone activities of this protein were lower than those of hPDI. Screening for hPDIR-binding proteins using a T7 phage display system revealed that alpha1-antitrypsin binds to hPDIR. Surface plasmon resonance experiments demonstrated that alpha1-antitrypsin interacts with hPDIR, but not with hPDI or human P5 (hP5). Interestingly, the rate of oxidative refolding of alpha1-antitrypsin with hPDIR was much higher than with hPDI or hP5. Thus, the substrate specificity of hPDIR differed from that associated with isomerase activity, and the contribution of the CSMC motif to the oxidative refolding of alpha1-antitrypsin was the most definite of the three (CSMC, CGHC, CPHC). Substitution of SM and PH in the CXXC motifs with GH increased isomerase activity and decreased oxidative refolding. In contrast, substitution of GH and PH with SM decreased isomerase activity and increased oxidative refolding. Because CXXC motif mutants lacking isomerase activity retain chaperone activity for the substrate rhodanese, it is clear that, similar to PDI and hP5, the isomerase and chaperone activities of hPDIR are independent. These results suggest that the central dipeptide of the CXXC motif is critical for both redox activity and substrate specificity.

Formation and transfer of disulphide bonds in living cells

Protein disulphide bonds are formed in the endoplasmic reticulum of eukaryotic cells and the periplasmic space of prokaryotic cells. The main pathways that catalyse the formation of protein disulphide bonds in prokaryotes and eukaryotes are remarkably similar, and they share several mechanistic features. The recent identification of new redox-active proteins in humans and yeast that mechanistically parallel the more established redox-active enzymes indicates that there might be further uncharacterized redox pathways throughout the cell.

Identification of a novel saturable endoplasmic reticulum localization mechanism mediated by the C-terminus of a Dictyostelium protein disulfide isomerase

Localization of soluble endoplasmic reticulum (ER) resident proteins is likely achieved by the complementary action of retrieval and retention mechanisms. Whereas the machinery involving the H/KDEL and related retrieval signals in targeting escapees back to the ER is well characterized, other mechanisms including retention are still poorly understood. We have identified a protein disulfide isomerase (Dd-PDI) lacking the HDEL retrieval signal normally found at the C terminus of ER residents in Dictyostelium discoideum. Here we demonstrate that its 57 residue C-terminal domain is necessary for intracellular retention of Dd-PDI and sufficient to localize a green fluorescent protein (GFP) chimera to the ER, especially to the nuclear envelope. Dd-PDI and GFP-PDI57 are recovered in similar cation-dependent complexes. The overexpression of GFP-PDI57 leads to disruption of endogenous PDI complexes and induces the secretion of PDI, whereas overexpression of a GFP-HDEL chimera induces the secretion of endogenous calreticulin, revealing the presence of two independent and saturable mechanisms. Finally, low-level expression of Dd-PDI but not of PDI truncated of its 57 C-terminal residues complements the otherwise lethal yeast TRG1/PDI1 null mutation, demonstrating functional disulfide isomerase activity and ER localization. Altogether, these results indicate that the PDI57 peptide contains ER localization determinants recognized by a conserved machinery present in D. discoideum and Saccharomyces cerevisiae.

Characterization of novel peptide agonists of the alpha mating factor of Saccharomyces cerevisiae

Alpha-factor [WHWLQLKPGQPMY], a secreted tridecapeptide pheromone, is required for mating between the a- and alpha-haploid mating types of Saccharomyces cerevisiae (MATa, MATalpha). New analogues of alpha-factor were synthesized and evaluated by morphogenesis assays and receptor binding studies. The Y(0)Nle(12)F(13) analogue [YWHWLQLKPGQPNleF] (MFN5) caused growth arrest and morphological alteration in MATa cells in a fashion identical to that of the native pheromone. Binding of (125)I-labeled MFN5 was saturable, and reversible as shown by equipotent label displacement by MFN5 and native alpha-mating factor. Scatchard analysis of equilibrium binding data on plasma membranes and intact cells indicated the existence of a single high-affinity binding site (K(d) = 6.4 x 10(-8)). Specific binding of (125)I-labeled MFN5 was significantly reduced by guanosine nucleotides. Affinity cross-linking of (125)I-labeled MFN5 to MATa cell membranes identified a specifically labeled 49-kDa protein. The novel synthetic alpha-factor analogue MFN5 can be easily iodinated and used as a probe for the alpha-factor receptor.

