The membrane topologies of Escherichia coli ExbB, ExbD and TonB are known, as is the membrane topology of Vibrio anguillarum TonB2, which has one transmembrane domain, with its amino terminus in the cytoplasm. Based on amino acid sequence homologies and in silico transmembrane domain predictions, the predicted membrane topologies of the Vibrio TonB2 system proteins in the inner membrane are shown.