High-performance microscope displays pores in the cell nucleus with greater precision

An active exchange takes place between the cell nucleus and the cytoplasm: Molecules are transported into the nucleus or from the nucleus into the cytoplasm. In a human cell, more than a million molecules are transported into the cell nucleus every minute. In the process, special pores embedded in the nucleus membrane act as transport gates. These nuclear pores are among the largest and most complex structures in the cell and comprise more than 200 individual proteins, which are arranged in a ring-like architecture. They contain a transportation channel, through which small molecules can pass unobstructed, while large molecules have to meet certain criteria to be transported. Now, for the first time, an University of Zurich research team headed by Professor Ohad Medalia has succeeded in displaying the spatial structure of the transport channel in the nuclear pores in high resolution (, "Structure and Gating of the Nuclear Pore Complex"). Nuclear Pores The nuclear pore complex is comprised of several layered rings: the cytoplasmic ring (gold), the spoke ring within the pore (blue) and the nucleoplasmic ring (green). (Image: University of Zurich) (click on image to enlarge) "Molecular gate" discovered in the pore channel For their study, the scientists used shock-frozen specimens of clawed frog oocytes. With the aid of cryo-electron microscopes, Medalia's team was able to display the miniscule nuclear pores, which were merely a ten thousandth of a millimeter in diameter, at a considerably higher resolution than ever before. As a result, they uncovered new details: "We discovered a previously unobserved structure inside the nuclear pore that forms a kind of molecular gate, which can only be opened by molecules that hold the right key," explains Medalia. This "molecular gate" is the so-called spoke ring, which is sandwiched between two other rings and extends inside the nuclear pores. The gate itself consists of a fine lattice, which enables small molecules to slip through unobstructed. The new, high-resolution presentation of the nuclear pore structure leads to a better understanding of why certain molecules are allowed to pass through the nuclear pores while others are turned away. It also helps improve our understanding of the development of some diseases that involve a defective transportation to the nuclear pores - such as intestinal, ovarian and thyroid cancer.
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The quantum spin Hall effect is also a fundamental property of light

Photons have neither mass nor charge, and so behave very differently from their massive counterparts, but they do share a property, called spin, which results in remarkable geometric and topological phenomena. The spin—a measure of the intrinsic angular momentum—can be thought of as an equivalent of the spin of a top. In the research published in ("Quantum spin Hall effect of light"), the team found that photons share with electrons a property related to spin—the quantum spin Hall effect. "We had previously done work looking at evanescent electromagnetic waves," says Konstantin Bliokh, who led the research, "and we realized the remarkable properties we found, an unusual transverse spin—was a manifestation of the fact that free-space light exhibits an intrinsic quantum spin Hall effect, meaning that evanescent waves with opposite spins will travel in opposite directions along an interface between two media." Evanescent waves propagate along the surface of materials, such as metals, at the interface with a vacuum, in the same way that ocean waves emerge at the interface between the air and the water, and they decay exponentially as they move away from the interface. The quantum spin Hall effect for electrons allows for the existence of an unusual type of material—called a topological insulator—which conducts electricity on the surface but not through the bulk of the material. The team was intrigued to learn that an analogy for these can be found for photons. Though light does not propagate through metals, it is known that it can propagate along interfaces between a metal and vacuum, in the form of so-called surface plasmons involving evanescent light waves. The group was able to show that the unusual transverse spin they found in evanescent waves was actually caused by the intrinsic quantum Hall effect of photons, and their findings also explain recent experiments that have shown spin-controlled unidirectional propagation of surface optical modes. Bliokh continues, "On a purely scientific level, this research deepens our understanding of the classical theory of light waves developed by James Clark Maxwell 150 years ago, and it could also lead to applications using optical devices that are based on the direction of spin." Franco Nori, who organized the project, says, "This work was made possible by the interdisciplinary nature of RIKEN, as we were able to bring together discoveries made in several different areas, to show that transverse spin, locked to the direction of propagation of waves, seems to be a universal feature of surface waves, even when they are of different nature."
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Interfering light waves produce unexpected forces

Few physical systems are better understood than the interference of two planar waves—like ripples on a pond. Proving that there are still secrets to be discovered even in such fundamentally well-known systems, RIKEN researchers Konstantin Bliokh, Aleksandr Bekshaev and Franco Nori have used theory to reveal a new, hidden force in this system that acts on particles in an unexpected way ("Transverse Spin and Momentum in Two-Wave Interference"). Two interfering plane waves (yellow arrows) exert a force (red) and torque (blue) on a small particle (yellow sphere) perpendicular to the interfering waves Figure 1: Two interfering plane waves (yellow arrows) exert a force (red) and torque (blue) on a small particle (yellow sphere) perpendicular to the interfering waves. (Licensed under CC BY 3.0 © 2014 A. Y. Bekshaev et al.) Two-dimensional waves have been studied for centuries: initially to understand the intrinsic behavior of waves and more recently to understand the fundamental mechanics of quantum physics. “The interference between two plane waves has always provided an important model for understanding the basic features of waves,” notes Bliokh. “It is difficult to find a simpler and more thoroughly studied system in physics. We show that such a basic system still exhibits unexpected and unusual features.” Recent research has showed that interfering planar waves can have unusual properties on a small scale. For over a century, waves such as light beams have been known to carry both momentum and angular momentum in the direction of the propagating wave and this momentum can be used to move and rotate small particles. This is consistent with the common understanding of photons as particles carrying momentum and spin. On the local scale in non-plane-wave optical fields, however, light can also impart forces and torques perpendicular to the light beam, counterintuitive to our everyday experience. These unusual effects have been noticed in highly confined near-field radiation known as evanescent waves, but so far they have not turned up in freely propagating light waves. In a comprehensive theoretical study, the scientists, from the RIKEN Center for Emergent Matter Science and Interdisciplinary Theoretical Science Research Group (iTHES), revisited the concept of two propagating waves interfering in the same plane. Their mathematical analysis of this system revealed that even this well-studied example of interfering waves can exert a force and torque on a small particle perpendicular to both waves (Fig. 1). Both the force and torque are strongly dependent on the polarization of the two interfering waves, which differs to the conventional experience of waves carrying the same momentum irrespective of their polarizations. The possibility of realizing such an effect in an actual experimental system and to potentially control it through parameters such as polarization is attractive and, Nori predicts, practically feasible. “Our findings offer a new vision for the fundamental properties of propagating optical fields and pave the way for novel optical manipulations of small particles.”
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