The nonactive site cysteine residues of yeast protein disulfide isomerase are not required for cell viability

Protein disulfide isomerase (PDI), the product of the essential PDI1 gene of Saccharomyces cerevisiae catalyzes oxidization of thiols, reduction of disulfide bonds, and isomerization of disulfides. It can also act as a chaperone to facilitate folding of denatured proteins. The protein has 6 cysteine (Cys) residues. Four of these Cys are part of the 2 thioredoxin-like catalytic sites (-CGHC-), one of which is located near the N- and the other near the C-terminus. In addition, it has 2 non-active site Cys near the N-terminus. The function of these non-active site Cys of yeast PDI is poorly understood. Whereas in yeast PDI, these Cys residues are in the vicinity of the N-terminal-most active site, in mammalian PDI their position is closer to the C-terminal-most active site. We have examined their role and that of the active site cysteines by constructing an extensive set of mutants in which the Cys were systematically replaced by Ser. As reported earlier, the N-terminal Cys of the two active sites sequences of yeast PDI were found to be required for cell viability, but mutation of the C-terminal Cys to Ser in the two active sites was not lethal. We found that replacement of the two non-active site Cys with Ser did not affect cell viability, but in the case of the double mutant in which both Cys were replaced by Ser the processing and secretion of CPY was impaired.

Ca(2+)-ATPases of Saccharomyces cerevisiae: diversity and possible role in protein sorting

The PMR1 gene of Saccharomyces cerevisiae is thought to encode a putative Ca(2+)-ATPase [1]. Membranes isolated from wild-type cells and from pmr1 null mutant of S. cerevisiae were fractionated on sucrose density gradients. In the pmr1 mutant we found a decrease in activity of the P-type ATPase and of ATP-dependent, protonophore-insensitive Ca2+ transport in light membranes, that comigrate with the Golgi marker GDPase. We conclude that the product of the PMR1 gene (Pmr1p) is indeed a Ca(2+)-ATPase of the Golgi and Golgi-like membranes. Surprisingly, the pmr1 null mutation abolished Ca(2+)-ATPase activity in Golgi and/or Golgi-like membranes only to 50% under conditions where they are separated from vacuolar membranes. This indicates that an additional Ca(2+)-ATPase is localized in Golgi and/or Golgi-like membranes. Moreover, a third Ca(2+)-ATPase is found in the ER and ER-like membranes. The data are consistent with the assumption that these Ca(2+)-ATPases are encoded by gene(s) different from PMR1. Disruption of PMR1 Ca(2+)-ATPase causes significant redistribution of enzyme activities and of total protein in compartments of the secretory pathway. A decrease in activity is observed for three integral membrane proteins: NADPH cytochrome c reductase, dolichyl phosphate mannose synthase, and Ca(2+)-ATPase, and also for total protein in Golgi, Golgi-like compartments and in vacuoles, whereas a corresponding increase of these activities is observed in endoplasmic reticulum and endoplasmic reticulum-like membranes. We assume that Ca(2+)-ATPases and sufficient Ca2+ gradients across the organellar membranes are important for the correct sorting of proteins to the various compartments of the secretory apparatus.

Overproduction of Mpd2p suppresses the lethality of protein disulfide isomerase depletion in a CXXC sequence dependent manner

The third multicopy suppressor gene of the PDI1 deletion from Saccharomyces cerevisiae, MPD2, was isolated and characterized. The MPD2 gene encodes a protein with a putative signal sequence, ER retention signal, and a disulfide isomerase active site like sequence. The amino acid sequence around the active site like sequence is similar to the thioredoxin-like domains of PDI and PDI related proteins, although the similarity is comparatively low. A delta-pdi1 strain over-producing Mpd2p showed slow growth and was sensitive to 1 mM dithiothreitol. Mpd2p can be detected in wild type cells and is a glycoprotein. Although the MPD2 gene was not essential for growth, overexpression of the gene partially restored the maturation defect of carboxypeptidase Y caused by the PDI1 deletion. Mutagenesis analysis revealed that Mpd2p can compensate for the loss of PDI with its CXXC sequence.

Active site mutations in yeast protein disulfide isomerase cause dithiothreitol sensitivity and a reduced rate of protein folding in the endoplasmic reticulum

Aspects of protein disulfide isomerase (PDI) function have been studied in yeast in vivo. PDI contains two thioredoxin-like domains, a and a', each of which contains an active-site CXXC motif. The relative importance of the two domains was analyzed by rendering each one inactive by mutation to SGAS. Such mutations had no significant effect on growth. The domains however, were not equivalent since the rate of folding of carboxypeptidase Y (CPY) in vivo was reduced by inactivation of the a domain but not the a' domain. To investigate the relevance of PDI redox potential, the G and H positions of each CGHC active site were randomly mutagenized. The resulting mutant PDIs were ranked by their growth phenotype on medium containing increasing concentrations of DTT. The rate of CPY folding in the mutants showed the same ranking as the DTT sensitivity, suggesting that the oxidative power of PDI is an important factor in folding in vivo. Mutants with a PDI that cannot perform oxidation reactions on its own (CGHS) had a strongly reduced growth rate. The growth rates, however, did not correlate with CPY folding, suggesting that the protein(s) required for optimal growth are dependent on PDI for oxidation. pdi1-deleted strains overexpressing the yeast PDI homologue EUG1 are viable. Exchanging the wild-type Eug1p C(L/I)HS active site sequences for C(L/I)HC increased the growth rate significantly, however, further highlighting the importance of the oxidizing function for optimal growth.

