Localization of the FK506-binding protein, FKBP 13, to the lumen of the endoplasmic reticulum

Accurate phenotypic classification and exome sequencing allow identification of novel genes and variants associated with adult-onset hearing loss

Adult-onset progressive hearing loss is a common, complex disease with a strong genetic component. Although to date over 150 genes have been identified as contributing to human hearing loss, many more remain to be discovered, as does most of the underlying genetic diversity. Many different variants have been found to underlie adult-onset hearing loss, but they tend to be rare variants with a high impact upon the gene product. It is likely that combinations of more common, lower impact variants also play a role in the prevalence of the disease. Here we present our exome study of hearing loss in a cohort of 532 older adult volunteers with extensive phenotypic data, including 99 older adults with normal hearing, an important control set. Firstly, we carried out an outlier analysis to identify genes with a high variant load in older adults with hearing loss compared to those with normal hearing. Secondly, we used audiometric threshold data to identify individual variants which appear to contribute to different threshold values. We followed up these analyses in a second cohort. Using these approaches, we identified genes and variants linked to better hearing as well as those linked to worse hearing. These analyses identified some known deafness genes, demonstrating proof of principle of our approach. However, most of the candidate genes are novel associations with hearing loss. While the results support the suggestion that genes responsible for severe deafness may also be involved in milder hearing loss, they also suggest that there are many more genes involved in hearing which remain to be identified. Our candidate gene lists may provide useful starting points for improved diagnosis and drug development.

Cytoplasmic FKBPs are involved in molting and metamorphosis through regulating the nuclear localization of EcR

Molting and metamorphosis are important physiological processes in insects that are tightly controlled by ecdysone receptor (EcR) through the 20-hydroxyecdysone (20E) signaling pathway. EcR is a steroid nuclear receptor (SR). Several FK506-binding proteins (FKBPs) have been identified from the mammal SR complex, and are thought to be involved in the subcellular trafficking of SR. However, their roles in insects are poorly understood. To explore whether FKBPs are involved in insect molting or metamorphosis, we injected an FKBP inhibitor (FK506) into a lepidopteran insect, Spodoptera litura, and found that molting was inhibited in 61.11% of the larvae, and that the time for larvae to pupate was significantly extended. A total of 10 FKBP genes were identified from the genome of S. litura and were clustered into 2 distinct groups, according to their subcellular localization, with FKBP13 and FKBP14 belonging to the endoplasmic reticulum (ER) group and with the other members belonging to the cytoplasmic (Cy) group. All the CyFKBPs were significantly upregulated in the prepupal or pupal stages, with the opposite being observed for the ER group members. FK506 completely blocked the transfer of EcR to the nucleus under 20E induction, and significantly downregulated the transcriptional expression of many 20E signaling genes. A similar phenomenon was observed after RNA interference of 2 CyFKBPs (FKBP45 and FKBP12b), but not for FKBP13. Taken together, our data indicate that the cytoplasmic FKBPs, especially FKBP45 and FKBP12b, mediate the nuclear localization of EcR, thereby regulating the 20E signaling and ultimately affecting molting and metamorphosis in insects.

FK506-binding protein 2 (FKBP13) inhibit Bax-induced apoptosis in Saccharomyces cerevisiae (yeast)

FK506-binding protein 2 (FKBP13) is a part of the immunophilin protein family involved in immunoregulation. It is also believed to operate as a factor in membrane cytoskeletal framework and as an ER chaperone. FKBP2 (FKBP13) and FKBP1 (FKBP12), known as immunophilins, are binding proteins for rapamycin and FK506, which are immunosuppressive drugs. It was suggested that immunophilin-like and immunophilin proteins play significant roles in regulating intracellular calcium and protein folding/sorting, acting as molecular chaperones. Within the 15 mammalian FKBPs known, FKBP1 is merely the only one proven to form complexes with rapamycin and FK506 in the cytosol and facilitate their T cells immunosuppressive effects, FKBP2 is a luminal protein of the endoplasmic reticulum (ER) and is reported to take part in protein folding in the ER. However, little is known about FKBP2 link with apoptosis (either as a pro or anti-apoptotic protein). In this study, FKPB2 protein was co-expressed with the pro-apoptotic protein Bax after a yeast-based human hippocampal cDNA library screening. The yeast strain carrying the Bax gene was transformed with an episomal 2-micron plasmid that encodes the HA-tagged FKBP2 gene. The resultant strain would allow co-expression of Bax and FKBP2 in yeast cells. The results presented here show that a protein involved in protein folding can play a role in protecting yeast cell from Bax-induced apoptosis.

Accurate phenotypic classification and exome sequencing allow identification of novel genes and variants associated with adult-onset hearing loss

Adult-onset progressive hearing loss is a common, complex disease with a strong genetic component. Although to date over 150 genes have been identified as contributing to human hearing loss, many more remain to be discovered, as does most of the underlying genetic diversity. Many different variants have been found to underlie adult-onset hearing loss, but they tend to be rare variants with a high impact upon the gene product. It is likely that combinations of more common, lower impact variants also play a role in the prevalence of the disease. Here we present our exome study of hearing loss in a cohort of 532 older adult volunteers with extensive phenotypic data, including 99 older adults with normal hearing, an important control set. Firstly, we carried out an outlier analysis to identify genes with a high variant load in older adults with hearing loss compared to those with normal hearing. Secondly, we used audiometric threshold data to identify individual variants which appear to contribute to different threshold values. We followed up these analyses in a second cohort. Using these approaches, we identified genes and variants linked to better hearing as well as those linked to worse hearing. These analyses identified some known deafness genes, demonstrating proof of principle of our approach. However, most of the candidate genes are novel associations with hearing loss. While the results support the suggestion that genes responsible for severe deafness may also be involved in milder hearing loss, they also suggest that there are many more genes involved in hearing which remain to be identified. Our candidate gene lists may provide useful starting points for improved diagnosis and drug development.

FK506-Binding Protein 2 Participates in Proinsulin Folding

Apart from chaperoning, disulfide bond formation, and downstream processing, the molecular sequence of proinsulin folding is not completely understood. Proinsulin requires proline isomerization for correct folding. Since FK506-binding protein 2 (FKBP2) is an ER-resident proline isomerase, we hypothesized that FKBP2 contributes to proinsulin folding. We found that FKBP2 co-immunoprecipitated with proinsulin and its chaperone GRP94 and that inhibition of FKBP2 expression increased proinsulin turnover with reduced intracellular proinsulin and insulin levels. This phenotype was accompanied by an increased proinsulin secretion and the formation of proinsulin high-molecular-weight complexes, a sign of proinsulin misfolding. FKBP2 knockout in pancreatic β-cells increased apoptosis without detectable up-regulation of ER stress response genes. Interestingly, FKBP2 mRNA was overexpressed in β-cells from pancreatic islets of T2D patients. Based on molecular modeling and an in vitro enzymatic assay, we suggest that proline at position 28 of the proinsulin B-chain (P28) is the substrate of FKBP2's isomerization activity. We propose that this isomerization step catalyzed by FKBP2 is an essential sequence required for correct proinsulin folding.

