THE STRUCTURE AND ORGANIZATION WITHIN THE MEMBRANE OF THE HELICES COMPOSING THE PORE-FORMING DOMAIN OF BACILLUS-THURINGIENSIS DELTA-ENDOTOXIN ARE CONSISTENT WITH AN UMBRELLA-LIKE STRUCTURE OF THE PORE

The aim of this study was to elucidate the mechanism of membrane inser tion and the structural organization of pores formed by Bacillus thuri ngiensis delta-endotoxin, We determined the relative affinities for me mbranes of peptides corresponding to the seven helices that compose th e toxin pore-forming domain, their modes of membrane interaction, thei r structures within membranes, and their orientations relative to the membrane normal, In addition, we used resonance energy transfer measur ements of all possible combinatorial pairs of membrane-bound helices t o map the network of interactions between helices in their membrane-bo und state. The interaction of the helices with the bilayer membrane wa s also probed by a Monte Carlo simulation protocol to determine lowest -energy orientations. Our results are consistent with a situation in w hich helices alpha 4 and alpha 5 insert into the membrane as a helical hairpin in an antiparallel manner, while the other helices lie on the membrane surface like the ribs of an umbrella (the ''umbrella model'' ). Our results also support the suggestion that (alpha 7 may serve as a binding sensor to initiate the structural rearrangement of the pore- forming domain.