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Single-molecule fluorescence techniques 1 , 2 , 3 are key for a number of applications, including DNA sequencing 4 , 5 , molecular and cell biology 6 , 7 and early diagnosis 8 . Unfortunately, observation of single molecules by diffraction-limited optics is restricted to detection volumes in the femtolitre range and requires pico- or nanomolar concentrations, far below the micromolar range where most biological reactions occur 2 . This limitation can be overcome using plasmonic nanostructures, which enable the confinement of light down to nanoscale volumes 9 , 10 , 11 , 12 , 13 . Although these nanoantennas enhance fluorescence brightness 14 , 15 , 16 , 17 , 18 , 19 , 20 , large background signals 20 , 21 , 22 and/or unspecific binding to the metallic surface 23 , 24 , 25 have hampered the detection of individual fluorescent molecules in solution at high concentrations. Here we introduce a novel ‘antenna-in-box’ platform that is based on a gap-antenna inside a nanoaperture. This design combines fluorescent signal enhancement and background screening, offering high single-molecule sensitivity (fluorescence enhancement up to 1,100-fold and microsecond transit times) at micromolar sample concentrations and zeptolitre-range detection volumes. The antenna-in-box device can be optimized for single-molecule fluorescence studies at physiologically relevant concentrations, as we demonstrate using various biomolecules. A plasmonic nanoantenna enables a thousand fold-enhanced fluorescence brightness allowing single-molecule analysis to be carried out in a zeptolitre volume at physiological concentrations.

In cells, the curved clathrin structures in vesicle budding are well characterized, while the flat ones remain poorly understood. Here, we reconstitute the flat assembly of ESCRT-0 protein HRS and clathrin onto lipid membranes in vitro. HRS forms gel-like protein condensates at micromolar concentrations in solutions. These condensates spread as a two-dimensional layer on negatively charged membranes and, together with clathrin, form multilayered coats. Importantly, the two-dimensional condensates spontaneously form only on membranes at HRS concentrations below 50 nM, its cytoplasmic concentration. Correlative cryo-electron tomography of HRS-labelled endosomes in cells reveals a multilayered structure containing a flat clathrin layer 16 nm away from the membrane, consistent with our in vitro findings. Cholesterol enhances HRS recruitment to the membrane both in cells and in supported bilayers. Furthermore, cholesterol promotes the phase separation of HRS onto membranes, which in turn concentrates cholesterol underneath. This positive feedback promotes the formation of HRS-clathrin microdomains that sorts reconstituted ubiquitinated cargoes. Altogether, our results show that the distinct architecture of ESCRT-0 is assembled by the two-dimensional phase-separation of HRS which drives the assembly of flat clathrin coats. The ESCRT-protein HRS undergoes two-dimensional phase separation on endosomal membranes to form gel-like condensates that recruit clathrin into multilayered, flat coats. Cholesterol enhances this process and drives stable cargo-sorting microdomains

Here, we study and compare the mechanisms of action of the GroEL/GroES and the TRiC chaperonin systems on MreB client protein variants extracted from E. coli . MreB is a homologue to actin in prokaryotes. Single-molecule fluorescence correlation spectroscopy (FCS) and time-resolved fluorescence polarization anisotropy report the binding interaction of folding MreB with GroEL, GroES and TRiC. Fluorescence resonance energy transfer (FRET) measurements on MreB variants quantified molecular distance changes occurring during conformational rearrangements within folding MreB bound to chaperonins. We observed that the MreB structure is rearranged by a binding-induced expansion mechanism in TRiC, GroEL and GroES. These results are quantitatively comparable to the structural rearrangements found during the interaction of β-actin with GroEL and TRiC, indicating that the mechanism of chaperonins is conserved during evolution. The chaperonin-bound MreB is also significantly compacted after addition of AMP-PNP for both the GroEL/ES and TRiC systems. Most importantly, our results showed that GroES may act as an unfoldase by inducing a dramatic initial expansion of MreB (even more than for GroEL) implicating a role for MreB folding, allowing us to suggest a delivery mechanism for GroES to GroEL in prokaryotes.

