An anti-PD-1-GITR-L bispecific agonist induces GITR clustering-mediated T cell activation for cancer immunotherapy.

Costimulatory receptors such as glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) play key roles in regulating the effector functions of T cells. In human clinical trials, however, GITR agonist antibodies have shown limited therapeutic effect, which may be due to suboptimal receptor clustering-mediated signaling. To overcome this potential limitation, a rational protein engineering approach is needed to optimize GITR agonist-based immunotherapies. Here we show a bispecific molecule consisting of an anti-PD-1 antibody fused with a multimeric GITR ligand (GITR-L) that induces PD-1-dependent and FcγR-independent GITR clustering, resulting in enhanced activation, proliferation and memory differentiation of primed antigen-specific GITR+PD-1+ T cells. The anti-PD-1-GITR-L bispecific is a PD-1-directed GITR-L construct that demonstrated dose-dependent, immunologically driven tumor growth inhibition in syngeneic, genetically engineered and xenograft humanized mouse tumor models, with a dose-dependent correlation between target saturation and Ki67 and TIGIT upregulation on memory T cells. Anti-PD-1-GITR-L thus represents a bispecific approach to directing GITR agonism for cancer immunotherapy.

VISTA is an activating receptor in human monocytes.

As indicated by its name, V-domain Ig suppressor of T cell activation (VISTA) is thought to serve primarily as an inhibitory protein that limits immune responses. VISTA antibodies can dampen the effects of several concomitantly elicited activation signals, including TCR and TLR activation, but it is currently unclear if VISTA agonism could singly affect immune cell biology. In this study, we discovered two novel VISTA antibodies and characterized their effects on human peripheral blood mononuclear cells by scRNA/CITE-seq. Both antibodies appeared to agonize VISTA in an Fc-functional manner to elicit transcriptional and functional changes in monocytes consistent with activation. We also used pentameric VISTA to identify Syndecan-2 and several heparan sulfate proteoglycan synthesis genes as novel regulators of VISTA interactions with monocytic cells, adding further evidence of bidirectional signaling. Together, our study highlights several novel aspects of VISTA biology that have yet to be uncovered in myeloid cells and serves as a foundation for future research.

Discovery of a Potent and Selective Covalent p300/CBP Inhibitor.

Aberrant gene activation driven by the histone acetyltransferases p300 and CREB binding protein (CBP) has been linked to several diseases, including cancers. Because of this, many efforts have been aimed toward the targeting of the closely related paralogues, p300 and CBP, but these endeavors have been exclusively directed toward noncovalent inhibitors. X-ray crystallography of A-485 revealed that both p300 and CBP possess a cysteine (C1450) near the active site, thus rendering covalent inhibition an attractive chemical approach. Herein we report the development of compound 2, an acrylamide-based inhibitor of p300/CBP that forms a covalent adduct with C1450. We demonstrated using mass spectrometry that compound 2 selectively targets C1450, and we also validated covalent binding using kinetics experiments and cellular washout studies. The discovery of covalent inhibitor 2 gives us a unique tool for the study of p300/CBP biology.

Hexavalent TRAIL Fusion Protein Eftozanermin Alfa Optimally Clusters Apoptosis-Inducing TRAIL Receptors to Induce On-Target Antitumor Activity in Solid Tumors.

