1.B.78. The DUF3374 Electron Transport-associated Porin (ETPorin) Family
The DUF3374 family includes large proteins (600 - 800 aas) with 20 - 24 putative β-TMSs. Some are annotated as decaheme-associated outer membrane proteins, and they bring up members of TC family 5.A.5 in TC Blast searches. See this family for description of these porins as well as the associated cytochromes that mediate electron flow from the cell surface to the cytoplasmic membrane (Beckwith et al. 2015).
Members of this protein family are integral proteins of the bacterial outer membrane, associated with multiheme c-type cytochromes involved in electron transfer. The MtrB protein of Shewanella oneidensis MR-1 (SO1776) forms a complex with 1:1:1 stochiometry with the small, periplasmic decaheme cytochrome MtrA and the large, surface-exposed decaheme cytochrome MtrC.
Homologs of the outer membrane beta-barrel protein MtrB of metal-reducing γ-proteobacteria contain a unique N-terminal CXXC motif that is missing from MtrB homologs of nonmetal-reducing bacteria. The N-terminal CXXC motif is required for dissimilatory metal reduction (Wee et al. 2014). Anaerobic growth experiments demonstrated that the first, but not the second conserved cysteine is required for metal reduction.