title: Needle length control and the secretion substrate specificity switch are only loosely coupled in the type III secretion apparatus of Shigella.
abstract: The type III secretion apparatus (T3SA), which is evolutionarily and structurally related to the bacterial flagellar hook basal body, is a key virulence factor used by many gram-negative bacteria to inject effector proteins into host cells. A hollow extracellular needle forms the injection conduit of the T3SA. Its length is tightly controlled to match specific structures at the bacterial and host-cell surfaces but how this occurs remains incompletely understood. The needle is topped by a tip complex, which senses the host cell and inserts as a translocation pore in the host membrane when secretion is activated. The interaction of two conserved proteins, inner-membrane Spa40 and secreted Spa32, respectively, in Shigella, is proposed to regulate needle length and to flick a type III secretion substrate specificity switch from needle components/Spa32 to translocator/effector substrates. We found that, as in T3SAs from other species, substitution N257A within the conserved cytoplasmic NPTH region in Spa40 prevented its autocleavage and substrate specificity switching. Yet, the spa40(N257A) mutant made only slightly longer needles with a few needle tip complexes, although it could not form translocation pores. On the other hand, Δspa32, which makes extremely long needles and also formed only few tip complexes, could still form some translocation pores, indicating that it could switch substrate specificity to some extent. Therefore, loss of needle length control and defects in secretion specificity switching are not tightly coupled in either a Δspa32 mutant or a spa40(N257A) mutant.
authors.affiliation: Schools of Cellular and Molecular Medicine and Biochemistry, University of Bristol, Bristol BS8 1TD, UK.
authors.forename: Da-Kang
authors.initials: DK
authors.lastname: Shen
authors.affiliation: Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadoaka, Suita, Osaka 565-0871, Japan.|Schools of Cellular and Molecular Medicine and Biochemistry, University of Bristol, Bristol BS8 1TD, UK.
authors.forename: Nao
authors.initials: N
authors.lastname: Moriya
authors.affiliation: Schools of Cellular and Molecular Medicine and Biochemistry, University of Bristol, Bristol BS8 1TD, UK.
authors.forename: Isabel
authors.initials: I
authors.lastname: Martinez-Argudo
authors.affiliation: Schools of Cellular and Molecular Medicine and Biochemistry, University of Bristol, Bristol BS8 1TD, UK.
authors.forename: Ariel J
authors.initials: AJ
authors.lastname: Blocker
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D058947:Bacterial Secretion Systems; D006801:Humans; D046911:Macromolecular Substances / Q000378:metabolism*; D008954:Models, Biological; D008956:Models, Chemical; D008958:Models, Molecular; D012760:Shigella / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; D046911:Macromolecular Substances