Insecticidal delta-endotoxin Cyt2Ba from Bacillus thuringiensis
High-resolution crystal structure of activated Cyt2Ba monomer (δ-endotoxin) from Bacillus thuringiensis subsp. israelensisHigh-resolution crystal structure of activated Cyt2Ba monomer (δ-endotoxin) from Bacillus thuringiensis subsp. israelensis
The Cyt family of proteins consists of delta-endotoxins expressed during sporulation of several subspecies of Bacillus thuringiensis. Its members possess insecticidal, hemolytic, and cytolytic activities through pore formation and attract attention due to their potential use as vehicles for targeted membrane destruction. The delta-endotoxins of subsp. israelensis include three Cyt species: a major Cyt1Aa and two minor proteins, Cyt2Ba and Cyt1Ca. A cleaved Cyt protein that lacks the N- and C-terminal segments forms a toxic monomer. Here, we describe the crystal structure of Cyt2Ba, cleaved at its amino and carboxy termini by bacterial endogenous protease(s). Overall, its fold resembles that of the previously described volvatoxin A2 and the nontoxic form of Cyt2Aa. The structural similarity between these three proteins may provide information regarding the mechanism(s) of membrane-perforating toxins.
High-resolution crystal structure of activated Cyt2Ba monomer from Bacillus thuringiensis subsp. israelensis., Cohen S, Dym O, Albeck S, Ben-Dov E, Cahan R, Firer M, Zaritsky A, J Mol Biol. 2008 Jul 25;380(5):820-7. Epub 2008 May 11. PMID:18571667
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
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The Overall Structure of Monomeric Cyt2BaThe Overall Structure of Monomeric Cyt2Ba
The crystal structure of the proteolytically activated, monomeric form of Cyt2Ba was solved to 1.8Å resolution. It is composed of a single domain of architecture with a surrounded by two layers representing a cytolysin fold. The sheet consists of six anti-parallel β-strands (β1-β6) flanked by an α-helix layer composed of α1, α2 on one side, and by a second α-helix layer composed of α3-α5 on the other. The four longest β-strands (β2-β5) of the central β-sheet have a modified Greek-key topology.
Comparison of Cyt2Ba with Structurally Related ProteinsComparison of Cyt2Ba with Structurally Related Proteins
Cyt2Ba shares only 16% sequence identity to VVA2 (1pp0), nevertheless they both adopt a cytolysin fold and their structure is very similar (see their ). A striking similarity is observed between the structures of the endogenously cleaved Cyt2Ba (gray) and the within the inactive protoxin of Cyt2Aa (monomers A and B of Cyt2Aa shown red and blue, respectively, the N- and C-termini are shown in spacefill representation). Each monomer of Cyt2Aa (1cby), consists of an extra β-strand at its N-terminus and α-helix at its C-terminus compared to the cleaved Cyt2Ba. The of Cyt2Aa is held together by the intertwined N-terminal strands from both monomers. The cleavage of Cyt2Aa the N and C termini segments, thereby preventing dimer formation and hence releasing a . Similarly, it was shown that in Cyt2Ba the proteolysis causes the removal of 34 amino acids at its N-terminal and 28 or 30 residues at its C-terminus forming the crystallized toxic monomer.
ConclusionsConclusions
The crystal structure of monomeric Cyt2Ba is the first structure of a toxic form of the Cyt family. Its structure is homologous to the corresponding region of Cyt2Aa and to that of VVA2. This structural comparison shows that the toxicity of Cyt2Ba, Cyt2Aa and VVA2 is an inherent property of the monomer and not the result of secondary structure rearrangement upon cleavage. A comprehensive understanding of the toxic activities of these proteins may not only broaden our understanding as to the cytolytic machinery of pore forming toxins but also help to design better membrane-active cytotoxins.
ReferenceReference
High-resolution crystal structure of activated Cyt2Ba monomer from Bacillus thuringiensis subsp. israelensis., Cohen S, Dym O, Albeck S, Ben-Dov E, Cahan R, Firer M, Zaritsky A, J Mol Biol. 2008 Jul 25;380(5):820-7. Epub 2008 May 11. PMID:18571667