Cyclosporin A inhibits an initial step in folding of transferrin within the endoplasmic reticulum

Cyclophilin BcCyp2 Regulates Infection-Related Development to Facilitate Virulence of the Gray Mold Fungus Botrytis cinerea

Cyclophilin (Cyp) and Ca²⁺/calcineurin proteins are cellular components related to fungal morphogenesis and virulence; however, their roles in mediating the pathogenesis of Botrytis cinerea, the causative agent of gray mold on over 1000 plant species, remain largely unexplored. Here, we show that disruption of cyclophilin gene BcCYP2 did not impair the pathogen mycelial growth, osmotic and oxidative stress adaptation as well as cell wall integrity, but delayed conidial germination and germling development, altered conidial and sclerotial morphology, reduced infection cushion (IC) formation, sclerotial production and virulence. Exogenous cyclic adenosine monophosphate (cAMP) rescued the deficiency of IC formation of the ∆Bccyp2 mutants, and exogenous cyclosporine A (CsA), an inhibitor targeting cyclophilins, altered hyphal morphology and prevented host-cell penetration in the BcCYP2 harboring strains. Moreover, calcineurin-dependent (CND) genes are differentially expressed in strains losing BcCYP2 in the presence of CsA, suggesting that BcCyp2 functions in the upstream of cAMP- and Ca²⁺/calcineurin-dependent signaling pathways. Interestingly, during IC formation, expression of BcCYP2 is downregulated in a mutant losing BcJAR1, a gene encoding histone 3 lysine 4 (H3K4) demethylase that regulates fungal development and pathogenesis, in B. cinerea, implying that BcCyp2 functions under the control of BcJar1. Collectively, our findings provide new insights into cyclophilins mediating the pathogenesis of B. cinerea and potential targets for drug intervention for fungal diseases.

Calnexin cycle - structural features of the ER chaperone system

The endoplasmic reticulum (ER) is the major folding compartment for secreted and membrane proteins and is the site of a specific chaperone system, the calnexin cycle, for folding N-glycosylated proteins. Recent structures of components of the calnexin cycle have deepened our understanding of quality control mechanisms and protein folding pathways in the ER. In the calnexin cycle, proteins carrying monoglucosylated glycans bind to the lectin chaperones calnexin and calreticulin, which recruit a variety of function-specific chaperones to mediate protein disulfide formation, proline isomerization, and general protein folding. Upon trimming by glucosidase II, the glycan without an inner glucose residue is no longer able to bind to the lectin chaperones. For proteins that have not yet folded properly, the enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) acts as a checkpoint by adding a glucose back to the N-glycan. This allows the misfolded proteins to re-associate with calnexin and calreticulin for additional rounds of chaperone-mediated refolding and prevents them from exiting the ERs. Here, we review progress in structural studies of the calnexin cycle, which reveal common features of how lectin chaperones recruit function-specific chaperones and how UGGT recognizes misfolded proteins.

Mitochondrial permeability transition in cardiac ischemia-reperfusion: whether cyclophilin D is a viable target for cardioprotection?

Growing number of studies provide strong evidence that the mitochondrial permeability transition pore (PTP), a non-selective channel in the inner mitochondrial membrane, is involved in the pathogenesis of cardiac ischemia-reperfusion and can be targeted to attenuate reperfusion-induced damage to the myocardium. The molecular identity of the PTP remains unknown and cyclophilin D is the only protein commonly accepted as a major regulator of the PTP opening. Therefore, cyclophilin D is an attractive target for pharmacological or genetic therapies to reduce ischemia-reperfusion injury in various animal models and humans. Most animal studies demonstrated cardioprotective effects of PTP inhibition; however, a recent large clinical trial conducted by international groups demonstrated that cyclosporine A, a cyclophilin D inhibitor, failed to protect the heart in patients with myocardial infarction. These studies, among others, raise the question of whether cyclophilin D, which plays an important physiological role in the regulation of cell metabolism and mitochondrial bioenergetics, is a viable target for cardioprotection. This review discusses previous studies to provide comprehensive information on the physiological role of cyclophilin D as well as PTP opening in the cell that can be taken into consideration for the development of new PTP inhibitors.

A potential role of the unfolded protein response in post-transplant cancer

Cancer is one of the major causes of mortality in organ transplant patients receiving immunosuppressive regimen based on Cyclosporin A (CsA). Organ transplantation and chronic immunosuppression are typically associated with skin cancers (both squamous cell carcinoma and melanoma) and renal cell carcinoma (RCC). Recent studies have shown that in addition to its immunosuppressive effects, accounted for by the inhibition of calcineurin and the modulation of the transcriptional programme of lymphocytes, CsA also directly stimulates the growth and aggressive behaviour of various cancer cells. Using renal carcinogenesis as an example, we discuss the current evidence for a role of cellular proteostasis, i.e. the regulation of the production, maturation and turnover of proteins in eukaryotic cells, in tumorigenesis arising under conditions of chronic immunosuppression. We present the recent studies showing that CsA induces the unfolded protein response (UPR) in normal and transformed kidney cells. We examine how the UPR might be important, considering in particular the genomic analyses showing the existence of a correlation between the levels of expression of the actors of the UPR, the chaperones of the endoplasmic reticulum (ER) and the aggressiveness of renal carcinoma. The UPR may offer a possible explanation for how immunosuppressive regimens based on CsA promote renal carcinogenesis. We discuss the opportunities offered by this biological knowledge in terms of screening, diagnosis and treatment of post-transplant cancers, and propose possible future translational studies examining the role of tumour proteostasis and the UPR in this context.

Cyclophilin inhibition as potential therapy for liver diseases

The cyclophilins are a group of proteins with peptidyl-prolyl isomerase enzymatic activity, localised in different cellular compartments and involved in a variety of functions related to cell metabolism and energy homeostasis, having enhanced expression in inflammation or malignancy. Cyclophilin A (CypA), the most abundantly expressed cyclophilin, is present mainly in the cytoplasm and is a host factor involved in the life cycle of multiple viruses. The extracellular fractions of CypA and CypB are potent pro-inflammatory mediators. CypD, located in mitochondria, is a key regulator of mitochondrial permeability transition pores, and is critical for necrotic cell death. Cyclosporines are the prototype cyclophilin inhibitors. Cyclic peptides, which bind and inhibit cyclophilins without having immunosuppressive properties, have been generated by chemical modifications of cyclosporin A. In addition, cyclophilin inhibitors that are structurally different from cyclosporines have been synthesized. The involvement of cyclophilins in the pathogenesis of different liver diseases has been established using both in vitro and in vivo investigations, thus indicating that cyclophilin inhibition may be of therapeutic benefit. This review summarises the evidence for potential therapeutic applications of non-immunosuppressive cyclophilin inhibitors, alone or in combination with other agents, in virus-induced liver diseases like hepatitis C, B or Delta, liver inflammation and fibrosis, acetaminophen-induced liver toxicity and hepatocellular carcinoma.

