Protein-protein interactions in the assembly of Shigella flexneri invasionplasmid antigens IpaB and IpaC into protein complexes

Shigella flexneri is a facultative intracellular bacterial pathogen that in vades human colonic epithelial cells by a process called pathogen-induced p hagocytosis. Pathogen entry requires three virulence plasmid-encoded protei ns called invasion plasmid antigens (Ipa) B, C and D which are secreted upo n bacterial contact with a host cell. Following their secretion, IpaB and I paC are found within a complex of proteins that may also contain IpaA and I paD. Previous work has shown that exogenously added recombinant IpaC is suf ficient for promoting the uptake of S. flexneri in gentamicin-protection as says. It is shown here that purified recombinant Ipa proteins can also be u sed to investigate the formation of Ipa protein complexes in vitro. The pro tein-protein contacts involved in the formation of Ipa complexes appear to include previously undescribed IpaC-IpaC interactions in addition to a stro ng association between IpaB and IpaC. IpaD does not appear to interact with either IpaB or IpaC in vitro although it is possible that recombinant IpaD forms homodimers that are stabilized by disulfide bridges involving this p rotein's single cysteine residue. This investigation represents the first c haracterization of the biochemistry of Ipa complex assembly. (C) 1998 Elsev ier Science B.V. All rights reserved.