The hsp90-related protein TRAP1 is a mitochondrial protein with distinct functional properties

Mitochondrial nitric oxide synthase

Nitric oxide (NO*) can bind to and inhibit the terminal enzyme of the mitochondrial respiratory chain, cytochrome c oxidase (complex IV). In vivo, NO* is made by the NO* synthase (NOS) family of enzymes, and considerable debate has recently arisen regarding a NOS inside mitochondria (termed 'mtNOS'). Such an enzyme is an intriguing proposition, since it affords unique organelle-based regulatory mechanisms for NO* synthesis, and has considerable implications for mitochondrial function. This review serves to discuss some of the current issues regarding mtNOS, such as its isoform identity, the availability of co-factors and substrates within the organelle, and potential physiological vs. pathological roles for the enzyme, all within the broader context of mitochondrial regulation by NO*.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Comparative analysis of the ATP-binding sites of Hsp90 by nucleotide affinity cleavage: a distinct nucleotide specificity of the C-terminal ATP-binding site

The 90-kDa heat shock protein (Hsp90) is a molecular chaperone that assists both in ATP-independent sequestration of damaged proteins, and in ATP-dependent folding of numerous targets, such as nuclear hormone receptors and protein kinases. Recent work from our lab and others has established the existence of a second, C-terminal nucleotide binding site besides the well characterized N-terminal, geldanamycin-sensitive ATP-binding site. The cryptic C-terminal site becomes open only after the occupancy of the N-terminal site. Our present work demonstrates the applicability of the oxidative nucleotide affinity cleavage in the site-specific characterization of nucleotide binding proteins. We performed a systematic analysis of the nucleotide binding specificity of the Hsp90 nucleotide binding sites. N-terminal binding is specific to adenosine nucleotides with an intact adenine ring. Nicotinamide adenine dinucleotides and diadenosine polyphosphate alarmones are specific N-terminal nucleotides. The C-terminal binding site is much more unspecific-it interacts with both purine and pirimidine nucleotides. Efficient binding to the C-terminal site requires both charged residues and a larger hydrophobic moiety. GTP and UTP are specific C-terminal nucleotides. 2',3'-O-(2,4,6-trinitrophenyl)-nucleotides (TNP-ATP, TNP-GTP) and pyrophosphate access the C-terminal binding site without the need for an occupied N-terminal site. Our data provide additional evidence for the dynamic domain-domain interactions of Hsp90, give hints for the design of novel types of specific Hsp90 inhibitors, and raise the possibility that besides ATP, other small molecules might also interact with the C-terminal nucleotide binding site in vivo.

A hydrophobic segment within the C-terminal domain is essential for both client-binding and dimer formation of the HSP90-family molecular chaperone

The alpha isoform of human 90-kDa heat shock protein (HSP90alpha) is composed of three domains: the N-terminal (residues 1-400); middle (residues 401-615) and C-terminal (residues 621-732). The middle domain is simultaneously associated with the N- and C-terminal domains, and the interaction with the latter mediates the dimeric configuration of HSP90. Besides one in the N-terminal domain, an additional client-binding site exists in the C-terminal domain of HSP90. The aim of the present study is to elucidate the regions within the C-terminal domain responsible for the bindings to the middle domain and to a client protein, and to define the relationship between the two functions. A bacterial two-hybrid system revealed that residues 650-697 of HSP90alpha were essential for the binding to the middle domain. An almost identical region (residues 657-720) was required for the suppression of heat-induced aggregation of citrate synthase, a model client protein. Replacement of either Leu665-Leu666 or Leu671-Leu672 to Ser-Ser within the hydrophobic segment (residues 662-678) of the C-terminal domain caused the loss of bindings to both the middle domain and the client protein. The interaction between the middle and C-terminal domains was also found in human 94-kDa glucose-regulated protein. Moreover, Escherichia coli HtpG, a bacterial HSP90 homologue, formed heterodimeric complexes with HSP90alpha and the 94-kDa glucose-regulated protein through their middle-C-terminal domains. Taken together, it is concluded that the identical region including the hydrophobic segment of the C-terminal domain is essential for both the client binding and dimer formation of the HSP90-family molecular chaperone and that the dimeric configuration appears to be similar in the HSP90-family proteins.

