Due to its favorable spectroscopic properties, Cd2+ is frequently used as a probe of Ca2+ sites in proteins. We investigate the ability of Cd2+ to act as a structural and functional surrogate of Ca2+ in protein–membrane interactions. C2 domain from protein kinase Cα (C2α) was chosen as a paradigm for the Ca2+-dependent phosphatidylserine-binding peripheral membrane domains. We identified the Cd2+-binding sites of C2α using NMR spectroscopy, determined the 1.6 Å crystal structure of Cd2+-bound C2α, and characterized metal-ion-dependent interactions between C2α and phospholipid membranes using fluorescence spectroscopy and ultracentrifugation experiments. We show that Cd2+ forms a tight complex with the membrane-binding loops of C2α but is unable to support its membrane-binding function. This is in sharp contrast with Pb2+, which is almost as effective as Ca2+ in driving the C2α-membrane association process. Our results provide the first direct evidence for the specific role of divalent metal ions in mediating protein–membrane interactions, have important implications for metal substitution studies in proteins, and illustrate the potential diversity of functional responses caused by toxic metal ions.

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Mn4+, Eu3+ co-doped magnesium lanthanum potassium tungstate (KLaMgWO6) phosphors have been prepared by a high-temperature solid-state reaction method. Partial substitution of K+ sites with La3+ and Li+ in the A site of KLaMgWO6: Mn4+ results in 9.7 times and 7.4 times increase in the intensity of Mn4+ emission at 697 nm under blue light excitation, respectively. Strong emission of Eu3+ (613 nm) is observed in the Mn4+, Eu3+ co-doped K0.3La1.233MgWO6 without weakening the emission of Mn4+, and the total emission intensity of the Mn4+, Eu3+ co-doped sample is enhanced by about 40% as compared with that of K0.3La1.233MgWO6:0.006Mn4+. More importantly, the fluorescence intensity ratio (I Mn/I Eu) in the Mn4+, Eu3+ co-doped K0.3La1.233MgWO6 phosphor increases monotonously not only with temperature but also with excitation wavelength within a certain range. The unique characteristics of the Mn4+, Eu3+ co-doped K0.3La1.233MgWO6 phosphor might provide a lot of possible application including plant growth lighting, optical temperature sensing, and wavelength detection.

Chloroplasts are essential plant organelles that divide by binary fission through a coordinated ring-shaped division machinery located both on the outside and inside of the chloroplast. The first step in chloroplast division is the assembly of an internal division ring (Z-ring) that is composed of the key filamentous chloroplast division proteins FtsZ1 and FtsZ2. How the individual FtsZ filaments assemble into higher-order structures to form the dividing Z-ring is not well understood and the most detailed insights have so far been gleaned from prokaryotic FtsZ. Here, we present in situ data of chloroplast FtsZ making use of a smaller ring-like FtsZ assembly termed mini-rings that form under well-defined conditions. Structured illumination microscopy (SIM) permitted their mean diameter to be determined as 208 nm and also showed that 68 % of these rings are terminally attached to linear FtsZ filaments. A correlative microscopy-compatible specimen preparation based on freeze substitution after high-pressure freezing is presented addressing the challenges such as autofluorescence and specific fluorescence attenuation. Transmission electron microscopy (TEM) and scanning TEM (STEM) imaging of thin sections exhibited ring-like densities that matched in size with the SIM data, and TEM tomography revealed insights into the molecular architecture of mini-rings demonstrating the following key features: (1) overall, a roughly bipartite split into a more ordered/curved and less ordered/curved half is readily discernible; (2) the density distribution in individual strands matches with the X-ray data, suggesting they constitute FtsZ protofilaments; (3) in the less ordered half of the ring, the protofilaments are able to assemble into higher-order structures such as double helices and supercoiled structures. Taken together, the data suggest that the state of existence of mini-rings could be described as metastable and their possible involvement in filament storage and Z-ring assembly is discussed.