EXTRACELLULAR TRANSPORT OF VIRG PROTEIN IN SHIGELLA

The ability of Shigella to spread within and between epithelial cells is a prerequisite for causing bacillary dysentery and requires the fun ction encoded by the virG gene on the large plasmid, The outer membran e VirG (IcsA) protein is essential for bacterial spreading by elicitin g polar deposition of filamentous actin (F-actin) in the cytoplasm of epithelial cells, Recent studies have indicated that an N-terminal 80- kDa VirG portion is exposed on the bacterial cell surface and released into the external medium, while the following 37-kDa C-terminal porti on is embedded in the outer membrane, although little is known about t he extracellular transport of the VirG protein, In this study, we atte mpted to elucidate the export pathway of VirG protein across the outer membrane and found that the C-terminal 37-kDa portion, termed VirG be ta-core, serves as the self transporter for the secretion of the prece ding 80-kDa portion from the periplasmic side of the outer membrane to the external side, Indeed, foreign polypeptides such as MalE or PhoA covalently linked to the N terminus of VirG beta-core were transported to the external side of the outer membrane, and it was further shown that the folding structure of the passenger polypeptide at the peripla smic side of the outer membrane interferes with its translocation, Ana lysis of the secondary structure of VirG beta-core predicted that the critical structural property was a beta-barrel channel consisting of a mphipathic antiparallel transmembrane beta-strands, interspersed by ha irpin turns and loops. These results thus strongly suggest that the se cretion of VirG protein from Shigella is similar to the export system utilized by the IgA protease of Neisseria.