MEMS-actuated terahertz metamaterials driven by phase-transition materials.

The non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO2). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO2, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems.

The GTPase activating protein Gyp7 regulates Rab7/Ypt7 activity on late endosomes.

Organelles of the endomembrane system contain Rab GTPases as identity markers. Their localization is determined by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). It remains largely unclear how these regulators are specifically targeted to organelles and how their activity is regulated. Here, we focus on the GAP Gyp7, which acts on the Rab7-like Ypt7 protein in yeast, and surprisingly observe the protein exclusively in puncta proximal to the vacuole. Mistargeting of Gyp7 to the vacuole strongly affects vacuole morphology, suggesting that endosomal localization is needed for function. In agreement, efficient endolysosomal transport requires Gyp7. In vitro assays reveal that Gyp7 requires a distinct lipid environment for membrane binding and activity. Overexpression of Gyp7 concentrates Ypt7 in late endosomes and results in resistance to rapamycin, an inhibitor of the target of rapamycin complex 1 (TORC1), suggesting that these late endosomes are signaling endosomes. We postulate that Gyp7 is part of regulatory machinery involved in late endosome function.

Structure of the metazoan Rab7 GEF complex Mon1-Ccz1-Bulli.

The endosomal system of eukaryotic cells represents a central sorting and recycling compartment linked to metabolic signaling and the regulation of cell growth. Tightly controlled activation of Rab GTPases is required to establish the different domains of endosomes and lysosomes. In metazoans, Rab7 controls endosomal maturation, autophagy, and lysosomal function. It is activated by the guanine nucleotide exchange factor (GEF) complex Mon1-Ccz1-Bulli (MCBulli) of the tri-longin domain (TLD) family. While the Mon1 and Ccz1 subunits have been shown to constitute the active site of the complex, the role of Bulli remains elusive. We here present the cryo-electron microscopy (cryo-EM) structure of MCBulli at 3.2 Å resolution. Bulli associates as a leg-like extension at the periphery of the Mon1 and Ccz1 heterodimers, consistent with earlier reports that Bulli does not impact the activity of the complex or the interactions with recruiter and substrate GTPases. While MCBulli shows structural homology to the related ciliogenesis and planar cell polarity effector (Fuzzy-Inturned-Wdpcp) complex, the interaction of the TLD core subunits Mon1-Ccz1 and Fuzzy-Inturned with Bulli and Wdpcp, respectively, is remarkably different. The variations in the overall architecture suggest divergent functions of the Bulli and Wdpcp subunits. Based on our structural analysis, Bulli likely serves as a recruitment platform for additional regulators of endolysosomal trafficking to sites of Rab7 activation.

Targeting of the Mon1-Ccz1 Rab guanine nucleotide exchange factor to distinct organelles by a synergistic protein and lipid code.

Activation of the small GTPase Rab7 by its cognate guanine nucleotide exchange factor Mon1-Ccz1 (MC1) is a key step in the maturation of endosomes and autophagosomes. This process is tightly regulated and subject to precise spatiotemporal control of MC1 localization, but the mechanisms that underly MC1 localization have not been fully elucidated. We here identify and characterize an amphipathic helix in Ccz1, which is required for the function of Mon-Ccz1 in autophagy, but not endosomal maturation. Furthermore, our data show that the interaction of the Ccz1 amphipathic helix with lipid packing defects, binding of Mon1 basic patches to positively charged lipids, and association of MC1 with recruiter proteins collectively govern membrane recruitment of the complex in a synergistic and redundant manner. Membrane binding enhances MC1 activity predominantly by increasing enzyme and substrate concentration on the membrane, but interaction with recruiter proteins can further stimulate the guanine nucleotide exchange factor. Our data demonstrate that specific protein and lipid cues convey the differential targeting of MC1 to endosomes and autophagosomes. In conclusion, we reveal the molecular basis for how MC1 is adapted to recognize distinct target compartments by exploiting the unique biophysical properties of organelle membranes and thus provide a model for how the complex is regulated and activated independently in different functional contexts.

Molecular insights into endolysosomal microcompartment formation and maintenance.

The endolysosomal system of eukaryotic cells has a key role in the homeostasis of the plasma membrane, in signaling and nutrient uptake, and is abused by viruses and pathogens for entry. Endocytosis of plasma membrane proteins results in vesicles, which fuse with the early endosome. If destined for lysosomal degradation, these proteins are packaged into intraluminal vesicles, converting an early endosome to a late endosome, which finally fuses with the lysosome. Each of these organelles has a unique membrane surface composition, which can form segmented membrane microcompartments by membrane contact sites or fission proteins. Furthermore, these organelles are in continuous exchange due to fission and fusion events. The underlying machinery, which maintains organelle identity along the pathway, is regulated by signaling processes. Here, we will focus on the Rab5 and Rab7 GTPases of early and late endosomes. As molecular switches, Rabs depend on activating guanine nucleotide exchange factors (GEFs). Over the last years, we characterized the Rab7 GEF, the Mon1-Ccz1 (MC1) complex, and key Rab7 effectors, the HOPS complex and retromer. Structural and functional analyses of these complexes lead to a molecular understanding of their function in the context of organelle biogenesis.

Structure of the Mon1-Ccz1 complex reveals molecular basis of membrane binding for Rab7 activation.