TRAIL can activate cell surface death receptors, resulting in potent tumor cell death via induction of the extrinsic apoptosis pathway. Eftozanermin alfa (ABBV-621) is a second generation TRAIL receptor agonist engineered as an IgG1-Fc mutant backbone linked to two sets of trimeric native single-chain TRAIL receptor binding domain monomers. This hexavalent agonistic fusion protein binds to the death-inducing DR4 and DR5 receptors with nanomolar affinity to drive on-target biological activity with enhanced caspase-8 aggregation and death-inducing signaling complex formation independent of FcγR-mediated cross-linking, and without clinical signs or pathologic evidence of toxicity in nonrodent species. ABBV-621 induced cell death in approximately 36% (45/126) of solid cancer cell lines in vitro at subnanomolar concentrations. An in vivo patient-derived xenograft (PDX) screen of ABBV-621 activity across 15 different tumor indications resulted in an overall response (OR) of 29% (47/162). Although DR4 (TNFSFR10A) and/or DR5 (TNFSFR10B) expression levels did not predict the level of response to ABBV-621 activity in vivo, KRAS mutations were associated with elevated TNFSFR10A and TNFSFR10B and were enriched in ABBV-621-responsive colorectal carcinoma PDX models. To build upon the OR of ABBV-621 monotherapy in colorectal cancer (45%; 10/22) and pancreatic cancer (35%; 7/20), we subsequently demonstrated that inherent resistance to ABBV-621 treatment could be overcome in combination with chemotherapeutics or with selective inhibitors of BCL-XL. In summary, these data provide a preclinical rationale for the ongoing phase 1 clinical trial (NCT03082209) evaluating the activity of ABBV-621 in patients with cancer. SIGNIFICANCE: This study describes the activity of a hexavalent TRAIL-receptor agonistic fusion protein in preclinical models of solid tumors that mechanistically distinguishes this molecular entity from other TRAIL-based therapeutics.

Epitope and Fc-Mediated Cross-linking, but Not High Affinity, Are Critical for Antitumor Activity of CD137 Agonist Antibody with Reduced Liver Toxicity.

CD137 (TNFRSF9, 4-1BB) agonist antibodies (mAb) have demonstrated potent antitumor activity with memory response while causing hepatotoxicity in mouse models. In clinical trials, the degrees of liver toxicity of anti-CD137 vary from grade 4 transaminitis (urelumab) to nonexistent (utomilumab). To exploit the antitumor potential of CD137 signaling, we identified a new class of CD137 agonist mAbs with strong antitumor potency without significant transaminitis in vivo compared with CD137 agonists previously reported. These mAbs are cross-reactive to mouse and cynomolgus monkey and showed cross-linking-dependent T-cell costimulation activity in vitro Antitumor efficacy was maintained in Fc gamma receptor (FcγR) III-deficient mice but diminished in FcγRIIB-deficient mice, suggesting the critical role for FcγRIIB to provide cross-linking in vivo Interestingly, a single dose of an affinity-reduced variant was sufficient to control tumor growth, but a higher affinity variant did not improve efficacy. These observations suggest that binding epitope and FcγR interaction, but not necessarily high affinity, are important for antitumor efficacy and reduced liver toxicity of CD137 mAb. Our study suggests the possibility of CD137 agonist therapy with improved safety profile in humans.

PARP1 Trapping by PARP Inhibitors Drives Cytotoxicity in Both Cancer Cells and Healthy Bone Marrow.

PARP inhibitors have recently been approved as monotherapies for the treatment of recurrent ovarian cancer and metastatic BRCA-associated breast cancer, and ongoing studies are exploring additional indications and combinations with other agents. PARP inhibitors trap PARP onto damaged chromatin when combined with temozolomide and methyl methanesulfonate, but the clinical relevance of these findings remains unknown. PARP trapping has thus far been undetectable in cancer cells treated with PARP inhibitors alone. Here, we evaluate the contribution of PARP trapping to the tolerability and efficacy of PARP inhibitors in the monotherapy setting. We developed a novel implementation of the proximity ligation assay to detect chromatin-trapped PARP1 at single-cell resolution with higher sensitivity and throughput than previously reported methods. We further demonstrate that the PARP inhibitor-induced trapping appears to drive single-agent cytotoxicity in healthy human bone marrow, indicating that the toxicity of trapped PARP complexes is not restricted to cancer cells with homologous recombination deficiency. Finally, we show that PARP inhibitors with dramatically different trapping potencies exhibit comparable tumor growth inhibition at MTDs in a xenograft model of BRCA1-mutant triple-negative breast cancer. These results are consistent with emerging clinical data and suggest that the inverse relationship between trapping potency and tolerability may limit the potential therapeutic advantage of potent trapping activity. IMPLICATIONS: PARP trapping contributes to single-agent cytotoxicity of PARP inhibitors in both cancer cells and healthy bone marrow, and the therapeutic advantage of potent trapping activity appears to be limited.