FKBP52 in Neuronal Signaling and Neurodegenerative Diseases: A Microtubule Story

The FK506-binding protein 52 (FKBP52) belongs to a large family of ubiquitously expressed and highly conserved proteins (FKBPs) that share an FKBP domain and possess Peptidyl-Prolyl Isomerase (PPIase) activity. PPIase activity catalyzes the isomerization of Peptidyl-Prolyl bonds and therefore influences target protein folding and function. FKBP52 is particularly abundant in the nervous system and is partially associated with the microtubule network in different cell types suggesting its implication in microtubule function. Various studies have focused on FKBP52, highlighting its importance in several neuronal microtubule-dependent signaling pathways and its possible implication in neurodegenerative diseases such as tauopathies (i.e., Alzheimer disease) and alpha-synucleinopathies (i.e., Parkinson disease). This review summarizes our current understanding of FKBP52 actions in the microtubule environment, its implication in neuronal signaling and function, its interactions with other members of the FKBPs family and its involvement in neurodegenerative disease.

Genome-wide characterization of peptidyl-prolyl cis-trans isomerases in Penicillium and their regulation by salt stress in a halotolerant P. oxalicum

Peptidyl-prolyl cis-trans isomerases (PPIases) are the only class of enzymes capable of cis-trans isomerization of the prolyl peptide bond. The PPIases, comprising of different families viz., cyclophilins, FK506-binding proteins (FKBPs), parvulins and protein phosphatase 2A phosphatase activators (PTPAs), play essential roles in different cellular processes. Though PPIase gene families have been characterized in different organisms, information regarding these proteins is lacking in Penicillium species, which are commercially an important fungi group. In this study, we carried out genome-wide analysis of PPIases in different Penicillium spp. and investigated their regulation by salt stress in a halotolerant strain of Penicillium oxalicum. These analyses revealed that the number of genes encoding cyclophilins, FKBPs, parvulins and PTPAs in Penicillium spp. varies between 7-11, 2-5, 1-2, and 1-2, respectively. The halotolerant P. oxalicum depicted significant enhancement in the mycelial PPIase activity in the presence of 15% NaCl, thus, highlighting the role of these enzymes in salt stress adaptation. The stress-induced increase in PPIase activity at 4 and 10 DAI in P. oxalicum was associated with higher expression of PoxCYP18. Characterization of PPIases in Penicillium spp. will provide an important database for understanding their cellular functions and might facilitate their applications in industrial processes through biotechnological interventions.

FK506-Binding Protein 13 Expression Is Upregulated in Interstitial Lung Disease and Correlated with Clinical Severity. A Potentially Protective Role

Pulmonary fibrosis is a progressive lung disease characterized by myofibroblast accumulation and excessive extracellular matrix deposition. We sought to investigate the role of FKBP13 (13-kD FK506-binding protein), an endoplasmic reticulum-resident molecular chaperone, in various forms of pulmonary fibrosis. We first characterized the gene and protein expression of FKBP13 in lung biopsy specimens from 24 patients with idiopathic pulmonary fibrosis and 17 control subjects. FKBP13 expression was found to be elevated in the fibrotic regions of idiopathic pulmonary fibrosis lung tissues and correlated with declining forced vital capacity and dyspnea severity. FKBP13 expression was also increased in lung biopsy specimens of patients with hypersensitivity pneumonitis, rheumatoid arthritis, and sarcoidosis-associated interstitial lung disease. We next evaluated the role of this protein using FKBP13^-/- mice in a bleomycin model of pulmonary fibrosis. Animals were assessed for lung function and histopathology at different stages of lung injury including the inflammatory (Day 7), fibrotic (Day 21), and resolution (Day 50) phases. FKBP13^-/- mice showed increased infiltration of inflammatory cells and cytokines at Day 7, increased lung elastance and fibrosis at Day 21, and impaired resolution of fibrosis at Day 50. These changes were associated with an increased number of cells that stained positive for TUNEL and cleaved caspase 3 in the FKBP13^-/- lungs, indicating a heightened cellular sensitivity to bleomycin. Our findings suggest that FKBP13 is a potential biomarker for severity of interstitial lung diseases and that it has a biologically relevant role in protecting mice against bleomycin-induced injury, inflammation, and fibrosis.

FKBP Ligands-Where We Are and Where to Go?

In recent years, many members of the FK506-binding protein (FKBP) family were increasingly linked to various diseases. The binding domain of FKBPs differs only in a few amino acid residues, but their biological roles are versatile. High-affinity ligands with selectivity between close homologs are scarce. This review will give an overview of the most prominent ligands developed for FKBPs and highlight a perspective for future developments. More precisely, human FKBPs and correlated diseases will be discussed as well as microbial FKBPs in the context of anti-bacterial and anti-fungal therapeutics. The last section gives insights into high-affinity ligands as chemical tools and dimerizers.

Retrospective analysis of the correlation between tacrolimus concentrations measured in whole blood and variations of blood cell counts in patients undergoing allogeneic haematopoietic stem cell transplantation

Objective

Tacrolimus is administered to patients undergoing haematopoietic stem cell transplantation (HSCT) as prophylaxis for graft-versus-host disease. As a high blood tacrolimus concentration within a narrow therapeutic range must be maintained after HSCT, therapeutic drug monitoring (TDM) is necessary. We investigated the correlation between blood tacrolimus concentration and blood cell count in HSCT patients to assess how changes in blood cell count affect tacrolimus TDM.

Methods

A retrospective analysis was performed for 24 patients who underwent allogeneic HSCT and received tacrolimus. The correlation between variations in blood tacrolimus concentration and blood cell count was evaluated for three consecutive weeks, starting 1 week after HSCT.

Results

Variations in blood tacrolimus concentration were significantly correlated with variations in red blood cell (RBC) count, haemoglobin level and haematocrit value, but not with variations in white blood cell or platelet counts. Further, the above variations were significantly correlated in patients undergoing cord blood transplantation and peripheral blood stem cell transplantation, but not in those undergoing bone marrow transplantation.

Conclusions

These findings demonstrate that RBC count is associated with variations in blood tacrolimus concentration, with the relevance of this association depending on the source of transfused stem cells. Thus, variations in RBC count might be useful for tacrolimus TDM.