Depletion of cyclophilins B and C leads to dysregulation of endoplasmic reticulum redox homeostasis

Protein folding within the endoplasmic reticulum is assisted by molecular chaperones and folding catalysts that include members of the protein-disulfide isomerase and peptidyl-prolyl isomerase families. In this report, we examined the contributions of the cyclophilin subset of peptidyl-prolyl isomerases to protein folding and identified cyclophilin C as an endoplasmic reticulum (ER) cyclophilin in addition to cyclophilin B. Using albumin and transferrin as models of cis-proline-containing proteins in human hepatoma cells, we found that combined knockdown of cyclophilins B and C delayed transferrin secretion but surprisingly resulted in more efficient oxidative folding and secretion of albumin. Examination of the oxidation status of ER protein-disulfide isomerase family members revealed a shift to a more oxidized state. This was accompanied by a >5-fold elevation in the ratio of oxidized to total glutathione. This "hyperoxidation" phenotype could be duplicated by incubating cells with the cyclophilin inhibitor cyclosporine A, a treatment that triggered efficient ER depletion of cyclophilins B and C by inducing their secretion to the medium. To identify the pathway responsible for ER hyperoxidation, we individually depleted several enzymes that are known or suspected to deliver oxidizing equivalents to the ER: Ero1αβ, VKOR, PRDX4, or QSOX1. Remarkably, none of these enzymes contributed to the elevated oxidized to total glutathione ratio induced by cyclosporine A treatment. These findings establish cyclophilin C as an ER cyclophilin, demonstrate the novel involvement of cyclophilins B and C in ER redox homeostasis, and suggest the existence of an additional ER oxidative pathway that is modulated by ER cyclophilins.

Orchestration of secretory protein folding by ER chaperones

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Cyclophilin A: a key player for human disease

Cyclophilin A (CyPA) is a ubiquitously distributed protein belonging to the immunophilin family. CyPA has peptidyl prolyl cis-trans isomerase (PPIase) activity, which regulates protein folding and trafficking. Although CyPA was initially believed to function primarily as an intracellular protein, recent studies have revealed that it can be secreted by cells in response to inflammatory stimuli. Current research in animal models and humans has provided compelling evidences supporting the critical function of CyPA in several human diseases. This review discusses recently available data about CyPA in cardiovascular diseases, viral infections, neurodegeneration, cancer, rheumatoid arthritis, sepsis, asthma, periodontitis and aging. It is believed that further elucidations of the role of CyPA will provide a better understanding of the molecular mechanisms underlying these diseases and will help develop novel pharmacological therapies.

Phosphorylation at serines 216 and 221 is important for Drosophila HeT-A Gag protein stability

Telomeres from Drosophila appear to be very different from those of other organisms - in size and the mechanism of their maintenance. In the absence of the enzyme telomerase, Drosophila telomeres are maintained by retrotransposition of three elements, HeT-A, TART, and TAHRE, but details of their transposition mechanisms are not known. Here we characterized some biochemical characteristics of the HeT-A Gag protein encoded by the HeT-A element to understand this mechanism. The HeT-A Gag protein when overexpressed in S2 cells was localized to the nucleus but was resistant to high salt, detergents and nuclease extraction treatments. Analysis of the HeT-A Gag protein by tandem mass spectrophotometry revealed that serines 216 and 221 are phosphorylated. Substituting these serines with alanine or aspartic acid by site-directed mutagenesis did not result in any changes in HeT-A Gag translocation across the nucleus, suggesting that phosphorylation of these sites is not associated with HeT-A Gag translocation, but time course experiments showed that these phosphorylation sites are important for Gag-protein stability.

Cyclophilin-mediated reactivation pathway of inactive adenosine kinase aggregates

Monomeric adenosine kinase (AdK), a pivotal salvage enzyme of the purine auxotrophic parasite, Leishmania donovani, tends to aggregate naturally or selectively in presence of ADP, leading to inactivation. A cyclophilin (LdCyP) from the parasite reactivated the enzyme by disaggregating it. We studied the aggregation pathway of AdK with or without ADP. Transmission electron microscopy revealed that ADP-induced aggregates, as opposed to annular or torus-shaped natural aggregates, were mostly amorphous with protofibril-like structures. Interestingly, only the natural aggregates bound thioflavin T with a KD of 3.33 μM, indicating cross β-sheet structure. Dynamic light scattering experiments indicated that monomers formed aggregates either upon prolonged storage or ADP exposure. ADP-aggregates were disaggregated by LdCyP with concomitant reactivation of the enzyme. The activity revived with decrease in the aggregate size. Displacement of ADP from the ADP-aggregated enzyme by LdCyP resulted in reactivation. CD-spectral studies suggested that, like the natural aggregates, ADP induced formation of β-sheet structure in the ADP-aggregates. However, unlike the natural aggregate, it could be reconverted to α-helical conformation upon addition of LdCyP. Based on the results, a regulatory mechanism through interplay of ADP and/or LdCyP interaction with the enzyme is envisaged and a pathway of AdK reactivation by LdCyP-chaperone is proposed.

Chaperone and foldase coexpression in the baculovirus-insect cell expression system

CONCLUSIONS

The BEVS has become widely utilized for production of recombinant proteins. However, protein aggregation and inefficient processing often limit yields, especially for secreted and membrane proteins. Since many proteins of pharmaceutical interest require similar posttranslational processing steps, engineering the folding, assembly, and secretion pathway may enhance the production of a wide variety of valuable complex proteins. Efforts should be undertaken to coexpress the relevant chaperones or foldases at low levels in concert with the final product to ensure the ideal folding and assembly environment. In the future, expression of oligosaccharide modifying enzymes and secretion factors may further improve secretion rates of assembled proteins and provide heterologous proteins with altered glycoforms. Also significant is the use of BEVS as an in vivo eucaryotic laboratory to study the fundamental roles of differnt chaperones, foldases, and secretion factors. The coexpression of chaperones and foldases will complement other approaches such as the development of alternative insect cell lines, promoters, and signal peptides to optimize the baculovirus-insect cell expression system for generating high yields of valuable proteins.

Cyclosporin a inhibits albumin synthesis in Huh7 cells

BACKGROUND/AIMS

Hypoalbuminemia occurs frequently in renal transplant recipients immediately after renal transplantation. We studied the regulation of hepatic albumin synthesis by cyclosporin A (CsA) in Huh7 cells.

