A virulence locus in *Pseudomonas aeruginosa* (HSI-I) encodes a protein secretion apparatus. This apparatus is regulated by RetS and LadS, which are global virulence regulators. The HSI-I locus is required for chronic infections in the rat lung and is likely involved in pathogenesis in cystic fibrosis (CF) patients. The apparatus exports Hcp1, a hexameric protein that forms rings with a 40-angstrom internal diameter. Hcp1 is detected in the pulmonary secretions of CF patients and specific antibodies against Hcp1 are present in their sera, suggesting its role in disease. The HSI-I locus is highly homologous to a group of genes found in many Gram-negative proteobacteria called the IcmF-associated homologous protein (IAHP) cluster. The HSI-I locus is required for the secretion of Hcp1, which is involved in extracellular protein targeting. The secretion of Hcp1 is dependent on the HSI-I components, including IcmF1 and ClpV1. ClpV1 is a ClpB-like AAA+ family protein that is likely to provide the energy for translocation of Hcp1. The function of ClpV1 is reflected in its subcellular localization, which is localized to discrete foci in the majority of cells. The punctate localization of ClpV1-GFP indicates the presence of an HSI-I-encoded secretion apparatus. IcmF1 and Hcp1 are required for punctate localization of ClpV1-GFP. The Hcp1 protein forms hexameric rings with a large internal diameter. The x-ray crystal structure of Hcp1 reveals that it forms two closely related hexameric rings. The Hcp1 ring interior is a 24-stranded β-barrel approximately 40 angstroms in diameter. The Hcp1 protein is highly conserved among Gram-negative proteobacteria, but shares little sequence homology with proteins of known structure. The Hcp1 protein is involved in the secretion of other macromolecules, possibly through the formation of a channel. The HSI-I locus is likely to play a general role in mediating host interactions. The findings support efforts to develop vaccines and therapeutics targeting Hcp1 or components of the HSI-I-encoded apparatus as treatments for chronic *P. aeruginosa* infections.A virulence locus in *Pseudomonas aeruginosa* (HSI-I) encodes a protein secretion apparatus. This apparatus is regulated by RetS and LadS, which are global virulence regulators. The HSI-I locus is required for chronic infections in the rat lung and is likely involved in pathogenesis in cystic fibrosis (CF) patients. The apparatus exports Hcp1, a hexameric protein that forms rings with a 40-angstrom internal diameter. Hcp1 is detected in the pulmonary secretions of CF patients and specific antibodies against Hcp1 are present in their sera, suggesting its role in disease. The HSI-I locus is highly homologous to a group of genes found in many Gram-negative proteobacteria called the IcmF-associated homologous protein (IAHP) cluster. The HSI-I locus is required for the secretion of Hcp1, which is involved in extracellular protein targeting. The secretion of Hcp1 is dependent on the HSI-I components, including IcmF1 and ClpV1. ClpV1 is a ClpB-like AAA+ family protein that is likely to provide the energy for translocation of Hcp1. The function of ClpV1 is reflected in its subcellular localization, which is localized to discrete foci in the majority of cells. The punctate localization of ClpV1-GFP indicates the presence of an HSI-I-encoded secretion apparatus. IcmF1 and Hcp1 are required for punctate localization of ClpV1-GFP. The Hcp1 protein forms hexameric rings with a large internal diameter. The x-ray crystal structure of Hcp1 reveals that it forms two closely related hexameric rings. The Hcp1 ring interior is a 24-stranded β-barrel approximately 40 angstroms in diameter. The Hcp1 protein is highly conserved among Gram-negative proteobacteria, but shares little sequence homology with proteins of known structure. The Hcp1 protein is involved in the secretion of other macromolecules, possibly through the formation of a channel. The HSI-I locus is likely to play a general role in mediating host interactions. The findings support efforts to develop vaccines and therapeutics targeting Hcp1 or components of the HSI-I-encoded apparatus as treatments for chronic *P. aeruginosa* infections.