Gram-negative bacteria need to be able to transport a large variety of macromolecules across their outer membranes. In Escherichia coli, the passage of the group 1 capsular polysaccharide is mediated by an integral outer membrane protein, Wza. The crystal structure of Wza, determined recently, reveals a novel transmembrane alpha-helical barrel and a large central cavity within the core of the vase-shaped protein complex. The structure has similarities with that of the secretin protein, PilQ, which mediates the transition of type IV pili across the outer membrane. We propose that the large internal chamber, which can accommodate the secreted assembled macromolecule, is likely to be a common feature found in other outer membrane proteins involved in secretion processes.