Activation of the GTPase Rab7/Ypt7 by its cognate guanine nucleotide exchange factor (GEF) Mon1-Ccz1 marks organelles such as endosomes and autophagosomes for fusion with lysosomes/vacuoles and degradation of their content. Here, we present a high-resolution cryogenic electron microscopy structure of the Mon1-Ccz1 complex that reveals its architecture in atomic detail. Mon1 and Ccz1 are arranged side by side in a pseudo-twofold symmetrical heterodimer. The three Longin domains of each Mon1 and Ccz1 are triangularly arranged, providing a strong scaffold for the catalytic center of the GEF. At the opposite side of the Ypt7-binding site, a positively charged and relatively flat patch stretches the Longin domains 2/3 of Mon1 and functions as a phosphatidylinositol phosphate-binding site, explaining how the GEF is targeted to membranes. Our work provides molecular insight into the mechanisms of endosomal Rab activation and serves as a blueprint for understanding the function of members of the Tri Longin domain Rab-GEF family.

Visible-Light Removable Photocaging Groups Accepted by MjMAT Variant: Structural Basis and Compatibility with DNA and RNA Methyltransferases.

Methylation and demethylation of DNA, RNA and proteins constitutes a major regulatory mechanism in epigenetic processes. Investigations would benefit from the ability to install photo-cleavable groups at methyltransferase target sites that block interactions with reader proteins until removed by non-damaging light in the visible spectrum. Engineered methionine adenosyltransferases (MATs) have been exploited in cascade reactions with methyltransferases (MTases) to modify biomolecules with non-natural groups, including first evidence for accepting photo-cleavable groups. We show that an engineered MAT from Methanocaldococcus jannaschii (PC-MjMAT) is 308-fold more efficient at converting ortho-nitrobenzyl-(ONB)-homocysteine than the wildtype enzyme. PC-MjMAT is active over a broad range of temperatures and compatible with MTases from mesophilic organisms. We solved the crystal structures of wildtype and PC-MjMAT in complex with AdoONB and a red-shifted derivative thereof. These structures reveal that aromatic stacking interactions within the ligands are key to accommodating the photocaging groups in PC-MjMAT. The enlargement of the binding pocket eliminates steric clashes to enable AdoMet analogue binding. Importantly, PC-MjMAT exhibits remarkable activity on methionine analogues with red-shifted ONB-derivatives enabling photo-deprotection of modified DNA by visible light.

Limitations of the transmitted photonic spin Hall effect through layered structure.

In this paper, we show theoretically that the spin-dependent transverse shift of the transmitted photonic spin Hall effect (SHE) through layered structure cannot exceed half of the incident beam waist. Exact conditions for obtaining the upper limit of the transmitted SHE are clarified in detail. In addition, different from the popular view in many investigations, we find that there is no positive correlation between the spin-dependent transverse displacement and the ratio between the Fresnel transmission coefficients (tp, ts). In contrast, the optimal transmission ratio is determined by the incident angle and the beam waist. Moreover, two conventional transmission structures are selected and studied in detail. The characteristics of the transverse displacements obtained are in very good agreement with our theoretical conclusions. These findings provide a deeper insight into the photonic spin Hall phenomena and offer a guide for future related research.

A conserved and regulated mechanism drives endosomal Rab transition.

Endosomes and lysosomes harbor Rab5 and Rab7 on their surface as key proteins involved in their identity, biogenesis, and fusion. Rab activation requires a guanine nucleotide exchange factor (GEF), which is Mon1-Ccz1 for Rab7. During endosome maturation, Rab5 is replaced by Rab7, though the underlying mechanism remains poorly understood. Here, we identify the molecular determinants for Rab conversion in vivo and in vitro, and reconstitute Rab7 activation with yeast and metazoan proteins. We show (i) that Mon1-Ccz1 is an effector of Rab5, (ii) that membrane-bound Rab5 is the key factor to directly promote Mon1-Ccz1 dependent Rab7 activation and Rab7-dependent membrane fusion, and (iii) that this process is regulated in yeast by the casein kinase Yck3, which phosphorylates Mon1 and blocks Rab5 binding. Our study thus uncovers the minimal feed-forward machinery of the endosomal Rab cascade and a novel regulatory mechanism controlling this pathway.

Tunable bilateral unidirectional transmissions in a grating and photonic crystal hybrid structure.

In this work, bilateral unidirectional transmissions (UDTs) with opposite transmission directions in one hybrid structure are realized using two different resonant mechanisms. The hybrid structure consists of a dielectric grating and a one-dimensional photonic crystal (PC) with a defect sandwiched at its center. One resonant mode is the defect mode of the PC enabling one UDT for one transmission direction. The other resonant mode is the grating guided mode resonance which introduces UDT for the opposite direction. Numerical calculations demonstrate that for each UDT, its transmittance difference, transmittance contrast ratio, and isolation degree can reach 90%, 100%, and 20%, respectively. In addition, the operation wavelength of each UDT as well as the wavelength interval between the two UDTs with opposite transmission directions can be tuned easily by adjusting structural parameters. This novel bilateral UDT creates potential for applications in both free space optics and optical circuits.

Programmable field localization and enhancement effects on a non-structured planar surface with a permittivity gradient.

We demonstrate electromagnetic field localization and enhancement effects on the non-structured planar surface of a two-dimensional gradient permittivity material. Surface plasmons are excited by a normally-incident Gaussian illumination beam and are confined to subwavelength rings on the surface of the gradient permittivity material. The performance of the surface is programmable by adjusting the permittivity distribution of the material and polarization of incident light. We show that field localization and enhancement effects can be realized at mid-infrared frequencies by conventional semiconductor materials with designed doping distributions. This demonstration suggests a compact and readily accessible platform for materials characterizations with spatially controlled illumination, providing a convenient approach to explore nanospectroscopy and light-matter interactions of nanomaterials, such as quantum dots, nanowires, and organic molecules.