Purification and Characterization of the Photoreducible b-type Cytochrome from Dictyostelium discoideum

In Vivo Assembly of a Dictyostelium Lamin Mutant Induced by Light, Mechanical Stress, and pH

We expressed Dictyostelium lamin (NE81) lacking both a functional nuclear localization signal and a CAAX-box for C-terminal lipid modification. This lamin mutant assembled into supramolecular, three-dimensional clusters in the cytosol that disassembled at the onset of mitosis and re-assembled in late telophase, thus mimicking the behavior of the endogenous protein. As disassembly is regulated by CDK1-mediated phosphorylation at serine 122, we generated a phosphomimetic S122E mutant called GFP-NE81-S122E-ΔNLSΔCLIM. Surprisingly, during imaging, the fusion protein assembled into cytosolic clusters, similar to the protein lacking the phosphomimetic mutation. Clusters disassembled again in the darkness. Assembly could be induced with blue but not green or near ultraviolet light, and it was independent of the fusion tag. Assembly similarly occurred upon cell flattening. Earlier reports and own observations suggested that both blue light and cell flattening could result in a decrease of intracellular pH. Indeed, keeping the cells at low pH also reversibly induced cluster formation. Our results indicate that lamin assembly can be induced by various stress factors and that these are transduced via intracellular acidification. Although these effects have been shown in a phosphomimetic CDK1 mutant of the Dictyostelium lamin, they are likely relevant also for wild-type lamin.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

Structure of the Thylakoids and Envelope Membranes of the Cyanelles of Cyanophora paradoxa

The cyanelles of Cyanophora paradoxa Korsch. are photosynthetically active obligate endosymbionts in which phycobiliproteins serve as the major accessory pigments. Freeze-fracture electron micrographs of thylakoids in isolated cyanelles reveal long parallel rows of particles covering most of the E-face, while a more random particle arrangement is evident in some areas. The center-to-center spacing of particles within these rows is about 10 nanometers. Their mean diameter was measured at 9.4 nanometers. The particles on the P-face have a mean diameter of 7.2 nanometers. Thylakoids that retained nearly the full complement of phycobiliproteins (determined spectrophotometrically and by gel electrophoresis) were isolated from the cyanelles. In thin sections of these preparations, rows of disc-shaped phycobilisomes are evident on the surface of the thylakoids. The spacing of the rows of phycobilisomes corresponds to that of the rows of E-face particles (approximately 45 nanometers, center to center). The periodicity of the disc-shaped phycobilisomes within a row is 10 nanometers suggesting a one-to-one association between phycobilisomes and E-face particles.In addition, visualization of the protoplasmic surface (PS) of isolated thylakoids by freeze-etch electron microscopy shows that rows of disc-shaped phycobilisomes are aligned directly above rows of particles exhibiting two subunits, presumably the P-surface projections of the 10-nanometer intramembrane particles. These observations, together with earlier studies indicating that the 10-nanometer E-face particles probably represent photosystem II (PSII) complexes, suggest that phycobilisomes are positioned on the thylakoid surface in direct contact with PSII centers within the thylakoid membrane.The inner envelope membrane of the cyanelles, observed in freeze-fracture replicas, resembles cyanobacterial plasma membranes and is dissimilar to the chloroplast envelope membranes of red or green algae. The envelope of isolated cyanelles exhibits two additional layers: (a) a 5- to 7-nanometer-thick layer that lies adjacent to the inner membrane and which seems to correspond to the peptidoglycan layer of cyanobacteria; and (b) a layer external to the purported peptidoglycan layer that exhibits fracture faces similar to those of the lipopolysaccharide layer of gram negative bacteria. Our findings indicate that the supramolecular architecture of cyanelles differs only slightly from free-living cyanobacteria to which they are presumably related.

Light-inducible Cytochrome Reduction in Membrane Preparations from Corn Coleoptiles: I. STABILIZATION AND SPECTRAL CHARACTERIZATION OF THE REACTION

Conditions for obtaining reproducible light-induced reduction of a b-type cytochrome in membrane fractions from coleoptiles of dark-grown Zea mays L. include a glucose-glucose oxidase system that lowers O(2) tension and generates H(2)O(2), substrate amounts of ethylenediaminetetraacetic acid which, in some manner, facilitates photoreduction by both added flavin and the endogenous photoreceptor and a sample temperature below 10 C. Cytochrome reduction could be obtained by photoexcitation of either a tightly bound endogenous receptor, which is probably a flavin, or added riboflavin, flavin mononucleotide, or flavin adenine dinucleotide. The latter flavin was the least effective. The endogenous photoreceptor appears to be rather firmly bound to the membranes, suggesting that this association may also exist in vivo. When any of the above four photoreceptors or methylene blue were used to sensitize the reaction, a cytochrome with a reduced alpha-band near 560 nanometers and a Soret difference peak near 429 nanometers was the electron acceptor. This cytochrome could be clearly distinguished spectrally from other cytochromes that predominated in the membrane preparations.