Identification of Selective Dual ROCK1 and ROCK2 Inhibitors Using Structure-Based Drug Design.

A HTS campaign identified compound 1, an excellent hit-like molecule to initiate medicinal chemistry efforts to optimize a dual ROCK1 and ROCK2 inhibitor. Substitution (2-Cl, 2-NH2, 2-F, 3-F) of the pyridine hinge binding motif or replacement with pyrimidine afforded compounds with a clean CYP inhibition profile. Cocrystal structures of an early lead compound were obtained in PKA, ROCK1, and ROCK2. This provided critical structural information for medicinal chemistry to drive compound design. The structural data indicated the preferred configuration at the central benzylic carbon would be ( R), and application of this information to compound design resulted in compound 16. This compound was shown to be a potent and selective dual ROCK inhibitor in both enzyme and cell assays and efficacious in the retinal nerve fiber layer model after oral dosing. This tool compound has been made available through the AbbVie Compound Toolbox. Finally, the cocrystal structures also identified that aspartic acid residues 176 and 218 in ROCK2, which are glutamic acids in PKA, could be targeted as residues to drive both potency and kinome selectivity. Introduction of a piperidin-3-ylmethanamine group to the compound series resulted in compound 58, a potent and selective dual ROCK inhibitor with excellent predicted drug-like properties.

PULSE SPR: A High Throughput Method to Evaluate the Domain Stability of Antibodies.

Downstream purification of therapeutic antibodies requires candidates to be stable under various stress conditions, such as low pH. Current approaches to assess the conformational or colloidal stability of biologics may require significant amounts of material, and the testing may occur over an extended period of time. Furthermore, typical methodologies for early stability testing often do not directly address low pH stability, but focus more on conditions suitable for long-term drug product storage. Here we report a high-throughput method that measures protonation-induced unfolding of ligand binding sites for stability evaluation by surface plasmon resonance or PULSE SPR. This method, which requires only several micrograms of sample, is highly sensitive to the structural integrity of ligand binding sites and correlates well with thermal and chemical conformational stability. Combined with ligands targeting different regions of antibodies, this method allows a comprehensive assessment of antibody domain stability. By applying PULSE SPR, we found that antibodies with different complementarity-determining regions (CDRs), frameworks, formats, and interchain disulfide bonds showed different stabilities under acidic conditions. Additionally, biologics that aggregated in solution also had poor low pH stability. Taken together, PULSE SPR is a high-throughput and low sample consumption method that can be adopted to evaluate antibody domain stability and aggregation at low pH, which are two important aspects of therapeutic biologics.

Author Correction: Discovery of a selective catalytic p300/CBP inhibitor that targets lineage-specific tumours.

In the originally published version of this Letter, the authors Arthur F. Kluge, Michael A. Patane and Ce Wang were inadvertently omitted from the author list. Their affiliations are: I-to-D, Inc., PO Box 6177, Lincoln, Massachusetts 01773, USA (A.F.K.); Mitobridge, Inc. 1030 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (M.A.P.); and China Novartis Institutes for BioMedical Research, No. 4218 Jinke Road, Zhangjiang Hi-Tech Park, Pudong District, Shanghai 201203, China (C.W.). These authors contributed to the interpretation of results and design of compounds. In addition, author 'Edward A. Kesicki' was misspelled as 'Ed Kesicki'. These errors have been corrected online.

ABT-165, a Dual Variable Domain Immunoglobulin (DVD-Ig) Targeting DLL4 and VEGF, Demonstrates Superior Efficacy and Favorable Safety Profiles in Preclinical Models.

