To promote viral entry, replication, release, and spread to neighboring cells, many cytolytic animal viruses encode proteins responsible for modification of host cell membrane permeability and for formation of ion channels in host cell membranes during their life cycles. In this study, we show that the envelope (E) protein of severe acute respiratory syndrome-associated coronavirus can induce membrane permeability changes when expressed in Escherichia coli. E protein expressed in bacterial and mammalian cells under reducing conditions existed as monomers, but formed homodimer- and homotrimer under non-reducing conditions. Site-directed mutagenesis studies revealed that two cysteine residues of the E protein were essential for oligomerization, leading to induction of membrane permeability. This is the first report demonstrating that a coronavirus-encoded protein could modify membrane permeability in E. coli cells. (C) 2004 Elsevier Inc. All rights reserved.