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Characterization of Prohibitin 1 as a Host Partner of Vibrio vulnificus RtxA1 Toxin. RtxA1 toxin, which results in cytoskeletal rearrangement, contact cytotoxicity, hemolysis, tissue invasion, and lethality in mice, is the most potent cytotoxic virulence factor of Vibrio vulnificus. Bioinformatics analysis of rtxA1 predicted 4 functional domains that presumably performed discrete functions during host cell killing. V. vulnificus RtxA1 has a unique domain designated as RtxA1-D2, corresponding to amino acids 1951-2574, which is absent in Vibrio cholerae multifunctional-autoprocessing repeats-in-toxin, suggesting that this domain confers specific biological functions to V. vulnificus RtxA1. HeLa cells expressing green fluorescent protein-RtxA1-D2 became round and lost their viability. A yeast 2-hybrid system identified prohibitin (PHB) 1 as the host partner of RtxA1-D2. The specific interaction of RtxA1-D2 with PHB1 was confirmed by performing immunoprecipitation. Interestingly, V. vulnificus RtxA1 up-regulated PHB1 expression on the cytoplasmic membrane of host cells. Extracellular signal-regulated kinase and p38 mitogen-activated protein kinase pathways were confirmed as being important in the up-regulation of PHB1 by using inhibitors. Down-regulation of PHB1 by small interfering RNAs decreased the cytotoxicity of RtxA1-D2 against HeLa cells. The pretreatment of an anti-PHB1 antibody impaired the cytotoxicity of V. vulnificus RtxA1. These results suggest that the involvement PHB1 in the RtxA1 cytotoxicity has significant implications for the pathogenesis of V. vulnificus infections.