The Function of FK506-Binding Protein 13 in Protein Quality Control Protects Plasma Cells from Endoplasmic Reticulum Stress-Associated Apoptosis

Plasma cells (PCs) are exposed to intense endoplasmic reticulum (ER) stress imposed by enormous rates of immunoglobulin (Ig) synthesis and secretion. Therefore, protein homeostasis is crucial for the survival of PCs, but its molecular mechanism remains largely unknown. Here, we found marked overexpression of FK506-binding protein 13 (FKBP13) in long-lived PCs from autoimmune mice and investigated its function using a plasmacytoma cell line secreting IgA. FKBP13 expression was induced largely in the lumen of ER in response to treatment with an ER stressor tunicamycin or overexpression of an adaptive unfolded protein response (UPR) protein X-box binding protein 1 (XBP1). Silencing of FKBP13 expression led to induction of molecules involved in the terminal UPR and ER stress-associated apoptosis. FKBP13 interacted with Ig, facilitated its ubiquitination, and lowered the extent of ER stress. FKBP13 overexpression caused a significant reduction in secreted IgA in plasmacytoma cells, and FKBP13 knockdown exerted an opposite effect. Rapamycin interfered with the interaction between FKBP13 and IgA and enhanced the amount of secreted IgA. Importantly, the level of FKBP13 was inversely correlated with the amount of secreted antibody in long-lived PCs from autoimmune mice. These results suggest that FKBP13 is a marker of long-lived PCs and a component of XBP1-dependent ER protein homeostasis. FKBP13 is likely to act as a molecular chaperone that delivers misfolded ER clients, including Ig, to ER-associated degradation, so reducing proteotoxic stress on the PC. Our data reveal a novel cytoprotective role for FKBP13 in long-lived PCs occurring at the expense of antibody production.

Golgi enzymes do not cycle through the endoplasmic reticulum during protein secretion or mitosis

The question of whether the Golgi complex is a stable compartment or is constantly regenerated from the endoplasmic reticulum (ER) is an important issue under debate. Using an ER trapping procedure and Golgi-specific O-linked glycosylation of a resident ER protein, this study demonstrates that Golgi enzymes do not cycle through the ER during secretion and mitosis.

Golgi-specific sialyltransferase (ST) expressed as a chimera with the rapamycin-binding domain of mTOR, FRB, relocates to the endoplasmic reticulum (ER) in cells exposed to rapamycin that also express invariant chain (Ii)-FKBP in the ER. This result has been taken to indicate that Golgi-resident enzymes cycle to the ER constitutively. We show that ST-FRB is trapped in the ER even without Ii-FKBP upon rapamycin addition. This is because ER-Golgi–cycling FKBP proteins contain a C-terminal KDEL-like sequence, bind ST-FRB in the Golgi, and are transported together back to the ER by KDEL receptor–mediated retrograde transport. Moreover, depletion of KDEL receptor prevents trapping of ST-FRB in the ER by rapamycin. Thus ST-FRB cycles artificially by binding to FKBP domain–containing proteins. In addition, Golgi-specific O-linked glycosylation of a resident ER protein occurs only upon artificial fusion of Golgi membranes with ER. Together these findings support the consensus view that there is no appreciable mixing of Golgi-resident enzymes with ER under normal conditions.

ER trapping reveals Golgi enzymes continually revisit the ER through a recycling pathway that controls Golgi organization

Whether Golgi enzymes remain localized within the Golgi or constitutively cycle through the endoplasmic reticulum (ER) is unclear, yet is important for understanding Golgi dependence on the ER. Here, we demonstrate that the previously reported inefficient ER trapping of Golgi enzymes in a rapamycin-based assay results from an artifact involving an endogenous ER-localized 13-kD FK506 binding protein (FKBP13) competing with the FKBP12-tagged Golgi enzyme for binding to an FKBP-rapamycin binding domain (FRB)-tagged ER trap. When we express an FKBP12-tagged ER trap and FRB-tagged Golgi enzymes, conditions precluding such competition, the Golgi enzymes completely redistribute to the ER upon rapamycin treatment. A photoactivatable FRB-Golgi enzyme, highlighted only in the Golgi, likewise redistributes to the ER. These data establish Golgi enzymes constitutively cycle through the ER. Using our trapping scheme, we identify roles of rab6a and calcium-independent phospholipase A2 (iPLA2) in Golgi enzyme recycling, and show that retrograde transport of Golgi membrane underlies Golgi dispersal during microtubule depolymerization and mitosis.

Improved and versatile viral 2A platforms for dependable and inducible high-level expression of dicistronic nuclear genes in Chlamydomonas reinhardtii

A significantly improved viral 2A peptide system for dependable high-level expression of dicistronic genes in Chlamydomonas reinhardtii has been developed. Data are presented demonstrating that use of an especially proficient 'extended FMDV 2A' coding region allows production of two independent protein products from a dicistronic gene with almost complete efficiency. Importantly, results are also presented that demonstrate the utility of this 2A system for efficient high-level expression of foreign genes in C. reinhardtii, which has not previously been reliably achievable in this algal model system. To expand the versatility of the 2A expression system, a number of commonly used selectable marker proteins were assessed for their compatibility with the extended FMDV 2A peptide. Additional experiments demonstrate the feasibility and utility of 2A-containing dicistronic systems that rely on a strong conditional promoter for transcriptional control and a low-expression marker gene for selection. This strategy allows easy and efficient delivery of genes of interest whose expression levels require regulation either to mitigate potential toxicity or allow differential expression under controlled experimental conditions. Finally, as an additional practical demonstration of the utility of the extended FMDV 2A system, confocal fluorescence microscopy is used to demonstrate that native and foreign proteins of interest bearing post-translational remnants of the extended FMDV 2A peptide localize correctly to various cellular compartments, including a striking demonstration of the almost exclusive localization of the Rubisco small subunit protein to the pyrenoid of the C. reinhardtii chloroplast in cells maintained under ambient CO2 concentrations.

The emerging role of peptidyl-prolyl isomerase chaperones in tau oligomerization, amyloid processing, and Alzheimer's disease

Peptidyl-prolyl cis/trans isomerases (PPIases), a unique family of molecular chaperones, regulate protein folding at proline residues. These residues are abundant within intrinsically disordered proteins, like the microtubule-associated protein tau. Tau has been shown to become hyperphosphorylated and accumulate as one of the two main pathological hallmarks in Alzheimer's disease, the other being amyloid beta (Ab). PPIases, including Pin1, FK506-binding protein (FKBP) 52, FKBP51, and FKBP12, have been shown to interact with and regulate tau biology. This interaction is particularly important given the numerous proline-directed phosphorylation sites found on tau and the role phosphorylation has been found to play in pathogenesis. This regulation then affects downstream aggregation and oligomerization of tau. However, many PPIases have yet to be explored for their effects on tau biology, despite the high likelihood of interaction based on proline content. Moreover, Pin1, FKBP12, FKBP52, cyclophilin (Cyp) A, CypB, and CypD have been shown to also regulate Ab production or the toxicity associated with Ab pathology. Therefore, PPIases directly and indirectly regulate pathogenic protein multimerization in Alzheimer's disease and represent a family rich in targets for modulating the accumulation and toxicity.