The covalent binding of [14C]acetaminophen to mouse hepatic microsomal proteins: the specific binding to calreticulin and the two forms of the thiol:protein disulfide oxidoreductases

Numerous in vitro studies have indicated that acetaminophen is activated by mouse hepatic microsomal cytochrome P450 to form N-acetylbenzoquinone imine. This in turn covalently binds through a Michael addition to protein sulfhydryl and amino groups. Although acetaminophen adducts of several cytosolic proteins have been purified after its administration in vivo, no adducts of specific microsomal proteins have been reported. We find that, after the in vitro incubation of mouse hepatic microsomes with [ring-14C] acetaminophen in the presence of an NADPH generating system, 95% of the bound radioactivity was associated with adducts to three intraluminal microsomal proteins: calreticulin and the two forms of thiol:protein disulfide oxidoreductase, Q2 and Q5. The acetaminophen bound to 0.35, 1.32, and 0.25 mol/mol of the three proteins, respectively. Sequencing of the 14C-labeled tryptic peptides indicated that the acetaminophen bound to lysine 103 of Q2, lysines 202, 209 or 210 and 354 of Q5 and lysines 233 or 239 of calreticulin. No adducts of cysteine residues were observed. Our data might suggest that acetaminophen hepatotoxicity results from the formation of the reactive metabolite within the endoplasmic reticulum. This then binds to these essential proteins and blocks the posttranslational modification of secretory and membrane proteins. This inhibition could then lead to cellular injury and death.

Protein disulfide isomerase: a multifunctional protein of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a resident enzyme of the endoplasmic reticulum (ER) that was discovered over three decades ago. Contemporary biochemical and molecular biology techniques have revealed that it is present in all eukaryotic cells studied and retained in the ER via a -KDEL or -HDEL sequence at its C-terminus. However, evidence is accumulating that in certain cell types, PDI can be found in other subcellular compartments, despite possessing an intact retention sequence. A wide range of studies has established that in presence of a redox pair, PDI acts catalytically to both form and reduce disulfide bonds, therefore acting as a disulfide isomerase. Recent studies have focused on the mechanism of the isomerization process and the precise role of the two active site sequences (-CGHC-) in the process. In addition, prokaryotes have been shown to possess a set of proteins that function in a similar fashion, being able to generate disulfide bonds on polypeptides translocated into the periplasmic space. Following the recent discovery that PDI binds peptides, coupled with earlier findings that PDI is a subunit of at least two enzymatic complexes (prolyl 4-hydroxylase and microsomal triglyceride transfer protein), it seems that it may serve functions other than merely that of a disulfide isomerase. In fact, it is now clear that PDI can facilitate protein folding independently of its disulfide isomerase activity. A major challenge for the future is to define mechanistically how it accomplishes isomerization and the relationship between this process and the protein folding steps that culminate in the final, fully mature protein.

Isolation and characterization of a yeast gene, MPD1, the overexpression of which suppresses inviability caused by protein disulfide isomerase depletion

MPD1, a yeast gene the overexpression of which suppresses the inviability caused by the loss of protein disulfide isomerase (PDI) was isolated and characterized. The MPD1 gene product retained a single disulfide isomerase active site sequence (APWCGHCK), an N-terminal putative signal sequence, and a C-terminal endoplasmic reticulum (ER) retention signal, and was a novel member of the PDI family. The gene product, identified in yeast extract, contained core size carbohydrates. MPD1 was not essential for growth, but overexpression of the gene suppressed the maturation defect of carboxypeptidase Y caused by PDI1 deletion, indicative of the related function to PDI in the yeast ER.

The cellular response to unfolded proteins: intercompartmental signaling

Both prokaryotic and eukaryotic cells respond to the accumulation of unfolded proteins by increasing the transcription of genes encoding molecular chaperones and other stress-responsive proteins. Different sets of genes are activated when particular cellular compartments are burdened with unfolded proteins. Cells thus maintain mechanisms to monitor changes in the concentration of unfolded proteins not only in the cytosol, but also in membrane-bound extracytoplasmic compartments. During the past year, work in yeast has identified a transmembrane receptor that appears to play a pivotal role in the regulation of protein folding. This receptor monitors the concentration of available chaperone molecules in the endoplasmic reticulum and transmits a signal to the cytosol to activate the transcription of nuclear genes encoding chaperones that are localized in the endoplasmic reticulum. Work using Escherichia coli suggests that prokaryotes also contain an intercompartmental 'unfolded protein' signaling pathway, in this case from the periplasmic space or outer membrane to the cytoplasm.

