Dust release from cold ring particles as a mechanism of spoke formation in Saturn's rings

Spokes in Saturn's rings are radially-extended structures consisting of dust grains. Although spacecraft and space telescope observations have revealed various detailed features of the spokes and their time variation, their formation mechanism is still under debate. Previous models examined charging mechanisms to attempt at explaining dust release from cm-sized ring particles; however, the attempt has been unsuccessful, because the electrostatic force caused by such charging mechanisms is much weaker than the cohesive force acting on dust grains at ordinary conditions in the ring environment. Here we propose a novel model for the formation of the spokes, where the temperature dependence of cohesion plays an essential role. Ring particles with a temperature below 60K adsorb an O2 ring atmosphere, which facilitates release of dust grains from them by a reduction in the cohesive force between the grains and the particles on the morning ansa. Then, intense electrostatic forces sufficient to overcome the cohesive force are generated on the surface of ring particles and the released dust grains form the structure of spokes. Our model explains observational features of the spokes including their longitudinal location, lifetime, radial expansion velocity, and seasonality.Comment: 37 pages, 7 figure

The hierarchical structure of the perichromosomal layer comprises Ki67, ribosomal RNAs, and nucleolar proteins

The perichromosomal layer (PCL) is a structure that surrounds mitotic chromosomes, found in both animal and plant cells. It comprises various proteins and RNAs, mainly derived from the nucleolus. Several functions for the PCL have been suggested; however, the mechanism of PCL organization during mitosis remains unclear. The localization of several nucleolar proteins to the PCL is reportedly dependent on pre-ribosomal RNAs and the marker of proliferation, Ki67, which is a major PCL-localized protein. Here we demonstrate that, although the removal of pre-ribosomal RNAs from the PCL causes PCL delocalization of several nucleolar proteins, it does not affect the localization of Ki67. Conversely, Ki67 depletion results in the dissociation of both pre-ribosomal RNAs and nucleolar proteins from the PCL, which indicates that Ki67 is required for the PCL accumulation of pre-ribosomal RNAs, to which several nucleolar proteins are associated. Given these findings, we propose a model for PCL organization that comprises three essential layers: the scaffolding protein Ki67, pre-ribosomal RNAs for linkage, and outer nucleolar proteins

Construction and Piezoelectric Properties of a Single-Peptide Nanotube Composed of Cyclic β-peptides with Helical Peptides on the Side Chains

To develop nanopiezoelectronics, it is necessary to investigate the relationship between the sizes and piezoelectric properties of the material. Peptide nanotubes (PNTs) composed of cyclic β-peptides have been studied as leading candidates for nanopiezoelectric materials. The current drawback of PNTs is aggregation to form a PNT bundle structure due to strong dipole–dipole interactions between PNTs. Here, we report the construction and piezoelectric properties of single PNTs without nonspecific aggregation by side-chain modification of helical peptides. A cyclic tri-β-peptide with a helical peptide was prepared by multiple-step liquid-phase peptide synthesis and assembled into PNTs by the vapor diffusion method. These nanotubes were characterized by polarized light microscopy and Fourier transform infrared (FTIR) spectroscopy. Additionally, atomic force microscopy (AFM) topographic images showed nanotubes with a height of 4 nm, which corresponds to the diameter of a PNT on a gold-coated mica substrate, indicating that a single PNT was prepared successfully. The converted piezoelectric response of a single PNT was determined to be 1.39 ± 0.12 pm/V. This value was consistent with that of a PNT bundle, which reveals that the piezoelectricity of PNTs is induced by deformation of their cyclic skeletons and is independent of the bundled structure. This finding not only demonstrates a new molecular design strategy to construct these smallest piezoelectric biomaterials by controlling the supramolecular hierarchical structures but also provides insights into the correlation between molecular assembly morphology and size-dependent piezoelectric properties

Parkin‐mediated ubiquitylation redistributes MITOL/March5 from mitochondria to peroxisomes

Ubiquitylation of outer mitochondrial membrane (OMM) proteins is closely related to the onset of familial Parkinson's disease. Typically, a reduction in the mitochondrial membrane potential results in Parkin‐mediated ubiquitylation of OMM proteins, which are then targeted for proteasomal and mitophagic degradation. The role of ubiquitylation of OMM proteins with non‐degradative fates, however, remains poorly understood. In this study, we find that the mitochondrial E3 ubiquitin ligase MITOL/March5 translocates from depolarized mitochondria to peroxisomes following mitophagy stimulation. This unusual redistribution is mediated by peroxins (peroxisomal biogenesis factors) Pex3/16 and requires the E3 ligase activity of Parkin, which ubiquitylates K268 in the MITOL C‐terminus, essential for p97/VCP‐dependent mitochondrial extraction of MITOL. These findings imply that ubiquitylation directs peroxisomal translocation of MITOL upon mitophagy stimulation and reveal a novel role for ubiquitin as a sorting signal that allows certain specialized proteins to escape from damaged mitochondria

Equivalent circuit analysis of a three-carrier electrolyte/electrode system

Perovskite type proton conductors are known to show non-monotonous transient responses due to non-ignorable contributions of holes and oxide ions as minor carriers. Efforts have been made to simulate the behavior of the three-carrier systems by numerical calculations1-4). In most cases, however, the calculation assumes reversible electrodes, and the results are not directly applicable for analyses of experimental results such as impedance spectra. The purpose of this study is to develop an equivalent circuit model of a three-carrier conductor as a simple but theoretically feasible tool to be used for practical analyses. In the modeling, charge carriers were assumed to be Hi•, VO••, and h•, for which the gradients of respective electrochemical potentials were taken as the driving forces in the following continuity equations, Please click Additional Files below to see the full abstract

Towards Trace-Based Array Contraction

Array contraction is a compilation optimization used to reduce the memory con-sumption, by shrinking the size of temporary arrays while preserving the correctness. The usualapproach to this problem is to perform a static analysis of the given program, creating overhead inthe compilation cycle. In this report, we take a look at exploiting execution traces of programs ofthe polyhedral model, in order to infer reduced sizes for the temporary arrays used during calcu-lations. We designed a five step process to reduce the storage requirements of a temporary arrayof a given scheduled program, in which we used an algorithm to deduce array access functionsfor which bounds are modulos of affine functions of parameters and counters of the program. Ourpreliminary results show reductions of an order of magnitude on several benchmarks examples fromthe polyhedral community.La contraction de tableau est une optimisation de compilation servant à amoindrir les coûts en mémoire, en réduisant la taille des tableaux temporaires sans en altérer l’exactitude du résultat. L’approche habituelle pour ce problème est l’analyse statique du programme, ce qui engendre plus de travail dans le cycle de compilation. Nous étudions les traces d’exécution de programmes du modèle polyédrique, afin d’en inférer des tailles réduites pour ces tableaux temporaires. Nous proposons une méthode en cinq étapes pour réaliser la contraction de tableaux sur un programme déjà ordonné, comprenant l’utilisation d’un algorithme pour déduire des fonctions d’accès aux tableaux affines. Nos résultats préliminaires comprennent des réductions d’un ordre de grandeur sur plusieurs exemples de la communauté polyédrique