Insecticidal delta-endotoxin Cyt2Ba from Bacillus thuringiensis: Difference between revisions

The [http://en.wikipedia.org/wiki/Crystal_structure crystal structure] of the [http://en.wikipedia.org/wiki/Proteolysis proteolytically] activated monomeric form of Cyt2Ba was determined at 1.8Å resolution. It consists of a single domain of  <scene name='Cyt2Ba/Alpha_beta/5'>α/β</scene> architecture with a <scene name='Cyt2Ba/Beta/2'>β-sheet</scene> (yellow) surrounded by 2 <scene name='Cyt2Ba/Alpha/2'>α-helical</scene> layers <font color='red'><b>(red)</b></font> forming a cytolysin fold. The [http://en.wikipedia.org/wiki/Beta_sheet β-sheet] is comprised of 6 anti-parallel β-strands (β1-β6). On one side of this sheet there is an [http://en.wikipedia.org/wiki/Alpha_helix α-helix] layer consisting of α1, α2; and on the other side  a second α-helix layer, composed of α3-α5. The β-strands β2-β5 of the central β-sheet have a modified Greek-key topology. The Greek key motif consists of four adjacent antiparallel strands and their linking loops. It consists of three antiparallel strands connected by hairpins, while the fourth is adjacent to the first and linked to the third by a longer loop [http://en.wikipedia.org/wiki/Beta_sheet]. <font color='gray'><b>Cyt2Ba (gray)</b></font> has only 16% sequence identity with <font color='red'><b>VVA2 (colored red,</b></font> [[1pp0]]), however they both have a cytolysin fold and their overall structure is very similar (see their <scene name='Cyt2Ba/Cyt2ba_vva/3'>structural alignment</scene>).

A remarkable similarity is observed between the structures of the endogenously cleaved Cyt2Ba <scene name='Cyt2Ba/Cyt2ba_monomer/2'>monomer</scene> <font color='gray'><b>(gray)</b></font> and the <scene name='Cyt2Ba/Alignment/2'>corresponding region</scene> <font color='red'><b>(red)</b></font> within the inactive protoxin  <scene name='Cyt2Ba/Dimer/2'>dimer</scene> of Cyt2Aa ([[1cby]], monomers <font color='red'><b>A</b></font> and <font color='blue'><b>B</b></font> of Cyt2Aa shown <font color='red'><b>red</b></font> and <font color='blue'><b>blue</b></font>, respectively, the N- and C-termini are shown in spacefilling representation). Although, [[1cby]] is a 1 chain structure, the biological relevant molecule for [[1cby]] can be assembled from the contents of the deposited coordinates by the application of crystallographic symmetry operations to give a dimer. It can be [http://www.ebi.ac.uk/pdbe/pqs/macmol/1cby.mmol downloaded]. Each monomer of Cyt2Aa ([[1cby]]), consists of an additional β-strand at its N-terminus and an additional α-helix at its C-terminus compared to the cleaved Cyt2Ba. The <scene name='Cyt2Ba/Dimer_mesh/12'>dimer interface</scene> of Cyt2Aa is held together by the intertwined N-terminal strands from both monomers. The cleavage of Cyt2Aa <scene name='Cyt2Ba/Dimer_mes/1'>removes</scene> the N- and C-terminal segments, prevents dimer formation and releases an <scene name='Cyt2Ba/Monomer_toxin/4'> active toxin monomer</scene>. Similarly, in Cyt2Ba the proteolysis causes the removal of 34 amino acids at its N-terminus and 28 or 30 residues at its C-terminus forming the crystallized toxic monomer.

The crystal structure of monomeric Cyt2Ba is the first structure of a [http://en.wikipedia.org/wiki/Toxicity toxic] form of the Cyt family. Its structure is [http://en.wikipedia.org/wiki/Homology_(biology) 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. Solving the 3D structure of these proteins extends the knowledge of the cytolytic machinery of pore-forming toxins and helps in designing novel membrane-active cytotoxins.