Subunit number and arrangement in Escherichia coli heat-labile enterotoxin

The Escherichia coli heat-labile enterotoxin (LT) was found to have the same subunit structure as cholera toxin, namely, one A subunit and five B subunits. Reaction with a bisimidate generated all the possible cross-linked derivatives of A5B: B,2B ... 5B and A, AB ... A5B. The isolated B component, coligenoid, contained five B subunits and showed some tendency of polymerize: with a bisimidate it became covalently connected into the set B ... 5B with lesser amounts of 6B ... 10B, etc. The subunit formulas of two independently prepared samples of LT were both proved to be A5B by cross-linking, but their B pentamers migrated at different rates on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, indicating that they have different conformations. The faster (R) form could be converted to a diffuse slower (C) form by incubating it at 50 degrees C or at 37 degrees C with 0.2 M galactose, which is the terminal sugar of ganglioside GM1, the natural receptor for LT. Cholera toxin resembled the R form more than the C form of LT.