Chemistry and biology of the immunophilins and their immunosuppressive ligands

Cyclosporin A, FK506, and rapamycin are inhibitors of specific signal transduction pathways that lead to T lymphocyte activation. These immunosuppressive agents bind with high affinity to cytoplasmic receptors termed immunophilins (immunosuppressant binding proteins). Studies in this area have focused on the structural basis for the molecular recognition of immunosuppressants by immunophilins and the biological consequences of their interactions. Defining the biological roles of this emerging family of receptors and their ligands may illuminate the process of protein trafficking in cells and the mechanisms of signal transmission through the cytoplasm.

A human peptidyl-prolyl isomerase essential for regulation of mitosis

The NIMA kinase is essential for progression through mitosis in Aspergillus nidulans, and there is evidence for a similar pathway in other eukaryotic cells. Here we describe the human protein Pin1, a peptidyl-prolyl cis/trans isomerase (PPIase) that interacts with NIMA. PPIases are important in protein folding, assembly and/or transport, but none has so far been shown to be required for cell viability. Pin1 is nuclear PPIase containing a WW protein interaction domain, and is structurally and functionally related to Ess1/Ptf1, an essential protein in budding yeast. PPIase activity is necessary for Ess1/Pin1 function in yeast. Depletion of Pin1/Ess1 from yeast or HeLa cells induces mitotic arrest, whereas HeLa cells overexpressing Pin1 arrest in the G2 phase of the cell cycle. Pin1 is thus an essential PPIase that regulates mitosis presumably by interacting with NIMA and attenuating its mitosis-promoting activity.

Cardiac defects and altered ryanodine receptor function in mice lacking FKBP12

FKBP12, a cis-trans prolyl isomerase that binds the immunosuppressants FK506 and rapamycin, is ubiquitously expressed and interacts with proteins in several intracellular signal transduction systems. Although FKBP12 interacts with the cytoplasmic domains of type I receptors of the transforming growth factor-beta (TGF-beta) superfamily in vitro, the function of FKBP12 in TGF-beta superfamily signalling is controversial. FKBP12 also physically interacts stoichiometrically with multiple intracellular calcium release channels including the tetrameric skeletal muscle ryanodine receptor (RyR1). In contrast, the cardiac ryanodine receptor, RyR2, appears to bind selectively the FKBP12 homologue, FKBP12.6. To define the functions of FKBP12 in vivo, we generated mutant mice deficient in FKBP12 using embryonic stem (ES) cell technology. FKBP12-deficient mice have normal skeletal muscle but have severe dilated cardiomyopathy and ventricular septal defects that mimic a human congenital heart disorder, noncompaction of left ventricular myocardium. About 9% of the mutants exhibit exencephaly secondary to a defect in neural tube closure. Physiological studies demonstrate that FKBP12 is dispensable for TGF-beta-mediated signalling, but modulates the calcium release activity of both skeletal and cardiac ryanodine receptors.

Pseudotyping human immunodeficiency virus type 1 (HIV-1) by the glycoprotein of vesicular stomatitis virus targets HIV-1 entry to an endocytic pathway and suppresses both the requirement for Nef and the sensitivity to cyclosporin A