Tumor necrosis factor signaling

A single mouse click on the topic tumor necrosis factor (TNF) in PubMed reveals about 50,000 articles providing one or the other information about this pleiotropic cytokine or its relatives. This demonstrates the enormous scientific and clinical interest in elucidating the biology of a molecule (or rather a large family of molecules), which began now almost 30 years ago with the description of a cytokine able to exert antitumoral effects in mouse models. Although our understanding of the multiple functions of TNF in vivo and of the respective underlying mechanisms at a cellular and molecular level has made enormous progress since then, new aspects are steadily uncovered and it appears that still much needs to be learned before we can conclude that we have a full comprehension of TNF biology. This review shortly covers some general aspects of this fascinating molecule and then concentrates on the molecular mechanisms of TNF signal transduction. In particular, the multiple facets of crosstalk between the various signalling pathways engaged by TNF will be addressed.

Isolation and identification of a novel mitochondrial metalloprotease (PreP) that degrades targeting presequences in plants

Most of the nuclear encoded mitochondrial precursor proteins contain an N-terminal extension called the presequence that carries targeting information and that is cleaved off after import into mitochondria. The presequences are amphiphilic, positively charged, membrane-interacting peptides with a propensity to form alpha-helices. Here we have investigated the proteolysis of the presequences that have been cleaved off inside mitochondria. A presequence derived from the overexpressed F(1)beta subunit of the ATP synthase and specific synthetic fluorescent peptides (Pep Tag Protease assay) have been shown to undergo rapid degradation catalyzed by a matrix located protease. We have developed a three-step chromatographic procedure including affinity and anion exchange chromatography for isolation of the protease from potato tuber mitochondria. Two-dimensional gel electrophoresis of the isolated proteolytically active fraction followed by electrospray ionization-mass spectrometry/mass spectrometry and data base searches allowed identification of the presequence peptide-degrading protease in Arabidopsis thaliana data base as a novel mitochondrial metalloendoprotease with a molecular mass of 105 kDa. The identified metalloprotease contains an inverted zinc-binding motif and belongs to the pitrilysin family.

Development of a purine-scaffold novel class of Hsp90 binders that inhibit the proliferation of cancer cells and induce the degradation of Her2 tyrosine kinase

The first published synthesis and characterization of a purine-scaffold library of hsp90 inhibitors is presented. The purine-scaffold represents a platform for the creation of easily synthesizable and derivatizable soluble molecules that are amenable for oral administration. The most active compound of the series (71) exhibits binding to hsp90 comparable to the natural product derivative 17AAG that is now in Phase I clinical trial as a cancer therapeutic. Induces the degradation of Her2 tyrosine kinase and arrests the MCF-7 breast cancer cell line at low micromolar concentrations (IC50=2 microM).

Phylogenetic relationships of organellar Hsp90 homologs reveal fundamental differences to organellar Hsp70 and Hsp60 evolution

In agreement with endosymbiont theory for the origin of organelles, mitochondria and chloroplasts (plastids) are universally accepted to have monophyletically arisen from within alpha-proteobacteria and cyanobacteria, respectively. Convincing particular evidence in support of this theory emerged from phylogenetic analysis of highly conserved, ubiquitous heat shock proteins (Hsps) chaperonin 60 and Hsp70. These apparently indispensable general chaperones have proven to be highly useful molecular tracers of organellar origin. Phylogenetic relationships of Hsp90--a less conserved and less widely distributed general chaperone--are reported here that are strikingly incongruent with canonical patterns of endosymbiotic ancestry. It appears that Hsp90 of chloroplasts derives from the endoplasmic reticulum-specific isoform while mitochondrial Hsp90 homologs affiliate with a eubacterial lineage other than alpha subdivision of proteobacteria. These data suggest that endosymbiont htpG genes, encoding Hsp90, have been either functionally displaced by pre-existing nuclear genes or completely lost during establishment of organelles and subsequently added to initial organellar complement.