Antiangiogenic therapy is a clinically validated modality in cancer treatment. To date, all approved antiangiogenic drugs primarily inhibit the VEGF pathway. Delta-like ligand 4 (DLL4) has been identified as a potential drug target in VEGF-independent angiogenesis and tumor-initiating cell (TIC) survival. A dual-specific biologic targeting both VEGF and DLL4 could be an attractive strategy to improve the effectiveness of anti-VEGF therapy. ABT-165 was uniquely engineered using a proprietary dual-variable domain immunoglobulin (DVD-Ig) technology based on its ability to bind and inhibit both DLL4 and VEGF. In vivo, ABT-165 induced significant tumor growth inhibition compared with either parental antibody treatment alone, due, in part, to the disruption of functional tumor vasculature. In combination with chemotherapy agents, ABT-165 also induced greater antitumor response and outperformed anti-VEGF treatment. ABT-165 displayed nonlinear pharmacokinetic profiles in cynomolgus monkeys, with an apparent terminal half-life > 5 days at a target saturation dose. In a GLP monkey toxicity study, ABT-165 was well-tolerated at doses up to 200 mg/kg with non-adverse treatment-related histopathology findings limited to the liver and thymus. In summary, ABT-165 represents a novel antiangiogenic strategy that potently inhibits both DLL4 and VEGF, demonstrating favorable in vivo efficacy, pharmacokinetic, and safety profiles in preclinical models. Given these preclinical attributes, ABT-165 has progressed to a phase I study. Mol Cancer Ther; 17(5); 1039-50. ©2018 AACR.

Quantitative ligand and receptor binding studies reveal the mechanism of interleukin-36 (IL-36) pathway activation.

IL-36 cytokines signal through the IL-36 receptor (IL-36R) and a shared subunit, IL-1RAcP (IL-1 receptor accessory protein). The activation mechanism for the IL-36 pathway is proposed to be similar to that of IL-1 in that an IL-36R agonist (IL-36α, IL-36ß, or IL-36γ) forms a binary complex with IL-36R, which then recruits IL-1RAcP. Recent studies have shown that IL-36R interacts with IL-1RAcP even in the absence of an agonist. To elucidate the IL-36 activation mechanism, we considered all possible binding events for IL-36 ligands/receptors and examined these events in direct binding assays. Our results indicated that the agonists bind the IL-36R extracellular domain with micromolar affinity but do not detectably bind IL-1RAcP. Using surface plasmon resonance (SPR), we found that IL-1RAcP also does not bind IL-36R when no agonist is present. In the presence of IL-36α, however, IL-1RAcP bound IL-36R strongly. These results suggested that the main pathway to the IL-36R·IL-36α·IL-1RAcP ternary complex is through the IL-36R·IL-36α binary complex, which recruits IL-1RAcP. We could not measure the binding affinity of IL-36R to IL-1RAcP directly, so we engineered a fragment crystallizable-linked construct to induce IL-36R·IL-1RAcP heterodimerization and predicted the binding affinity during a complete thermodynamic cycle to be 74 µm The SPR analysis also indicated that the IL-36R antagonist IL-36Ra binds IL-36R with higher affinity and a much slower off rate than the IL-36R agonists, shedding light on IL-36 pathway inhibition. Our results reveal the landscape of IL-36 ligand and receptor interactions, improving our understanding of IL-36 pathway activation and inhibition.

Discovery of a selective catalytic p300/CBP inhibitor that targets lineage-specific tumours.

The dynamic and reversible acetylation of proteins, catalysed by histone acetyltransferases (HATs) and histone deacetylases (HDACs), is a major epigenetic regulatory mechanism of gene transcription and is associated with multiple diseases. Histone deacetylase inhibitors are currently approved to treat certain cancers, but progress on the development of drug-like histone actyltransferase inhibitors has lagged behind. The histone acetyltransferase paralogues p300 and CREB-binding protein (CBP) are key transcriptional co-activators that are essential for a multitude of cellular processes, and have also been implicated in human pathological conditions (including cancer). Current inhibitors of the p300 and CBP histone acetyltransferase domains, including natural products, bi-substrate analogues and the widely used small molecule C646, lack potency or selectivity. Here, we describe A-485, a potent, selective and drug-like catalytic inhibitor of p300 and CBP. We present a high resolution (1.95 Å) co-crystal structure of a small molecule bound to the catalytic active site of p300 and demonstrate that A-485 competes with acetyl coenzyme A (acetyl-CoA). A-485 selectively inhibited proliferation in lineage-specific tumour types, including several haematological malignancies and androgen receptor-positive prostate cancer. A-485 inhibited the androgen receptor transcriptional program in both androgen-sensitive and castration-resistant prostate cancer and inhibited tumour growth in a castration-resistant xenograft model. These results demonstrate the feasibility of using small molecule inhibitors to selectively target the catalytic activity of histone acetyltransferases, which may provide effective treatments for transcriptional activator-driven malignancies and diseases.