Ziploc-ing the structure: Triple helix formation is coordinated by rough endoplasmic reticulum resident PPIases

BACKGROUND

Protein folding is crucial for proteins' specific functions and is facilitated by various types of enzymes and molecular chaperones. The peptidyl prolyl cis/trans isomerases (PPIase) are one of these families of enzymes. They ubiquitously exist inside the cell and there are eight PPIases in the rough endoplasmic reticulum (rER), a compartment where the folding of most secreted proteins occurs.

SCOPE OF REVIEW

We review the functional and structural aspects of individual rER resident PPIases. Furthermore, we specifically discuss the role of these PPIases during collagen biosynthesis, since collagen is the most abundant protein in humans, is synthesized in the rER, and contains a proportionally high number of proline residues.

MAJOR CONCLUSIONS

The rER resident PPIases recognize different sets of substrates and facilitate their folding. Although they are clearly catalysts for protein folding, they also have more broad and multifaceted functions. We propose that PPIases coordinate collagen biosynthesis in the rER.

GENERAL SIGNIFICANCE

This review expands our understanding of collagen biosynthesis by explaining the influence of novel indirect mechanisms of regulating folding and this is also explored for PPIases. We also suggest future directions of research to obtain a better understanding of collagen biosynthesis and functions of PPIases in the rER. This article is part of a Special Issue entitled Proline-directed Foldases: Cell Signaling Catalysts and Drug Targets.

Structure of human peptidyl-prolyl cis-trans isomerase FKBP22 containing two EF-hand motifs

The FK506-binding protein (FKBP) family consists of proteins with a variety of protein-protein interaction domains and versatile cellular functions. It is assumed that all members are peptidyl-prolyl cis-trans isomerases with the enzymatic function attributed to the FKBP domain. Six members of this family localize to the mammalian endoplasmic reticulum (ER). Four of them, FKBP22 (encoded by the FKBP14 gene), FKBP23 (FKBP7), FKBP60 (FKBP9), and FKBP65 (FKBP10), are unique among all FKBPs as they contain the EF-hand motifs. Little is known about the biological roles of these proteins, but emerging genetics studies are attracting great interest to the ER resident FKBPs, as mutations in genes encoding FKBP10 and FKBP14 were shown to cause a variety of matrix disorders. Although the structural organization of the FKBP-type domain as well as of the EF-hand motif has been known for a while, it is difficult to conclude how these structures are combined and how it affects the protein functionality. We have determined a unique 1.9 Å resolution crystal structure for human FKBP22, which can serve as a prototype for other EF hand-containing FKBPs. The EF-hand motifs of two FKBP22 molecules form a dimeric complex with an elongated and predominantly hydrophobic cavity that can potentially be occupied by an aliphatic ligand. The FKBP-type domains are separated by a cleft and their putative active sites can catalyze isomerazation of two bonds within a polypeptide chain in extended conformation. These structural results are of prime interest for understanding biological functions of ER resident FKBPs containing EF-hand motifs.

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.

Large FK506-binding proteins shape the pharmacology of rapamycin

The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.

Identification and comparative analysis of the peptidyl-prolyl cis/trans isomerase repertoires of H. sapiens, D. melanogaster, C. elegans, S. cerevisiae and Sz. pombe

The peptidyl-prolyl cis/trans isomerase (PPIase) class of proteins comprises three member families that are found throughout nature and are present in all the major compartments of the cell. Their numbers appear to be linked to the number of genes in their respective genomes, although we have found the human repertoire to be smaller than expected due to a reduced cyclophilin repertoire. We show here that whilst the members of the cyclophilin family (which are predominantly found in the nucleus and cytoplasm) and the parvulin family (which are predominantly nuclear) are largely conserved between different repertoires, the FKBPs (which are predominantly found in the cytoplasm and endoplasmic reticulum) are not. It therefore appears that the cyclophilins and parvulins have evolved to perform conserved functions, while the FKBPs have evolved to fill ever-changing niches within the constantly evolving organisms. Many orthologous subgroups within the different PPIase families appear to have evolved from a distinct common ancestor, whereas others, such as the mitochondrial cyclophilins, appear to have evolved independently of one another. We have also identified a novel parvulin within Drosophila melanogaster that is unique to the fruit fly, indicating a recent evolutionary emergence. Interestingly, the fission yeast repertoire, which contains no unique cyclophilins and parvulins, shares no PPIases solely with the budding yeast but it does share a majority with the higher eukaryotes in this study, unlike the budding yeast. It therefore appears that, in comparison with Schizosaccharomyces pombe, Saccharomyces cerevisiae is a poor representation of the higher eukaryotes for the study of PPIases.

Identification of Conus peptidylprolyl cis-trans isomerases (PPIases) and assessment of their role in the oxidative folding of conotoxins

Peptidylprolyl cis-trans isomerases (PPIases) are ubiquitous proteins that catalyze the cis-trans isomerization of prolines. A number of proteins, such as Drosophila rhodopsin and the human immunodeficiency viral protein HIV-1 Gag, have been identified as endogenous substrates for PPIases. However, very little is known about the interaction of PPIases with small, disulfide-rich peptides. Marine cone snails synthesize a wide array of cysteine-rich peptides, called conotoxins, many of which contain one or more prolines or hydroxyprolines. To identify whether PPIase-associated cis-trans isomerization of these residues affects the oxidative folding of conotoxins, we identified, sequenced, and expressed three functionally active isoforms of PPIase from the venom gland of Conus novaehollandiae, and we characterized their ability to facilitate oxidative folding of conotoxins in vitro. Three conotoxins, namely mu-GIIIA, mu-SIIIA, and omega-MVIIC, derived from two distinct toxin gene families were assayed. Conus PPIase significantly increased the rate of appearance of the native form of mu-GIIIA, a peptide containing three hydroxyprolines. In contrast, the presence of PPIase had no effect on the folding of mu-SIIIA and omega-MVIIC, peptides containing no or one proline residue, respectively. We further showed that an endoplasmic reticulum-resident PPIase isoform facilitated folding of mu-GIIIA more efficiently than two cytosolic isoforms. This is the first study to demonstrate PPIase-assisted folding of conotoxins, small disulfide-rich peptides with unique structural properties.

Mapping organelle proteins and protein complexes in Drosophila melanogaster

Many proteins within eukaryotic cells are organized spatially and functionally into membrane bound organelles and complexes. A protein's location thus provides information about its function. Here, we apply LOPIT, a mass-spectrometry based technique that simultaneously maps proteins to specific subcellular compartments, to Drosophila embryos. We determine the subcellular distribution of hundreds of proteins, and protein complexes. Our results reveal the potential of LOPIT to provide average snapshots of cells.

