The profound stability of porin trimers is a product of tight monomer interactions. The C termini and strand 16 are essential to trimer formation. Points of contact include the barrel walls and peripheral contacts between L1 and L5. Salt bridges are also formed with arginines in L3. Many of the channel's functional properties stem from its loops.
Some loops pack together to form a hydrophilic umbrella structure over the channel opening .
It is inferred that these loops protect the channel and screen solutes based upon size and charge. Contributors to the constriction with the barrel wall are highly conserved Pro, Glu, Phe, Gly residues at the tip of loop 3 as well as Asp 113 and Glu 117 . The constriction zone determines a pore's selectivity and absolute solute size limitation. Although there is no recognized sequence homology between maltoporin and OMPF, there are a number of folding similarities. Successive stands are connected though periplasmic b -hairpin turns and irregular cell surface loops .