The oligomeric state of Bacillus thuringiensis Cry toxins in solution

The molecular mass of different Cry toxins produced by Bacillus thuringiens is bacteria was estimated by size-exclusion chromatography and non-denaturi ng polyacrylamide gel electrophoresis at neutral and alkaline pH in order t o assess the existence of oligomers in solution. We found that Cry1Aa, Cry1 Ac, Cry1C, Cry1D and Cry3A toxins exist in solution as a mixture of monomer and high molecular mass aggregates with an apparent molecular mass greater than 600 kDa, that depend on the time elapsed between toxin activation and analysis. Aggregation of toxins by disulfide bonds is unlikely because agg regates are also observed in samples incubated with DTT. These data show th at the Cry toxins studied do not form oligomers of less than ten subunits i n solution and suggest that oligomer formation may occur after the toxin bi nds to the receptor and inserts into the membrane. (C) 1999 Published by El sevier Science B.V. All rights reserved.