We studied fibril formation in a family of peptides based on PHF6 (VQIVYK), a short peptide segment found in the microtubule binding region of tau protein. N-Acetylated peptides AcVYK-amide (AcVYK), AcIVYKamide (AcPHF4), AcQIVYK-amide (AcPHF5), and AcV-QIVYK-amide (AcPHF6) rapidly formed straight filaments in the presence of 0.15 M NaCl, each composed of two laterally aligned protofilaments ϳ5 nm in width. X-ray fiber diffraction showed the omnipresent sharp 4.7-Å reflection indicating that the scattering objects are likely elongated along the hydrogen-bonding direction in a cross-␤ conformation, and Fourier transform IR suggested the peptide chains were in a parallel (AcVYK, AcPHF6) or antiparallel (AcPHF4, AcPHF5) ␤-sheet configuration. The dipeptide N-acetyl-YK-amide (AcYK) formed globular structures ϳ200 nm to 1 m in diameter. The polymerization rate, as measured by thioflavin S binding, increased with the length of the peptide going from AcYK 3 AcPHF6, and peptides that aggregated most rapidly displayed CD spectra consistent with ␤-sheet structure. There was a 3-fold decrease in rate when Val was substituted for Ile or Gln, nearly a 10-fold decrease when Ala was substituted for Tyr, and an increase in polymerization rate when Glu was substituted for Lys. Twisted filaments, composed of four laterally aligned protofilaments (9 -19 nm width, ϳ90 nm halfperiodicity), were formed by mixing AcPHF6 with AcVYK. Taken together these results suggest that the core of PHF6 is localized at VYK, and the interaction between small amphiphilic segments of tau may initiate nucleation and lead to filaments displaying paired helical filament morphology.

In eukaryotic organisms, the orthologs of the DEAD-box RNA helicase Ded1p from yeast and DDX3 from human form a well-defined subfamily that is characterized by high sequence conservation in their helicase core and their N- and C- termini. Individual members of this Ded1/DDX3 subfamily perform multiple functions in RNA metabolism in both nucleus and cytoplasm. Ded1/DDX3 subfamily members have also been implicated in cellular signaling pathways and are targeted by diverse viruses. In this review, we discuss the considerable body of work on the biochemistry and biology of these proteins, including the recently discovered link of human DDX3 to tumorigenesis.

At least nine inherited neurodegenerative diseases, including Huntington's, are caused by poly(L-glutamine) (polyGln, polyQ) expansions > 35-40 repeats in widely or ubiquitously expressed proteins. Except for their expansions, these proteins have no sequence homologies, and their functions mostly remain unknown. Although each disease is characterized by a distinct pathology specific to a subset of neuronal cells, the formation of neuronal intranuclear aggregates containing protein with an expanded polyQ is the hallmark and common feature to most polyQ disorders. The neurodegeneration is thought to be caused by a toxic gain of function that occurs at the protein level and depends on the length of the expansion: Longer repeats cause earlier age of onset and more severe symptoms. To address whether there is a structural difference between polyQ having < 40 versus > 40 residues, we undertook an X-ray fiber diffraction study of synthetic polyQ peptides having varying numbers of residues: Ac-Q8-NH2, D2Q15K2, K2Q28K2, and K2Q45K2. These particular lengths bracket both the range of normalcy (9-36 repeats) and the pathological (45 repeats), and therefore could be indicative of the structural changes expected in expanded polyQ domains. Contrary to expectations of different length-dependent morphologies, we accounted for all the X-ray patterns by slablike, beta-sheet structures, approximately 20 A thick in the beta-chain direction, all having similar monoclinic lattices. Moreover, the slab thickness indicates that K2Q45K2, rather than forming a water-filled nanotube, must form multiple reverse turns.

The presence of disulfide bonds can be detected unambiguously only by X-ray crystallography, and otherwise must be inferred by chemical methods. In this study we demonstrate that 13C NMR chemical shifts are diagnostic of disulfide bond formation, and can discriminate between cysteine in the reduced (free) and oxidized (disulfide bonded) state. A database of cysteine 13C C(alpha) and C(beta) chemical shifts was constructed from the BMRB and Sheffield databases, and published journals. Statistical analysis indicated that the C(beta) shift is extremely sensitive to the redox state, and can predict the disulfide-bonded state. Further, chemical shifts in both states occupy distinct clusters as a function of secondary structure in the C(alpha)/C(beta) chemical shift map. On the basis of these results, we provide simple ground rules for predicting the redox state of cysteines; these rules could be used effectively in NMR structure determination, predicting new folds, and in protein folding studies.

DEAD-box proteins utilize ATP to bind and remodel RNA and RNA-protein complexes. All DEAD-box proteins share a conserved core that consists of two RecA-like domains. The core is flanked by subfamily-specific extensions of idiosyncratic function. The Ded1/DDX3 subfamily of DEAD-box proteins is of particular interest as members function during protein translation, are essential for viability, and are frequently altered in human malignancies. Here, we define the function of the subfamily-specific extensions of the human DEAD-box protein DDX3. We describe the crystal structure of the subfamily-specific core of wild-type DDX3 at 2.2 Å resolution, alone and in the presence of AMP or nonhydrolyzable ATP. These structures illustrate a unique interdomain interaction between the two ATPase domains in which the C-terminal domain clashes with the RNA-binding surface. Destabilizing this interaction accelerates RNA duplex unwinding, suggesting that it is present in solution and inhibitory for catalysis. We use this core fragment of DDX3 to test the function of two recurrent medulloblastoma variants of DDX3 and find that both inactivate the protein in vitro and in vivo. Taken together, these results redefine the structural and functional core of the DDX3 subfamily of DEADbox proteins.DEAD-box proteins are ATP-dependent RNA-binding proteins that remodel RNA structures and RNA-protein complexes, stably clamp RNA, and promote fluidity within RNA granules (1-3). The human DEAD-box protein DDX3 (encoded by DDX3X) and its yeast ortholog Ded1p have been implicated in numerous functions including translation initiation (4 -12). Messenger RNA molecules containing especially long or structured 5Ј leader sequences are particularly sensitive to DDX3 activity (6,7,12,13). DDX3X is frequently mutated in numerous cancer types (5), such as chronic lymphocytic leukemia (14 -16), natural killer/T-cell lymphoma (17), head and neck squamous cell carcinoma (18,19), and lung cancer (20). DDX3X is also one of the most frequently mutated genes in the highly malignant brain tumor medulloblastoma (21-24). In medulloblastoma, many mutations are predicted to inactivate DDX3, and some have been demonstrated to diminish activities in vitro (17,25).DEAD-box proteins are defined by 12 different motifs that function in ATP binding or hydrolysis and RNA binding, or couple ATP and RNA binding (1). Outside of these conserved motifs, each DEAD-box protein subfamily has unique tails that lie N-or C-terminal to the helicase core and contain elements that define the unique properties of that subfamily. For example, DDX21 has a GUCT domain in its C-terminal extension (26, 27), DDX5 has tandem P68HR domains in its C-terminal extension, and DDX43 has a KH1 domain in its N-terminal extension. However, as the tails of each DEAD-box protein subfamily are idiosyncratic, whereas the cores are very similar (28,29), it is essential to study individual subfamilies of DEAD-box proteins in detail to understand the role of subfamily-specific tails.DDX3 is a member of the Ded1/DDX...