The Aβ42 peptide rapidly aggregates to form oligomers, protofibils and fibrils en route to the deposition of amyloid plaques associated with Alzheimer's disease. We show that low temperature and low salt can stabilize disc-shaped oligomers (pentamers) that are significantly more toxic to murine cortical neurons than protofibrils and fibrils. We find that these neurotoxic oligomers do not have the β-sheet structure characteristic of fibrils. Rather, the oligomers are composed of loosely aggregated strands whose C-terminus is protected from solvent exchange and which have a turn conformation placing Phe19 in contact with Leu34. On the basis of NMR spectroscopy, we show that the structural conversion of Aβ42 oligomers to fibrils involves the association of these loosely aggregated strands into β-sheets whose individual β-strands polymerize in a parallel, in-register orientation and are staggered at an inter-monomer contact between Gln15 and Gly37.

Amyloid fibrils associated with Alzheimer's disease and a wide range of other neurodegenerative diseases have a cross beta-sheet structure, where main chain hydrogen bonding occurs between beta-strands in the direction of the fibril axis. The surface of the beta-sheet has pronounced ridges and grooves when the individual beta-strands have a parallel orientation and the amino acids are in-register with one another. Here we show that in Abeta amyloid fibrils, Met35 packs against Gly33 in the C-terminus of Abeta40 and against Gly37 in the C-terminus of Abeta42. These packing interactions suggest that the protofilament subunits are displaced relative to one another in the Abeta40 and Abeta42 fibril structures. We take advantage of this corrugated structure to design a new class of inhibitors that prevent fibril formation by placing alternating glycine and aromatic residues on one face of a beta-strand. We show that peptide inhibitors based on a GxFxGxF framework disrupt sheet-to-sheet packing and inhibit the formation of mature Abeta fibrils as assayed by thioflavin T fluorescence, electron microscopy, and solid-state NMR spectroscopy. The alternating large and small amino acids in the GxFxGxF sequence are complementary to the corresponding amino acids in the IxGxMxG motif found in the C-terminal sequence of Abeta40 and Abeta42. Importantly, the designed peptide inhibitors significantly reduce the toxicity induced by Abeta42 on cultured rat cortical neurons.

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The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) hypothesis suggests that pairs of proteins known as vesicle (v-) SNAREs and target membrane (t-) SNAREs interact specifically to control and mediate intracellular membrane fusion events. Here, cells expressing the interacting domains of v- and t-SNAREs on the cell surface were found to fuse spontaneously, demonstrating that SNAREs are sufficient to fuse biological membranes.

The role of highly conserved aromatic residues surrounding the zinc binding site of human carbonic anhydrase II (CAII) in determining the metal ion binding specificity of this enzyme has been examined by mutagenesis. Residues F93, F95, and W97 are located along a beta-strand containing two residues that coordinate zinc, H94 and H96, and these aromatic amino acids contribute to the high zinc affinity and slow zinc dissociation rate constant of CAII [Hunt, J. A., and Fierke, C. A. (1997) J. Biol. Chem. 272, 20364-20372]. Substitutions of these aromatic amino acids with smaller side chains enhance the copper affinity (up to 100-fold) while decreasing the affinity of both cobalt and zinc, thereby altering the metal binding specificity up to 10(4)-fold. Furthermore, the free energy of the stability of native CAII, determined by solvent-induced denaturation, correlates positively with increased hydrophobicity of the amino acids at positions 93, 95, and 97 as well as with cobalt and zinc affinity. Conversely, increased copper affinity correlates with decreased protein stability. Zinc specificity is therefore enhanced by formation of the native enzyme structure. These data suggest that the hydrophobic cluster in CAII is important for orienting the histidine residues to stabilize metals bound with a distorted tetrahedral geometry and to destabilize the trigonal bipyramidal geometry of bound copper. Knowledge of the structural factors that lead to high metal ion specificity will aid in the design of metal ion biosensors and de novo catalytic sites.