METHODS

Huh7 cells were incubated with various concentrations of CsA for 4, 8, 16, and 24 hours. Albumin was measured in Huh7 cell-conditioned medium by sandwich enzyme-linked immunosorbent assay and Western blot. Albumin mRNA expression was analyzed by Northern blotting in CsA-treated cells.

RESULTS

CsA (10(-7)-10(-4) M) inhibited albumin synthesis in Huh7 cells in a dose- dependent manner. A Western blot analysis for albumin in the conditioned medium released from CsA-treated (10(-7)-10(-5) M) cells also showed significant inhibition of albumin synthesis in a dose-dependent manner. Vehicle (olive oil) did not affect albumin synthesis. In contrast, a Northern blot analysis revealed no inhibition of albumin mRNA expression by CsA at any time point from 1-24 hours, indicating that the inhibition of albumin synthesis occurred at the translational level.

CONCLUSIONS

Our results suggest that inhibition of hepatic albumin synthesis by high dose CsA contributes to the hypoalbuminemia in renal transplant recipients.

Structural basis of cyclophilin B binding by the calnexin/calreticulin P-domain

Little is known about how chaperones in the endoplasmic reticulum are organized into complexes to assist in the proper folding of secreted proteins. One notable exception is the complex of ERp57 and calnexin that functions as part the calnexin cycle to direct disulfide bond formation in N-glycoproteins. Here, we report three new complexes composed of the peptidyl prolyl cis-trans-isomerase cyclophilin B and any of the lectin chaperones: calnexin, calreticulin, or calmegin. The 1.7 Å crystal structure of cyclophilin with the proline-rich P-domain of calmegin reveals that binding is mediated by the same surface that binds ERp57. We used NMR titrations and mutagenesis to measure low micromolar binding of cyclophilin to all three lectin chaperones and identify essential interfacial residues. The immunosuppressant cyclosporin A did not affect complex formation, confirming the functional independence of the P-domain binding and proline isomerization sites of cyclophilin. Our results reveal the P-domain functions as a unique protein-protein interaction domain and implicate a peptidyl prolyl isomerase as a new element in the calnexin cycle.

Cyclosporin A and atherosclerosis--cellular pathways in atherogenesis

Cyclosporin A (CsA) is an immunosuppressant drug widely used in organ transplant recipients and people with autoimmune disorders. Long term treatment with CsA is associated with many side effects including hyperlipidemia and an increased risk of atherosclerosis. While its immunosuppressive effects are closely linked to its effects on T cell activation via the inhibition of the nuclear factor of activated T cells (NFAT) pathway, the precise mechanisms underlying its cardiovascular effects appear to involve multiple pathways additional to those relevant for immunosuppression. These include inhibition of calcineurin activity and intracellular cyclophilin peptidylprolyl isomerase and chaperone activities, inhibition of pro-inflammatory extracellular cyclophilin A, and NFAT-independent transcriptional effects. CsA demonstrates complex effects on lipoprotein metabolism and bile acid production, and affects endothelial cells, smooth muscle cells and macrophages, all of which are critical to the atherosclerotic process. Interpretation of the available data is hampered as many experimental models are used to study the effects of CsA in vivo and in vitro, leading to diverse and often contradictory findings. In this review we will describe the cellular mechanisms related to CsA-induced hyperlipidemia and atherosclerosis, with a focus on identifying pro-atherogenic pathways that are distinct from those relevant to its immunosuppressant effects. The potential of CsA analogues to avoid such sequelae will be discussed.

The role of corneal crystallins in the cellular defense mechanisms against oxidative stress

The refracton hypothesis describes the lens and cornea together as a functional unit that provides the proper ocular transparent and refractive properties for the basis of normal vision. Similarities between the lens and corneal crystallins also suggest that both elements of the refracton may also contribute to the antioxidant defenses of the entire eye. The cornea is the primary physical barrier against environmental assault to the eye and functions as a dominant filter of UV radiation. It is routinely exposed to reactive oxygen species (ROS)-generating UV light and molecular O(2) making it a target vulnerable to UV-induced damage. The cornea is equipped with several defensive mechanisms to counteract the deleterious effects of UV-induced oxidative damage. These comprise both non-enzymatic elements that include proteins and low molecular weight compounds (ferritin, glutathione, NAD(P)H, ascorbate and alpha-tocopherol) as well as various enzymes (catalase, glucose-6-phosphate dehydrogenase, glutathione peroxidase, glutathione reductase, and superoxide dismutase). Several proteins accumulate in the cornea at unusually high concentrations and have been classified as corneal crystallins based on the analogy of these proteins with the abundant taxon-specific lens crystallins. In addition to performing a structural role related to ocular transparency, corneal crystallins may also contribute to the corneal antioxidant systems through a variety of mechanisms including the direct scavenging of free radicals, the production of NAD(P)H, the metabolism and/or detoxification of toxic compounds (i.e. reactive aldehydes), and the direct absorption of UV radiation. In this review, we extend the discussion of the antioxidant defenses of the cornea to include these highly expressed corneal crystallins and address their specific capacities to minimize oxidative damage.

Amino acid residues of Leishmania donovani cyclophilin key to interaction with its adenosine kinase: biological implications

Cyclophilins (CyPs), by interacting with a variety of proteins, often modulate their biological activities and thus have been implicated in several cellular functions. However, mechanisms that determine such interactions are poorly understood. We earlier reported that an endoplasmic reticulum (ER)-located cyclophilin (LdCyP) from the purine auxotrophic parasitic protozoan Leishmania donovani reactivated its adenosine kinase (AdK). The AdK-reactivating property of LdCyP was however abolished at high ionic strength but not by nonionic detergents. Modeling of LdCyP, based on its crystal structure solved at 1.97 A resolution, revealed several solvent-exposed hydrophobic and charged residues. Mutagenesis of several of such solvent-exposed residues was performed and their corresponding activities with regard to their (i) AdK reactivation property, (ii) ability to form complex with the enzyme, (iii) capacity to induce red shift in the intrinsic tryptophan fluorescence maxima of AdK, and (iv) efficiency to withdraw the ADP inhibition from the AdK-mediated reaction were compared to the wild-type protein. Results indicated that while the replacement of R147 with either A or D severely impaired all of the above characteristics displayed by the wild-type LdCyP, the effect of mutating K114 and K153 was although relatively less but nevertheless noticeable. Alteration of other exposed hydrophobic and charged residues apparently did not have any discernible effect. Under the condition of cellular stress, the ER-located LdCyP is released into the cytoplasm with concomitant increase both in the specific activity of the cytosol-resident AdK and the uptake of radiolabeled Ado into the cells. These experiments, besides demonstrating the importance of the positive charge, identified R147 as the most crucial residue in the LdCyP-AdK interaction and provide evidence for the stress-induced retrograde translocation of LdCyP from the ER to the cytoplasm. A possible implication of this interaction in the life cycle of the parasite is proposed.