Membrane association of the Arabidopsis ARF exchange factor GNOM involves interaction of conserved domains

The GNOM protein plays a fundamental role in Arabidopsis thaliana development by regulating endosome-to-plasma membrane trafficking required for polar localization of the auxin efflux carrier PIN1. GNOM is a family member of large ARF guanine nucleotide exchange factors (ARF-GEFs), which regulate vesicle formation by activating ARF GTPases on specific membranes in animals, plants, and fungi. However, apart from the catalytic exchange activity of the SEC7 domain, the functional significance of other conserved domains is virtually unknown. Here, we show that a distinct N-terminal domain of GNOM mediates dimerization and in addition interacts heterotypically with two other conserved domains in vivo. In contrast with N-terminal dimerization, the heterotypic interaction is essential for GNOM function, as mutations abolishing this interaction inactivate the GNOM protein and compromise its membrane association. Our results suggest a general model of large ARF-GEF function in which regulated changes in protein conformation control membrane association of the exchange factor and, thus, activation of ARFs.

Developmental regulation and coordinate reexpression of FKBP65 with extracellular matrix proteins after lung injury suggest a specialized function for this endoplasmic reticulum immunophilin

AFKBP65 (65-kDa FK506-binding protein) is an endoplasmic reticulum (ER)-localized peptidyl-prolyl cis-trans isomerase predicted to play a role in the folding and trafficking of secretory proteins. In previous studies, we have shown that FKBP65 is developmentally regulated and associates with the extracellular matrix protein, tropoelastin, during its maturation and transport through the ER. In this study, we show that FKBP65 is expressed in the lung with the same developmental pattern as tropoelastin and other matrix proteins. To test the hypothesis that FKBP65 is upregulated at times when extracellular matrix proteins are being actively synthesized and assembled, adult mice were treated with bleomycin to cause reinitiation of matrix protein production during the ensuing development of pulmonary fibrosis. After bleomycin instillation, FKBP65 expression was reactivated in the lung with a pattern similar to that observed for tropoelastin and type I collagen. Using human lung fibroblast cultures, we showed that FKBP65 does not undergo the unfolded protein response, a response associated with an upregulation of resident ER proteins that occurs after increased ER stress. When fibroblasts were treated with transforming growth factor (TGF)-beta1, which is upregulated during the development of pulmonary fibrosis and known to induce matrix production, FKBP65 expression and synthesis was also increased. Similar to type I collagen and tropoelastin, this response was completely inhibited in a dose-dependent manner by GGTI-298, a geranylgeranyl transferase I inhibitor. Treatment of fibroblasts with an inhibitor of ribonucleic acid (RNA) polymerase II after TGF-beta1 treatment showed that the effect of TGF-beta1 was not because of increased stabilization of the FKBP65 messenger RNA. In summary, we have shown that FKBP65 is highly expressed in lung development, downregulated in the adult, and can be reactivated in a coordinated manner with extracellular matrix proteins after lung injury. The expression pattern of FKBP65, which is atypical for general ER foldases, suggests that FKBP65 has a distinct set of developmentally regulated protein ligands. The response to injury, which may be in part a direct response to TGF-beta1, assures the presence of FKBP65 in the ER of cells actively producing components of the extracellular matrix.

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 novel calcineurin inhibitor ISA247: a more potent immunosuppressant than cyclosporine in vitro

ISA247 is a novel cyclosporine analog. In this study we compare, in vitro, the effects of ISA247 on immune function with those of cyclosporine. Whole blood from cynomolgus monkeys (n = 5) was incubated with different concentrations of ISA247 or cyclosporine and stimulated with different mitogens in culture medium. Lymphocyte proliferation was assessed by [3H]-TdR incorporation assay and by flow cytometry. Flow cytometry was also used to assess production of intracellular cytokines by T cells and expression of T cell activation surface antigens. The concentration of drug necessary to attain 50% of the maximum effect (EC50) was subsequently calculated. EC50 values for ISA247 were lower than for cyclosporine, and the differences were statistically significant for lymphocyte proliferation, T cell cytokine production, and expression of all T cell activation surface antigens but one. We conclude that ISA247 suppresses diverse immune functions more potently than cyclosporine in vitro.

The FK506 binding protein 13 kDa (FKBP13) interacts with the C-chain of complement C1q

Background

The pharmacological action of specific immunosuppressants is mediated by immunophilins. While cyclosporin A binds to cyclophilins, FK506/tacrolimus, rapamycin, and others bind to FK506 binding proteins (FKBPs). Different physiological actions of immunophilins were described but their genuine function, however, remains elusive and is still under investigation. A yeast two-hybrid screen was performed using the FK506 binding protein 13 kDa (FKBP13) as a bait and a fetal liver expression library as a prey.

Results

The C-chain of complement C1q (C1q-C) was detected to interact with FKBP13 in the yeast two-hybrid system and in a protein complementation assay. Neither FKBP12, FKBP25, FKBP52 nor the unrelated immunophilin CypA did react with C1q-C in the yeast system stressing the specificity of the interaction. Binding of C1q-C to FKBP13 could not be prevented in the presence of FK506, demonstrating that possibly other regions than the binding pocket of the drug are responsible for the interaction of the two proteins.

Conclusion

It is concluded that exclusively FKBP13 but no other FKBPs tested so far interact with the C-chain of complement C1q in the two different assays and further work will be initiated to investigate the physiological relevance of the interaction.

Interaction of FK506-binding protein 13 with brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1): effects of FK506

BIG1 and BIG2 are brefeldin A-inhibited guanine nucleotide-exchange proteins that activate ADP-ribosylation factors (ARFs), critical components of vesicular trafficking pathways. These proteins can exist in macromolecular complexes and move between Golgi membranes and cytosol. In the BIG1 molecule, a centrally located Sec7 domain is responsible for ARF activation, but functions of other regions are largely unknown. Yeast two-hybrid screens of a human placenta cDNA library with BIG1 cDNA constructs revealed specific interaction of the N-terminal region (amino acids 1-331) with FK506-binding protein 13 (FKBP13). The association was confirmed by immunoprecipitation of both endogenous BIG1 and FKBP13 from Jurkat T cells with antibodies against either one. Binding of BIG1, BIG2, and ARF to cell membranes in vitro was increased by guanosine 5'-[gamma-thio]triphosphate, and further increases were induced by FK506. Incubation of Jurkat T cells with FK506 increased binding of BIG1, BIG2, and ARF to Golgi and other membranes in a time- and concentration-dependent manner, without effects on clathrin or gamma-adaptin binding. Binding of BIG1, BIG2, and ARF to membranes was also increased by L-732,531, an agonist structurally related to FK506, but was not increased by a related antagonist, L-685,818, nor by cyclosporin A or rapamycin. These findings are consistent with a role for FKBP13 and FK506 in vesicular trafficking, influencing ARF activity through their guanine nucleotide-exchange proteins.

