General diffusion porins usually assemble as a trimer in the membrane, and the transmembrane core of these proteins is composed exclusively of beta strands.

It is lined almost exclusively with charged amino acyl residues arranged on opposite sides of the channel, creating a transversal electric field across the pore.

The eyelet has a local surplus of negative charges from four glutamic acid and seven aspartic acid residues (in contrast to one histidine, two lysine and three arginine residues) is partially compensated for by two bound calcium atoms, and this asymmetric arrangement of molecules is thought to have an influence in the selection of molecules that can pass through the channel[3].

Some osmoporins, such as OmpC, forms a complex with alpha-helical transmembrane protein MlaA to maintain outer membrane lipid asymmetry.

(TC# 1.B.3) The SP family includes the well characterized maltoporin of E. coli for which the three-dimensional structures with and without its substrate have been obtained by X-ray diffraction.

The Rhodobacter PorCa Protein, the only well characterized member of the RPP family, was the first porin to yield its three-dimensional structure by X-ray crystallography.