Not All Bacterial Outer-Membrane Proteins Are β-Barrels

The discovery of Wza, an octomeric helical barrel integral bacterial outer-transmembrane protein, has challenged the widely held understanding that all integral outer-membrane proteins of Gram-negative bacteria are closed β-barrels composed of transmembrane β- strands. Wza is a member of the Outer-Membrane Polysaccharide Exporter family and our bioinformatics analysis suggests that other members of the family may also contain outer-membrane transmembrane segments that are helical. A review of the literature indicates that in addition to Wza, outer-membrane core complex proteins of the type IV secretion systems also contain transmembrane segments that are helical.