The channel-forming protein proaerolysin remains a dimer at low concentrations in solution

Proaerolysin, the preform of the channel-forming protein aerolysin, is secr eted as a dimer by Aeromonas sp, The protein also exists as a dimer in the crystal, as well as in solution, at least at concentrations in the region o f 500 mug/ml. Recently it has been argued that proaerolysin becomes monomer ic at concentrations below 100 mug/ml and that only the monomeric form of t he protoxin can bind to cell surface receptors (Fivaz, M,, Velluz, DI,-C,, and van der Goot, F. G. (1999) J. Biol. Chem. 274, 37705-37708), Here we sh ow, using non-denaturing polyacrylamide electrophoresis, chemical cross-lin king, and analytical ultracentrifugation, that proaerolysin remains dimeric at the lowest concentrations of the protein that we measured (less than 5 mug/ml) and that the dimeric protoxin is quite capable of receptor binding.