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.

Immunophilins may limit calcineurin inhibition by cyclosporine and tacrolimus at high drug concentrations

BACKGROUND

The immunosuppressive drugs cyclosporine (CsA) and tacrolimus (FK506 or FK) are qualitatively similar but differ in molar potency. Both drugs sterically inhibit the phosphatase activity of calcineurin (CN) but differ in molar potency. In our study we explored whether differential inhibition of CN explained the differences in molar potency of FK versus CsA.

METHODS

We compared their effects on NFATC2 dephosphorylation using Western analysis, interferon-gamma production using ELISA, and CN phosphatase activity using the CN assay in human peripheral blood leukocytes (PBL) and mouse spleen cell suspension.

RESULTS

The FK concentration inhibiting 50% (IC50) of all three activities was approximately 0.2 microg/ml in human PBL, versus 5-20 microg/ml for CsA. Although inhibition of interferon-gamma secretion and NFATC2 dephosphorylation was complete, inhibition of CN phosphatase activity was incomplete with both drugs at saturation, particularly with FK. Inhibition of CN phosphatase activity was incomplete whether FK treatment was in vivo in mouse or in vitro in various human and mouse tissues, especially brain. Exogenous FKBP12 or CyPA increased CN phosphatase inhibition, suggesting that incomplete inhibition of CN phosphatase activity reflected limiting amounts of active immunophilin.

CONCLUSIONS

These data contradict the prevailing assumption that immunophilins are abundant and not limiting for inhibition of CN by CsA or FK. Further, the observation that FK and CsA completely inhibit immune function without completely inhibiting CN suggests that the inhibition of immune function is not mediated by general CN inhibition but by inhibition of a subset of CN which is critical for lymphocyte activation.

Biochemical analysis of mouse FKBP60, a novel member of the FKPB family

We have identified mouse and human FKBP60, a new member of the FKBP gene family. FKBP60 shares strongest homology with FKBP65 and SMAP. FKBP60 contains a hydrophobic signal peptide at the N-terminus, 4 peptidyl-prolyl cis/trans isomerase (PPIase) domains and an endoplasmic reticulum retention motif (HDEL) at the C-terminus. Immunodetection of HA-tagged FKBP60 in NIH-3T3 cells suggests that FKBP60 is segregated to the endoplasmic reticulum. Northern blot analysis shows that FKBP60 is predominantly expressed in heart, skeletal muscle, lung, liver and kidney. With N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide as a substrate, recombinant GST-FKBP60 is shown to accelerate effectively the isomerization of the peptidyl-prolyl bond. This isomerization activity is inhibited by FK506. mFKBP60 binds Ca2+ in vitro, presumably by its C-terminal EF-hand Ca2+ binding motif, and is phosphorylated in vivo. hFKBP60 has been mapped to 7p12 and/or 7p14 by fluorescence in situ hybridization (FISH).

Molecular cloning, characterization, and chromosomal localization of FKBP23, a novel FK506-binding protein with Ca2+-binding ability

We have identified and characterized a cDNA encoding a novel FK506-binding protein (FKBP), named FKBP23, from mouse heart by the signal sequence trap method. The deduced amino acid sequence has significant homology to other FKBP family members around the peptidylprolyl cis-trans-isomerase motifs. FKBP23 also has two Ca2+-binding (EF-hand) motifs, and purified FKBP23 protein was shown to have Ca2+-binding ability. This is the first report of a Ca2+-binding FKBP. FKBP23 is a glycoprotein retained in the endoplasmic reticulum by its carboxyl-terminal tetrapeptide His-Asp-Glu-Leu, as demonstrated by immunostaining, retention, and deglycosylation assays. FKBP23 mRNA is expressed most strongly in heart, lung, and testis, beginning at day 8.5 of embryonic development. The FKBP23 gene was mapped to mouse chromosome 2.

The 13-kD FK506 binding protein, FKBP13, interacts with a novel homologue of the erythrocyte membrane cytoskeletal protein 4.1

We have identified a novel generally expressed homologue of the erythrocyte membrane cytoskeletal protein 4.1, named 4.1G, based on the interaction of its COOH-terminal domain (CTD) with the immunophilin FKBP13. The 129-amino acid peptide, designated 4.1G-CTD, is the first known physiologic binding target of FKBP13. FKBP13 is a 13-kD protein originally identified by its high affinity binding to the immunosuppressant drugs FK506 and rapamycin (Jin, Y., M.W. Albers, W.S. Lane, B.E. Bierer, and S.J. Burakoff. 1991. Proc. Natl. Acad. Sci. USA. 88:6677- 6681); it is a membrane-associated protein thought to function as an ER chaperone (Bush, K.T., B.A. Henrickson, and S.K. Nigam. 1994. Biochem. J. [Tokyo]. 303:705-708). We report the specific association of FKBP13 with 4.1G-CTD based on yeast two-hybrid, in vitro binding and coimmunoprecipitation experiments. The histidyl-proline moiety of 4.1G-CTD is required for FKBP13 binding, as indicated by yeast experiments with truncated and mutated 4.1G-CTD constructs. In situ hybridization studies reveal cellular colocalizations for FKBP13 and 4.1G-CTD throughout the body during development, supporting a physiologic role for the interaction. Interestingly, FKBP13 cofractionates with the red blood cell homologue of 4.1 (4.1R) in ghosts, inside-out vesicles, and Triton shell preparations. The identification of FKBP13 in erythrocytes, which lack ER, suggests that FKBP13 may additionally function as a component of membrane cytoskeletal scaffolds.

Chicken FK506-binding protein, FKBP65, a member of the FKBP family of peptidylprolyl cis-trans isomerases, is only partially inhibited by FK506

The chicken FK506-binding protein FKBP65, a peptidylprolyl cis-trans isomerase, is a rough endoplasmic reticulum protein that contains four domains homologous to FKBP13, another rough endoplasmic reticulum PPIase. Analytical ultracentrifugation suggests that in FKBP65 these four domains are arranged in a linear extended structure with a length of about 26 nm and a diameter of about 3 nm. All four domains are therefore expected to be accessible to substrates. The specificity of FKBP65 towards a number of peptide substrates was determined. The specific activity of FKBP65 is generally lower than that of FKBP12 when expressed as a per domain activity. The substrate specificity of FKBP65 also differs from that of FKBP12. Inhibition studies show that only one of the four domains can be inhibited by FK506, a powerful inhibitor of all other known FKBPs. Furthermore, the same domain seems to be susceptible to inhibition by cyclosporin A. No other FKBPs were shown to be inhibited by cyclosporin A. It is also shown that FKBP65 can catalyse the re-folding of type III collagen in vitro with a kcat/Km = 4.3 x 10(3) M-1.s-1.