Molecular and functional characterization of a cyclophilin with antifungal activity from Chinese cabbage

An antifungal protein that inhibits the growth of filamentous fungal pathogens was isolated from Chinese cabbage (Brassica campestris L. ssp. pekinensis) by affinity chromatography on Affi-gel blue gel and ion exchange chromatography on CM-Sepharose. The N-terminal amino acid sequence of the protein was highly homologous to that of plant cyclophilins and consequently the protein was denoted as C-CyP. To understand the antifungal activity of C-CyP, we isolated a cDNA encoding its gene from a Chinese cabbage leaf cDNA library. The Chinese cabbage genome bears more than one C-CyP gene copy and C-CyP mRNA is highly expressed in all tissues except the seeds. Recombinant C-CyP catalyzed the cis-trans inter-conversion of the Ala-Pro bond of the substrate, which indicates this protein has peptidyl-prolyl cis-trans isomerase activity. It also inhibited the growth of several fungal pathogens.

Reversal of ADP-mediated aggregation of adenosine kinase by cyclophilin leads to its reactivation

Cyclophilins have been implicated in several important cellular functions. Our earlier results showed that reactivation of adenosine kinase (AdK) by CyP (LdCyP) from the parasitic protozoa Leishmania donovani is accompanied with disaggregation of the enzyme [Chakraborty, A., et al. (2002) J. Biol. Chem. 277, 47451-47460; Chakraborty, A., et al. (2004) Biochemistry 43, 11862-11872]. However, it remained to be known why the enzyme displayed progressive inhibition during the time-dependent reaction and what LdCyP does to prevent and/or reverse the inhibition. Herein, we demonstrate that one of its reaction products, ADP but not AMP, facilitates the formation of AdK aggregates, leading to its inactivation. Further studies revealed that LdCyP reactivates the enzyme by withdrawing the ADP inhibition. To investigate the molecular mechanism, the intrinsic tryptophan fluorescence and polarization of AdK were monitored in the presence of either LdCyP or ADP and in combination thereof. Whereas in the presence of LdCyP the tryptophan fluorescence emission maxima of AdK exhibited a red shift, ADP had a quenching effect. However, both the red shift and quenching became less noticeable when one (W234) of the two tryptophan residues of AdK was altered, indicating W234 fluorescence is relatively more sensitive to both LdCyP and ADP binding. Kinetic measurements indicated that LdCyP-facilitated reactivation of AdK is accompanied with a concomitant increase in the KD of ADP but not of AMP. Interestingly, addition of myokinase (MK) and pyruvate kinase (PK) along with phosphoenolpyruvate, either singly or in conjunction, to the AdK reaction mixture led to its reactivation. The effect of PK but not of MK could be substituted by CyP and vice versa. Taken together, the results suggest that LdCyP-induced reactivation occurs due to conformational reorientation of AdK in a manner that decreases the affinity of the enzyme for ADP with consequent relief from the ADP-mediated aggregation.

Versatility of the endoplasmic reticulum protein folding factory

The endoplasmic reticulum (ER) is dedicated to import, folding and assembly of all proteins that travel along or reside in the secretory pathway of eukaryotic cells. Folding in the ER is special. For instance, newly synthesized proteins are N-glycosylated and by default form disulfide bonds in the ER, but not elsewhere in the cell. In this review, we discuss which features distinguish the ER as an efficient folding factory, how the ER monitors its output and how it disposes of folding failures.

The endoplasmic reticulum as a protein-folding compartment

The lumen of the endoplasmic reticulum (ER) provides a dynamic and efficient environment for the folding of proteins destined for secretion and for a variety of cellular compartments and membranes. Usually, the folding process begins on the nascent chains and is completed minutes or hours later during assembly of oligomers. It is assisted by molecular chaperones and folding enzymes, some of which are unique to the ER. Quality control and selective degradation systems ensure only conformationally mature proteins are transported from the ER.

Isomerase-Independent Chaperone Function of Cyclophilin Ensures Aggregation Prevention of Adenosine Kinase Both in vitro and under in vivo Conditions

Using inactive aggregates of adenosine kinase (AdK) from Leishmania donovani as the model substrate, we recently demonstrated that a cyclophilin (LdCyP) from the same source in an isomerase-independent fashion reactivated the enzyme in vitro by disaggregating its inactive oligomers [Chakraborty et al. (2002) J. Biol. Chem. 277, 47451-47460]. Besides disrupting preformed aggregates, LdCyP also prevents reaggregation of the newly formed active protein that is generated after productive refolding from its urea-denatured state. To investigate possible physiological implications of such phenomena, a unique expression system that simultaneously induces both AdK and LdCyP in naturally AdK-deficient Escherichia coli, was developed. Both in vitro and in vivo experiments revealed that oligomerization is an inherent property of this particular enzyme. In vivo protein cross-linking studies, activity determination analysis and Ado phosphorylation experiments carried out in cells coexpressing both the proteins unequivocally demonstrated that, similar to the phenomena observed in vitro, aggregates of the enzyme formed in vivo are able to interact with both LdCyP and its N-terminal truncated form (N(22-88)DEL LdCyP) in a crowded intracellular environment, resulting in aggregation prevention and reactivation of the enzyme. Our results indicate that the isomerase-independent chaperone function of LdCyP, detected in vitro, participates in vivo as well to keep aggregation-prone proteins in a monomeric state. Furthermore, analogous to yeast/bacterial two-hybrid systems, development of this simple coexpression system may help in the confirmation of interaction of two proteins under simulated in vivo conditions.

Deletion of the GPI pre-anchor sequence in human p97—a general approach for generating the soluble form of GPI-linked proteins

Melanotransferrin, also named p97, belongs to the transferrin-like group of iron-binding proteins. Unlike the other members of this family, p97 exists in two forms-one soluble form and one attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor. The GPI-linked form plays a role in the uptake of iron, while the soluble form of p97 has the unique ability of traversing the blood-brain barrier and may be utilized to deliver drug conjugates into the brain. To investigate these possibilities, a recombinant soluble form of p97 from the GPI-linked p97 protein is required. The approach involved sequential deletions of the p97 GPI pre-anchor sequence (PAS) up to the putative site of cleavage/attachment, releasing p97 from attachment to the GPI-anchor and rendering it soluble. Transfection of the p97 deletion constructs into both the CHO and BHK TK(-) cells was performed with the aim of optimizing the production of p97 by utilizing the cell characteristics unique to each cell line. Altering the GPI PAS resulted in the generation of a recombinant soluble form that was secreted at significantly higher rates than from the full-length expressing cell lines. Increases were from 22 x 10(-9) to 241 x 10(-9)microg/cell/h for expression in the CHO cell system and from 220 x 10(-9) to 4970 x 10(-9)microg/cell/h for the BHK system. Furthermore, there appeared to be differences in the secretion rates between the various deletions suggesting the need for closer examination of the C-terminus in achieving maximum production of the altered proteins. The results of this study are likely applicable for expressing soluble forms of other GPI-linked proteins.

