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

{{Structure

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The Cyt family of proteins consists of delta-endotoxins expressed during sporulation of several subspecies of ''B. 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-endotoxin of subsp. ''israelensis'' includes three Cyt species, a major Cyt1Aa and two minor proteins Cyt2Ba and Cyt1Ca. Cleaved Cyt protein that lacks the N- and C-terminal segments form toxic monomers. The crystal structure of Cyt2Ba, cleaved at its amino and carboxy terminus by bacterial endogenous protease(s) is presented. Overall, its fold resembles that of the previously described Volvatoxin A2 (VVA2) and the non-toxic form of Cyt2A.  

The structural similarity between these three proteins may provide information regarding the mechanism(s) of membrane perforating toxins.

<applet load='Cyt2Baa.pdb' size='350' frame='true' align='left' caption=Cyt2Ba' />

===The 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  <scene name='Cyt2Ba/Alpha_beta/2'>α/β</scene> architecture with a <scene name='Cyt2Ba/Beta/1'>β-sheet</scene> surrounded by two <scene name='Cyt2Ba/Alpha/1'>α-helical</scene> 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.

 

Cyt2Ba shares only 16% sequence identity to VVA2 ([[1pp0]]), nevertheless they both adopt a cytolysin fold and their structure is very similar (see their <scene name='Cyt2Ba/Cyt2ba_vva/2'>structural alignment</scene>).

A striking similarity is observed between the structures of the endogenously cleaved Cyt2Ba <scene name='Cyt2Ba/Dimer_mesh/2'>monomer</scene> (gray) and the  <scene name='Cyt2Ba/Dimer_mesh/4'>corresponding region</scene> within the inactive protoxin  <scene name='Cyt2Ba/Dimer_mesh/6'>dimer</scene> 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  <scene name='Cyt2Ba/Dimer_mesh/7'>dimer interface</scene> of Cyt2Aa is held together by the intertwined N-terminal strands from both monomers. The cleavage of Cyt2Aa removes the <scene name='Cyt2Ba/Dimer_mesh/9'>N and C termini segments</scene>, thereby preventing dimer formation and hence releasing a <scene name='Cyt2Ba/Monomer_toxin/2'>monomer active toxin</scene>. 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.