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).



Cohen, S., O. Dym, S. Albeck, E. Ben-Dov, R. Cahan, M. Firer, and A. Zaritsky. (2008). High-resolution crystal structure of activated Cyt2Ba monomer from Bacillus thuringiensis subsp. israelensis. J. Mol. Biol. 380: 820-827.

Manasherob, R., A. Zaritsky, Y. Metzler, E. Ben-Dov, M. Itsko, and I. Fishov. (2003). Compaction of the Escherichia coli nucleoid caused by Cyt1Aa. Microbiology 149: 3553-3564.

Tharad, S., J.L. Toca-Herrera, B. Promdonkoy, and C. Krittanai. (2016). Bacillus thuringiensis Cyt2Aa2 toxin disrupts cell membranes by forming large protein aggregates. Biosci Rep 36:.

Weng, Y.-P., Y.-P. Lin, C.-I. Hsu, and J.-Y. Lin. (2004). Functional domains of a pore-forming cardiotoxic protein, volvatoxin A2. J. Biol. Chem. 279: 6805-6814.


TC#NameOrganismal TypeExample
1.C.71.1.1The Cyt1Aa δ endotoxinBacteriaCyt1Aa of Bacillus thuringiensis subsp. israelensis (P0A382)

The Cyt2Aa δ endotoxin of 259 aas.  Cyt2Aa2 binds and aggregates on the lipid membrane leading to the formation of non-specific holes and disruption of the cell membrane (Tharad et al. 2016). The crystal structure is available (PDB 3RON).


Cyt2Aa of Bacillus thuringensis (Q04470)


Type 2Ba cytolytic δ-endotoxin of 263 aas, Cyt2Ba.  Kills the larvae of dipteran insects by making pores in the epithelial cell membrane of the insect midgut.  The x-ray structure has been solved (PDB 2RCI) (Cohen et al. 2008).

Cry2Ba of Bacillus thuringiensis


Uncharacterized toxin of 164 aas.

Toxin of Clostridium kluyveri


TC#NameOrganismal TypeExample
1.C.71.2.1The volvatoxin A2 precursorFungusVolvatoxin A2 precursor of Volvariella volvacea (Q6USC4)

Delta endotoxin, CytB of 206 aas

CytB of Gloeophyllum trabeum (Brown rot fungus)


Delta endotoxin CytB-like protein, ENDO, of 198 aas.

CytB of Rhizoctonia solani


Uncharacterized toxin of 178 aas

Toxin of Fomitopsis pinicola (Brown rot fungus)