1.B.23 The Cyanobacterial Porin (CBP) Family
Two tandemly lying homologous genes of Synechococcus PCC 6301, somA and somB, are present within a single operon. They encode two functionally characterized outer membrane porins of the CBP family that are proposed to form 14- or 16-stranded β-barrels (Hansel and Tadros 1998). The two proteins are 50% identical to each other, and they are homologous to six proteins encoded within the completely sequenced genome of Synechocystis PCC 6803. All of these sequences share an N-terminal motif with similarity to 'S-layer homology' (SLH) domains. These domains probably form periplasmic extensions that may act as linkers connecting the outer membrane to the peptidoglycan layer.
The two functionally characterized CBP family members, SomA and SomB, have been shown to form pores in lipid bilayers with single-channel conductances (Hansel and Tadros 1998). Another gene product, Slr0042, is present in an operon with genes encoding the bicarbonate-specific ABC transporter and may therefore be a HCO3- porin.