Mechanistic Dissection of PARP1 Trapping and the Impact on In Vivo Tolerability and Efficacy of PARP Inhibitors.

UNLABELLED: Poly(ADP-ribose) polymerases (PARP1, -2, and -3) play important roles in DNA damage repair. As such, a number of PARP inhibitors are undergoing clinical development as anticancer therapies, particularly in tumors with DNA repair deficits and in combination with DNA-damaging agents. Preclinical evidence indicates that PARP inhibitors potentiate the cytotoxicity of DNA alkylating agents. It has been proposed that a major mechanism underlying this activity is the allosteric trapping of PARP1 at DNA single-strand breaks during base excision repair; however, direct evidence of allostery has not been reported. Here the data reveal that veliparib, olaparib, niraparib, and talazoparib (BMN-673) potentiate the cytotoxicity of alkylating agents. Consistent with this, all four drugs possess PARP1 trapping activity. Using biochemical and cellular approaches, we directly probe the trapping mechanism for an allosteric component. These studies indicate that trapping is due to catalytic inhibition and not allostery. The potency of PARP inhibitors with respect to trapping and catalytic inhibition is linearly correlated in biochemical systems but is nonlinear in cells. High-content imaging of γH2Ax levels suggests that this is attributable to differential potentiation of DNA damage in cells. Trapping potency is inversely correlated with tolerability when PARP inhibitors are combined with temozolomide in mouse xenograft studies. As a result, PARP inhibitors with dramatically different trapping potencies elicit comparable in vivo efficacy at maximum tolerated doses. Finally, the impact of trapping on tolerability and efficacy is likely to be context specific. IMPLICATIONS: Understanding the context-specific relationships of trapping and catalytic inhibition with both tolerability and efficacy will aid in determining the suitability of a PARP inhibitor for inclusion in a particular clinical regimen.

Characterization of ABT-806, a Humanized Tumor-Specific Anti-EGFR Monoclonal Antibody.

Despite clinical efficacy, current approved agents targeting EGFR are associated with on-target toxicities as a consequence of disrupting normal EGFR function. MAb 806 is a novel EGFR antibody that selectively targets a tumor-selective epitope suggesting that a mAb 806-based therapeutic would retain antitumor activity without the on-target toxicities associated with EGFR inhibition. To enable clinical development, a humanized variant of mAb 806 designated ABT-806 was generated and is currently in phase 1 trials. We describe the characterization of binding and functional properties of ABT-806 compared with the clinically validated anti-EGFR antibody cetuximab. ABT-806 binds the mutant EGFRvIII with high affinity and, relative to cetuximab, exhibits increased potency against glioblastoma multiforme cell line and patient-derived xenografts expressing this form of the receptor. ABT-806 also inhibits the growth of squamous cell carcinoma xenograft models expressing high levels of wild-type EGFR, associated with inhibition of EGFR signaling, although higher doses of ABT-806 than cetuximab are required for similar activity. ABT-806 enhances in vivo potency of standard-of-care therapies used to treat glioblastoma multiforme and head and neck squamous cell carcinoma. An indium-labeled version of ABT-806, [(111)In]-ABT-806, used to investigate the relationship between dose and receptor occupancy, revealed greater receptor occupancy at lowers doses in an EGFRvIII-expressing model and significant uptake in an orthotopic model. Collectively, these results suggest that ABT-806 may have antitumor activity superior to cetuximab in EGFRvIII-expressing tumors, and similar activity to cetuximab in tumors highly overexpressing wild-type EGFR with reduced toxicity.