Selective retention of secretory proteins in the yeast endoplasmic reticulum by treatment of cells with a reducing agent

We have used four glycoproteins as markers to study how disulfide bond formation and protein folding effect the intracellular transport of proteins in yeast. Under normal conditions, the vacuolar enzyme carboxypeptidase Y (CPY) and the secretory stress-protein hsp150 acquired disulfide bonds in the endoplasmic reticulum (ER). Treatment of living cells with the reducing agent dithiothreitol (DTT) prevented disulfide formation of newly synthesized CPY and hsp150, resulting in retention of the proteins in the ER. When DTT was removed, the sulfhydryls were reoxidized, and the transport of the proteins to their correct destinations was resumed. Even mature CPY, located in the vacuole, could be reduced with DTT, and reoxidized after removal of the drug. DTT treatment blocked intracellular transport of hsp150 only when present during the synthesis and translocation of the protein. Reduction of folded hsp150, accumulated in the ER due to a sec block prior to DTT treatment, did not inhibit its secretion. The Kar2p/BiP protein, a component of the ER lumen, was found to be associated with fully translocated reduced hsp150, but not with native hsp150, suggesting that Kar2p/BiP may be involved in the putative retention mechanism. The cysteine-free pro-alpha-factor, and invertase which was shown to have free sulfhydryls, were secreted and modified similarly in the presence and absence of DTT, showing that the secretory pathway of yeast functioned under reducing conditions.

Identification of genomic sequences that mediate the induction of the endoplasmic reticulum stress protein, ERp72, by protein traffic

ERp72, a resident protein of the endoplasmic reticulum (ER) is both a stress protein and a member of the protein disulfide isomerase family of proteins. Analysis of the murine ERp72 promoter region revealed the presence of potential transcriptional control elements characteristic of the promoters of mammalian ER proteins. These include multiple CCAAT elements and Sp1 and AP-2 consensus sequences. Functional analysis of mutations in the ERp72 promoter and 5'-flanking region revealed an 82-bp fragment that is sufficient to mediate the stimulation observed for ERp72 either by stress or by the expression of incompletely assembled immunoglobulin mu heavy chain in the ER. This 82-bp fragment contains two CCAAT elements but little additional homology to protein traffic-responsive sequences of other members of the ER stress family. This suggests that the ERp72 gene contains a novel element that is the target of an intracellular signaling pathway initiated by protein traffic in the ER.

The essential function of yeast protein disulfide isomerase does not reside in its isomerase activity

Protein disulfide isomerase (PDI) is believed to function in vivo by catalyzing the isomerization of disulfide bonds in proteins and thereby facilitating their folding. In S. cerevisiae PDI is encoded by an essential gene. Deletion of nearly one-third of the C-terminal residues of PDI altered PDI's cellular localization but not cell viability. Further deletions resulted in lethality, but these truncated proteins still exhibited PDI activity in vitro. Cells carrying a variant PDI in which both-CGHC-active sites were disrupted were viable. However, these cells exhibited a delay in the disulfide bond formation and transport of carboxypeptidase Y. In vitro enzyme assays revealed that disruption of both sites abolished catalytic activity. These results indicate that PDI catalyzes disulfide bond formation both in vivo and in vitro and that the integrity of the active sites is required for catalysis. However, this catalytic activity is not essential for yeast viability.

A novel primary Ca(2+)-transport system from Saccharomyces cerevisiae

A novel primary Ca(2+)-transport system in membranes from Saccharomyces cerevisiae is described. Ca2+ transport is strictly dependent on the presence of ATP; other nucleotides like GTP, UTP and CTP do not efficiently (< 10% of the rate of ATP) drive uptake. Transport is inhibited by sodium vanadate with an IC50 of 130 microM, but is insensitive to carbonylcyanide p-trifluoromethoxy-phenylhydrazone, valinomycin, gramicidin or calmodulin. Ca2+ accumulates in a free form and can be readily released by the Ca2+ ionophore A-23187 or by osmotic shock. The apparent Km values of transport activity for free Ca2+ was determined to be 0.11 microM and 5 microM for Mg.ATP, respectively. Taken together the results indicate that the Ca2+ transport described here does not belong to the plasma-membrane-type Ca(2+)-ATPase family but rather to the family of endomembrane-type ATPases. Cell-fractionation studies of crude membranes on sucrose gradient centrifugation have shown that the Ca(2+)-transport activity separates from marker enzymes for endoplasmic reticulum, vacuole, or plasma membrane and migrates with GDPase activity, a marker for the yeast Golgi complex.