1.C.73 The Pseudomonas Exotoxin A (P-ExoA) Family Exposure to low endosomal pH during internalization of Pseudomonas exotoxin A (PE) triggers membrane insertion of its translocation domain. This process is a prerequisite for PE translocation to the cytosol where it inactivates protein synthesis. Although hydrophobic helices enable membrane insertion of related bacterial toxins such as diphtheria toxin, the PE translocation domain is devoid of hydrophobic stretches. The structural features triggering acid-induced membrane insertion of PE have recently been elucidated (Méré et al., 2005). At neutral pH, a Trp is buried in a hydrophobic pocket, closed by the smallest α-helix of the translocation domain. Upon acidification, protonation of the Asp that is the N-cap residue of the helix leads to its destabilization, enabling Trp side chain insertion into the endosome membrane (Méré et al., 2005). This tryptophan-based membrane insertion system is similar to the membrane-anchoring mechanism of human annexin-V. P-ExoA is 613 aas long and consists of 3 structural/functional domains. Domain I binds to the α2-macroglobulin/low density lipoprotein receptor-related protein, enabling internalization via receptor-mediated endocytosis. Domain II then mediates translocation into the cytosol of the entire toxin or of a carboxyl-terminal fragment generated by furin proteolysis and encompassing domain III and most of domain II. Finally, domain III catalyzes the ADP ribosylation of elongation factor 2, thereby inhibiting protein synthesis and killing the cell. P-ExoA has only one homologue in the current (7/05) NCBI database, a hypothetical exotoxin A from Vibrio cholerae. It is 666 aas long and exhibits 33% identity with P-ExoA throughout almost all of its length. A C-terminal domain (residues 436-502) shows significant sequence similarity (27% identity) with a region (42-167) of diphtheria toxin from C. diphtheriae (TC #1.C.7.1.1) as well as a region (116-239) with 22% identity with a region (314-420) of a putative exported protein from Yersinia species such as Y. pestis (806 aas; AAS62938).