1.A.63 The Ignicoccus Outer Membrane α-helical Porin (I-OMP) Family

The membrane protein Imp1227 (Ignicoccus outer membrane protein; Imp1227) is the main protein constituent of the unique outer sheath of the hyperthermophilic, chemolithoautotrophic Archaeum Ignicoccus hospitalis. This outer sheath is the so far only known example for an asymmetric bilayer among the Archaea and is named 'outer membrane'. With its molecular mass of only 6.23 kDa (85 aas), Imp1227 is found to be incorporated into the outer membrane in the form of large, stable complexes. When separated by SDS-PAGE, they exhibit apparent masses of about 150, 50, 45 and 35 kDa. Dissociation into the monomeric form is achieved by treatment with SDS-containing solutions at temperatures at or above 113 degrees C. Electron micrographs of negatively stained samples confirmed that isolated membranes are tightly packed with round complexes, about 7 nm in diameter, with a central, stain-filled 2 nm pore; a local two-dimensional crystalline arrangement in the form of small patches can be detected by tomographic reconstruction (Burghardt et al., 2007). The comparison of the nucleotide and amino acid sequence of Imp1227 with public databases showed no reliable similarities with known proteins. Using secondary structure prediction and molecular modelling, a single α-helical hydrophobic transmembrane segment (>20 aas in length) is present; for the oligomer, a ring-shaped nonamer with a central 2 nm pore is a likely arrangement.

The proposed transport reaction catalyzed by Imp1227 is:

small molecules (out) ⇌ small molecules (in)