Not all Bacterial Outer Membrane Proteins are β-Barrels

The discovery of WzA, an octomeric helical barrel integral bacterial outer membrane 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 a 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.