A SNARE-adaptor interaction is a new mode of cargo recognition in clathrin-coated vesicles

Soluble NSF attachment protein receptors (SNAREs) are type II transmembrane proteins that have critical roles in providing the specificity and energy for transport-vesicle fusion and must therefore be correctly partitioned between vesicle and organelle membranes. Like all other cargo, SNAREs need to be sorted into the forming vesicles by direct interaction with components of the vesicles' coats. Here we characterize the molecular details governing the sorting of a SNARE into clathrin-coated vesicles, namely the direct recognition of the three-helical bundle H(abc) domain of the mouse SNARE Vti1b by the human clathrin adaptor epsinR (EPNR, also known as CLINT1). Structures of each domain and of their complex show that this interaction (dissociation constant 22 muM) is mediated by surface patches composed of approximately 15 residues each, the topographies of which are dependent on each domain's overall fold. Disruption of the interface with point mutations abolishes the interaction in vitro and causes Vti1b to become relocalized to late endosomes and lysosomes. This new class of highly specific, surface-surface interaction between the clathrin coat component and the cargo is distinct from the widely observed binding of short, linear cargo motifs by the assembly polypeptide (AP) complex and GGA adaptors and is therefore not vulnerable to competition from standard motif-containing cargoes for incorporation into clathrin-coated vesicles. We propose that conceptually similar but mechanistically different interactions will direct the post-Golgi trafficking of many SNAREs.     

Inhibition of nociceptors by TRPV1-mediated entry of impermeant sodium channel blockers

Most local anaesthetics used clinically are relatively hydrophobic molecules that gain access to their blocking site on the sodium channel by diffusing into or through the cell membrane. These anaesthetics block sodium channels and thereby the excitability of all neurons, not just sensory neurons. We tested the possibility of selectively blocking the excitability of primary sensory nociceptor (pain-sensing) neurons by introducing the charged, membrane-impermeant lidocaine derivative QX-314 through the pore of the noxious-heat-sensitive TRPV1 channel. Here we show that charged sodium-channel blockers can be targeted into nociceptors by the application of TRPV1 agonists to produce a pain-specific local anaesthesia. QX-314 applied externally had no effect on the activity of sodium channels in small sensory neurons when applied alone, but when applied in the presence of the TRPV1 agonist capsaicin, QX-314 blocked sodium channels and inhibited excitability. Inhibition by co-applied QX-314 and capsaicin was restricted to neurons expressing TRPV1. Injection of QX-314 together with capsaicin into rat hindpaws produced a long-lasting (more than 2 h) increase in mechanical and thermal nociceptive thresholds. Long-lasting decreases in pain sensitivity were also seen with regional injection of QX-314 and capsaicin near the sciatic nerve; however, in contrast to the effect of lidocaine, the application of QX-314 and capsaicin together was not accompanied by motor or tactile deficits.     

Adaptive immunity maintains occult cancer in an equilibrium state

The capacity of immunity to control and shape cancer, that is, cancer immunoediting, is the result of three processes that function either independently or in sequence: elimination (cancer immunosurveillance, in which immunity functions as an extrinsic tumour suppressor in naive hosts); equilibrium (expansion of transformed cells is held in check by immunity); and escape (tumour cell variants with dampened immunogenicity or the capacity to attenuate immune responses grow into clinically apparent cancers). Extensive experimental support now exists for the elimination and escape processes because immunodeficient mice develop more carcinogen-induced and spontaneous cancers than wild-type mice, and tumour cells from immunodeficient mice are more immunogenic than those from immunocompetent mice. In contrast, the equilibrium process was inferred largely from clinical observations, including reports of transplantation of undetected (occult) cancer from organ donor into immunosuppressed recipients. Herein we use a mouse model of primary chemical carcinogenesis and demonstrate that equilibrium occurs, is mechanistically distinguishable from elimination and escape, and that neoplastic cells in equilibrium are transformed but proliferate poorly in vivo. We also show that tumour cells in equilibrium are unedited but become edited when they spontaneously escape immune control and grow into clinically apparent tumours. These results reveal that, in addition to destroying tumour cells and sculpting tumour immunogenicity, the immune system of a naive mouse can also restrain cancer growth for extended time periods.     