Extracellular influx of calcium or release of calcium from intracellular stores have been shown to activate mammalian TRPA1 as well as to sensitize and desensitize TRPA1 electrophilic activation. Calcium binding sites on both intracellular N- and C-termini have been proposed. Here, we demonstrate based on fluorescence correlation spectroscopy (FCS), Forster resonance energy transfer (FRET) and bilayer patch-clamp studies, a direct calmodulin-independent action of calcium on the purified human TRPA1 (hTRPA1), causing structural changes and activation of hTRPA1 with and without its N-terminal ankyrin repeat domain (N-ARD). Thus, calcium can activate hTRPA1 by direct interaction with binding sites outside the N-ARD.

Zero-mode waveguides (ZMWs) are confining light into attoliter volumes, enabling single molecule fluorescence experiments at physiological micromolar concentrations. Among the fluorescence spectroscopy techniques that can be enhanced by ZMWs, F\"orster resonance energy transfer (FRET) is one of the most widely used in life sciences. Combining zero-mode waveguides with FRET provides new opportunities to investigate biochemical structures or follow interaction dynamics at micromolar concentration with single molecule resolution. However, prior to any quantitative FRET analysis on biological samples, it is crucial to establish first the influence of the ZMW on the FRET process. Here, we quantify the FRET rates and efficiencies between individual donor-acceptor fluorophore pairs diffusing in aluminum zero-mode waveguides. Aluminum ZMWs are important structures thanks to their commercial availability and the large literature describing their use for single molecule fluorescence spectroscopy. We also compare the results between ZMWs milled in gold and aluminum, and find that while gold has a stronger influence on the decay rates, the lower losses of aluminum in the green spectral region provide larger fluorescence brightness enhancement factors. For both aluminum and gold ZMWs, we observe that the FRET rate scales linearly with the isolated donor decay rate and the local density of optical states (LDOS). Detailed information about FRET in ZMWs unlocks their application as new devices for enhanced single molecule FRET at physiological concentrations.

Nanophotonics achieves accurate control over the luminescence properties of a single quantum emitter by tailoring the light-matter interaction at the nanoscale and modifying the local density of optical states (LDOS). This paradigm could also benefit to Förster resonance energy transfer (FRET) by enhancing the near-field electromagnetic interaction between two fluorescent emitters. Despite the wide applications of FRET in nanosciences, using nanophotonics to enhance FRET remains a debated and complex challenge. Here, we demonstrate enhanced energy transfer within single donor-acceptor fluorophore pairs confined in gold nanoapertures. Experiments monitoring both the donor and the acceptor emission photodynamics at the single molecule level clearly establish a linear dependence of the FRET rate on the LDOS in nanoapertures. These findings are applied to enhance the FRET rate in nanoapertures up to six times, demonstrating that nanophotonics can be used to intensify the near-field energy transfer and improve the biophotonic applications of FRET.

Single-molecule approaches to biology offer a powerful new vision to elucidate the mechanisms that underpin the functioning of living cells. However, conventional optical single molecule spectroscopy techniques such as Förster fluorescence resonance energy transfer (FRET) or fluorescence correlation spectroscopy (FCS) are limited by diffraction to the nanomolar concentration range, far below the physiological micromolar concentration range where most biological reaction occur. To breach the diffraction limit, zero mode waveguides and plasmonic antennas exploit the surface plasmon resonances to confine and enhance light down to the nanometre scale. The ability of plasmonics to achieve extreme light concentration unlocks an enormous potential to enhance fluorescence detection, FRET and FCS. Single molecule spectroscopy techniques greatly benefit from zero mode waveguides and plasmonic antennas to enter a new dimension of molecular concentration reaching physiological conditions. The application of nano-optics to biological problems with FRET and FCS is an emerging and exciting field, and is promising to reveal new insights on biological functions and dynamics.