Human immunodeficiency virus type 1 (HIV-1) normally enters cells by direct fusion with the plasma membrane. In this report, HIV-1 particles capable of infecting cells through an endocytic pathway are described. Chimeric viruses composed of the HIV-1 core and the envelope glycoprotein of vesicular stomatitis virus (VSV-G) were constructed and are herein termed HIV-1(VSV) pseudotypes. HIV-1(VSV) pseudotypes were 20- to 130-fold more infectious than nonpseudotyped HIV-1. Infection by HIV-1(VSV) pseudotypes was markedly diminished by ammonium chloride and concanamycin A, a selective inhibitor of vacuolar H+ ATPases, demonstrating that these viruses require endosomal acidification to achieve productive infection. HIV-1 is thus capable of performing all of the viral functions necessary for infection when entry is targeted to an endocytic route. Maximal HIV-1 infectivity requires the presence of the viral Nef protein and the cellular protein cyclophilin A (CyPA) during virus assembly. Pseudotyping by VSV-G markedly suppressed the requirement for Nef. HIV-1(VSV) particles were also resistant to inhibition by cyclosporin A; however, the deleterious effect of a gag mutation inhibiting CyPA incorporation was not relieved by VSV-G. These results suggest that Nef acts at a step of the HIV-1 life cycle that is either circumvented or facilitated by targeting virus entry to an endocytic pathway. The findings also support the hypothesis that Nef and CyPA enhance HIV-1 infectivity through independent processes and demonstrate a mechanistic difference between reduction of HIV-1 infectivity by cyclosporin A and gag mutations that decrease HIV-1 incorporation of CyPA.

Identification of cyclophilin as a proinflammatory secretory product of lipopolysaccharide-activated macrophages

We have isolated an 18-kDa peptide (designated sp18, for 18-kDa secreted protein) from the conditioned medium of lipopolysaccharide-stimulated RAW 264.7 murine macrophages. Purified sp18 had in vivo inflammatory activity and in vitro chemotactic activity for human peripheral blood polymorphonuclear leukocytes and monocytes. Surprisingly, N-terminal sequencing and tryptic mapping studies revealed that sp18 and cyclophilin, an intracellular protein that binds the immunosuppressive drug cyclosporin A, are highly homologous. The in vitro chemotactic activity of sp18 on monocytes was blocked by cyclosporin A but not by cyclosporin H, a structural analog of cyclosporin A that does not bind cyclophilin. Like purified porcine cyclophilin, mouse sp18 exhibited peptidyl-prolyl cis-trans isomerase activity. Medium conditioned by lipopolysaccharide-stimulated resident peritoneal exudate macrophages isolated from C57BL/6 mice contained substantially higher levels of sp18/cyclophilin than medium conditioned by nonstimulated macrophages. The observation that sp18/cyclophilin exhibits proinflammatory activity and is secreted by macrophages in response to endotoxin suggests that this protein may function as a cytokine, and invites the hypothesis that the immunosuppressive action of cyclosporin A results in part from interaction with an extracellular form of cyclophilin released as a mediator of immune and inflammatory functions.

Molecular chaperone machines: chaperone activities of the cyclophilin Cyp-40 and the steroid aporeceptor-associated protein p23

Molecular chaperones are essential proteins that participate in the regulation of steroid receptors in eukaryotes. The steroid aporeceptor complex contains the molecular chaperones Hsp90 and Hsp70, p48, the cyclophilin Cyp-40, and the associated proteins p23 and p60. In vitro folding assays showed that Cyp-40 and p23 functioned as molecular chaperones in a manner similar to that of Hsp90 or Hsp70. Although neither Cyp-40 nor p23 could completely refold an unfolded substrate, both proteins interacted with the substrate to maintain a nonnative folding-competent intermediate. Thus, the steroid aporeceptor complexes have multiple chaperone components that maintain substrates in an intermediate folded state.

Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription

Cyclophilin A (CyPA) is incorporated into human immunodeficiency virus type 1 (HIV-1) virions via contact with the Gag polyprotein. Genetic or pharmacologic disruption of CyPA incorporation causes a quantitative reduction in virion infectivity with no discernible effects on virion assembly or on endogenous reverse transcriptase activity. Instead, the reduction of virion-associated CyPA is accompanied by a parallel, quantitative decrease in the initiation of viral DNA synthesis after infection of T cells. The infectivity of CyPA-deficient virions is not restored by pseudotyping with Env of amphotropic murine leukemia virus, demonstrating that CyPA is not required for the HIV-1-Env-CD4 interaction. These results indicate that CyPA is required for an early step in the HIV-1 life cycle following receptor binding and membrane fusion but preceding reverse transcription. CyPA is the first cellular protein other than the cell surface receptor shown to be required for an early event in the life cycle of a retrovirus.

