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)


 

References:

Burghardt, T., D.J. Näther, B. Junglas, H. Huber, and R. Rachel. (2007). The dominating outer membrane protein of the hyperthermophilic Archaeum Ignicoccus hospitalis: a novel pore-forming complex. Mol. Microbiol. 63: 166-176.
Examples:

TC#NameOrganismal TypeExample
1.A.63.1.1

The α-helical pore-forming outer membrane nanomeric porin, Imp1227 or Ihomp1, of 85 aas and one TMS. The membrane protein Imp1227 is the main protein constituent of the unique outer sheath of the hyperthermophilic, chemolithoautotrophic archaeum Ignicoccus hospitalis. With its molecular mass of only 6.23 kDa, Imp1227 is found to be incorporated into the outer membrane to form large, stable complexes. When separated by SDS-PAGE, they exhibit apparent masses of about 150, 50, 45 and 35 kDa.  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 seen by tomographic reconstruction. Using secondary structure predictions and molecular modelling, an alpha-helical transmembrane domain is proposed; for the oligomer, a ring-shaped nonamer with a central 2 nm pore was a likely arrangement.

Archaea

Imp1227 of Ignicoccus hospitalis (A8ABZ0)

 
1.A.63.1.2

Uncharacterized protein of 78 aas and 1 N-terminal TMS.

UP of Crenarchaeota archaeon

 
Examples:

TC#NameOrganismal TypeExample
1.A.63.2.1

Transmembrane DUF4845 protein with 120 aas and one TMS.

Proteobacteria

Transmembrane protein of Acidovorax sp. KKS102

 
1.A.63.2.2

Uncharacterized protein of 129 aas and 1 TMS

UP of Congregibacter litoralis

 
1.A.63.2.3

Uncharacterized protein of 130 aas and 1 TMS/

UP of Legionella pneumophila

 
1.A.63.2.4

Uncharacterized DUF4845 domain-containing protein of 192 aas and 1 N-terminal TMS.

UP of Thiobacillus sp.

 
1.A.63.2.5

Uncharacterized DUF4845 domain-containing protein of 131 aas and 1 N-terminal TMS.

UP of Cupriavidus sp.

 
1.A.63.2.6

Uncharacterized DUF4845 domain-containing protein of 92 aas and possibly two TMSs, N- and C-terminal.

UP of Betaproteobacteria bacterium

 
1.A.63.2.7

Uncharacterized DUF4845 domain-containing protein of 122 aas and 2 TMSs, one N-terminal and possibly a second, near the C-terminus of the protein.

UP of Halobacteria archaeon