Background pRb2/p130 plays a key role in cell proliferation and is a considerable progress about expression patterns of pRb2/p130 in number of malignancies. However, pRb2/p130 expression and its significance in rectal cancer remain unknown. The purpose of the present study was to investigate pRb2/p130 protein patterns and their correlations with clinicopathological and biological factors in rectal cancer patients with or without preoperative radiotherapy (RT). Patient/Methods pRb2/p130 protein was examined by immunohistochemistry in 130 primary tumors, along with the corresponding 61 distant normal mucosa specimens, 85 adjacent normal mucosa specimens, 34 lymph node metastases, and 93 primary tumor biopsies from rectal cancer patients who participated in a Swedish clinical trial of preoperative RT. Results The pRb2/p130 protein was mainly localized in the cytoplasm of tumor cells. In nonradiated cases, the lack of pRb2/p130 was related to advanced tumor-node-metastases stage, poorer differentiation, weak fibrosis, less inflammatory infiltration, higher Ki-67, and positive Cox-2 expression (p < 0.05). In radiated cases, the lack of pRb2/p130 was related to nonstaining of Cox-2 and survivin (p < 0.05). pRb2/p130 protein in primary tumors tended to be increased after RT (27% vs 16%, p = 0.07). Conclusion pRb2/p130 was mainly localized in the cytoplasm rather than in the nucleus in rectal cancer. After RT, pRb2/p130 protein seems to be increased in primary tumors, and further the relationship of the pRb2/p130 with the clinicopathological and biological variables changed compared to the nonradiated cases. However, we did not find that the pRb2/p130 was directly related to RT, tumor recurrence, and patients' survival.

Amongst the different clathrin structures in mammalian cells, bi-layered clathrin coat colocalizing with endosomal sorting complex required for transport (ESCRT)-0 remains one of the most ambiguous. Despite being observed for the first time twenty years ago, their structure and how they are assembled remains unknown. Here, we reconstituted in vitro the ESCRT-0 clathrin assembly onto various types of membranes. The ESCRT-0 protein HRS, a known clathrin adaptor on endosomes, was found to form protein condensates. These condensates spread into a thin layer on PI(3)P-rich membranes. Platinum replica electron microscopy revealed that, surprisingly, the assembly of clathrin was different depending on the HRS phase. Protein droplets recruited clathrin as a dense, curved lattice, with many cage-like structures. On two-dimensional condensates, HRS recruited clathrin as a dense flat assembly. Two-dimensional HRS-clathrin condensates promoted the clustering of cholesterol in the underlying membrane, while cholesterol enhanced PI(3)P- dependent HRS recruitment on the membrane. On free-standing membranes, two-dimensional HRS-clathrin condensates promoted membrane flattening. Overall, these results show that a two- dimensional HRS condensate creates a unique membrane structure for sorting cargo molecules, defining a new mechanism in membrane trafficking processes.

In cells, the curved clathrin structures in vesicle budding are well characterized, while the flat ones remain poorly understood. We reconstituted the flat assembly of ESCRT-0 protein HRS and clathrin onto lipid membranes in vitro. HRS was found to form protein condensates. These condensates spread as a two-dimensional layer on negatively charged membranes and promoted the assembly of clathrin into a flat coat. Correlative cryo-tomography of HRS-labeled endosomes revealed a pure hexagonal lattice, consistent with flat clathrin structures. Cholesterol enhanced HRS recruitment to the membrane both in cells and in supported bilayers. Furthermore, cholesterol promoted the phase separation of HRS onto membranes, which in turn concentrated cholesterol underneath. This positive feedback promoted the formation of HRS-clathrin microdomains that sorted reconstituted ubiquitinated cargoes. Altogether, our results show that the unique architecture of ESCRT-0 is assembled by the two-dimensional phase-separation of HRS which drives the assembly of flat clathrin coats.Competing Interest StatementThe authors have declared no competing interest.Footnotes* The manuscript was updated with high-speed atomic force microscopy data and subtomogram averaging of in situ cryo-tomography data. Figures and the main text were updated to include the new data.