The cyclophilin homolog ninaA is required in the secretory pathway

In Drosophila, the major rhodopsin Rh1 is synthesized in endoplasmic reticulum (ER)-bound ribosomes of the R1-R6 photoreceptor cells and is then transported to the rhabdomeres where it functions in phototransduction. Mutations in the cyclophilin homolog ninaA lead to a 90% reduction in Rh1 opsin. Cyclophilins have been shown to be peptidyl-prolyl cis-trans isomerases and have been implicated in catalyzing protein folding. We now show that mutations in the ninaA gene severely inhibit opsin transport from the ER, leading to dramatic accumulations of ER cisternae in the photoreceptor cells. These results demonstrate that ninaA functions in the ER. Interestingly, ninaA and Rh1 also colocalize to secretory vesicles, suggesting that Rh1 may require ninaA as it travels through the distal compartments of the secretory pathway. These results are discussed in relation to the possible role of cyclophilins in protein folding and intracellular protein trafficking.

Immunophilins in protein folding and immunosuppression

Lymphocyte activation requires the transmission of signals from molecules at the plasma membrane to nuclear signals that regulate gene expression. In recent years, several immunosuppressive compounds have been used as probes to identify important and potentially novel molecules involved in lymphocyte signal transduction processes. The immunosuppressants cyclosporin A (CsA), FK506, and rapamycin have been studied in particular detail. Two distinct classes of immunosuppressant binding proteins have been identified, and collectively termed immunophilins. The cyclophilin family of immunophilins binds CsA, whereas the FK506-binding protein (FKBP) family binds FK506 and rapamycin. This review will discuss both the endogenous functions of immunophilins as well as their roles in mediating immunosuppression.

ProOmpA is stabilized for membrane translocation by either purified E. coli trigger factor or canine signal recognition particle

We have isolated large amounts of E. coli outer-membrane protein A precursor (proOmpA). Purified proOmpA is active in membrane assembly, and this assembly is saturable with respect to the precursor protein. A proOmpA-Sepharose matrix allows affinity isolation of trigger factor, a soluble, 63,000 dalton monomeric protein that stabilizes proOmpA in assembly competent form. Comparison of trigger factor's amino-terminal sequence with those in a computer data bank and with those encoded by sec genes, as well as groEL and heat shock gene dnaK, suggests that trigger factor is encoded by a previously undescribed gene. Trigger factor and proOmpA form a 1:1 complex that can be isolated by gel filtration. Purified canine signal recognition particle (SRP) can also stabilize proOmpA for membrane insertion. This postribosomal activity of SRP suggests a unifying theme in protein translocation mechanisms.

Proline cis-trans isomerization controls autoinhibition of a signaling protein

Autoinhibition is being widely used in nature to repress otherwise constitutive protein activities and is typically regulated by extrinsic factors. Here we show that autoinhibition can be controlled by an intrinsic intramolecular switch afforded by prolyl cis-trans isomerization. We find that a proline on the linker tethering the two SH3 domains of the Crk adaptor protein interconverts between the cis and trans conformation. In the cis conformation, the two SH3 domains interact intramolecularly, thereby forming the basis of an autoinhibitory mechanism. Conversely, in the trans conformation Crk exists in an extended, uninhibited conformation that is marginally populated but serves to activate the protein upon ligand binding. Interconversion between the cis and trans, and, hence, of the autoinhibited and activated conformations, is accelerated by the action of peptidyl-prolyl isomerases. Proline isomerization appears to make an ideal switch that can regulate the kinetics of activation, thereby modulating the dynamics of signal response.

