Porins form aqueous channels that aid the diffusion of small hydrophilic molecules across the outer membrane of Gram-negative bacteria. The crystal structures of matrix porin and phosphoporin both reveal trimers of identical subunits, each subunit consisting of a 16-stranded anti-parallel beta-barrel containing a pore. A long loop inside the barrel contributes to a constriction of the channel where the charge distribution affects ion selectivity. The structures explain at the molecular level functional characteristics and their alterations by known mutations.

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The higher stability of sIGPS compared with eIGPS seems to be the result of several improved interactions. These include a larger number of salt bridges, stabilization of alpha helices and strengthening of both polypeptide chain termini and solvent-exposed loops.