Unique behavior of a dictyostelium homologue of TRAP-1, coupling with differentiation of D. discoideum cells

Dd-TRAP1 is a Dictyostelium homologue of TRAP-1, a human protein that binds to the type 1 tumor necrosis factor (TNF) receptor. TRAP-1 has a putative mitochondrial localization sequence and shows significant homology to members of the HSP90 family. Although TRAP-1 is mainly localized to mitochondria in several mammalian cells, in certain tissues it is also localized at specific extramitochondrial sites. In Dictyostelium cells, Dd-TRAP1 is predominantly located in the cell membrane/cortex during growth and just after starvation. Double staining of vegetatively growing cells with the anti-Dd-TRAP1 antibody and TRITC-phalloidin has demonstrated colocalization of Dd-TRAP1 and F-actin at the leading edge of cortical protrusions such as pseudopodes. Coupled with differentiation, however, Dd-TRAP1 located at the cortical region is translocated to mitochondria in spite of the absence of the mitochondrial localization sequence at its N-terminus. The translocation of this protein raises interesting and fundamental questions regarding possible mechanisms by which Dd-TRAP1 is involved in cellular differentiation.

The assembly and intermolecular properties of the hsp70-Hop-hsp90 molecular chaperone complex

The highly coordinated interactions of several molecular chaperones, including hsp70 and hsp90, are required for the folding and conformational regulation of a variety of proteins in eukaryotic cells, such as steroid hormone receptors and many other signal transduction regulators. The protein called Hop serves as an adaptor protein for hsp70 and hsp90 and is thought to optimize their functional cooperation. Here we characterize the assembly of the hsp70-Hop-hsp90 complex and reveal interactions that cause conformational changes between the proteins in the complex. We found that hsp40 plays an integral role in the assembly by enhancing the binding of hsp70 to the Hop complex. This is accomplished by stimulating the conversion of hsp70-ATP to hsp70-ADP, the hsp70 conformation favored for Hop binding. The hsp70-Hop-hsp90 complex is highly dynamic, as has been observed previously for hsp90 in its interaction with client proteins. Nonetheless, hsp90 binds with high affinity to Hop (K(d) = 90 nm), and this binding is not affected by hsp70. hsp70 binds with lower affinity to Hop (K(d) = 1.3 microm) on its own, but this affinity is increased (K(d) = 250 nm) in the presence of hsp90. hsp90 also reduces the number of hsp70 binding sites on the Hop dimer from two sites in the absence of hsp90 to one site in its presence. Hop can inhibit the ATP binding and p23 binding activity of hsp90, yet this can be reversed if hsp70 is present in the complex. Taken together, our results suggest that the assembly of hsp70-Hop-hsp90 complexes is selective and influences the conformational state of each protein.

ATPases as drug targets: learning from their structure

ATPases are involved in several cellular functions, and are at the origin of various human diseases. They are therefore attractive drug targets, and various ATPase inhibitors are already on the market. However, most of these drugs are active without binding directly to the nucleotide-binding site. An alternative strategy to inhibit ATPases is to design competitive ATP inhibitors. This approach, which has been used successfully to design protein-kinase inhibitors, depends on the structure of the nucleotide-binding site. This review describes the structural features of the nucleotide-binding site of various ATPases and analyses how this structural information can be exploited for drug discovery.

