A Mechanical Force Contributes to the "Osmotic Swelling" of Brush Border Membrane Vesicles
Martin Kirouac 1, Vincent Vachon 1, Mélanie Fortier 1, Marie-Claude Trudel 1, Alfred Berteloot 1, Jean-Louis Schwartz 1 and Raynald Laprade 1*
1 Université de Montréal
* To whom correspondence should be addressed. E-mail: raynald.laprade{at}umontreal.ca.
Submitted on May 9, 2006
Revised on July 10, 2006
Accepted on 18 July 2006
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Abstract |
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Brush border membrane vesicles and an osmotic swelling assay have been used extensively to monitor the pore-forming activity of Bacillus thuringiensis toxins. Following a hypertonic shock, Manduca sexta midgut brush border membrane vesicles shrink rapidly and re-swell partially to a volume that depends on membrane permeability and toxin concentration rather than re-gaining their original volume as expected from theoretical models. Because efflux of buffer from the vesicles, as they shrink, could contribute to this phenomenon, vesicles were mixed with a hypertonic solution of the buffer with which they were loaded. Under these conditions, they are not expected to re-swell since the same solute is present on both sides of the membrane. Nevertheless, with several buffers, vesicles re-swelled readily, an observation which demonstrates the involvement of an additional restoration force. Re-swelling also occurred when, in the absence of toxin, the buffers were replaced by glucose, a solute which diffuses readily across the membrane, but did not occur with rat liver microsomes, despite their permeability to glucose. Unexpected swelling was also observed with rabbit jejunum brush border membrane vesicles suggesting that the cytoskeleton, present in brush border membrane vesicles but absent from microsomes, could be responsible for the restoration force.
Key Words:
Bacillus thuringiensis, Brush border membrane vesicles, Light-scattering assay, Manduca sexta, Membrane permeability, Pore-forming toxin