Structure reveals function of the dual variable domain immunoglobulin (DVD-Ig™) molecule.

Several bispecific antibody-based formats have been developed over the past 25 years in an effort to produce a new generation of immunotherapeutics that target two or more disease mechanisms simultaneously. One such format, the dual-variable domain immunoglobulin (DVD-Ig™), combines the target binding domains of two monoclonal antibodies via flexible naturally occurring linkers, which yields a tetravalent IgG - like molecule. We report the structure of an interleukin (IL)12-IL18 DVD-Ig™ Fab (DFab) fragment with IL18 bound to the inner variable domain (VD) that reveals the remarkable flexibility of the DVD-Ig™ molecule and how the DVD-Ig™ format can function to bind four antigens simultaneously. An understanding of how the inner variable domain retains function is of critical importance for designing DVD-Ig™ molecules, and for better understanding of the flexibility of immunoglobulin variable domains and linkers, which may aid in the design of improved bi- and multi-specific biologics in general.

Design and generation of DVD-Ig™ molecules for dual-specific targeting.

The dual variable domain immunoglobulin (DVD-Ig™) protein is a new type of dual-specific IgG. As a novel therapeutic class, the great potential of the DVD-Ig protein is to simultaneously target two mediators of disease by a single pharmaceutical entity. The molecule contains an Fc region and constant regions in a configuration similar to a conventional IgG; however, the DVD-Ig protein is unique in that each arm of the molecule contains two variable domains (VDs). The VDs within an arm are linked in tandem and can possess different binding specificities. Here, we discuss critical design features of the DVD-Ig protein and describe a methodology for cloning, expressing, and purifying the molecules.

Ligand association rates to the inner-variable-domain of a dual-variable-domain immunoglobulin are significantly impacted by linker design.

The DVD-Ig (TM) protein is a dual-specific immunoglobulin. Each of the two arms of the molecule contains two variable domains, an inner variable domain and an outer variable domain linked in tandem, each with binding specificity for different targets or epitopes. One area of on-going research involves determining how the proximity of the outer variable domain affects the binding of ligands to the inner variable domain. To explore this area, we prepared a series of DVD-Ig proteins with binding specificities toward TNFα and an alternate therapeutic target. Kinetic measurements of TNFα binding to this series of DVD-Ig proteins were used to probe the effects of variable domain position and linker design on ligand on- and off-rates. We found that affinities for TNFα are generally lower when binding to the inner domain than to the outer domain and that this loss of affinity is primarily due to reduced association rate. This effect could be mitigated, to some degree, by linker design. We show several linker sequences that mitigate inner domain affinity losses in this series of DVD-Ig proteins. Moreover, we show that single chain proteolytic cleavage between the inner and outer domains, or complete outer domain removal, can largely restore inner domain TNFα affinity to that approaching the reference antibody. Taken together, these results suggest that a loss of affinity for inner variable domains in this set of DVD-Ig proteins may be largely driven by simple steric hindrance effects and can be reduced by careful linker design.

Structure of the SH3 domain of human osteoclast-stimulating factor at atomic resolution.