Requirement for cyclophilin A for the replication of vesicular stomatitis virus New Jersey serotype

Several host proteins have been shown to play key roles in the life-cycle of vesicular stomatitis virus (VSV). We have identified an additional host protein, cyclophilin A (CypA), a chaperone protein possessing peptidyl cis-trans prolyl-isomerase activity, as one of the cellular factors required for VSV replication. Inhibition of the enzymatic activity of cellular CypA by cyclosporin A (CsA) or SDZ-211-811 resulted in a drastic inhibition of gene expression by VSV New Jersey (VSV-NJ) serotype, while these drugs had a significantly reduced effect on the genome expression of VSV Indiana (VSV-IND) serotype. Overexpression of a catalytically inactive mutant of CypA resulted in the reduction of VSV-NJ replication, suggesting a requirement for functional CypA for VSV-NJ infection. It was also shown that CypA interacted with the nucleocapsid (N) protein of VSV-NJ and VSV-IND in infected cells and was incorporated into the released virions of both serotypes. VSV-NJ utilized CypA for post-entry intracellular primary transcription, since inhibition of CypA with CsA reduced primary transcription of VSV-NJ by 85-90 %, whereas reduction for VSV-IND was only 10 %. Thus, it seems that cellular CypA binds to the N protein of both serotypes of VSV. However, it performs an obligatory function on the N protein activity of VSV-NJ, while its requirement is significantly less critical for VSV-IND N protein function. The different requirements for CypA by two serologically different viruses belonging to the same family has highlighted the utilization of specific host factors during their evolutionary lineages.

A Single-domain Cyclophilin from Leishmania donovaniReactivates Soluble Aggregates of Adenosine Kinase by Isomerase-independent Chaperone Function

Disaggregation and reactivation of aggregated proteins by chaperones is well established. However, little is known regarding such kind of function of single-domain small cyclophilins (CyPs). Here we demonstrate that, with increasing concentrations, fully active adenosine kinase (AdK) of Leishmania donovani tends to form soluble aggregates, resulting in inactivation. Using this inactive enzyme as the substrate, it is shown that a CyP from L. donovani (LdCyP) alone can cause complete disaggregation, leading to reactivation of the enzyme. The reactivating ability of LdCyP remains unaffected even in the presence of cyclosporin A and macromolecular crowding agents. The reactivation occurs noncatalytically and is reversible. A truncated LdCyP, devoid of 88 amino acids from the N terminus, is found to be required in near stoichiometric proportion to reactivate AdK, suggesting essentiality of the C-terminal region. Gel filtration and light-scattering experiments together with protein cross-linking studies revealed that both full-length LdCyP and the truncated form directly interact with AdK and convert oligomeric forms of the enzyme to monomeric state. Homology modeling studies suggest that the exposed hydrophobic residues of LdCyP, by interacting with solvent-accessible hydrophobic surface of AdK, pull apart its aggregated inactive oligomers to functional monomers. Clearly, the results are consistent with the interpretation that the higher efficiency of the truncated LdCyP is most likely due to increased exposure of the hydrophobic residues on its surface. These observations, besides establishing L. donovani AdK as one of the model enzymes to study aggregation-disaggregation of proteins, raise the possibility that single-domain small CyPs, under physiological conditions, may regulate the activity of aggregation-prone proteins by ensuring their disaggregation.

Structural and biological characterisation of the gut-associated cyclophilin B isoforms from Caenorhabditis elegans

The free-living nematode Caenorhabditis elegans expresses 18 cyclophilin isoforms, eight of which are conserved single domain forms, comprising two closely related secreted or type B forms (CYP-5 and CYP-6). Recombinant CYP-5 has been purified, crystallised and the X-ray structure solved to a resolution of 1.75A. The detailed molecular architecture most strongly resembles the structure of human cyclophilin B with conserved changes in loop structure and N and C-terminal extensions. Interestingly, the active site pocket is occupied by a molecule of dithiothreitol though this has little effect on the geometry of the active site which is similar to other cyclophilin structures. The peptidyl-prolyl isomerase activity of CYP-5 has been characterised against the substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide, and gives a k(cat)/K(m) value of 3.6x10(6)M(-1)s(-1) that compares with a value of 6.3x10(6)M(-1)s(-1) for human cyclophilin B. The immunosuppressive drug cyclosporin A binds and inhibits CYP-5 with an IC(50) value of 50nM, which is comparable to the value of 84nM found for human cyclophilin B. CYP-6 has 67% sequence identity with CYP-5 and a molecular model was built based on the CYP-5 crystal structure. The model shows that CYP-5 and CYP-6 are likely to have very similar structures, but with a markedly increased number of negative charges distributed around the surface of CYP-6. The spatial expression patterns of the cyclophilin B isoforms were examined using transgenic animals carrying a LacZ reporter fusion to these genes, and both cyp-5 and cyp-6 are found to be expressed in an overlapping fashion in the nematode gut. The temporal expression pattern of cyp-5 was further determined and revealed a constitutive expression pattern, with highest abundance levels being found in the embryo.

Lack of abundance of cytoplasmic cyclosporin A-binding protein renders free-living Leishmania donovani resistant to cyclosporin A

The majority of the effects of cyclosporin A (CsA) on cells is caused by the inhibition of phosphatase activity of calcineurin (CN) by the cyclophilin A (CyPA)-CsA complex formed in the cytoplasm. Although CsA inhibits the proliferation of a large number of parasites, not all are susceptible. The presence of structurally altered CyPA with lower affinity for CsA had been suggested to be the cause of resistance. We report here the identification and cloning of a high affinity CsA-binding protein (LdCyP) from Leishmania donovani, a trypanosomatid parasite that is naturally resistant to CsA. The translated LdCyP consists of 187 amino acids with a cleavable 21-amino acid hydrophobic NH(2)-terminal extension. Modeling studies confirmed that all the residues of human CyPs responsible for interaction with CsA are sequentially and conformationally conserved in LdCyP. The purified recombinant protein displayed biochemical parameters comparable to human CyPs. Reverse transcription-polymerase chain reaction analysis confirmed that LdCyP was abundantly expressed. Immunoblot experiments and direct CsA binding studies revealed that LdCyP located in the subcellular organelles constituted the bulk of the CsA binding activity present in L. donovani, whereas the level of binding activity in the cytosol was conspicuously low. CsA selectively facilitated the secretion of LdCyP in the culture medium. Based on these results, it is concluded that the insensitivity of L. donovani to CsA is probably due to the paucity of CsA binding activity in the cytoplasm of the parasite. We suggest that LdCyP, located in the secretory pathway, may function as a chaperone by binding to membrane proteins rather than as the mediator of CN inhibition.