A Nucleotide-dependent molecular switch controls ATP binding at the C-terminal domain of Hsp90. N-terminal nucleotide binding unmasks a C-terminal binding pocket

In vivo function of the molecular chaperone Hsp90 is ATP-dependent and requires the full-length protein. Our earlier studies predicted a second C-terminal ATP-binding site in Hsp90. By applying direct biochemical approaches, we mapped two ATP-binding sites and unveiled the C-terminal ATP-binding site as the first example of a cryptic chaperone nucleotide-binding site, which is opened by occupancy of the N-terminal site. We identified an N-terminal gamma-phosphate-binding motif in the middle domain of Hsp90 similar to other GHKL family members. This motif is adjacent to the phosphate-binding region of the C-terminal ATP-binding site. Whereas novobiocin disrupts both C- and N-terminal nucleotide binding, we found a selective C-terminal nucleotide competitor, cisplatin, that strengthens the Hsp90-Hsp70 complex leaving the Hsp90-p23 complex intact. Cisplatin may provide a pharmacological tool to dissect C- and N-terminal nucleotide binding of Hsp90. A model is proposed on the interactions of the two nucleotide-binding domains and the charged region of Hsp90.

Regulation of heat shock protein 90 ATPase activity by sequences in the carboxyl terminus

Hsp90, in addition to being an abundant and pivotal cytoplasmic chaperone protein, has been shown to be a weak ATPase. In an effort to characterize the ATPase activity of hsp90, we have observed marked differences in activities among various species of hsp90. Chicken or human hsp90 hydrolyzed ATP with a k(cat) of 0.02 min(-1) and a K(m) greater than 300 microm. In contrast, yeast hsp90 and TRAP1, an hsp90 homologue found in mitochondria, were 10-100-fold more active as ATPases. Sedimentation studies confirmed that all are dimeric proteins. Chicken hsp90 mutants were then analyzed to identify regions within the protein that influence ATPase activity. A truncation mutant of chicken hsp90, N1-573, was found to be monomeric, and yet the catalytic efficiency (k(cat)/K(m)) was greater than 100 times that of the full-length protein (k(cat) of 0.24 min(-1) and K(m) of 60 microm). In contrast, an internal deletion mutant, Delta661-677, was also monomeric but failed to hydrolyze ATP. Finally, deletion of the last 30 amino acids resulted in a dimeric protein with an ATPase activity very similar to full-length hsp90. These data indicate that sequences within the last one-fourth of hsp90 regulate ATP hydrolysis.

HSP90 as a new therapeutic target for cancer therapy: the story unfolds

Current anticancer drug development strategies involve identifying novel molecular targets which are crucial for tumourigenesis. The molecular chaperone heat shock protein (HSP) 90 is of interest as an anticancer drug target because of its importance in maintaining the conformation, stability and function of key oncogenic client proteins involved in signal transduction pathways leading to proliferation, cell cycle progression and apoptosis, as well as other features of the malignant phenotype such as invasion, angiogenesis and metastasis. The natural product HSP90 inhibitors geldanamycin and radicicol exert their antitumour effect by inhibiting the intrinsic ATPase activity of HSP90, resulting in degradation of HSP90 client proteins via the ubiquitin proteosome pathway. Anticancer selectivity may derive from the simultaneous combinatorial effects of HSP90 inhibitors on multiple cancer targets and pathways. 17-allylamino, 17-demethoxygeldanamycin (17AAG), a geldanamycin derivative, showed good activity and cancer selectivity in preclinical models and has now progressed to Phase I clinical trial in cancer patients with encouraging initial results. Phase II trials including combination studies with cytotoxic agents are now being planned and these should allow the therapeutic activity of 17AAG to be determined. Second generation HSP90 inhibitors may be designed to overcome some of the drawbacks of 17AAG, including limited oral bioavailability and solubility. They could also be engineered to target specific functions of HSP90, which may not only provide greater molecular selectivity and clinical benefit but may also increase understanding of the complex functions of this molecular chaperone. HSP90 inhibitors provide proof of concept for drugs directed at HSP90 and protein folding and this principle may be applicable to other medical conditions involving protein aggregation and stability.

