1.B.55 The Poly Acetyl Glucosamine Porin (PgaA) Family

The linear homopolymer poly-beta-1,6-N-acetyl-D-glucosamine (beta-1,6-GlcNAc; PGA) serves as an adhesin for the maintenance of biofilm structural stability in diverse eubacteria. Its function in Escherichia coli K-12 requires the gene products of the pgaABCD operon, all of which are necessary for biofilm formation. PgaC is an apparent glycosyltransferase that is required for PGA synthesis, and PgaD is also needed for PGA formation. Deletion of genes for the predicted outer membrane proteins PgaA and PgaB did not prevent PGA synthesis but did block its export at the cell poles, the initial attachment site for biofilm formation (Itoh et al., 2008). PgaA contains a predicted beta-barrel porin and a superhelical domain containing tetratricopeptide repeats, which may mediate protein-protein interactions. It may form the outer membrane secretin for PGA. PgaB contains predicted carbohydrate binding and polysaccharide N-deacetylase domains. Overexpression of pgaB increases the primary amine content (glucosamine) of PGA. Site-directed mutations targeting the N-deacetylase catalytic activity of PgaB blocked PGA export and biofilm formation, implying that N-deacetylation promotes PGA export through the PgaA porin.

The generalized reaction catalyzed by PgaA is:

deacetylated PGA (periplasm) → deacetylated PGA (cell surface)

This family belongs to the Outer Membrane Pore-forming Protein I (OMPP-I) Superfamily .



Acheson, J.F., Z.S. Derewenda, and J. Zimmer. (2019). Architecture of the Cellulose Synthase Outer Membrane Channel and Its Association with the Periplasmic TPR Domain. Structure. [Epub: Ahead of Print]

Cerca, N. and K.K. Jefferson. (2008). Effect of growth conditions on poly-N-acetylglucosamine expression and biofilm formation in Escherichia coli. FEMS Microbiol. Lett. 283: 36-41.

Itoh, Y., J.D. Rice, C. Goller, A. Pannuri, J. Taylor, J. Meisner, T.J. Beveridge, J.F. Preston, 3rd, and T. Romeo. (2008). Roles of pgaABCD genes in synthesis, modification, and export of the Escherichia coli biofilm adhesin poly-β-1,6-N-acetyl-D-glucosamine. J. Bacteriol. 190: 3670-3680.

Kenedy MR., Luthra A., Anand A., Dunn JP., Radolf JD. and Akins DR. (2014). Structural modeling and physicochemical characterization provide evidence that P66 forms a beta-barrel in the Borrelia burgdorferi outer membrane. J Bacteriol. 196(4):859-72.

Zogaj, X., M. Nimtz, M. Rohde, W. Bokranz, and U. Römling. (2001). The multicellular morphotypes of Salmonella typhimurium and Escherichia coli produce cellulose as the second component of the extracellular matrix. Mol. Microbiol. 39: 1452-1463.


TC#NameOrganismal TypeExample

The β-barrel porin with a superhelical domain containing tetratricopeptide repeats, PgaA or YcdS; exports (deacetylated) poly β-1,6-N-acetyl glucosamine (PGA), a biofilm adhesin that may also play a role in immune evasion (Itoh et al., 2008; Cerca and Jefferson 2008).

Gram-negative γ-proteobacteria

PgaA of E. coli (P69434)


PgaA homologue


PgaA homologue of Burkholderia cepacia


PgaA homologue


PgaA homologue of Neisseria wadsworthii


TC#NameOrganismal TypeExample

Putative porin of 604 aas


Porin of Blastopirellula marina


Uncharacterized protein of 555 aas and 20 predicted beta strands with an N-terminal TMS.


UP of Candidatus Nitroospira defluvii


TRP-repeat containing protein of 492 aas and 16 predicted beta strands.


Trp repeat protein of Geobacter daltonii


TC#NameOrganismal TypeExample

Cellulose synthase operon, protein C of 1157 aas and up to 18 C-terminal (720 - 1157 aas) β-strands with a single N-terminal α-TMS, BcsC or YhjL (Zogaj et al. 2001). Translocation across the outer membrane occurs through the BcsC porin, which extends into the periplasm via 19 tetra-tricopeptide repeats (TPR). Acheson et al. 2019 presented the crystal structure of a truncated BcsC, encompassing the last TPR repeat and the complete outer membrane channel domain, revealing a 16-stranded, β barrel pore architecture. The pore is blocked by an extracellular gating loop, while the extended C terminus inserts deeply into the channel and positions a conserved Trp residue near its extracellular exit. The channel is lined with hydrophilic and aromatic residues suggesting a mechanism for facilitated cellulose diffusion based on aromatic stacking and hydrogen bonding (Acheson et al. 2019).


BcsC of E. coli


Uncharacterized protein of 963 aas


UP of Leptospirillum ferriphilum


TC#NameOrganismal TypeExample

YaiO family outer membrane beta-barrel protein of 409 aas and 1 N-terminal TMS.

YaiO of Candidatus Atribacteria bacterium (marine sediment metagenome)


YaiO family outer membrane beta-barrel protein of 409 aas and 1 N-terminal TMS.

YaiO of Algoriphagus aquimarinus


YaiO family outer membrane beta-barrel protein of 400 aas and 1 N-terminal TM

YaiO of Candidatus Sulfopaludibacter sp. SbA6