Bacillus thuriingensis

[Joseph G. Kunkel]

Davide wrote:
> 
> Can anyone explain why the Bt protein used in transgenic Zea mais is toxic
> in some lepidoptera?

>From URL:
http://nightshade.cit.cornell.edu/coursepak/LAMBERT.html

"Mode of action of the insecticidal crystal proteins
What happens after ingestion of spores and crystals? After an insect
larva ingests Bt spores containing crystals, several scenarios are
possible. Because the alkaline environment of the  midgut of most insect
larvae is unfavorable for spore germination and multiplication of Bt, it
is most likely that the spores remain intact, but the crystal proteins
solubilize and are converted into toxic fragments that cause structural
deformation of midgut epithelial cells, disintegration of the
microvillar membrane, and destruction of the midgut ..."

>From URL:
http://www.bio.cam.ac.uk/dept/biochem/UTOs/Ellar.html

"During sporulation, Bacillus thuringiensis (Bt) synthesises a
cytoplasmic inclusion containing one or more proteins that are toxic to
insect larvae. Upon completion of sporulation the parent bacterium lyses
to release the spore and the inclusion. In these inclusions, the toxins
exist as inactive protoxins. When the inclusions are ingested by insect
larvae, they dissolve in the midgut and the protoxin is converted to the
active form by 'trimming' with gut proteases. Work in this laboratory
over the past ten years has indicated that activated toxin then binds to
and inserts into the membrane of midgut epithelial cells via
insect-specific receptors, creating transmembrane leakage pores that
cause cell swelling and lysis and death of the insect. Due to their high
specificity for these unique receptors on the membrane of the gut
epithelial cells, these toxins (delta-endotoxins) are harmless to
non-target insects and the end-user and are compatible with integrated
pest management programmes. The fact that they are proteins ensures that
they are readily biodegraded."


-- 
--------------------
Joseph G. Kunkel, Professor
Biology Department             joe at bio.umass.edu
University of Massachusetts    http://www.bio.umass.edu/biology/kunkel
Amherst MA 01003