First-passage times in complex scale-invariant media

How long does it take a random walker to reach a given target point? This quantity, known as a first-passage time (FPT), has led to a growing number of theoretical investigations over the past decade. The importance of FPTs originates from the crucial role played by first encounter properties in various real situations, including transport in disordered media, neuron firing dynamics, spreading of diseases or target search processes. Most methods of determining FPT properties in confining domains have been limited to effectively one-dimensional geometries, or to higher spatial dimensions only in homogeneous media. Here we develop a general theory that allows accurate evaluation of the mean FPT in complex media. Our analytical approach provides a universal scaling dependence of the mean FPT on both the volume of the confining domain and the source-target distance. The analysis is applicable to a broad range of stochastic processes characterized by length-scale-invariant properties. Our theoretical predictions are confirmed by numerical simulations for several representative models of disordered media, fractals, anomalous diffusion and scale-free networks.     

Coupling superconducting qubits via a cavity bus

Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine, and several examples of two-qubit interactions and gates have been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gate operations between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a 'quantum bus', which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip.     

Radio-frequency scanning tunnelling microscopy

The scanning tunnelling microscope (STM) relies on localized electron tunnelling between a sharp probe tip and a conducting sample to attain atomic-scale spatial resolution. In the 25-year period since its invention, the STM has helped uncover a wealth of phenomena in diverse physical systems--ranging from semiconductors to superconductors to atomic and molecular nanosystems. A severe limitation in scanning tunnelling microscopy is the low temporal resolution, originating from the diminished high-frequency response of the tunnel current readout circuitry. Here we overcome this limitation by measuring the reflection from a resonant inductor-capacitor circuit in which the tunnel junction is embedded, and demonstrate electronic bandwidths as high as 10 MHz. This approximately 100-fold bandwidth improvement on the state of the art translates into fast surface topography as well as delicate measurements in mesoscopic electronics and mechanics. Broadband noise measurements across the tunnel junction using this radio-frequency STM have allowed us to perform thermometry at the nanometre scale. Furthermore, we have detected high-frequency mechanical motion with a sensitivity approaching approximately 15 fm Hz(-1/2). This sensitivity is on par with the highest available from nanoscale optical and electrical displacement detection techniques, and the radio-frequency STM is expected to be capable of quantum-limited position measurements.     

Drosophila TCTP is essential for growth and proliferation through regulation of dRheb GTPase

Cellular growth and proliferation are coordinated during organogenesis. Misregulation of these processes leads to pathological conditions such as cancer. Tuberous sclerosis (TSC) is a benign tumour syndrome caused by mutations in either TSC1 or TSC2 tumour suppressor genes. Studies in Drosophila and other organisms have identified TSC signalling as a conserved pathway for growth control. Activation of the TSC pathway is mediated by Rheb (Ras homologue enriched in brain), a Ras superfamily GTPase. Rheb is a direct target of TSC2 and is negatively regulated by its GTPase-activating protein activity. However, molecules required for positive regulation of Rheb have not been identified. Here we show that a conserved protein, translationally controlled tumour protein (TCTP), is an essential new component of the TSC-Rheb pathway. Reducing Drosophila TCTP (dTCTP) levels reduces cell size, cell number and organ size, which mimics Drosophila Rheb (dRheb) mutant phenotypes. dTCTP is genetically epistatic to Tsc1 and dRheb, but acts upstream of dS6k, a downstream target of dRheb. dTCTP directly associates with dRheb and displays guanine nucleotide exchange activity with it in vivo and in vitro. Human TCTP (hTCTP) shows similar biochemical properties compared to dTCTP and can rescue dTCTP mutant phenotypes, suggesting that the function of TCTP in the TSC pathway is evolutionarily conserved. Our studies identify TCTP as a direct regulator of Rheb and a potential therapeutic target for TSC disease.