Protein connectivity in chemotaxis receptor complexes

The chemotaxis sensory system allows bacteria such as Escherichia coli to swim towards nutrients and away from repellents. The underlying pathway is remarkably sensitive in detecting chemical gradients over a wide range of ambient concentrations. Interactions among receptors, which are predominantly clustered at the cell poles, are crucial to this sensitivity. Although it has been suggested that the kinase CheA and the adapter protein CheW are integral for receptor connectivity, the exact coupling mechanism remains unclear. Here, we present a statistical-mechanics approach to model the receptor linkage mechanism itself, building on nanodisc and electron cryotomography experiments. Specifically, we investigate how the sensing behavior of mixed receptor clusters is affected by variations in the expression levels of CheA and CheW at a constant receptor density in the membrane. Our model compares favorably with dose-response curves from in vivo Förster resonance energy transfer (FRET) measurements, demonstrating that the receptor-methylation level has only minor effects on receptor cooperativity. Importantly, our model provides an explanation for the non-intuitive conclusion that the receptor cooperativity decreases with increasing levels of CheA, a core signaling protein associated with the receptors, whereas the receptor cooperativity increases with increasing levels of CheW, a key adapter protein. Finally, we propose an evolutionary advantage as explanation for the recently suggested CheW-only linker structures

The magnetic and electric transverse spin density of spatially confined light

When a beam of light is laterally confined, its field distribution can exhibit points where the local magnetic and electric field vectors spin in a plane containing the propagation direction of the electromagnetic wave. The phenomenon indicates the presence of a non-zero transverse spin density. Here, we experimentally investigate this transverse spin density of both magnetic and electric fields, occurring in highly-confined structured fields of light. Our scheme relies on the utilization of a high-refractive-index nano-particle as local field probe, exhibiting magnetic and electric dipole resonances in the visible spectral range. Because of the directional emission of dipole moments which spin around an axis parallel to a nearby dielectric interface, such a probe particle is capable of locally sensing the magnetic and electric transverse spin density of a tightly focused beam impinging under normal incidence with respect to said interface. We exploit the achieved experimental results to emphasize the difference between magnetic and electric transverse spin densities.Comment: 7 pages, 4 figure

Trends in critical infrastructure protection in Germany

Critical Infrastructures failures cause harmful consequences to the population, because they disrupt the supply of necessary goods and services. The failures pose an indirect threat, as they will regularly be triggered by natural hazards, technical failure/human error or intentional acts. In the risk analyses on the national level in Germany, Critical Infrastructure failures are qualitatively described to estimate their impacts on society. Critical Infrastructure Protection is seen as a joint task of many different stakeholders. Rules and regulations with different degrees of compulsion build the framework for their cooperation, and a strategy is in place that promotes the trustful exchange of information among all the relevant stakeholders. The most important stakeholder groups are public authorities, infrastructure operators, and the population. An example is given on how a joint risk management of public authorities and infrastructure operators may be performed, and the cooperation of public authorities and the population is discussed. As Civil Protection covers the entire risk and crisis management cycle with its phases prevention, preparedness, response and recovery, the article ends with examples of the support, which the German Federal Offi ce of Civil Protection and Disaster Assistance and the Federal Ministry of the Interior offer for other stakeholders in order to achieve well-protected infrastructures and, in consequence, well-protected citizens

Spin-Orbit Coupling and the Evolution of Transverse Spin

We investigate the evolution of transverse spin in tightly focused circularly polarized beams of light, where spin-orbit coupling causes a local rotation of the polarization ellipses upon propagation through the focal volume. The effect can be explained as a relative Gouy-phase shift between the circularly polarized transverse field and the longitudinal field carrying orbital angular momentum. The corresponding rotation of the electric transverse spin density is observed experimentally by utilizing a recently developed reconstruction scheme, which relies on transverse-spin-dependent directional scattering of a nano-probe.Comment: 4 pages, 2 figure

Peter Grzybek (1957-2019)

Obituary: Peter Grzybek (1957-2019

Peter Grzybek (1957-2019)

Obituary: Peter Grzybek (1957-2019