Cooperation of enzymatic and chaperone functions of trigger factor in the catalysis of protein folding

The trigger factor of Escherichia coli is a prolyl isomerase and accelerates proline-limited steps in protein folding with a very high efficiency. It associates with nascent polypeptide chains at the ribosome and is thought to catalyse the folding of newly synthesized proteins. In its enzymatic mechanism the trigger factor follows the Michaelis-Menten equation. The unusually high folding activity of the trigger factor originates from its tight binding to the folding protein substrate, as reflected in the low Km value of 0.7 microM. In contrast, the catalytic constant kcat is small and shows a value of 1.3 s(-1) at 15 degrees C. An unfolded protein inhibits the trigger factor in a competitive fashion. The isolated catalytic domain of the trigger factor retains the full prolyl isomerase activity towards short peptides, but in a protein folding reaction its activity is 800-fold reduced and no longer inhibited by an unfolded protein. Unlike the prolyl isomerase site, the polypeptide binding site obviously extends beyond the FKBP domain. Together, this suggests that the good substrate binding, i.e. the chaperone property, of the intact trigger factor is responsible for its high efficiency as a catalyst of proline-limited protein folding.

A cyclophilin function in Hsp90-dependent signal transduction

Cpr6 and Cpr7, the Saccharomyces cerevisiae homologs of cyclophilin-40 (CyP-40), were shown to form complexes with Hsp90, a protein chaperone that functions in several signal transduction pathways. Deletion of CPR7 caused severe growth defects when combined with mutations that decrease the amount of Hsp90 or Sti1, another component of the Hsp90 chaperone machinery. The activities of two heterologous Hsp90-dependent signal transducers expressed in yeast, glucocorticoid receptor and pp60(v-src) kinase, were adversely affected by cpr7 null mutations. These results suggest that CyP-40 cyclophilins play a general role in Hsp90-dependent signal transduction pathways under normal growth conditions.

Regulation of the permeability transition pore, a voltage-dependent mitochondrial channel inhibited by cyclosporin A

Mitochondria from a variety of sources possess a regulated inner membrane channel, the permeability transition pore (MTP), which is responsible for the 'permeability transition', a sudden permeability increase to solutes with molecular masses < or = 1500 Da, most easily observed after Ca2+ accumulation. The MTP is a voltage-dependent channel blocked by cyclosporin A with Ki in the nanomolar range. The MTP open probability is regulated by both the membrane potential and matrix pH. The probability of pore opening increases as the membrane is depolarized, while it decreases as matrix pH is decreased below 7.3 through reversible protonation of histidine residues. Many physiological and pathological effectors, including Ca2+ and ADP, modulate MTP operation directly through changes of the gating potential rather than indirectly through changes of the membrane potential (Petronilli, V., Cola, C., Massari, S., Colonna, R. and Bernardi, P. (1993) J. Biol. Chem. 268, 21939-21945). Here we present recent work from our laboratory indicating that (i) the voltage sensor comprises at least two vicinal thiols whose oxidation-reduction state affects the MTP gating potential; as the couple becomes more oxidized the gating potential increases; conversely, as it becomes more reduced the gating potential decreases; (ii) that MTP opening is fully reversible, as mitochondria maintain volume homeostasis through several cycles of pore opening/closure; and (iii) that the mechanism of MTP inhibition by cyclosporin A presumably involves a mitochondrial cyclophilin but does not utilize a calcineurin-dependent pathway.

Immunophilins and the nervous system

The search for immunosuppressant drugs to increase the success of organ transplantation led to the discovery of the immunophilins, proteins that interface with a range of signal transduction systems inside cells, especially in the nervous and immune systems. Here we review how these interesting molecules work and consider their therapeutic potential.

