The seven-residue peptide N-acetyl-Lys-Leu-Val-Phe-Phe-Ala-Glu-NH(2), called A beta(16-22) and representing residues 16-22 of the full-length beta-amyloid peptide associated with Alzheimer's disease, is shown by electron microscopy to form highly ordered fibrils upon incubation of aqueous solutions. X-ray powder diffraction and optical birefringence measurements confirm that these are amyloid fibrils. The peptide conformation and supramolecular organization in A beta(16-22) fibrils are investigated by solid state (13)C NMR measurements. Two-dimensional magic-angle spinning (2D MAS) exchange and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) measurements indicate a beta-strand conformation of the peptide backbone at the central phenylalanine. One-dimensional and two-dimensional spectra of selectively and uniformly labeled samples exhibit (13)C NMR line widths of <2 ppm, demonstrating that the peptide, including amino acid side chains, has a well-ordered conformation in the fibrils. Two-dimensional (13)C-(13)C chemical shift correlation spectroscopy permits a nearly complete assignment of backbone and side chain (13)C NMR signals and indicates that the beta-strand conformation extends across the entire hydrophobic segment from Leu17 through Ala21. (13)C multiple-quantum (MQ) NMR and (13)C/(15)N rotational echo double-resonance (REDOR) measurements indicate an antiparallel organization of beta-sheets in the A beta(16-22) fibrils. These results suggest that the degree of structural order at the molecular level in amyloid fibrils can approach that in peptide or protein crystals, suggest how the supramolecular organization of beta-sheets in amyloid fibrils can be dependent on the peptide sequence, and illustrate the utility of solid state NMR measurements as probes of the molecular structure of amyloid fibrils. A beta(16-22) is among the shortest fibril-forming fragments of full-length beta-amyloid reported to date, and hence serves as a useful model system for physical studies of amyloid fibril formation.

HIV integrase is the enzyme responsible for inserting the viral DNA into the host chromosome; it is essential for HIV replication. The crystal structure of the catalytically active core domain (residues 50 to 212) of HIV-1 integrase was determined at 2.5 A resolution. The central feature of the structure is a five-stranded beta sheet flanked by helical regions. The overall topology reveals that this domain of integrase belongs to a superfamily of polynucleotidyl transferases that includes ribonuclease H and the Holliday junction resolvase RuvC. The active site region is identified by the position of two of the conserved carboxylate residues essential for catalysis, which are located at similar positions in ribonuclease H. In the crystal, two molecules form a dimer with a extensive solvent-inaccessible interface of 1300 A2 per monomer.

Hundreds of acetyltransferases exist. All use a common acetyl donor--acetyl coenzyme A--and each exhibits remarkable specificity for acetyl acceptors, which include small molecules and proteins. Analysis of the primary sequences of these enzymes indicates that they can be sorted into several superfamilies. This review covers the three-dimensional structures of members of one of these superfamilies, now referred to in the literature as the GCN5-related N-acetyltransferases (GNAT), reflecting the importance of one functional category, the histone acetyltransferases. Despite the diversity of substrate specificities, members of the GNAT superfamily demonstrate remarkable similarity in protein topology and mode of acetyl coenzyme A binding, likely reflecting a conserved catalytic mechanism.

Dynamin is an atypical GTPase that catalyzes membrane fission during clathrin-mediated endocytosis. The mechanisms of dynamin’s basal and assembly-stimulated GTP hydrolysis are unknown, though both are indirectly influenced by the GTPase effector domain (GED). Here we present the 2.0Å resolution crystal structure of a minimal GTPase-GED fusion protein (GG) constructed from human dynamin 1, which has dimerized in the presence of the transition state mimic GDP.AlF4−. The structure reveals dynamin’s catalytic machinery and explains how assembly-stimulated GTP hydrolysis is achieved through G domain dimerization. A sodium ion present in the active site suggests that dynamin uses a cation to compensate for the developing negative charge in the transition state in the absence of an arginine finger. Structural comparison to the rat dynamin G domain reveals key conformational changes that promote G domain dimerization and stimulated hydrolysis. The structure of the GG dimer provides new insight into the mechanisms underlying dynamin-catalyzed membrane fission.

HIV-1 integrase is an essential enzyme in the life cycle of the virus, responsible for catalyzing the insertion of the viral genome into the host cell chromosome; it provides an attractive target for antiviral drug design. The previously reported crystal structure of the HIV-1 integrase core domain revealed that this domain belongs to the superfamily of polynucleotidyltransferases. However, the position of the conserved catalytic carboxylic acids differed from those observed in other enzymes of the class, and attempts to crystallize in the presence of the cofactor, Mg 2؉ , were unsuccessful. We report here three additional crystal structures of the core domain of HIV-1 integrase mutants, crystallized in the presence and absence of cacodylate, as well as complexed with Mg 2؉ . These three crystal forms, containing between them seven independent core domain structures, demonstrate the unambiguous extension of the previously disordered helix ␣4 toward the amino terminus from residue M154 and show that the catalytic E152 points in the general direction of the two catalytic aspartates, D64 and D116. In the vicinity of the active site, the structure of the protein in the absence of cacodylate exhibits significant deviations from the previously reported structures. These differences can be attributed to the modification of C65 and C130 by cacodylate, which was an essential component of the original crystallization mixture. We also demonstrate that in the absence of cacodylate this protein will bind to Mg 2؉ , and could provide a satisfactory platform for binding of inhibitors.

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Serotonin N-acetyltransferase (AANAT) controls the daily rhythm in melatonin synthesis. When isolated from tissue, AANAT copurifies with isoforms epsilon and zeta of 14-3-3. We have determined the structure of AANAT bound to 14-3-3zeta, an association that is phosphorylation dependent. AANAT is bound in the central channel of the 14-3-3zeta dimer, and is held in place by extensive interactions both with the amphipathic phosphopeptide binding groove of 14-3-3zeta and with other parts of the central channel. Thermodynamic and activity measurements, together with crystallographic analysis, indicate that binding of AANAT by 14-3-3zeta modulates AANAT's activity and affinity for its substrates by stabilizing a region of AANAT involved in substrate binding.

Summary The GTPase dynamin catalyzes membrane fission. Though this process requires dynamin assembly, G domain dimerization and stimulated GTP hydrolysis, the underlying structural interactions and conformational changes remain a mystery. Here we present the GMPPCP-bound structures of the truncated human dynamin 1 helical polymer at 12.2Å and a fusion protein linking human dynamin 1’s catalytic G domain to its GTPase effector domain (GG) at 2.2Å. Newly resolved density features in the polymer reconstruction and the unique conformation of GGGMPPCP allowed us to position crystallized dynamin fragments in the assembled structure and define their connectivity. The resulting model shows that G domain dimers only form between tetramers in sequential rungs of the dynamin helix. Using chemical crosslinking, we demonstrate that dynamin tetramers are dimers of domain-swapped dimers. Structural comparison of GGGMPPCP to the GG transition-state complex identifies a hydrolysis-dependent powerstroke that may play a role in membrane remodeling events necessary for fission.