In vitro reconstitution of functional hepadnavirus reverse transcriptase with cellular chaperone proteins

Initiation of reverse transcription in hepadnaviruses (hepatitis B viruses) depends on the specific binding of an RNA signal (the packaging signal, epsilon) on the pregenomic RNA template by the viral reverse transcriptase (RT) and is primed by the RT itself (protein priming). We have previously shown that the RT-epsilon interaction and protein priming require the cellular heat shock protein, Hsp90. However, additional host factors required for these reactions remained to be identified. We now report that five cellular chaperone proteins, all known cofactors of Hsp90, were sufficient to reconstitute a duck hepatitis B virus RT active in epsilon binding and protein priming in vitro. Four proteins, Hsp90, Hsp70, Hsp40, and Hop, were required for reconstitution of RT activity, and the fifth protein, p23, further enhanced the kinetics of reconstitution. RT activation by the chaperone proteins is a dynamic process dependent on ATP hydrolysis and the Hsp90 ATPase activity. Thus, our results have defined a minimal complement of host factors necessary and sufficient for RT activation. Furthermore, this defined in vitro reconstitution system has now paved the way for future biochemical and structural studies to elucidate the mechanisms of RT activation and chaperone functions.

Domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90 molecular chaperone

In the present study, we investigated the domain structure and domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90. Limited proteolysis of recombinant HtpG, revealed three major tryptic sites, i.e. Arg7-Gly8, Arg336-Glu337 and Lys552-Leu553, of which the latter two were located at the positions equivalent to the major cleavage sites of human HSP90alpha. A similar pattern was obtained by papain treatment under nondenaturing conditions but not under denaturing conditions. Thus, HtpG consists of three domains, i.e. Domain A, Met1-Arg336; domain B, Glu337-Lys552; and domain C, Leu553-Ser624, as does HSP90. The domains of HtpG were expressed and their interactions were estimated on polyacrylamide gel electrophoresis under nondenaturing conditions. As a result, two kinds of domain-domain interactions were revealed: domain B interaction with domain A of the same polypeptide and domain C of one partner with domain B of the other in the dimer. Domain B could be structurally and functionally divided into two subdomains, the N-terminal two-thirds (subdomain BI) that interacted with domain A and the C-terminal one-third (subdomain BII) that interacted with domain C. The C-terminal two-thirds of domain A, i.e. Asp116-Arg336, were sufficient for the binding to domain B. We finally propose the domain organization of an HtpG dimer.

Dephosphorylated hypoxia-inducible factor 1alpha as a mediator of p53-dependent apoptosis during hypoxia

Under hypoxia, HIF-1alpha binds to aryl hydrocarbon receptor nuclear translocator (ARNT, also called HIF-1beta) to activate expression of genes important for cell survival. Alternatively, HIF-1alpha can bind to the tumor suppressor p53 and promote p53-dependent apoptosis. Here we show that the opposite functions of HIF-1alpha are distinguished by its phosphorylation status. Two distinguishable forms of HIF-1alpha, phosphorylated and dephosphorylated, were induced during hypoxia-induced apoptosis. The phosphorylated HIF-1alpha was the major form that bound to ARNT. Ectopically expressed ARNT was consistently able to enhance HIF-1alpha phosphorylation in a binding-dependent manner. In contrast, the dephosphorylated HIF-1alpha was the major form that bound to p53. Depletion of the dephosphorylated HIF-1alpha, by using the Hsp90 inhibitor geldanamycin A that had little effect on the phosphorylated HIF-1alpha expression, suppressed p53 induction and subsequent apoptosis. Depletion of dephosphorylated HIF-1alpha also prevented hypoxia-induced nuclear accumulation of HDM2, a negative regulator of p53. Our results indicate that the functions of HIF-1alpha varied with its phosphorylation status and that dephosphorylated HIF-1alpha mediated apoptosis by binding to and stabilizing p53.

Hsp90: a specialized but essential protein-folding tool

Hsp90 is unique among molecular chaperones. The majority of its known substrates are signal transduction proteins, and recent work indicates that it uses a novel protein-folding strategy.

