title: Oligomeric states of the Shigella translocator protein IpaB provide structural insights into formation of the type III secretion translocon.
abstract: The Shigella flexneri Type III secretion system (T3SS) senses contact with human intestinal cells and injects effector proteins that promote pathogen entry as the first step in causing life threatening bacillary dysentery (shigellosis). The Shigella Type III secretion apparatus (T3SA) consists of an anchoring basal body, an exposed needle, and a temporally assembled tip complex. Exposure to environmental small molecules recruits IpaB, the first hydrophobic translocator protein, to the maturing tip complex. IpaB then senses contact with a host cell membrane, forming the translocon pore through which effectors are delivered to the host cytoplasm. Within the bacterium, IpaB exists as a heterodimer with its chaperone IpgC; however, IpaB's structural state following secretion is unknown due to difficulties isolating stable protein. We have overcome this by coexpressing the IpaB/IpgC heterodimer and isolating IpaB by incubating the complex in mild detergents. Interestingly, preparation of IpaB with n-octyl-oligo-oxyethylene (OPOE) results in the assembly of discrete oligomers while purification in N,N-dimethyldodecylamine N-oxide (LDAO) maintains IpaB as a monomer. In this study, we demonstrate that IpaB tetramers penetrate phospholipid membranes to allow a size-dependent release of small molecules, suggesting the formation of discrete pores. Monomeric IpaB also interacts with liposomes but fails to disrupt them. From these and additional findings, we propose that IpaB can exist as a tetramer having inherent flexibility, which allows it to cooperatively interact with and insert into host cell membranes. This event may then lay the foundation for formation of the Shigella T3SS translocon pore.
authors: Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma 74078, USA.
authors: Nicholas E
authors: NE
authors: Dickenson
authors: Shyamal P
authors: SP
authors: Choudhari
authors: Philip R
authors: PR
authors: Adam
authors: Ryan M
authors: RM
authors: Kramer
authors: Sangeeta B
authors: SB
authors: Joshi
authors: C Russell
authors: CR
authors: Middaugh
authors: Wendy L
authors: WL
authors: Picking
authors: William D
authors: WD
authors: Picking
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D004405:Dysentery, Bacillary / Q000382:microbiology; D054884:Host-Pathogen Interactions; D006801:Humans; D008081:Liposomes / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism; D011487:Protein Conformation; D055503:Protein Multimerization; D055550:Protein Stability; D021381:Protein Transport; D012763:Shigella flexneri / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology
chemical_list: D001426:Bacterial Proteins; D008081:Liposomes; D010743:Phospholipids; C087447:ipaB protein, Shigella