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"). 
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 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.
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.”