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.
This family belongs to the Outer Membrane Pore-forming Protein I (OMPP-I) Superfamily .
References:
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.
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.
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.
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]
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.
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.
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:.
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.
Examples:
TC#	Name	Organismal Type	Example
1.B.23.1.1	SomA porin	Cyanobacteria	*SomA of Synechococcus* PCC 6301**

1.B.23.1.10	Putative outer membrane porin	Firmicutes with an outer membrane	*OMP of Megasphaera elsdenii* (G0VLV3)**

1.B.23.1.11	Arginine/agmatine porin, AaxA (Smith and Graham 2008).	Chlamydiae	AaxA of Chlamydia (Chlamydiophila) pneumoniae

1.B.23.1.12	OprB homologue of 430 aas	Proteobacteria	OprB homologue of Thermovibrio ammonificans

1.B.23.1.13	OprB homologue of 408 aas	Verrucomicrobia	OprB homologue of Coraliomargarita akajimensis

1.B.23.1.14	OprB homologue	Nitrospirae	OprB homologue of Thermodesulfovibrio yellowstonii

1.B.23.1.15	OprB homologue of 435 aas	Acidobacteria	OprB homologue of Terriglobus saanensis

1.B.23.1.16	Putative porin, OprB, of 494 aas	Verrucomicrobia	OprB of Akkermansia muciniphila

1.B.23.1.17	Putative porin of 488 aas	Proteobacteria	Putative porin of Magnetospirillum magneticum

1.B.23.1.18	Putative porin of 470 aas in the outer membrane of an atypical firmicute	Firmicutes	Putative porin of Megasphaera elsdenii

1.B.23.1.19	Putative porin	Planctomycetes	Porin of Candidatus Kuenenia stuttgartiensis

1.B.23.1.2	*Putative HCO₃^-/Fe⁺³ porin, slr0042 gene product of 576 aas (Qiu et al.* 2018).**	Cyanobacteria	*Slr0042 of Synechocystis* PCC6803**

1.B.23.1.20	Putative porin of 500 aas	Synergistetes	*Putative porin of Synergistes* sp. 3_1_syn1**

1.B.23.1.21	OprB of 415 aas	Verrucomicrobia	OprB of Pedosphaera parvula

1.B.23.1.22	Carbohydrate-selective porin OprB of 405 aas	Verrucomicrobia	OprB of Akkermansia muciniphila

1.B.23.1.23	*Porin (residues 250 - 433) with an N-terminal S-layer domain, SLH, residues 1 - 250 (Kalmokoff et al.* 2009).**		Outer membrane porin of Mitsuokella multacida

1.B.23.1.24	Major envelope protein, Mep45, of 432 aas and 1 N-terminal TMS. Mep45 contains two distinct domains: an N-terminal S-layer homologous (SLH) domain that protrudes into the periplasm and binds to peptidoglycan, and the remaining C-terminal transmembrane domain that forms a non-selective pore able to transport molecules of <600 Da (Kojima et al. 2016). The estimated pore radius is 0.58 nm; truncation of the SLH domain does not affect the channel.		Mep45 of Selenomonas ruminantium

1.B.23.1.25	*Porin of 546 aas. Capable of facilitating the uptake of ferric iron (Qiu et al.* 2018).**		*Porin of Synechocystis* sp. strain PCC 6803**

1.B.23.1.3	Major Omp, OmpM1	Firmicute with outer membrane	OmpM1 of Selenomonas sputigena

1.B.23.1.4	Putative porin	Firmicute	Putative porin of Thermosinus carboxydivorans

1.B.23.1.5	S-layer domain protein	Firmicutes	S-layer domain protein of Selenomonas noxia

1.B.23.1.6	Outer membrane porin homologue	Firmicutes	*OMP homologue of Veillonella atypica* (E1L471)**

1.B.23.1.7	Outer membrane porin homologue	Firmicutes with outer membrane	*OMP homologue of Megasphaera micronuciformis* (E2ZDM6)**

1.B.23.1.8	Outer membrane porin homologue	Firmicutes	*OMP of Anaeroglobus geminatus* (G9YIN5)**

1.B.23.1.9	Putative outer membrane porin	Cyanobacteria	*OMP of Fischerella* sp. JSC-11 (G6FN26)**

Examples:
TC#	Name	Organismal Type	Example
1.B.23.2.1	Putative porin	Acidobacteria	Putative porin of Holophaga foetida

1.B.23.2.2	Putative porin	Acidobacteria	Porin of Holophaga foetida

1.B.23.2.3	Putative porin	Acidobacteria	Porin of Holophaga foetida

Examples:
TC#	Name	Organismal Type	Example
1.B.23.3.1	Putative porin	Fibrobacteres	Porin of Fibrobacter succinogenes

1.B.23.3.2	Putative porin	Fibrobacteres	Porin of Fibrobacter succinogenes

1.B.23.3.3	Putative porin	Fibrobacteres	Porin of Fibrobacter succinogenes

1.B.23.3.4	Putative porin of 371 aas	Fibrobacteres	Porin of Fibrobacter succinogenes

Examples:
TC#	Name	Organismal Type	Example
1.B.23.4.1	Uncharacterized protein of 518 aas	Chlorobi	UP of Chloroherpeton thalassium

1.B.23.4.2	Uncharacterized protein of 704 aas with 23 putative β-strains and one N-terminal α-TMS.	Bacteroidetes	UP of Flavobacterium psychrophilum