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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.

References associated with 1.B.23 family:

Hansel, A. and M.H. Tadros. (1998). Characterization of two pore-forming proteins isolated from the outer membrane of Synechococcus PCC 6301. Curr. Microbiol. 36: 321-326. 9608742
Hansel, A., F. Pattus, U.J. Jürgens and M.H. Tadros (1998). Cloning and characterization of the genes coding for two porins in the unicellular cyanobacterium Synechococcus PCC 6301. Biochim. Biophys. Acta 1399: 31-39. 9714719
Kalmokoff, M.L., J.W. Austin, T.D. Cyr, M.A. Hefford, R.M. Teather, and L.B. Selinger. (2009). Physical and genetic characterization of an outer-membrane protein (OmpM1) containing an N-terminal S-layer-like homology domain from the phylogenetically Gram-positive gut anaerobe Mitsuokella multacida. Anaerobe 15: 74-81. 19344649
Kojima, S., K. Hayashi, S. Tochigi, T. Kusano, J. Kaneko, and Y. Kamio. (2016). Peptidoglycan-associated outer membrane protein Mep45 of rumen anaerobe Selenomonas ruminantium forms a non-specific diffusion pore via its C-terminal transmembrane domain. Biosci. Biotechnol. Biochem. 1-6. [Epub: Ahead of Print] 27310312
Lupas, A., H. Engelhardt, J. Peters, U. Santarius, S. Volker and W. Baumeister (1994). Domain structure of the Acetogenium kivui surface layer revealed by electron crystallography and sequence analysis. J. Bacteriol. 176: 1224-1233. 8113161
Omata, T., S. Gohta, Y. Takahashi, Y. Harano and S. Maeda (2001). Involvement of a CbbR homolog in low CO2-induced activation of the bicarbonate transporter operon in cyanobacteria. J. Bacteriol. 183: 1891-1898. 11222586
Qiu, G.W., W.J. Lou, C.Y. Sun, N. Yang, Z.K. Li, D.L. Li, S.S. Zang, F.X. Fu, D.A. Hutchins, H.B. Jiang, and B.S. Qiu. (2018). Outer Membrane Iron Uptake Pathways in the Model Cyanobacterium Synechocystis sp. Strain PCC 6803. Appl. Environ. Microbiol. 84:. 30076192
Smith, C.B. and D.E. Graham. (2008). Outer and inner membrane proteins compose an arginine-agmatine exchange system in Chlamydophila pneumoniae. J. Bacteriol. 190: 7431-7440. 18790867