Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM). The performance of these devices is currently limited by the modest (< approximately 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to approximately 220% at room temperature and approximately 300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of approximately 85%, which rivals that previously observed only using half-metallic ferromagnets. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 degrees C) will accelerate the development of new families of spintronic devices.

SUMMARY AAA+ unfoldases denature and translocate polypeptides into associated peptidases. We report direct observations of mechanical, force-induced protein unfolding by the ClpX unfoldase from E. coli, alone, and in complex with the ClpP peptidase. ClpX hydrolyzes ATP to generate mechanical force and translocate polypeptides through its central pore. Threading is interrupted by pauses that are found to be off the main translocation pathway. ClpX’s translocation velocity is force dependent, reaching a maximum of 80 aa/s near-zero force and vanishing at around 20 pN. ClpX takes 1, 2, or 3 nm steps, suggesting a fundamental step-size of 1 nm and a certain degree of intersubunit coordination. When ClpX encounters a folded protein, it either overcomes this mechanical barrier or slips on the polypeptide before making another unfolding attempt. Binding of ClpP decreases the slip probability and enhances the unfolding efficiency of ClpX. Under the action of ClpXP, GFP unravels cooperatively via a transient intermediate.

Proteins are synthesized by the ribosome and generally must fold to become functionally active. Although it is commonly assumed that the ribosome affects the folding process, this idea has been extremely difficult to demonstrate. We have developed an experimental system to investigate the folding of single ribosome-bound stalled nascent polypeptides with optical tweezers. In T4 lysozyme, synthesized in a reconstituted in vitro translation system, the ribosome slows the formation of stable tertiary interactions and the attainment of the native state relative to the free protein. Incomplete T4 lysozyme polypeptides misfold and aggregate when free in solution, but they remain folding-competent near the ribosomal surface. Altogether, our results suggest that the ribosome not only decodes the genetic information and synthesizes polypeptides, but also promotes efficient de novo attainment of the native state.

Protein synthesis rates can affect gene expression and the folding and activity of the translation product. Interactions between the nascent polypeptide and the ribosome exit tunnel represent one mode of regulating synthesis rates. The SecM protein arrests its own translation, and release of arrest at the translocon has been proposed to occur by mechanical force. Using optical tweezers, we demonstrate that arrest of SecM-stalled ribosomes can indeed be rescued by force alone and that the force needed to release stalling can be generated in vivo by a nascent chain folding near the ribosome tunnel exit. We formulate a kinetic model describing how a protein can regulate its own synthesis by the force generated during folding, tuning ribosome activity to structure acquisition by a nascent polypeptide.

The contribution of co-translational chaperone functions to protein folding is poorly understood. Ribosome-associated trigger factor (TF) is the first molecular chaperone encountered by nascent polypeptides in bacteria. Here we show, using fluorescence spectroscopy to monitor TF function and structural rearrangements in real time, that TF interacts with ribosomes and translating polypeptides in a dynamic reaction cycle. Ribosome binding stabilizes TF in an open, activated conformation. Activated TF departs from the ribosome after a mean residence time of approximately 10 s, but may remain associated with the elongating nascent chain for up to 35 s, allowing entry of a new TF molecule at the ribosome docking site. The duration of nascent-chain interaction correlates with the occurrence of hydrophobic motifs in translating polypeptides, reflecting a high aggregation propensity. These findings can explain how TF prevents misfolding events during translation and may provide a paradigm for the regulation of nucleotide-independent chaperones.

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Using neutron diffraction, 170 Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb 2 Ti 2 O 7 , where the Yb 31 S 0 1͞2 ground state has planar anisotropy. Below ϳ0.24 K, the temperature of the known specific-heat l transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb 31 spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime. DOI: 10.1103/PhysRevLett.88.077204 PACS numbers: 75.40. -s, 75.25. +z, 76.75. +i, 76.80. +y Geometrically derived magnetic frustration arises when the spatial arrangement of the spins is such that it prevents the simultaneous minimization of all interaction energies [1 -4]. In the crystallographically ordered pyrochlore structure compounds R 2 Ti 2 O 7 , the rare earth ions ͑R͒ form a sublattice of corner sharing tetrahedra and a number of these compounds have been observed to exhibit frustration related behavior [5][6][7][8][9][10][11][12]. The low temperature magnetic behavior associated with a particular rare earth depends on the properties of the crystal field ground state and on the origin (exchange/dipole), size, and sign of the interionic interactions. For example, the recently identified spin-ice configuration [6,10,11] has been linked with an Ising-like anisotropy and a net ferromagnetic interaction. Most of the published work on the pyrochlores have concerned rare earth ions with Ising-like characteristics [6 -12] and there has also been some interest in the properties of weakly anisotropic Gd 31 [5]. Less attention has been paid to the case, considered here, where the ion has planar anisotropy.To date, in systems where geometrical frustration may be present, two different low temperature magnetic ground states have been considered. First, under the influence of the frustration the system does not experience a magnetic phase transition and remains in a collective paramagnetic state with the spin fluctuations persisting as T ! 0 [3,4,[6][7][8][9][10][11]13]. Second, a long range ordered state is reached through a phase transition which may be first order [5,14,15]. Our results for Yb 2 Ti 2 O 7 , obtained using neutron diffraction, 170 Yb Mössbauer spectroscopy, and muon spin relaxation (mSR), evidence a novel scenario: there is a first-order transition which does not correspond to a transition from a paramagnetic state to a (long or short range) magnetically ordered state. The transition chiefly concerns the time domain, and involves an abrupt slowing down of the dynamics of short range correlated spins; below the transition temperature, these spins continue to fluctuate at a temperature independent rate.We have established the background magnetic characteristics for Yb 2 Ti 2 O 7 in a separate study [16,17]. The Yb 31 ion crystal field ground state is a very well isolated Kramers doubl...

Highlights d How the ribosome modulates nascent chain folding switches during elongation d Sequential domain-wise folding reduces misfolding d Co-translational folding can be reversed by an unexpected unfolding pathway d Protection of folded domains is an unanticipated chaperone function