title: Crystal structure of the parasporin-2 Bacillus thuringiensis toxin that recognizes cancer cells.
abstract: Parasporin-2 is a protein toxin that is isolated from parasporal inclusions of the Gram-positive bacterium Bacillus thuringiensis. Although B. thuringiensis is generally known as a valuable source of insecticidal toxins, parasporin-2 is not insecticidal, but has a strong cytocidal activity in liver and colon cancer cells. The 37-kDa inactive nascent protein is proteolytically cleaved to the 30-kDa active form that loses both the N-terminal and the C-terminal segments. Accumulated cytological and biochemical observations on parasporin-2 imply that the protein is a pore-forming toxin. To confirm the hypothesis, we have determined the crystal structure of its active form at a resolution of 2.38 A. The protein is unusually elongated and mainly comprises long beta-strands aligned with its long axis. It is similar to aerolysin-type beta-pore-forming toxins, which strongly reinforce the pore-forming hypothesis. The molecule can be divided into three domains. Domain 1, comprising a small beta-sheet sandwiched by short alpha-helices, is probably the target-binding module. Two other domains are both beta-sandwiches and thought to be involved in oligomerization and pore formation. Domain 2 has a putative channel-forming beta-hairpin characteristic of aerolysin-type toxins. The surface of the protein has an extensive track of exposed side chains of serine and threonine residues. The track might orient the molecule on the cell membrane when domain 1 binds to the target until oligomerization and pore formation are initiated. The beta-hairpin has such a tight structure that it seems unlikely to reform as postulated in a recent model of pore formation developed for aerolysin-type toxins. A safety lock model is proposed as an inactivation mechanism by the N-terminal inhibitory segment.
authors.affiliation: Biological Information Research Center, AIST, Tsukuba, Ibaraki 305-8566, Japan. toshi-akiba@aist.go.jp
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Akiba
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Abe
authors.forename: Sakae
authors.initials: S
authors.lastname: Kitada
authors.forename: Yoshitomo
authors.initials: Y
authors.lastname: Kusaka
authors.forename: Akio
authors.initials: A
authors.lastname: Ito
authors.forename: Tokio
authors.initials: T
authors.lastname: Ichimatsu
authors.forename: Hideki
authors.initials: H
authors.lastname: Katayama
authors.forename: Tetsuyuki
authors.initials: T
authors.lastname: Akao
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Higuchi
authors.forename: Eiichi
authors.initials: E
authors.lastname: Mizuki
authors.forename: Michio
authors.initials: M
authors.lastname: Ohba
authors.forename: Ryuta
authors.initials: R
authors.lastname: Kanai
authors.forename: Kazuaki
authors.initials: K
authors.lastname: Harata
mesh_terms: D000595:Amino Acid Sequence; D001413:Bacillus thuringiensis / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D018360:Crystallography, X-Ray; D030562:Databases, Protein; D004354:Drug Screening Assays, Antitumor; D004731:Endotoxins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017510:Protein Folding; D012694:Serine / Q000235:genetics / Q000378:metabolism; D013912:Threonine / Q000235:genetics / Q000378:metabolism
chemical_list: D004731:Endotoxins; C515506:parasporin-2, Bacillus thuringiensis; D013912:Threonine; D012694:Serine