> Pathogénie microbienne moléculaire - INSERM U389
• Summary
• Saga Shigella
• Objectives
• Cell Biology
• Inflammation
• Immunology
• Vaccines
1 - Genetics of the Shigella flexneri invasive phenotype.
Permanent scientist: Claude Parsot (Chef de Laboratoire, Institut Pasteur). Post-doctoral scientists : Maria Mavris, Anne-Laure Page, Kaïs Jamoussi, Dong Wong Kim. Technician : Elisabeth Ageron.
A 30-kb “pathogenicity island” (PAI) on the 214kb virulence plasmid of S.flexneri is required for entry into epithelial cells. This PAI is composed of two loci transcribed in opposite directions. One comprises two operons (mxi and spa) encoding a type III secreton that translocates effector proteins into the epithelial cell membrane and cytoplasm. These effector proteins are encoded by the other locus, essentially the ipa operon ( , ). Two proteins, IpaB and IpaC, that are chaperoned in the bacterial cytoplasm by IpgC ( ), are secreted upon contact with the epithelial cell surface ( ) and form a pore in its membrane that serves to translocate other effector proteins such as IpaA and IpgD ( , ). The IpaB-IpaC complex is also instrumental to promote entry of the bacterium( ).
More recent contributions encompass (i) morphological description and partial composition of the type III secreton ( , ); ii) characterization of the induction of secretion ( , ) ; (iii) identification of the network of interactions between chaperons and effector proteins and its role in the protection and timing of secretion of these effector proteins ( , ). Also, by combining sequence and annotation of the S.flexneri virulence plasmid ( ) and identification of the secreted proteins, the potential for Shigella to secrete about 20 proteins, in addition to the Ipa proteins; has been demonstrated. These other proteins are likely to play a specific role in bacteial-cell interactions. The genes encoding several of these proteins, that are located outside the PAI, are controlled at the transcriptional level by activation of the type III secreton ( ). The molecular basis of this regulatory process has been characterized.
These results provide a general framework of the function of the type III secretion pathway in S. flexneri. At 37°C, the Mxi-Spa apparatus is produced and assembled in an inactive form. The IpaA-D, IpgB, and IpgD proteins are produced and stored in the cytoplasm, most of them in association with specific chaperones. Upon contact of bacteria with host cells, the Mxi-Spa apparatus is activated, allowing delivery of bacterial invasins within or beyond the membrane of eukaryotic cells. The IpgC chaperone then activates the transcriptional activator MxiE, leading to expression of a second set of secreted effectors, the functions of which now need to be elucidated.