Gram-negative bacteria have developed a variety of secretion pathways to secrete toxins and enzymes into the extracellular medium. These pathways are very different with respect to their functional mechanism and complexity, and each system has its own advantages and limitations, regarding the number, size, folding state and fate of their substrates. Pseudomonas aeruginosa secretes many different proteins into the extracellular medium, using at least four secretion pathways. Most of the exoproteins are secreted via the type II system, composed of the 12 Xcp proteins. The only outer membrane protein of the system, XcpQ, belongs to a large family of proteins, designated secretins, which participate in a variety of different transport processes. Other Xcp proteins, XcpT-X, show homology to the subunits of the retractile type IV pili. Further analogies between the type II system and the assembly of retractile pili suggest a mechanism for type II secretion, in which a pilus-like structure, composed of XcpT-X, facilitates the transport of exoproteins through the channel formed by the secretin XcpQ.