1.C.105 The Bacillus thuringiensis Vegetative Insecticidal Protein-3 (Vip3) Family

Bacillus thuringiensis is an entomopathogenic bacterium producing parasporal proteinaceous insecticidal crystal inclusions during sporulation. Many strains are capable of also expressing other insecticidal proteins called Vip during the vegetative growing phase. Vip3A proteins have activity against certain Lepidoptera species through a unique mechanism of action which emphasized their possible use in resistance management strategies against resistant pests. Thirteen vip3Aa genes were identified in strains belonging to 10 different B. thuringiensis serovars. Three intra-subclass variants of vip3Aa genes could be differentiated (Sauka et al. 2012). This is the first report where variants of a single subclass of insecticidal genes were distinguished following PCR-RFLP.

The entomopathogenic bacterium, B. thuringiensis, produces parasporal crystalline inclusions during sporulation containing insecticidal proteins such as Cry and Cyt proteins [Sauka and Benintende, 2008], but many strains also produce other kinds of insecticidal proteins during the vegetative growing phase that do not form crystals and these are called Vip (vegetative insecticidal proteins) [Estruch et al., 1996]. This type of proteins include Vip1, Vip2 and Vip3. Vip1 (TC#1.C.42) and Vip2 are the components of binary toxins that have coleopteran specificity, whereas Vip3 proteins have activity against a wide variety of lepidopteran pests [Estruch et al., 1996; Warren, 1997]. To date, these three groups of Vip proteins can also be classified into 9 subgroups, 27 classes and 92 subclasses according to their amino acid sequence similarity (http://www.lifesci.sussex.ac.uk/home/Neil_Crickmore/Bt/vip.html).

Among these proteins, Vip3A do not share homology with other known proteins and act against lepidopteran larvae through a unique mechanism of action [Bhalla et al., 2005; Estruch et al., 1996; Lee et al. 2003, 2006]. These proteins act by binding to specific receptors, different when compared to that of Cry, located in the midgut epithelium of susceptible insect species. Then, cation-specific pores are formed that disrupt ion flow in the midgut, causing paralysis and death [Lee et al., 2006].