The architecture of functional modules in the Hsp90 co-chaperone Sti1/Hop.

Sti1/Hop is a modular protein required for the transfer of client proteins from the Hsp70 to the Hsp90 chaperone system in eukaryotes. It binds Hsp70 and Hsp90 simultaneously via TPR (tetratricopeptide repeat) domains. Sti1/Hop contains three TPR domains (TPR1, TPR2A and TPR2B) and two domains of unknown structure (DP1 and DP2). We show that TPR2A is the high affinity Hsp90-binding site and TPR1 and TPR2B bind Hsp70 with moderate affinity. The DP domains exhibit highly homologous α-helical folds as determined by NMR. These, and especially DP2, are important for client activation in vivo. The core module of Sti1 for Hsp90 inhibition is the TPR2A-TPR2B segment. In the crystal structure, the two TPR domains are connected via a rigid linker orienting their peptide-binding sites in opposite directions and allowing the simultaneous binding of TPR2A to the Hsp90 C-terminal domain and of TPR2B to Hsp70. Both domains also interact with the Hsp90 middle domain. The accessory TPR1-DP1 module may serve as an Hsp70-client delivery system for the TPR2A-TPR2B-DP2 segment, which is required for client activation in vivo.

BclxL changes conformation upon binding to wild-type but not mutant p53 DNA binding domain.

p53 can induce apoptosis through mitochondrial membrane permeabilization by interaction of its DNA binding region with the anti-apoptotic proteins BclxL and Bcl2. However, little is known about the action of p53 at the mitochondria in molecular detail. By using NMR spectroscopy and fluorescence polarization we characterized the binding of wild-type and mutant p53 DNA binding domains to BclxL and show that the wild-type p53 DNA binding domain leads to structural changes in the BH3 binding region of BclxL, whereas mutants fail to induce such effects due to reduced affinity. This was probed by induced chemical shift and residual dipolar coupling data. These data imply that p53 partly achieves its pro-apoptotic function at the mitochondria by facilitating interaction between BclxL and BH3-only proteins in an allosteric mode of action. Furthermore, we characterize for the first time the binding behavior of Pifithrin-mu, a specific small molecule inhibitor of the p53-BclxL interaction, and present a structural model of the protein-ligand complex. A rather unusual behavior is revealed whereby Pifithrin-mu binds to both sides of the protein-protein complex. These data should facilitate the rational design of more potent specific BclxL-p53 inhibitors.

Rational Design of Cyclic Peptide Inhibitors of U2AF Homology Motif (UHM) Domains To Modulate Pre-mRNA Splicing.

U2AF homology motifs (UHMs) are atypical RNA recognition motif domains that mediate critical protein-protein interactions during the regulation of alternative pre-mRNA splicing and other processes. The recognition of UHM domains by UHM ligand motif (ULM) peptide sequences plays important roles during early steps of spliceosome assembly. Splicing factor 45 kDa (SPF45) is an alternative splicing factor implicated in breast and lung cancers, and splicing regulation of apoptosis-linked pre-mRNAs by SPF45 was shown to depend on interactions between its UHM domain and ULM motifs in constitutive splicing factors. We have developed cyclic peptide inhibitors that target UHM domains. By screening a focused library of linear and cyclic peptides and performing structure-activity relationship analysis, we designed cyclic peptides with 4-fold improved binding affinity for the SPF45 UHM domain compared to native ULM ligands and 270-fold selectivity to discriminate UHM domains from alternative and constitutive splicing factors. These inhibitors are useful tools to modulate and dissect mechanisms of alternative splicing regulation.

Structural analysis of the interaction between Hsp90 and the tumor suppressor protein p53.

In eukaryotes, the essential dimeric molecular chaperone Hsp90 is required for the activation and maturation of specific substrates such as steroid hormone receptors, tyrosine kinases and transcription factors. Hsp90 is involved in the establishment of cancer and has become an attractive target for drug design. Here we present a structural characterization of the complex between Hsp90 and the tumor suppressor p53, a key mediator of apoptosis whose structural integrity is crucial for cell-cycle control. Using biophysical methods, we show that the human p53 DNA-binding domain interacts with multiple domains of yeast Hsp90. p53 binds to the Hsp90 C-terminal domain in its native-like state in a charge-dependent manner, but it also associates weakly with binding sites in the middle and the N-terminal domains. The fine-tuned interplay between several Hsp90 domains provides the interactions required for efficient chaperoning of p53.

Design, synthesis, and functionalization of dimeric peptides targeting chemokine receptor CXCR4.

The chemokine receptor CXCR4 is a critical regulator of inflammation and immune surveillance, and it is specifically implicated in cancer metastasis and HIV-1 infection. On the basis of the observation that several of the known antagonists remarkably share a C(2) symmetry element, we constructed symmetric dimers with excellent antagonistic activity using a derivative of a cyclic pentapeptide as monomer. To optimize the binding affinity, we investigated the influence of the distance between the monomers and the pharmacophoric sites in the synthesized constructs. The affinity studies in combination with docking computations support a two-site binding model. In a final step, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was introduced as chelator for (radio-)metals, thus allowing to exploit these compounds as a new group of CXCR4-binding peptidic probes for molecular imaging and endoradiotherapeutic purposes. Both the DOTA conjugates and some of their corresponding metal complexes retain good CXCR4 affinity, and one (68)Ga labeled compound was studied as PET tracer.

