title: Binding affects the tertiary and quaternary structures of the Shigella translocator protein IpaB and its chaperone IpgC.
abstract: Shigella flexneri uses its type III secretion system (T3SS) to promote invasion of human intestinal epithelial cells as the first step in causing shigellosis, a life-threatening form of dysentery. The Shigella type III secretion apparatus (T3SA) consists of a basal body that spans the bacterial envelope and an exposed needle that injects effector proteins into target cells. The nascent Shigella T3SA needle is topped with a pentamer of the needle tip protein invasion plasmid antigen D (IpaD). Bile salts trigger recruitment of the first hydrophobic translocator protein, IpaB, to the tip complex where it senses contact with a host membrane. In the bacterial cytoplasm, IpaB exists in a complex with its chaperone IpgC. Several structures of IpgC have been determined, and we recently reported the 2.1 Å crystal structure of the N-terminal domain (IpaB(74.224)) of IpaB. Like IpgC, the IpaB N-terminal domain exists as a homodimer in solution. We now report that when the two are mixed, these homodimers dissociate and form heterodimers having a nanomolar dissociation constant. This is consistent with the equivalent complexes copurified after they had been co-expressed in Escherichia coli. Fluorescence data presented here also indicate that the N-terminal domain of IpaB possesses two regions that appear to contribute additively to chaperone binding. It is also likely that the N-terminus of IpaB adopts an alternative conformation as a result of chaperone binding. The importance of these findings within the functional context of these proteins is discussed.
authors.affiliation: Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, OK 74078, USA.
authors.forename: Philip R
authors.initials: PR
authors.lastname: Adam
authors.forename: Mrinalini K
authors.initials: MK
authors.lastname: Patil
authors.forename: Nicholas E
authors.initials: NE
authors.lastname: Dickenson
authors.forename: Shyamal
authors.initials: S
authors.lastname: Choudhari
authors.forename: Michael
authors.initials: M
authors.lastname: Barta
authors.forename: Brian V
authors.initials: BV
authors.lastname: Geisbrecht
authors.forename: Wendy L
authors.initials: WL
authors.lastname: Picking
authors.forename: William D
authors.initials: WD
authors.lastname: Picking
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002151:Calorimetry / Q000379:methods; D002942:Circular Dichroism; D005454:Fluorescence Polarization; D031541:Fluorescence Resonance Energy Transfer; D018832:Molecular Chaperones / Q000737:chemistry* / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D001426:Bacterial Proteins; D018832:Molecular Chaperones; D010446:Peptide Fragments; C087447:ipaB protein, Shigella; C094580:IpgC protein, Shigella