SHORT COMMUNICATION INHIBITION OF POTASSIUM-GRADffiNT-DRIVEN PHENYLALANINE UPTAKE IN LARVAL LYMANTRIA DISPAR MIDGUT BY TWO BACILLUS THURINGIENSIS DELTA- ENDOTOXINS CORRELATES WITH THE ACTIVITY OF THE TOXINS AS GYPSY MOTH LARVICIDES

During speculation, Bacillus thuringiensis produces parasporal inclusions with insecticidal activity. The parasporal inclusions produced by most subspecies of B. thuringiensis are active only against the larvae of a few lepidopteran insects. Lepidopteran-active parasporal inclusions are usually bipyramidal crystals composed of one or more 130x lCP-l^Ox 10 Mr polypeptides. These polypeptides are designated as protoxins. The complete insecticidal activity of each protoxin resides in a 55 x 10-70 x 10 Mr protease-resistant toxin which results from solubilization and partial digestion of the crystals in the larval midgut (Aronson et al. 1987). The target of lepidopteran-active B. thuringiensis toxins is the brush-border membrane of larval lepidopteran midgut (Liithy et al. 1986). This insect cell membrane contains specific high-affinity receptors, of unknown normal physiological function, for B. thuringiensis toxins (Hofmann et al. 1988a; VanRie et al. 19906). After binding to the membrane receptors, the toxin or the toxin-receptor complex forms a pore that is the primary lesion in the mode of action of these cytolytic toxins (Wolfersberger, 1990a). In many cases, the larvicidal activity of a B. thuringiensis toxin has been found to correlate directly with the concentration and/or affinity of receptors for the toxin in the larval insect midgut (Hofmann et al. 19886; VanRie et al. 1989, 19906). These observations led to the suggestion that receptor binding was the primary determinant of a toxin's larvicidal activity (VanRie et al. 1990a). However, toxin alone at very high concentrations is able to form pores in artificial lipid membranes that contain no receptors (Slatin et al. 1990) and certain toxins are specifically bound with high affinity by brush-border membrane vesicles (BBMV) prepared from midguts of insects against which the toxins show little or no larvicidal activity (VanRie et al. 1990a; Wolfersberger, 19906). The latter observation, in particular, favors the suggestion (Wolfersberger, 19906) that the ability of a toxin to form a membrane pore is a more important determinant of its larvicidal activity than its binding characteristics.