Osteoclast-stimulating factor (OSF) is an intracellular signaling protein, produced by osteoclasts themselves, that enhances osteoclast formation and bone resorption. It is thought to act via an Src-related signaling pathway and contains SH3 and ankyrin-repeat domains which are involved in protein-protein interactions. As part of a structure-based anti-bone-loss drug-design program, the atomic resolution X-ray structure of the recombinant human OSF SH3 domain (hOSF-SH3) has been determined. The domain, residues 12-72, yielded crystals that diffracted to the ultrahigh resolution of 1.07 A. The overall structure shows a characteristic SH3 fold consisting of two perpendicular beta-sheets that form a beta-barrel. Structure-based sequence alignment reveals that the putative proline-rich peptide-binding site of hOSF-SH3 consists of (i) residues that are highly conserved in the SH3-domain family, including residues Tyr21, Phe23, Trp49, Pro62, Asn64 and Tyr65, and (ii) residues that are less conserved and/or even specific to hOSF, including Thr22, Arg26, Thr27, Glu30, Asp46, Thr47, Asn48 and Leu60, which might be key to designing specific inhibitors for hOSF to fight osteoporosis and related bone-loss diseases. There are a total of 13 well defined water molecules forming hydrogen bonds with the above residues in and around the peptide-binding pocket. Some of those water molecules might be important for drug-design approaches. The hOSF-SH3 structure at atomic resolution provides an accurate framework for structure-based design of its inhibitors.

High resolution crystal structure of human Rab9 GTPase: a novel antiviral drug target.

Rab GTPases and their effectors facilitate vesicular transport by tethering donor vesicles to their respective target membranes. Rab9 mediates late endosome to trans-Golgi transport and has recently been found to be a key cellular component for human immunodeficiency virus-1, Ebola, Marburg, and measles virus replication, suggesting that it may be a novel target in the development of broad spectrum antiviral drugs. As part of our structure-based drug design program, we have determined the crystal structure of a C-terminally truncated human Rab9 (residues 1-177) to 1.25-A resolution. The overall structure shows a characteristic nucleotide binding fold consisting of a six-stranded beta-sheet surrounded by five alpha-helices with a tightly bound GDP molecule in the active site. Structure-based sequence alignment of Rab9 with other Rab proteins reveals that its active site consists of residues highly conserved in the Rab GTPase family, implying a common catalytic mechanism. However, Rab9 contains seven regions that are significantly different in conformation from other Rab proteins. Some of those regions coincide with putative effector-binding sites and switch I and switch II regions identified by structure/sequence alignments. The Rab9 structure at near atomic resolution provides an excellent model for structure-based antiviral drug design.

Reversible amyloid formation by the p53 tetramerization domain and a cancer-associated mutant.

The tetramerization domain for wild-type p53 (p53tet-wt) and a p53 mutant, R337H (p53tet-R337H), associated with adrenocortical carcinoma (ACC) in children, can be converted from the soluble native state to amyloid-like fibrils under certain conditions. Circular dichroism, Fourier transform infrared spectroscopy and staining with Congo red and thioflavin T showed that p53tet-wt and p53tet-R337H adopt an alternative beta-sheet conformation (p53tet-wt-beta and p53tet-R337H-beta, respectively), characteristic of amyloid-like fibrils, when incubated at pH 4.0 and elevated temperatures. Electron micrographs showed that the alternative conformations for p53tet-wt (p53tet-wt-beta) and p53tet-R337H (p53tet-R337H-beta) were supramolecular structures best described as "molecular ribbons". FT-IR analysis demonstrated that the mechanism of amyloid-like fibril formation involved unfolding of the p53tet-wt beta-strands, followed by unfolding of the alpha-helices, followed finally by formation of beta-strand-containing structures that other methods showed were amyloid-like ribbons. The mutant, p53tet-R337H, had a significantly higher propensity to form amyloid-like fibrils. Both p53tet-wt (pH 4.0) and p53tet-R337H (pH 4.0 and 5.0), when incubated at room temperature (22 degrees C) for one month, were converted to molecular ribbons. In addition, p53tet-R337H, and not p53tet-wt, readily formed ribbons at pH 4.0 and 37 degrees C over 20 hours. Interestingly, unlike other amyloid-forming proteins, p53tet-wt-beta and p53tet-R337H-beta disassembled and refolded to the native tetramer conformation when the solution pH was raised from 4.0 to 8.5. Although fibril formation at pH 4.0 was concentration and temperature-dependent, fibril disassembly at pH 8.5 was independent of both. Finally, we propose that the significantly higher propensity of the mutant to form ribbons, compared to the wild-type, may provide a possible mechanism for the observed nuclear accumulation of p53 in ACC cells and other cancerous cells.