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Insecticidal crystal proteins (ICPs) from Bacillus thuringiensis have been used as biopesticides for the last 35 years. B. thuringiensis is a gram-positive bacterium which produces proteinaceous inclusions during sporulation; these inclusions can be distinguished as distinctively shaped crystals by phase-contrast microscopy. The inclusions are composed of proteins known as ICPs, Cry proteins, or d-endotoxins, which are highly toxic to a wide variety of important agricultural and healthrelated insect pests as well as other invertebrates. Due to their high specificity and their safety for the environment, ICPs are a valuable alternative to chemical pesticides for control of insect pests in agriculture and forestry and in the home. It has been proposed that the rational use of B. thuringiensis toxins will provide a variety of alternatives for insect control and for coping with the problem of insect resistance to pesticides. Intensive screening programs have identified strains of B. thuringiensis from soil samples, plant surfaces, dead insects, and stored grains from all over the world. The isolated strains show a wide range of specificity against different insect orders (Lepidoptera, Diptera, Coleoptera, Hymenoptera, Homoptera, Phthiraptera or Mallophaga, and Acari) and other invertebrates (Nemathelminthes, Platyhelminthes, and Sarcomastigorphora) (13). Currently 45 different serotypes have been catalogued, representing a total number of 58 serovars (28). Many of the ICP genes have been cloned, sequenced, and classified as cry and cty genes. The first classification was based on insecticidal activity (23), with the different Cry proteins denoting ICPs toxic to various insect and invertebrate groups as follows: CryI toxic to lepidopterans, CryII toxic to lepidopterans and dipterans, CryIII toxic to coleopterans, CryIV toxic to dipterans, and CryV and CryVI toxic to nematodes (14). Novel cry genes isolated recently have created some problems for this classification scheme, especially genes that were homologous to known genes but displayed different specificities and genes that had dual specificity. Recently, a novel nomenclature has been proposed based exclusively on amino acid identity (7). To date, over 50 cry gene sequences have been determined and classified into 15 families (7). The cry genes code for proteins with a range of molecular masses from 50 to 140 kDa. Upon ingestion by the susceptible target, the protoxins are solubilized and proteolytically processed to release the toxic fragment (23). During proteolytic activation, peptides are removed from both aminoand carboxyl-terminal ends of the protoxin. For the 130to 140-kDa protoxins, the carboxyl-terminal proteolytic activation removes half of the molecule, resulting in an active toxin fragment of 60 to 70 kDa. A generally accepted model for Cry toxin action is that it is a multistage process. First, the activated toxin binds to receptors located on the apical microvillus membrane of epithelial midgut cells (6, 22, 49). After the toxin binds the receptor, it is thought that there is a change in the toxin conformation allowing toxin insertion into the membrane. Oligomerization of the toxin follows, and this oligomer then forms a pore that leads to osmotic cell lysis (26a, 30, 32, 40). Receptor binding is a key factor in specificity. Two different insect proteins have been identified as receptors for Cry toxins, the 120-kDa aminopeptidase N Cry1Ac toxin-binding protein purified from brush border vesicles of Manduca sexta, Heliothis virescens, and Lymantria dispar (20, 25, 41, 48) and the 210-kDa cadherin-like glycoprotein Cry1Ab toxin-binding protein purified from M. sexta membranes (47). Specific binding involves two steps, one that is reversible and one that is irreversible. Recent data suggest that toxicity correlates with irreversible binding (31). Irreversible binding might be related to insertion of the toxin into the membrane but could also reflect a tighter interaction of the toxin with the receptor. The crystal structures of Cry3A (coleopteran-specific) and Cry1Aa (lepidopteran-specific) toxins have been reported (21, 30). The Cry3A protoxin has a molecular mass of 70 kDa and does not contain the large carboxyl-terminal extension contained in the Cry1Aa toxin. The crystal structure of Cry1Aa toxin was determined from the activated toxin fragment. Both toxins share 36% amino acid identity, and the two structures show high overall similarity (21). Both are globular molecules containing three distinct domains connected by single linkers. Domain I extends from residues 33 to 253 in Cry1Aa and from residues 58 to 290 in Cry3A; it is a seven a-helix bundle in which a central helix (helix a-5) is completely surrounded by six outer helices (Fig. 1A). This domain has been implicated in the channel formation in the membrane. The six a-helices are amphipathic and are long enough to span the 30-A-thick hydrophobic region of a membrane bilayer. Point mutations in the region encoding the central a-5 helix of the Cry1Ac toxin (residues 163 to 170) drastically affect toxicity without affecting binding to larval midgut vesicles (52). Residues 265 to 461 in Cry1Aa and 291 to 500 in Cry3A form domain II (Fig. 1B). Domain II consists of three antiparallel b-sheets with similar topologies packed around a hydrophobic core. This domain represents the most divergent part in structure between the two toxin molecules (21). This domain has been described as the specificity-determining domain, since reciprocal hybrid genes between closely related toxins * Mailing address: Department of Microbiology, Instituto de Biotecnologia, Universidad Nacional Autonoma de Mexico, Apdo. Postal 510-3, Cuernavaca, Morelos, Mexico. Phone: (52) (5) 622-7635. Fax: (52) (73) 17-2388. E-mail: bravo@ibt.unam.mx.

Phylogenetic relationships of Bacillus thuringiensis delta-endotoxin family proteins and their functional domains