The Hsp90 family of proteins in Arabidopsis thaliana

The 90-kDa heat shock protein (Hsp90) is an essential molecular chaperone in eukaryotic cells, with key roles in the folding and activation of proteins involved in signal transduction and control of the cell cycle. A search for Hsp90 sequences in the Arabidopsis thaliana genome revealed that this family includes 7 members. The AtHsp90-1 through AtHsp90-4 proteins constitute the cytoplasmic subfamily, whereas the AtHsp90-5, AtHsp90-6, and AtHsp90-7 proteins are predicted to be within the plastidial, mitochondrial, and endoplasmic reticulum compartments, respectively. The deduced amino acid sequences of each of the cytoplasmic proteins contains the highly conserved C-terminal pentapeptide MEEVD. All of the AtHsp90 sequences include a conserved adenosine triphosphate-binding domain, whereas only the cytoplasmic and endoplasmic reticulum-resident sequences include an adjacent charged linker domain that is common in mammalian and yeast sequences. The occurrence of multiple AtHsp90 proteins in the cytoplasm and of family members in other subcellular compartments suggests a range of specific functions and target polypeptides.

Dimerization and N-terminal domain proximity underlie the function of the molecular chaperone heat shock protein 90

Heat shock protein (hsp)90 functions in a complex chaperoning pathway where its activity is modulated by ATP and by interaction with several co-chaperones. One co-chaperone, p23, binds selectively to the ATP-bound state of hsp90. However, the isolated ATP-binding domain of hsp90 does not bind p23. In an effort to identify the p23-binding domain, we have constructed a series of hsp90 deletion mutants fused with glutathione-S-transferase (GST). Full-length GST-hsp90 is able to bind p23, and also, to chaperone assembly of progesterone receptor complexes. Truncations from the C terminus of GST-hsp90 reveal a C-terminal boundary for the p23-binding domain at approximately residue 490. This fragment contains, in order, the ATP-binding domain, a highly charged region, and 203 residues beyond the charged region. p23 binding is unaffected by deletion of the charged region, indicating that two noncontiguous regions of hsp90 are involved in p23 binding. These regions are only effective when hsp90 is in a dimeric state as shown by loss of p23 binding upon removal of GST or as shown by use of FK506-binding protein12-hsp90 constructs that form dimers and bind p23 only in the presence of a bivalent drug. Thus, p23 binding requires an hsp90 dimer with close proximity between N-terminal regions of hsp90 and a conformation specified by ATP.

Hsp90 chaperone activity requires the full-length protein and interaction among its multiple domains

Hsp90 is an abundant and ubiquitous protein involved in a diverse array of cellular processes. Mechanistically we understand little of the apparently complex interactions of this molecular chaperone. Recently, progress has been made in assigning some of the known functions of hsp90, such as nucleotide binding and peptide binding, to particular domains within the protein. We used fragments of hsp90 and chimeric proteins containing functional domains from hsp90 or its mitochondrial homolog, TRAP1, to study the requirements for this protein in the folding of firefly luciferase as well as in the prevention of citrate synthase aggregation. In agreement with others who have found peptide binding and limited chaperone ability in fragments of hsp90, we see that multiple fragments from hsp90 can prevent the aggregation of thermally denatured citrate synthase, a measure of passive chaperoning activity. However, in contrast to these results, the luciferase folding assay was found to be much more demanding. Here, folding is mediated by hsp70 and hsp40, requires ATP, and thus is a measure of active chaperoning. Hsp90 and the co-chaperone, Hop, enhance this process. This hsp90 activity was only observed using full-length hsp90 indicating that the cooperation of multiple functional domains is essential for active, chaperone-mediated folding.

Immunoelectron microscopy provides evidence that tumor necrosis factor receptor-associated protein 1 (TRAP-1) is a mitochondrial protein which also localizes at specific extramitochondrial sites

