MtrB-PioB is a family of bacterial putative outer membrane porins. This family, is secreted as part of the pio (phototrophic iron oxidation) operon that has been found to couple the oxidation of ferrous iron [Fe(II)] to reductive CO2 fixation using light energy. PioABC is found in Rhodopseudomonas palustris and MtrB-PioB is likely to be a beta-barrel porin. Similar to other outer membrane porins, PioB and MtrB are predicted to have long loops protruding into the extracellular space and short turns on the periplasmic side [1].

1: The pio operon is essential for phototrophic Fe(II) oxidation in Rhodopseudomonas palustris TIE-1. Jiao Y, Newman DK; J Bacteriol. 2007;189:1765-1773. PMID:17189359