SUMMARY A cycle of palmitoylation/depalmitoylation of H-Ras mediates bidirectional trafficking between the Golgi apparatus and the plasma membrane but nothing is known about how this cycle is regulated. We show that the prolyl isomerase (PI) FKBP12 binds to H-Ras in a palmitoylation-dependent fashion and promotes depalmitoylation. A variety of inhibitors of the PI activity of FKBP12, including FK506, rapamycin and cycloheximide, increase steady-state palmitoylation. FK506 inhibits retrograde trafficking of H-Ras from the plasma membrane to the Golgi in a proline 179-dependent fashion, augments early GTP-loading of Ras in response to growth factors, and promotes H-Ras dependent neurite outgrowth from PC12 cells. These data demonstrate that FKBP12 regulates H-Ras trafficking by promoting depalmitoylation through cis-trans isomerization of a peptidyl-prolyl bond in proximity to the palmitoylated cysteines.

Edited by Wilhelm JustKeywords: GD2 Monoclonal antibody Immunotherapy Neuroblastoma Melanoma Sarcoma Breast cancer stem cell a b s t r a c t Ganglioside GD2 is highly expressed on neuroectoderm-derived tumors and sarcomas, including neuroblastoma, retinoblastoma, melanoma, small cell lung cancer, brain tumors, osteosarcoma, rhabdomyosarcoma, Ewing's sarcoma in children and adolescents, as well as liposarcoma, fibrosarcoma, leiomyosarcoma and other soft tissue sarcomas in adults. Since GD2 expression in normal tissues is restricted to the brain, which is inaccessible to circulating antibodies, and in selected peripheral nerves and melanocytes, it was deemed a suitable target for systemic tumor immunotherapy. Anti-GD2 antibodies have been actively tested in clinical trials for neuroblastoma for over the past two decades, with proven safety and efficacy. The main limitations have been acute pain toxicity associated with GD2 expression on peripheral nerve fibers and the inability of antibodies to treat bulky tumor. Several strategies have been developed to reduce pain toxicity, including bypassing complement activation, using blocking antibodies, or targeting of O-acetyl-GD2 derivative that is not expressed on peripheral nerves. To enhance anti-tumor efficacy, anti-GD2 monoclonal antibodies and fragments have been engineered into immunocytokines, immunotoxins, antibody drug conjugates, radiolabeled antibodies, targeted nanoparticles, T-cell engaging bispecific antibodies, and chimeric antigen receptors. The challenges of these approaches will be reviewed to build a perspective for next generation anti-GD2 therapeutics in cancer therapy.

Bacterial ribonuclease P (RNase P) is a ribonucleoprotein complex composed of one catalytic RNA (PRNA) and one protein subunit (P protein) that together catalyze the 59 maturation of precursor tRNA. High-resolution X-ray crystal structures of the individual P protein and PRNA components from several species have been determined, and structural models of the RNase P holoenzyme have been proposed. However, holoenzyme models have been limited by a lack of distance constraints between P protein and PRNA in the holoenzyme-substrate complex. Here, we report the results of extensive cross-linking and affinity cleavage experiments using single-cysteine P protein variants derivatized with either azidophenacyl bromide or 5-iodoacetamido-1,10-o-phenanthroline to determine distance constraints and to model the Bacillus subtilis holoenzyme-substrate complex. These data indicate that the evolutionarily conserved RNR motif of P protein is located near (<15 Å ) the pre-tRNA cleavage site, the base of the pre-tRNA acceptor stem and helix P4 of PRNA, the putative active site of the enzyme. In addition, the metal binding loop and N-terminal region of the P protein are proximal to the P3 stem-loop of PRNA. Studies using heterologous holoenzymes composed of covalently modified B. subtilis P protein and Escherichia coli M1 RNA indicate that P protein binds similarly to both RNAs. Together, these data indicate that P protein is positioned close to the RNase P active site and may play a role in organizing the RNase P active site.