PET of CXCR4 expression by a (68)Ga-labeled highly specific targeted contrast agent.

UNLABELLED: The overexpression of the chemokine receptor CXCR4 plays an important role in oncology, since together with its endogenous ligand, the stromal cell-derived factor (SDF1-α), CXCR4 is involved in tumor development, growth, and organ-specific metastasis. As part of our ongoing efforts to develop highly specific CXCR4-targeted imaging probes and with the aim to assess the suitability of this ligand for first proof-of-concept studies in humans, we further evaluated the new (68)Ga-labeled high-affinity cyclic CXCR4 ligand, (68)Ga-CPCR4-2 (cyclo(D-Tyr(1)-[NMe]-D-Orn(2)-[4-(aminomethyl) benzoic acid,(68)Ga-DOTA]-Arg(3)-2-Nal(4)-Gly(5))). METHODS: Additional biodistribution and competitions studies in vivo, dynamic PET studies, and investigations on the metabolic stability and plasma protein binding were performed in nude mice bearing metastasizing OH1 human small cell lung cancer xenografts. CXCR4 expression on OH1 tumor sections was determined by immunohistochemical staining. RESULTS: (nat)Ga-CPCR4-2 exhibits high CXCR4 affinity with a half maximum inhibitory concentration of 4.99 ± 0.72 nM. (68)Ga-CPCR4-2 showed high in vivo stability and high and specific tumor accumulation, which was reduced by approximately 80% in competition studies with AMD3100. High CXCR4 expression in tumors was confirmed by immunohistochemical staining. (68)Ga-CPCR4-2 showed low uptake in nontumor tissue and particularly low kidney accumulation despite predominant renal excretion, leading to high-contrast delineation of tumors in small-animal PET studies. CONCLUSION: The small and optimized cyclic peptide CPCR4-2 labeled with (68)Ga is a suitable tracer for targeting and imaging of human CXCR4 receptor expression in vivo. The high affinity for CXCR4, its in vivo stability, and the excellent pharmacokinetics recommend the further evaluation of (68)Ga-CPCR4-2 in a proof-of-concept study in humans.

18O stable isotope labeling, quantitative model experiments, and molecular dynamics simulation studies on the trans-specific degradation of the bitter tasting iso-alpha-acids of beer.

The typical bitterness of fresh beer is well-known to decrease in intensity and to change in quality with increasing age. This phenomenon was recently shown to be caused by the conversion of bitter tasting trans-iso-alpha-acids into lingering and harsh bitter tasting tri- and tetracyclic degradation products such as tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol. Interestingly, the formation of these compounds was shown to be trans-specific and the corresponding cis-iso-alpha-acids were found to be comparatively stable. Application of 18O stable isotope labeling as well as quantitative model studies combined with LC-MS/MS experiments, followed by computer-based molecular dynamics simulations revealed for the first time a conclusive mechanism explaining the stereospecific transformation of trans-iso-alpha-acids into the tri- and tetracyclic degradation products. This transformation was proposed to be induced by a proton-catalyzed carbon/carbon bond formation between the carbonyl atom C(1') of the isohexenoyl moiety and the alkene carbon C(2'') of the isoprenyl moiety of the trans-iso-alpha-acids.

Differential analysis of Saccharomyces cerevisiae mitochondria by free flow electrophoresis.

One major problem concerning the electrophoresis of mitochondria is the heterogeneity of mitochondrial appearance especially under pathological conditions. We show here the use of zone electrophoresis in a free flow electrophoresis device (ZE-FFE) as an analytical sensor to discriminate between different yeast mitochondrial populations. Impairment of the structural properties of the organelles by hyperosmotic stress resulted in broad separation profiles. Conversely untreated mitochondria gave rise to homogeneous populations reflected by sharp separation profiles. Yeast mitochondria with altered respiratory activity accompanied by a different outer membrane proteome composition could be discriminated based on electrophoretic deflection. Proteolysis of the mitochondrial surface proteome and the deletion of a single major protein species of the mitochondrial outer membrane altered the ZE-FFE deflection of these organelles. To demonstrate the usefulness of ZE-FFE for the analysis of mitochondria associated with pathological processes, we analyzed mitochondrial fractions from an apoptotic yeast strain. The cdc48(S565G) strain carries a mutation in the CDC48 gene that is an essential participant in the endoplasmic reticulum-associated protein degradation pathway. Mutant cells accumulate polyubiquitinated proteins in microsomal and mitochondrial extracts. Subsequent ZE-FFE characterization could distinguish a mitochondrial subfraction specifically enriched with polyubiquitinated proteins from the majority of non-affected mitochondria. This result demonstrates that ZE-FFE may give important information on the specific properties of subpopulations of a mitochondrial preparation allowing a further detailed functional analysis.