Identification of tropoelastin as a ligand for the 65-kD FK506-binding protein, FKBP65, in the secretory pathway

The folding and trafficking of tropoelastin is thought to be mediated by intracellular chaperones, although the identity and role of any tropoelastin chaperone remain to be determined. To identify proteins that are associated with tropoelastin intracellularly, bifunctional chemical cross-linkers were used to covalently stabilize interactions between tropoelastin and associated proteins in the secretory pathway in intact fetal bovine auricular chondrocytes. Immunoprecipitation of tropoelastin from cell lysates after cross-linking and analysis by SDS-PAGE showed the presence of two proteins of approximately 74 kD (p74) and 78 kD (p78) that coimmunoprecipitated with tropoelastin. Microsequencing of peptide fragments from a cyanogen bromide digest of p78 identified this protein as BiP and sequence analysis identified p74 as the peptidyl-prolyl cis-trans isomerase, FKPB65. The appearance of BiP and FKBP65 in the immunoprecipitations could be enhanced by the addition of brefeldin A (BFA) and N-acetyl-leu-leu-norleucinal (ALLN) to the culture medium for the final 4 h of labeling. Tropoelastin accumulates in the fused ER/Golgi compartment in the presence of BFA if its degradation is inhibited by ALLN (Davis, E.C., and R.P. Mecham. 1996. J. Biol. Chem. 271:3787-3794). The use of BFA and other secretion-disrupting agents suggests that the association of tropoelastin with FKBP65 occurs in the ER. Results from this study provide the first identification of a ligand for an FKBP in the secretory pathway and suggest that the prolyl cis-trans isomerase activity of FKBP65 may be important for the proper folding of the proline-rich tropoelastin molecule before secretion.

Effects of the immunosuppressive drugs CsA and FK506 on intracellular signalling and gene regulation

The isolation of Cyclosporin A (CsA) from cultures of the fungus Tolypocladium inflatum and its subsequent elucidation of immunosuppressive properties by Borel et al. (1) was of great clinical consequence. In the early 80s CsA was introduced in the field of organ transplantation resulting in extraordinary improvements of graft survival. CsA has become a first choice drug for patients with allograft organs. The discovery of FK506 by Kino et al. (2) as a novel immuno-suppressant and its introduction into clinics in 1989 (3) extended the available regimen for immunosuppressive therapy. Yet despite their advantages both CsA and FK506 display unwanted side effects and a possible preference of one drug over another remains controversial (4, 5). Although identification of the involvement of the transcription factor NF-AT was an important step forward (6), it has become clear that immunosuppressant action is more complex. CsA and FK506 selectively interact with certain cellular signal transduction pathways. This review briefly describes these effects on signal transduction. We further concentrate on the major known effect of these immunosuppressants, namely the inhibition of the PP2B phosphatase calcineurin. In addition we provide a compilation of effects of CsA and FK506 on gene expression at the level of transcription.

A novel plant peptidyl-prolyl-cis-trans-isomerase (PPIase): cDNA cloning, structural analysis, enzymatic activity and expression

A novel cDNA encoding for a peptidyl-prolyl-cis-trans-isomerase (PPIase) belonging to the FK506-binding protein (FKBP) family was isolated from wheat. It contains an open reading frame of 559 amino acids and it represents the first plant FKBP-PPIase to be cloned. It possesses a unique sequence which is composed of three FKPB-like domains, in addition to a putative tetratricopeptide repeat (TPR) motif and a calmodulin-binding site. The recombinant FKBP-PPIase expressed in and purified from Escherichia coli exhibits PPIase activity that is efficiently inhibited by the immunosuppressive drugs FK506 and rapamycin. Northern blot analysis showed that wheat FKBP was found mainly in young tissues. Polyclonal antibodies revealed the presence of cross-reacting proteins in embryos, roots and shoots. The unique structural features, the enzymatic activity and the presence of putative isoforms in wheat tissues indicate the possibility of the involvement of wheat PPIase in essential biological functions, similar to other members of the FKBP gene family.

Perturbations in maturation of secretory proteins and their association with endoplasmic reticulum chaperones in a cell culture model for epithelial ischemia

The effects of ischemia on the maturation of secretory proteins are not well understood. Among several events that occur during ischemia-reperfusion are a rapid and extensive decrease in ATP levels and an alteration of cellular oxidative state. Since the normal folding and assembly of secretory proteins are mediated by endoplasmic reticulum (ER) molecular chaperones, the function of which depends on ATP and maintenance of an appropriate redox environment, ischemia might be expected to perturb folding of secretory proteins. In this study, whole animal and cultured cell models for the epithelial ischemic state were used to examine this possibility. After acute kidney ischemia, marked increases in the mRNA levels of the ER chaperones glucose-regulated protein (grp)78/immunoglobulin-binding protein (BiP), grp94, and ER protein (ERp)72 were noted. Likewise, when cellular ATP was depleted to less than 10% of control with antimycin A, mRNA levels of BiP, ERp72, and grp94 were increased in kidney and thyroid epithelial cell culture models. Since the signal for the up-regulation of these stress proteins is believed to be the accumulation of misfolded/misassembled secretory proteins in the ER, their induction after ischemia in vivo and antimycin treatment of cultured cells suggests that maturation of secretory proteins in the ER lumen might indeed be perturbed. To analyze the effects of antimycin A on the maturation of secretory proteins, we studied the fate of thyroglobulin (Tg), a large oligomeric secretory glycoprotein, the folding and assembly of which seems to require a variety of ER chaperones. Treatment of cultured thyroid epithelial cells with antimycin A greatly inhibited ( > 90%) the secretion of Tg. Sucrose density gradient analysis revealed that in antimycin A-treated cells Tg associates into large macromolecular complexes which, by immunofluorescence, appeared to localize to the ER. Furthermore, coimmunoprecipitation studies after antimycin A treatment demonstrated that Tg stably associates with BiP, grp94, and ERp72. Together, our results suggest that a key cellular lesion in ischemia is the misfolding of secretory proteins as they transit the ER, and this leads not only to increased expression of ER chaperones but also to their stable association with and the subsequent retention of at least some misfolded secretory proteins.

