This domain is found in the protoxins portion of insecticidal proteins (parasporins, or Cry proteins) such as those from Bacillus thuringiensis (Bt) Cry1Ac. The protoxin portion comprise a proteolytically labile C-terminal segment (sometimes referred to as the protoxin domain). This is domain V in Cry1Ac from B. thuringiensis. One of the four protoxin domains (D-IV through D-VII). Domains V and VII are beta-rolls (similar to D-II or D-III) that closely resemble carbohydrate-binding modules (CBM) found in sugar hydrolases, however, it is difficult to guess which particular carbohydrates (if any) may serve as their ligands because residues on the putative sugar-binding interfaces are conserved neither in sequence nor in local structure. Structural analysis indicate that there are putative disulfide crosslinking at the dimer interface mediated by cysteines within 783-823 region of this domain which together with other cysteines creates a three-dimensional network of cross-links across the crystal which may play a role in stabilizing mature Bt Cry1Ac [1].

1: Structure of the full-length insecticidal protein Cry1Ac reveals intriguing details of toxin packaging into in vivo formed crystals. Evdokimov AG, Moshiri F, Sturman EJ, Rydel TJ, Zheng M, Seale JW, Franklin S; Protein Sci. 2014;23:1491-1497. PMID:25139047