Cyclophilin A is required for the replication of group M human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus SIV(CPZ)GAB but not group O HIV-1 or other primate immunodeficiency viruses

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein binds to cyclophilin A and incorporates this cellular peptidyl prolyl-isomerase into virions. Disruption of cyclophilin A incorporation, either by gag mutations or by cyclosporine A, inhibits virion infectivity, indicating that cyclophilin A plays an essential role in the HIV-1 life cycle. Using assays for packaging of cyclophilin A into virions and for viral replication sensitivity to cyclosporine A, as well as information gleaned from the alignment of Gag residues encoded by representative viral isolates, we demonstrate that of the five lineages of primate immunodeficiency viruses, only HIV-1 requires cyclophilin A for replication. Cloned viral isolates from clades A, B, and D of HIV-1 group M, as well as a phylogenetically related isolate from chimpanzee, all require cyclophilin A for replication. In contrast, the replication of two outlier (group O) HIV-1 isolates is unaffected by concentrations of cyclosporine A which disrupt cyclophilin A incorporation into virions, indicating that these viruses are capable of replicating independently of cyclophilin A. These studies identify the first phenotypic difference between HIV-1 group M and group O and are consistent with phylogenetic studies suggesting that the two HIV-1 groups were introduced into human populations via separate zoonotic transmission events.

Protein folding and the regulation of signaling pathways

A growing number of intracellular signaling molecules are found associated with components of the cellular protein folding machinery. In this minireview we suggest that the same ancient cellular process that promotes the folding and assembly of nascent proteins plays a pivotal role in signal transduction by promoting the regulated folding or assembly and disassembly of mature signaling molecules between active and inactive states. Members of the protein folding machinery mediate the activity of various kinases, receptors, and transcription factors. These may be poised in late stages of folding or assembly until upstream signaling events trigger their biogenesis into activated molecules.

Influence of site specifically altered Mip proteins on intracellular survival of Legionella pneumophila in eukaryotic cells

Legionella pneumophila, the causative agent of Legionnaires' disease, is able to survive intracellularly in eukaryotic cells such as monocytes, macrophages, and protozoan organisms. The Mip (macrophage infectivity potentiator) protein represents a factor of L. pneumophila necessary for optimal intracellular survival. Interestingly, Mip belongs to the substance class of FK 506-binding proteins and exhibits peptidyl-prolyl cis/trans isomerase (PPIase) activity that can be inhibited by the immunosuppressant FK506. In order to identify amino acids most likely to be involved in the enzymatic activity of Mip, site-directed mutagenized Mip proteins were constructed and characterized. It was shown that an Asp-142 to Leu-142 mutation and a Tyr-185 to Ala-185 substitution resulted in strongly reduced PPIase activity of the recombinant Mip proteins (5.3 and 0.6% of the activity of the wild-type Mip, respectively). Genes coding for the wild-type and for site-directed-mutagenized Mip proteins were used to complement three different Mip-negative mutants of the L. pneumophila Corby, Philadelphia I, and Wadsworth. While Mip protein expression could be restored in the corresponding complementants, significant Mip-specific PPIase activity could be detected only in Mip mutants complemented with wild-type mip genes. To investigate the influence of the PPIase activity of Mip on intracellular survival of L. pneumophila, invasion assays were performed using the macrophage-like cell line U937, human blood monocytes, and Acanthamoeba castellanii. The Mip-negative mutants were approximately 50- to 100-fold less infective for A. castellanii and for human mononuclear phagocytes in vitro compared with their isogenic Mip-positive parental strains. The wild-type invasion rate could be restored by introducing an intact copy of the mip gene into Mip-negative strains. In addition, no differences in intracellular survival were observed between the wild-type isolates and the Legionella strains exhibiting strongly reduced PPIase activity. These data indicated that the enzymatic activity of Mip does not contribute to intracellular survival of L. pneumophila.