Cyclosporin A inhibits creatine uptake by altering surface expression of the creatine transporter

The immunosuppressive drug cyclosporin A (CsA) inhibited the hCRT-1 cDNA-induced creatine uptake in Xenopus oocytes and the endogenous creatine uptake in cultured C(2)C(12) muscle cells in a dose- and time-dependent manner. FK506, another potent immunosuppressant, was unable to mimic the effect of CsA suggesting that the inhibitory effect of CsA was specific. To delineate the mechanism underlying, we investigated the effect of CsA on the K(m) and V(max) of creatine transport and also on the cell surface distribution of the creatine transporter. Although CsA treatment did not affect the K(m) (20-24 microm) for creatine, it significantly decreased the V(max) of creatine uptake in both oocytes and muscle cells. CsA treatment reduced the cell surface expression level of the creatine transporter in the muscle cells by approximately 60% without significantly altering its total expression level, and the reduction in the cell surface expression paralleled the decrease in creatine uptake. Taken together, our results suggest that CsA inhibited creatine uptake by altering the surface abundance of the creatine transporter. We propose that CsA impairs the targeting of the creatine transporter by inhibiting the function of an associated cyclophilin, resulting in an apparent loss in surface expression of the creatine transporter. Our results also suggest that prolonged exposure to CsA may result in chronically creatine-depleted muscle, which may be a cause for the development of CsA-associated clinical myopathies in organ transplant patients.

Developmental regulation of FKBP65. An ER-localized extracellular matrix binding-protein

FKBP65 (65-kDa FK506-binding protein) is a member of the highly conserved family of intracellular receptors called immunophilins. All have the property of peptidyl-prolyl cis-trans isomerization, and most have been implicated in folding and trafficking events. In an earlier study, we identified that FKBP65 associates with the extracellular matrix protein tropoelastin during its transport through the cell. In the present study, we have carried out a detailed investigation of the subcellular localization of FKBP65 and its relationship to tropoelastin. Using subcellular fractionation, Triton X-114 phase separation, protease protection assays, and immunofluorescence microscopy (IF), we have identified that FKBP65 is contained within the lumen of the endoplasmic reticulum (ER). Subsequent IF studies colocalized FKBP65 with tropoelastin and showed that the two proteins dissociate before reaching the Golgi apparatus. Immunohistochemical localization of FKBP65 in developing lung showed strong staining of vascular and airway smooth muscle cells. Similar areas stained positive for the presence of elastic fibers in the extracellular matrix. The expression of FKBP65 was investigated during development as tropoelastin is not expressed in adult tissues. Tissue-specific expression of FKBP65 was observed in 12-d old mouse tissues; however, the pattern of expression of FKBP65 was not restricted to those tissues expressing tropoelastin. This suggests that additional ligands for FKBP65 likely exist within the ER. Remarkably, in the adult tissues examined, FKBP65 expression was absent or barely detectable. Taken together, these results support an ER-localized FKBP65-tropoelastin interaction that occurs specifically during growth and development of tissues.

Transferrins: iron release from lactoferrin

Iron loss in vitro by the iron scavenger bovine lactoferrin was investigated in acidic media in the presence of three different monoanions (NO(3)(-), Cl(-) and Br(-)) and one dianion (SO(4)(2-)). Holo and monoferric C-site lactoferrins lose iron in acidic media (pH< or =3.5) by a four-step mechanism. The first two steps describe modifications in the conformation affecting the whole protein, which occur also with apolactoferrin. These two processes are independent of iron load and are followed by a third step consisting of the gain of two protons. This third step is kinetically controlled by the interaction with two Cl(-), Br(-) and NO(3)(-) or one SO(4)(2-). In the fourth step, iron loss is under the kinetic control of a slow gain of two protons; third-order rate-constants k(2), 4.3(+/-0.2)x10(3), 3.4(+/-0.5)x10(3), 3.3(+/-0.5)x10(3) and 1.5(+/-0.5)x10(3) M(-2) s(-1) when the protein is in interaction with SO(4)(2-), NO(3)(-), Cl(-) or Br(-), respectively. This step is accompanied by the loss of the interaction with the anions; equilibrium constant K(2), 20+/-5 mM, 1.0(+/-0.2)x10(-1), 1.5(+/-0.5)x10(-1) and 1.0(+/-0.3)x10(-1) M(2), for SO(4)(-), NO(3)(-), Cl(-) and Br(-), respectively. This mechanism is very different from that determined in mildly acidic media at low ionic strength (micro<0.5) for the iron transport proteins, serum transferrin and ovotransferrin, with which no prior change in conformation or interaction with anions is required. These differences may result from the fact that in the transport proteins, the interdomain hydrogen bonds that consolidate the closed conformation of the iron-binding cleft occur between amino acid side-chain residues that can protonate in mildly acidic media. With bovine lactoferrin, most of the interdomain hydrogen bonds involved in the C-site and one of those involved in the N-site occur between amino acid side-chain residues that cannot protonate. The breaking of the interdomain H-bond upon protonation can trigger the opening of the iron cleft, facilitating iron loss in serum transferrin and ovotransferrin. This situation is, however, different in lactoferrin, where iron loss requires a prior change in conformation. This can explain why lactoferrin does not lose its iron load in acidic media and why it is not involved in iron transport in acidic endosomes.

Isolation, characterization and targeted disruption of mouse ppia: cyclophilin A is not essential for mammalian cell viability

Cyclophilins (CyPs) are a family of proteins found in organisms ranging from prokaryotes to humans. These molecules exhibit peptidyl-prolyl isomerase activity in vitro, suggesting that they influence the conformation of proteins in cells. CyPs also bind with varying affinities to the immunosuppressive drug cyclosporin A (CsA), a compound used clinically to prevent allograft rejection. The founding member of the family, cyclophilin A (CyPA), is an abundant, ubiquitously expressed protein of unknown function that binds with nanomolar affinity to CsA. Here, we describe the isolation and characterization of mouse Ppia (mPpia), the gene encoding CyPA. Ppia was isolated using a PCR screen that distinguishes the expressed gene from multiple pseudogenes present in the mouse genome. mPpia consists of 5 exons and 4 introns spanning roughly 4.5 kb and maps to chromosome 11 near the centromere. Sequence analysis of a 369-bp fragment from the proximal promoter region of mPpia revealed the presence of a TATA box and sites recognized by several transcriptional regulators, including Sp1, AP-2, GATA factors, c-Myb, and NF-IL-6. This region is sufficient to drive high-level reporter gene expression in transfected cells. Both copies of Ppia were disrupted in murine embryonic stem (ES) cells via gene targeting. Ppia(-/-) ES cells grow normally and differentiate into hematopoeitic precursor cells in vitro, indicating that CyPA is not essential for mammalian cell viability.

