1.C.71 The Cytolytic Delta Endotoxin (Cyt1/2) Family

Cyt1Aa kills the larvae of dipteran insects by making cation-selective pores in the epithelial cell membrane of the insect mid gut. The pores are 1-2 nm in diameter. They lead to osmotic lysis of the cells. The toxin acts on mosquitoes and black flies. It is active after proteolytic processing. Cyt1Aa exhibits 4 putative TMSs and is 249 aas long.

This crystal protein is produced during sporulation of Bacillus thuringiensis. It accumulates both as an inclusion and as part of the spore coat. This family is the Cyt1/Cyt2 family of SwissProt. Cyt1Aa is homologous to Cyt2Aa from B. thuringiensis. Cyt2Aa has a single pore-forming domain composed of two outer layers of α-helical hairpins wrapped around mixed β-sheets. Cyt1Aa and Cyt2Aa are 41% identical, 61% similar in amino acid sequence. These toxins are distantly related (19% identity, 40% similarity) to the fungal (mushroom) toxin, volvatoxin A2, from Volvariella volvacea (Weng et al., 2004). In volvatoxin A2, the N-terminal domain is responsible for oligomerization, and the C-terminal domain is responsible for membrane binding and insertion. Prepore oligomeric complex formation appears to precede membrane insertion (Weng et al., 2004).

The reaction catalyzed by Cyt1Aa is:

cations (in) cations (out).