The tumor necrosis factor receptor-associated protein 1 (TRAP-1) interacts with a variety of proteins involved in diverse functions. We have used quantitative immunogold electron microscopy and biochemical analysis to evaluate the subcellular distribution of TRAP-1 in rat tissues. Immunofluorescence employing a polyclonal antibody raised to human recombinant TRAP-1 reveals specific staining of mitochondria and nuclear region in mammalian cells. Western blot analysis of purified rat liver mitochondrial subfractions with the TRAP-1 antibody reveals that the cross-reactive protein (M(r) approximately 80 kDa) is mainly present in the matrix compartment. Immunogold labeling of rat tissue sections embedded in LR Gold resin shows strong labeling of mitochondria in all the tissues examined (viz., liver, heart, pancreas, kidney, spleen, anterior pituitary gland). Additionally, specific and significant labeling with TRAP-1 antibody was also observed in certain tissues in a number of nonmitochondrial locations, including pancreatic zymogen granules, insulin secretory granules, cardiac sarcomeres, and nuclei of pancreatic and heart cells, and on the cell surface of blood vessel endothelial cells. Western blot analysis showed that a cross-reactive protein of similar molecular mass as TRAP-1 is present in purified pancreatic zymogen granules. Immunogold labeling was prevented in all tissues by preadsorption of the TRAP-1 antibody with the purified recombinant TRAP-1 protein. These observations and the fact that TRAP-1 is synthesized with a typical mitochondrial targeting presequence strongly indicate that TRAP-1 is primarily a mitochondrial matrix protein. The localization of this protein at specific extramitochondrial sites raises interesting and fundamental questions regarding the possible mechanisms by which these proteins are translocated to such sites.

The small heat shock protein Hsp22 of Drosophila melanogaster is a mitochondrial protein displaying oligomeric organization

Drosophila melanogaster has four main small heat shock proteins (Hsps), D. melanogaster Hsp22 (DmHsp22), Hsp23 (DmHsp23), Hsp26 (DmHsp26), and Hsp27 (DmHsp27). These proteins, although they have high sequence homology, show distinct developmental expression patterns. The function(s) of each small heat shock protein is unknown. DmHsp22 is shown to localize in mitochondria both in D. melanogaster S2 cells and after heterologous expression in mammalian cells. Fractionation of mitochondria indicates that DmHsp22 resides in the mitochondrial matrix, where it is found in oligomeric complexes, as shown by sedimentation and gel filtration analysis and by cross-linking experiments. Deletion analysis using a DmHsp22-EGFP construct reveals that residues 1-17 and an unknown number of residues between 17-28 are necessary for import. Site-directed mutagenesis within a putative mitochondrial motif (WRMAEE) at positions 8-13 shows that the first four residues are necessary for mitochondrial localization. Immunoprecipitation results indicate that there is no interaction between DmHsp22 and the other small heat shock proteins. The mitochondrial localization of this small Hsp22 of Drosophila and its high level of expression in aging suggests a role for this small heat shock protein in protection against oxidative stress.

dtrap-1 encodes a novel member of the heat shock super family of proteins and is expressed in derivatives of all three germ layers during Drosophila embryogenesis

Heat shock proteins (Hsps) comprise a highly conserved superfamily of proteins that are required for stress tolerance in living cells. At physiological conditions, these proteins act as chaperones during protein folding and protein assembly processes (Parsell and Lindquist, 1993. Annu. Rev. Genet. 27, 427-496). Members of the subfamily of Hsp90 proteins were shown to be additionally involved in the presentation and structural modification of components of diverse cellular signal transduction pathways including steroid hormone reception and regulatory kinase activities (Pratt, 1993. J. Biol. Chem. 268, 21455-21458); Cutforth et al., 1994. Cell 77, 1027-1036; van der Straten et al., 1997. EMBO J. 16, 1961-1997; Hunter and Poon, 1997. Trends Cell Biol. 7, 157-161). We have identified a Drosophila gene, called dtrap-1, which encodes a Hsp of a novel subfamily that is related to the Hsp90 family of proteins. During oogenesis dtrap-1 is expressed in nurse cells and its transcripts accumulate in the oocyte. The maternal transcripts remain in the egg but rapidly degrade during early embryogenesis, except in the posterior pole region. Zygotic expression is initiated after the onset of gastrulation showing dynamic patterns of transcripts in the developing mid- and hindgut as well as a subset of mesoderm derivatives.