Cyt proteins have received little study in comparison to Cry proteins, as they typically only occur in mosquitocidal strains of Bt and are not used in transgenic crops for insect control. Their mode of action will therefore be discussed only briefly here. As far as is known, Cyt proteins do not require a protein receptor but, instead, bind directly to the nonglycosylated lipid portion of the microvillar membrane. Once within the membrane, they appear to aggregate, forming lipid faults that cause an osmotic imbalance that results in cell lysis.32 Cyt1A plays an important role in the biology of B. thuringiensis subsp. israelensis, a species in which it is known that this protein synergizes the toxicity of the Cry4 and Cry11 proteins and delays the development of mosquito resistance to these.33 Cyt proteins likely play a similar role in other strains in which they occur, such as the PG14 isolate of B. thuringiensis subsp. morrisoni.
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