Positively charged peptide segments of 16 and 18 residues were inserted at a
periplasmic turn of the porin from Rhodobacter blasticus in order to form an
electric field-dependent plug. The X-ray diffraction analysis of a mutant
confirmed that the structure of the porin had remained intact and that the
insert was mobile. Incorporation experiments of single molecules into lipid
bilayers showed that the distribution of electric conduction increments depended
on the field polarity. The observed distributions are explained if the porin
molecules enter the bilayer preferentially with their periplasmic surface first.
Furthermore, the conduction of membrane-incorporated porin mutants changed
reproducibly on field reversal showing asymmetries of reverse similar 15%, while
the wild-type remained constant. This asymmetry is most likely caused by the
electric field pressing the charged insert onto the pore eyelet in one field
direction and removing it from the eyelet in the other. The results encourage
attempts to improve the inserts in order to eventually reach diode
characteristics.
