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

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/5'>α/β</scene> architecture with a <scene name='Cyt2Ba/Beta/2'>β-sheet</scene> surrounded by two <scene name='Cyt2Ba/Alpha/2'>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/3'>structural alignment</scene>).

A striking similarity is observed between the structures of the endogenously cleaved Cyt2Ba <scene name='Cyt2Ba/Cyt2ba_monomer/2'>monomer</scene> (gray) and the <scene name='Cyt2Ba/Alignment/2'>corresponding region</scene> within the inactive protoxin  <scene name='Cyt2Ba/Dimer/2'>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/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 termini segments, thereby preventing dimer formation and hence releasing a <scene name='Cyt2Ba/Monomer_toxin/4'>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.