Molecular cloning, DNA sequence analysis, and biochemical characterization of a novel 65-kDa FK506-binding protein (FKBP65)

We have identified a mouse gene encoding a 65-kDa protein (FKBP65) that shares homology with members of the FK506-binding protein (FKBP) class of immunophilins. Predicted amino acid sequence shows that this protein shares significant homology with FKBP12 (46%), FKBP13 (43%), FKBP25 (35%), and FKBP52 (26%). FKBP65 contains four predicted peptidylprolyl cistrans-isomerase (PPIase) signature domains, and, although similar in size, is distinct from FKBP52 (also identified as FKBP59, hsp56, or HBI), which contains three FKBP12-like PPIase domains. With N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide as the substrate, recombinant FKBP65 is shown to accelerate the isomerization of the prolyl peptide bond with a catalytic efficiency similar to other family members. This isomerization activity is inhibited by FK506 and rapamycin, but is not sensitive to Cyclosporin A. Based on Northern blot analysis, FKBP65 mRNA transcripts are present in lung, spleen, heart, brain, and testis. A polyclonal antibody, raised against a COOH-terminal peptide (amino acid residues 566-581), was used to immunoprecipitate FKBP65 from NIH3T3 cells and demonstrate that FKBP65 is a glycoprotein. In addition, [32P]orthophosphate labeling experiments show that FKBP65 is also a phosphoprotein. These results suggest that FKBP65 is a new FKBP family member.

Mechanism of action of the immunosuppressant rapamycin

Rapamycin has potent immunosuppressive properties reflecting its ability to disrupt cytokine signaling that promotes lymphocyte growth and differentiation. In IL-2-stimulated T cells, rapamycin impedes progression through the G1/S transition of the proliferation cycle, resulting in a mid-to-late G1 arrest. Two major biochemical alterations underlie this mode of action. The first one affects the phosphorylation/activation of the p70 S6 kinase (p70s6k), an early event of cytokine-induced mitogenic response. By inhibiting this enzyme, whose major substrate is the 40S ribosomal subunit S6 protein, rapamycin reduces the translation of certain mRNA encoding for ribosomal proteins and elongation factors, thereby decreasing protein synthesis. A second, later effect of rapamycin in IL-2-stimulated T cells is an inhibition of the enzymatic activity of the cyclin-dependent kinase cdk2-cyclin E complex, which functions as a crucial regulator of G1/S transition. This inhibition results from a prevention of the decline of the p27 cdk inhibitor, that normally follows IL-2 stimulation. To mediate these biochemical alterations, rapamycin needs to bind to intracellular proteins, termed FKBP, thereby forming a unique effector molecular complex. However, neither(p70s6k) inhibition, nor p27-induced cdk2-cyclin E inhibition are directly caused by the FKBP-rapamycin complex. Instead, this complex physically interacts with a novel protein, designated "mammalian target of rapamycin" (mTOR), which has sequence homology with the catalytic domain of phosphatidylinositol kinases and may therefore be itself a kinase. mTOR may act upstream of (p70s6K) and cdk2-cyclin E in a linear or bifurcated pathway of growth regulation. Molecular dissection of this pathway should further unravel cytokine-mediated signaling processes and help devise new immunosuppressants.

FK506 binding protein mutational analysis. Defining the surface residue contributions to stability of the calcineurin co-complex

The 12- and 13-kDa FK506 binding proteins (FKBP12 and FKBP13) are cis-trans peptidyl-prolyl isomerases that bind the macrolides FK506 (Tacrolimus) and rapamycin (Sirolimus). The FKBP12.FK506 complex is immunosuppressive, acting as an inhibitor of the protein phosphatase calcineurin. We have examined the role of the key surface residues of FKBP12 and FKBP13 in calcineurin interactions by generating substitutions at these residues by site-directed mutagenesis. All mutants are active catalysts of the prolyl isomerase reaction, and bind FK506 or rapamycin with high affinity. Mutations at FKBP12 residues Asp-37, Arg-42, His-87, and Ile-90 decrease calcineurin affinity of the mutant FKBP12.FK506 complex by as much as 2600-fold in the case of I90K. Replacement of three FKBP13 surface residues (Gln-50, Ala-95, and Lys-98) with the corresponding homologous FKBP12 residues (Arg-42, His-87, and Ile-90) generates an FKBP13 variant that is equivalent to FKBP12 in its affinity for FK506, rapamycin, and calcineurin. These results confirm the role of two loop regions of FKBP12 (residues 40-44 and 84-91) as part of the effector face that interacts with calcineurin.

FK506 binding protein 12 mediates sensitivity to both FK506 and rapamycin in murine mast cells

The immunosuppressive drugs FK506 and rapamycin bind to a family of intracellular proteins termed FK506-binding proteins (FKBP). FK506 and rapamycin inhibit lymphocyte-activation pathways by forming complexes with an FKBP; subsequently, the drug/FKBP complexes interact with target molecules involved in signal transduction. A key target of FK506/FKBP12 complexes is calcineurin, a calcium- and calmodulin-dependent serine/threonine phosphatase. In mammalian cells, rapamycin treatment is associated with inhibition of the activity of several cellular serine/threonine kinases, including p70 S6 kinase. These kinases may function in signaling pathways involving TOR gene producs, which have been shown to interact with rapamycin/FKBP12 complexes in vitro. To determine if FKBP12 mediates the effects of both FK506 and rapamycin in mammalian cells, we overexpressed FKBP12 in a murine mast cell line. Increased expression of FKBP12 resulted in increased sensitivity to FK506 and rapamycin, as measured by inhibition of calcineurin activity and p70 S6 kinase activity, respectively. In contrast, overexpression of FKBP25 had no effect on sensitivity to either drug. Two distinct point mutations in FKBP12, one altering a hydrophobic residue within the drug-binding pocket and the other changing a charged surface residue of FKBP12, abrogated its ability to mediate sensitivity to FK506 and rapamycin. These results establish that FKBP12 can mediate sensitivity to both FK506 and rapamycin in mammalian cells.

Induction of the FK506-binding protein, FKBP13, under conditions which misfold proteins in the endoplasmic reticulum

In order to determine whether the endoplasmic reticulum (ER) luminal FK506-binding protein, FKBP13, shares properties of ER molecular chaperones, MDCK cells were treated with either tunicamycin or Ca2+ ionophores. By Northern-blot analysis, tunicamycin resulted in a 2-fold rise in FKBP13 mRNA, whereas ionophores (A23187 and ionomycin) caused a more impressive rise in FKBP13 mRNA (up to 5-fold with ionomycin). Actinomycin D chase experiments in ionomycin-treated cells revealed no change in the half-life of FKBP13 mRNA, indicating that the increase in FKBP13 mRNA observed was not due to greater message stability. Moreover, sequencing of the 5' flanking region of the gene for murine FKBP13 revealed significant similarity to similar regions in human BiP (immunoglobulin-binding protein) and the human glucose-regulated protein grp94, including a 37 bp sequence in FKBP13 with approximately 50% identity with the unfolded protein response element of the BiP gene. Together, these data suggest a role for FKBP13 in ER protein folding.