The HIV-1 Env protein signal sequence retards its cleavage and down-regulates the glycoprotein folding

Secretory proteins and most membrane proteins are synthesized with a signal sequence that is usually cleaved from the nascent polypeptide chain, during its transport, into the lumen of the endoplasmic reticulum (ER). We have analyzed the kinetics of the cleavage of the HIV-1 Env protein signal sequence from gp160 and gp120 in HeLa, BHK, and Jurkat cells. Furthermore, we have determined the effects of this cleavage on the association of the gp160 and gp120 glycoproteins with the ER protein calnexin and the effects of the signal sequence cleavage on protein folding. The cleavage of the HIV-1 Env protein signal sequence on both gp160 and gp120 occurred very slowly in all three cell lines with a t(1/2) of 45-60 min. The core glycosylated and signal-sequence-retained forms of gp160 and gp120 associated with calnexin while the signal-sequence-cleaved forms of gp160 and gp120 had disassociated from calnexin and correctly folded as determined by their ability to associate with the CD4 cellular receptor. Further analysis of the folding state of gp160 and gp120 in nonreducing SDS-PAGE revealed that the signal-sequence-retained and calnexin-associated forms of gp160 and gp120 migrated as broad, diffuse bands, whereas the signal-sequence-cleaved or CD4-associated forms of gp160 and gp120 migrated as single sharper bands. The cause of this retardation in the rate of folding and intracellular transport of HIV-1 glycoproteins was localized to their signal sequences by fusing the vesicular stomatitis virus G protein with the HIV-1 Env protein signal sequence and expressing this chimeric protein in mammalian cells. The HIV-1 Env protein signal sequence on the VSV-G protein also confers a reduced rate of cleavage and slow intracellular transport and folding of the chimeric G protein. These results provide direct evidence that in vivo the HIV-1 glycoprotein signal sequence inhibits the folding of HIV-1 Env protein. Our data also suggest a direct correlation between the rate of the signal sequence cleavage and protein folding.

Upregulation of interleukin-10 and inhibition of alloantigen responses by transferrin and transferrin-derived glycans

Previous studies have shown that critically timed administration of transferrin (Tf) facilitates induction of immunologic unresponsiveness. Here, we determined in mixed leukocyte culture (MLC) and in concanavalin A (ConA)-driven cultures the effect of exogenous Tf and Tf-derived glycans (Tf-Gly) on lymphocyte proliferation. In cultures of human blood lymphocytes, Tf inhibited selectively alloantigen-driven proliferation in MLC, but not ConA-stimulated lymphocyte proliferation. Deglycosylation of Tf abrogated the inhibitory effect of Tf on alloantigen-induced lymphocyte proliferation, and, consistent with a role for glycans, an effect qualitatively and quantitatively similar to Tf was exerted by purified Tf-Gly. Glycans isolated from other proteins, for example, immunoglobulin G (IgG) or fibrinogen, failed to inhibit alloantigen-induced proliferation selectively. Rather, they suppressed lymphocyte proliferation in a non-specific manner. Determination of cytokines in MLC supernatant showed a downregulation of interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), IL-2, and IL-12 (p40), along with an upregulation of IL-10, a pattern entirely consistent with the observed effects of Tf and Tf-Gly on alloantigen-induced lymphocyte proliferation. The effect of Tf on MLC was directly IL-10-dependent. IL-10 levels were inversely correlated with lymphocyte proliferation and CD86 expression. Neutralization of IL-10 by anti-IL-10 monoclonal antibody (mAb) blocked the effect of Tf. The MLC-modulating effect of Tf (or Tf-Gly) was not dependent upon the Tf receptor CD71 but appeared to be mediated by a Gly-responsive receptor. These data suggest a role of Tf, and, in particular, Tf-Gly, in allo-interactions that is independent from the role of Tf in iron metabolism, and appears to involve co-stimulatory signals.

Yeast flavin-containing monooxygenase is induced by the unfolded protein response

Flavin-containing monooxygenase from yeast (yFMO) carries out the O(2)- and NADPH-dependent oxidation of biological thiols, including oxidizing glutathione to glutathione disulfide. FMO provides a large fraction of the oxidizing necessary for proper folding of disulfide bond-containing proteins; deletion of the enzyme reduces proper folding of endogenous carboxypeptidase Y by about 40%. The enzyme is not essential to cell viability because other enzymes can generate a significant fraction of the oxidizing equivalents required by the cell. However, yFMO is vital to the yeast response to reductive stress. FMO1 deletion mutants grow poorly under reductive stress, and carboxypeptidase Y activity is less than 10% of that in a stressed wild type. The FMO1 gene appears to be under control of an unfolded protein response element and is inducible by factors, such as reductive stress, that elicit the unfolded protein response. Reductive stress can increase yFMO activity at least 6-fold. This increased activity allows the cell to process endogenous disulfide bond-containing proteins and also to allow correct folding of disulfide-bonded proteins expressed from multicopy plasmids. The unfolded protein response is mediated by the Hac1p transcription factor that mediates virtually all of the induction of yFMO triggered by exogenous reducing agents.

Protein folding in the ER

The endoplasmic reticulum (ER) is a major protein folding compartment for secreted, plasma membrane and organelle proteins. Each of these newly-synthesized polypeptides folds in a deterministic process, affected by the unique conditions that exist in the ER. An understanding of protein folding in the ER is a fundamental biomolecular challenge at two levels. The first level addresses how the amino acid sequence programs that polypeptide to efficiently arrive at a particular fold out of a multitude of alternatives, and how different sequences obtain similar folds. At the second level are the issues introduced by folding not in the cytosol, but in the ER, including the risk of aggregation in a molecularly crowded environment, accommodation of post-translational modifications and the compatibility with subsequent intracellular trafficking. This review discusses both the physicochemical and cell biological constraints of folding, which are the challenges that the ER molecular chaperones help overcome.

Import and processing of heart mitochondrial cyclophilin D

Cyclophilins are a family of cyclosporin-A-binding proteins which catalyse rotation about prolyl peptide bonds. A mitochondrial isoform in mammalian cells, cyclophilin D, is a component of the permeability transition pore that is formed by the adenine nucleotide translocase and the voltage-dependent anion channel at contact sites between the inner and outer membrane. This study investigated the submitochondrial location of cyclophilin D by following the fate of radiolabelled protein following import. Precursor [(35)S]cyclophilin D was expressed in vitro from a PCR-generated cDNA. The precursor was imported by rat heart mitochondria and processed in a single step to a 21-kDa protein that was identical (SDS/PAGE) to an in vitro expressed mature protein and a cyclophilin D purified from rat heart mitochondria. No further modification of the mature protein could be demonstrated. Fractionation of mitochondria following import established that cyclophilin D locates only to the matrix. It is concluded that cyclophilin D binding to the permeability transition pore must occur at the inner face of the mitochondrial inner membrane.

