Omp85Tt from Thermus thermophilus HB27:: an ancestral type of the omp85 protein family

Proteins belonging to the Omp85 family are involved in the assembly of P-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 Omp85Tt existed mainly as a monomer, composed of two stable protease-resistant modules. Circular dichroism spectroscopy indicated predominantly R-sheet secondary structure. Electron microscopy of negatively stained lipid-embedded Omp85Tt revealed ring-like structures with a central cavity of similar to 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 similar to 0.4 nS and similar to 0.65 nS, respectively.