Secretion by Trypanosoma cruzi of a peptidyl-prolyl cis-trans isomerase involved in cell infection

Macrophage infectivity potentiators are membrane proteins described as virulence factors in bacterial intracellular parasites, such as Legionella and Chlamydia. These factors share amino acid homology to eukaryotic peptidyl-prolyl cis-trans isomerases that are inhibited by FK506, an inhibitor of signal transduction in mammalian cells with potent immunosuppressor activity. We report here the characterization of a protein released into the culture medium by the infective stage of the protozoan intracellular parasite Trypanosoma cruzi. The protein possesses a peptidyl-prolyl cis-trans isomerase activity that is inhibited by FK506 and its non-immunosuppressing derivative L-685,818. The corresponding gene presents sequence homology with bacterial macrophage infectivity potentiators. The addition of the protein, produced heterologously in Escherichia coli, to cultures of trypomastigotes and simian epithelial or HeLa cells enhances invasion of the mammalian cells by the parasites. Antibodies raised in mice against the T.cruzi isomerase greatly reduce infectivity. A similar reduction of infectivity is obtained by addition to the cultures of FK506 and L-685,818. We concluded that the T.cruzi isomerase is involved in cell invasion.

Immunophilins: structure-function relationship and possible role in microbial pathogenicity

Immunophilins are housekeeping proteins present in a wide variety of organisms. Members of two protein superfamilies, cyclophilins (Cyps) and FK506-binding proteins (FKBPs) belong to this class of immunophilins. Despite the fact that the amino acid sequences of Cyp and FKBPs do not exhibit noticeable homology to each other, proteins of both classes are able to ligate immunosuppressive peptide derivatives. Cyps form complexes with the cyclic undercapeptide cyclosporin A and FKBPs are able to bind FK506 as well as rapamycin, both of which have a pipecolyl bond within their structure. In a ligand-bound form, immunophilins interfere with signal transduction in T cells. In addition, immunophilins have peptidyl prolyl cis-trans isomerase (PPlase) activity and are able to accelerate the rate of conformational events in proline-containing polypeptides. Microorganisms produce proteins that exhibit extensive sequence homologies to cyclophilins and FKBPs of higher organisms and which have considerable PPlase catalytic activity. While cyclophilins seem to be present in most if not all microbial species investigated, FKBPs are produced by yeasts as well as by a number of pathogenic bacteria, such as Legionella pneumophila, Chlamydia trachomatis and Neisseria meningitidis. The Mip protein of L. pneumophila is a virulence factor that plays an essential role in the ability of the bacteria to survive and multiply in phagocytic cells. Some results are summarized on the structure and putative functions of immunophilins and place special emphasis on the contribution of these polypeptides to the virulence of pathogenic microorganisms.

FK-506 and cyclosporin A: immunosuppressive mechanism of action and beyond

Cyclosporin A and FK-506 are important therapeutic agents that have found widespread use in preventing graft rejection during tissue transplantation. Research efforts aimed at elucidating the molecular mechanism of action of these drugs have, in addition to defining their immunosuppressive functions, led to the identification of two new gene families whose products may function as components of several diverse signal transduction pathways. In the presence of the immunosuppressive drugs, some members of the receptor families interact with the Ca2+/calmodulin-dependent protein phosphatase 2B, also known as calcineurin. Inhibition of phosphatase activity may effect several downstream biochemical processes. In this way, cyclosporin A and FK-506 have proved to be useful probes of signaling events in both lymphocytic and other cell types.

Identification of two CyP-40-like cyclophilins in Saccharomyces cerevisiae, one of which is required for normal growth

We report the analysis of two Saccharomyces cerevisiae cyclophilins, Cpr6 and Cpr7, identified by their ability to interact in vivo with the transcriptional regulator Rpd3. Both cyclophilins have an extended carboxy-terminal region containing a three-unit tetratricopeptide repeat (TPR) motif and share significant amino acid identity with the mammalian cyclophilin CyP-40. Neither CPR6 nor CPR7 is essential but deletion of CPR7 results in a significant impairment of the rate of cell division. This is the first demonstration that a member of the cyclophilin family is required for normal cell growth.