A highly conserved nematode protein folding operon in Caenorhabditis elegans and Caenorhabditis briggsae

In the free-living model nematode, Caenorhabditis elegans, a protein-folding co-transcribed gene pair has previously been described. The degree and form of trans-splicing, orientation and spacing of the genes, and the co-ordinate co-expression of protein folding catalysts in the nematode's hypodermis indicated this to be a functionally important operon. This gene pair has now been cloned and compared in the related organism Caenorhabditis briggsae to identify evolutionarily conserved, functionally important features. The corresponding C. briggsae gene pair was found to share the operon-specific features, including sequence homology blocks in the upstream 5' flanking regions. The intergenic regions were not conserved. The homology block closest to the translational initiation codon of the upstream gene was found to contain a known Ceanorhabbitis promoter element site, and may therefore be an important cis-regulatory region directing the hypodermis-specific expression of this operon gene of C. elegans. This study also provides further confirmation of the high degree of chromosomal synteny between these nematode species.

Yeast flavin-containing monooxygenase generates oxidizing equivalents that control protein folding in the endoplasmic reticulum

The flavin-containing monooxygenase from yeast (yFMO) catalyzes the O2- and NADPH-dependent oxidations of biological thiols, including oxidation of glutathione to glutathione disulfide (GSSG). Glutathione and GSSG form the principle redox buffering system in the cell, with the endoplasmic reticulum (ER) being more oxidizing than the cytoplasm. Proper folding of disulfide-bonded proteins in the ER depends on an optimum redox buffer ratio. Here we show that yFMO is localized to the cytoplasmic side of the ER membrane. We used a gene knockout strain and expression vectors to show that yFMO has a major effect on the generation of GSSG transported into the ER. The enzyme is required for the proper folding, in the ER, of test proteins with disulfide bonds, whereas those without disulfide bonds are properly folded independently of yFMO in the ER or in the cytoplasm.

Analysis of the neurotrophic effects of GPI-1046 on neuron survival and regeneration in culture and in vivo

The putative neurotrophic effects of the immunophilin ligand GPI-1046 were evaluated in established experimental systems of neuron survival and axon growth in vitro and in vivo. GPI-1046 marginally increased neurite outgrowth of chick dorsal root ganglia in culture under conditions where a very robust effect of nerve growth factor was seen. GPI-1046 failed to protect dopaminergic neurons from 1-methyl-4-phenylpyridinium in culture or to protect cultured cortical neurons from experimentally induced apoptosis in vitro. In adult rats in vivo, daily administration of GPI-1046 (10 mg/kg, s.c.) for three days enhanced the maximal regeneration distance of both motor and large myelinated sensory axons measured using an electrophysiological assay. However, detailed morphometric analysis of these animals failed to provide evidence for an increase in axon numbers in GPI-1046-treated animals. The ability of GPI-1046 to promote the recovery of dopaminergic function following unilateral 6-hydroxydopamine lesions of the substantia nigra was also tested in rats. In the first study, the duration of amphetamine (3 mg/kg, s.c.)-induced circling, but not the maximal number of rotations, was significantly reduced in animals treated with GPI-1046 for five days (10 mg/kg/day). In a second study, testing the effects of delayed GPI-1046 administration, chronic treatment with GPI-1046 (10 mg/kg/day) for two weeks, beginning one month after surgery, did not alter circling responses. Morphometric analysis failed to reveal any changes in either the density of tyrosine hyroxylase-positive fibres in dopaminergic target areas or in cell numbers in the substantia nigra in both experiments. Thus, while GPI-1046 produced marginal effects on neurite outgrowth in dorsal root ganglia cultures and on functional paramaters of nerve regeneration in vivo, we failed to obtain evidence in support of the notion of a general neuroprotective effect of the compound or for an effect on morphologic nerve regeneration in vivo.

High-molecular-weight FK506-binding proteins are components of heat-shock protein 90 heterocomplexes in wheat germ lysate

In animal cell lysates the multiprotein heat-shock protein 90 (hsp90)-based chaperone complexes consist of hsp70, hsp40, and p60. These complexes act to convert steroid hormone receptors to their steroid-binding state by assembling them into heterocomplexes with hsp90, p23, and one of several immunophilins. Wheat germ lysate also contains a hsp90-based chaperone system that can assemble the glucocorticoid receptor into a functional heterocomplex with hsp90. However, only two components of the heterocomplex-assembly system, hsp90 and hsp70, have thus far been identified. Recently, purified mammalian p23 preadsorbed with JJ3 antibody-protein A-Sepharose pellets was used to isolate a mammalian p23-wheat hsp90 heterocomplex from wheat germ lysate (J.K. Owens-Grillo, L.F. Stancato, K. Hoffmann, W.B. Pratt, and P. Krishna [1996] Biochemistry 35: 15249-15255). This heterocomplex was found to contain an immunophilin(s) of the FK506-binding class, as judged by binding of the radiolabeled immunosuppressant drug [3H]FK506 to the immune pellets in a specific manner. In the present study we identified the immunophilin components of this heterocomplex as FKBP73 and FKBP77, the two recently described high-molecular-weight FKBPs of wheat. In addition, we present evidence that the two FKBPs bind hsp90 via tetratricopeptide repeat domains. Our results demonstrate that binding of immunophilins to hsp90 via tetratricopeptide repeat domains is a conserved protein interaction in plants. Conservation of this protein-to-protein interaction in both plant and animal cells suggests that it is important for the biological action of the high-molecular-weight immunophilins.

Antibody catalysis of peptidyl-prolyl cis-trans isomerization in the folding of RNase T1

An antibody generated to an alpha-keto amide containing hapten 1 catalyzes the cis-trans isomerization of peptidyl-prolyl amide bonds in peptides and in the protein RNase T1. The antibody-catalyzed peptide isomerization reaction showed saturation kinetics for the cis-substrate, Suc-Ala-Ala-Pro-Phe-pNA, with a kcat/Km value of 883 s-1.M-1; the reaction was inhibited by the hapten analog 13 (Ki = 3. 0 +/- 0.4 microM). Refolding of denatured RNase T1 to its native conformation also was catalyzed by the antibody, with the antibody-catalyzed folding reaction inhibitable both by the hapten 1 and hapten analog 13. These results demonstrate that antibodies can catalyze conformational changes in protein structure, a transformation involved in many cellular processes.