The Moraxella IgD-binding protein MID/Hag is an oligomeric autotransporter

The immunoglobulin D (IgD)-binding protein MID/Hag of the human respiratory pathogen Moraxella catarrhalis is an outer membrane protein of approximately 200kDa belonging to the autotransporter family. MID also functions as an adhesin and hemagglutinin. In the present paper, the ultrastructure of MID was mapped. Using a series of Escherichia coli transformants, the last 210 aa of the C-terminal region were shown to translocate protein MID through the outer membrane suggesting that MID has a beta-barrel structure comprising of 10 transmembrane beta-sheets. Electron microscopy mapping with gold-labelled specific antibodies, and partial unravelling using guanidine hydrochloride showed that the rest of the MID protein forms an approximately 120nm long, fibrillar structure in which the individual monomers fold back on themselves to expose a globular distal domain at their tips comprising both the IgD-binding (MID962-1200) and adhesive (MID764-913) regions. This positions their N-termini close to the C-terminal membrane spanning domains. Mass measurements by scanning transmission electron microscopy (STEM) verified that the MID molecule is an oligomer.     

Omp85(Tt) from Thermus thermophilus HB27: an ancestral type of the Omp85 protein family

Proteins belonging to the Omp85 family are involved in the assembly of beta-barrel outer membrane proteins or in the translocation of proteins across the outer membrane in bacteria, mitochondria, and chloroplasts. The cell envelope of the thermophilic bacterium Thermus thermophilus HB27 is multilayered, including an outer membrane that is not well characterized. Neither the precise lipid composition nor much about integral membrane proteins is known. The genome of HB27 encodes one Omp85-like protein, Omp85(Tt), representing an ancestral type of this family. We overexpressed Omp85(Tt) in T. thermophilus and purified it from the native outer membranes. In the presence of detergent, purified Omp85(Tt) existed mainly as a monomer, composed of two stable protease-resistant modules. Circular dichroism spectroscopy indicated predominantly beta-sheet secondary structure. Electron microscopy of negatively stained lipid-embedded Omp85(Tt) revealed ring-like structures with a central cavity of approximately 1.5 nm in diameter. Single-channel conductance recordings indicated that Omp85(Tt) forms ion channels with two different conducting states, characterized by conductances of approximately 0.4 nS and approximately 0.65 nS, respectively.     

Photosynthetic Membranes of Synechocystis or Plants Convert Sunlight to Photocurrent through Different Pathways due to Different Architectures

Thylakoid membranes contain the redox active complexes catalyzing the light-dependent reactions of photosynthesis in cyanobacteria, algae and plants. Crude thylakoid membranes or purified photosystems from different organisms have previously been utilized for generation of electrical power and/or fuels. Here we investigate the electron transferability from thylakoid preparations from plants or the cyanobacterium Synechocystis. We show that upon illumination, crude Synechocystis thylakoids can reduce cytochrome c. In addition, this crude preparation can transfer electrons to a graphite electrode, producing an unmediated photocurrent of 15 μA/cm². Photocurrent could be obtained in the presence of the PSII inhibitor DCMU, indicating that the source of electrons is Q_A, the primary Photosystem II acceptor. In contrast, thylakoids purified from plants could not reduce cyt c, nor produced a photocurrent in the photocell in the presence of DCMU. The production of significant photocurrent (100 μA/cm²) from plant thylakoids required the addition of the soluble electron mediator DCBQ. Furthermore, we demonstrate that use of crude thylakoids from the D1-K238E mutant in Synechocystis resulted in improved electron transferability, increasing the direct photocurrent to 35 μA/cm². Applying the analogous mutation to tobacco plants did not achieve an equivalent effect. While electron abstraction from crude thylakoids of cyanobacteria or plants is feasible, we conclude that the site of the abstraction of the electrons from the thylakoids, the architecture of the thylakoid preparations influence the site of the electron abstraction, as well as the transfer pathway to the electrode. This dictates the use of different strategies for production of sustainable electrical current from photosynthetic thylakoid membranes of cyanobacteria or higher plants.     

Occurrence and Ultrastructure of a Variant (rho) Form of Mycoplasma

The ultrastructure of a variant (rho) form of Mycoplasma is described. The rho-forms are characterized by dark-ground light microscopy as relatively rigid, unbranched, filamentous organisms with discoidal swellings, and by electron microscopy by the presence of an intracytoplasmic axial fiber extending throughout the length of the cell and associated with a terminal structure of characteristic appearance. In negatively stained preparations the fiber presents a pattern of transverse light and dark major bands, the dark band being divided by a central minor light band. The periodicity of the banding varies from 12.0 to 14.5 nm, and the width of the fiber varies from 40 to 120 nm. The fiber appears to be composed of fibrils aligned parallel to the long axis. The evidence indicates that the fiber contains protein and is devoid of nucleic acid. rho-Forms were commonly found in Mycoplasma strains derived from goats and occasionally in bovine strains. They may have a wider distribution, as the growth medium was shown to be important both for the expression of the rho-character and for the selection of the rho-variant. The functional significance, if any, of the fiber and the terminal structure is unknown.