Secondary structure of a channel-forming protein: porin from E. coli outer membranes

Porin from Escherichia coli outer membranes has been analysed by high angle diffuse X-ray diffraction, and by attenuated total reflection infrared spectroscopy. These methods demonstrate independently that the majority of the polypeptide backbone is arranged in anti-parallel beta-pleated sheet structure. The average length of the beta-strands, which are oriented nearly normal to the membrane plane, is estimated to be 10-12 residues, independent of the method used. Although the details of strand arrangement (beta-barrels or stacked sheets) are not as yet known, porin represents the first transmembrane protein for which beta-structure has been established unequivocally.