Cry protein

Taken together, these data indicate that light can cause an increase in neuronal activity in the SCN of wC -deficient mice, and that the absence of CRY proteins does not prevent light signalling from the retina to the SCN. Moreover, the ability of the / Crj-deBcient SCN to respond to light exposure electrophysiologically is lost upon ageing. 

The mammalian molecular oscillator, similar to that in Drosophila and Neurospora, is composed of two interconnected feedback loops, one within the negative limb and one within the positive limb. The members of the positive limb (CLOCK and BMALl) activate transcription of the genes encoding the negative limb components (cryptochromes and period proteins). PER and CRY proteins then form heterotypic protein complexes that are translocated into the nucleus, and once these complexes reach a critical threshold level, they suppress the activity of CLOCK and BMALL As a consequence, the concentration of CRY and PER proteins falls below the threshold required for autorepression, and a new cycle of CryjPer transcription can initiate (for review, see Reppert Weaver 2002). 

Reduced binding of toxin is a primary mechanism of insect resistance to Cry proteins (Ferre and Van Rie, 2002). Resistance of the Indian meal moth to Bt was due to a 50-fold reduction in affinity of the membrane receptor (from brush borer membrane vesicles of larval midguts) for 5-endotoxin. In pinkbollworms, Bt resistance is associated with reduced binding of CrylAb to the brush borer membrane vesicle. Similarly, CrylAc resistance in diamondback moths was demonstrated to be due to dramatically reduced target binding. 

Proteolytic activation of Cry toxins is critical not only for protoxin activation, but has also implications for toxin specificity [59,60], receptor binding [61], and insect resistance [62,63]. The absence of a major gut protease in Plodia interpmctdla correlated with its resistance to Cry 1 Ac [62]. Moreover, rapid degradation of Cry toxins was associated with the loss of sensitivity of S instar 5. litoralis larvae to Cry 1C [64], and serine protease inhibitors enhanced the toxicity of some Cry proteins up to 20-fold [65]. 

Early studies showed a correlation between the biological toxicities of Cry proteins and their abilities to bind to midgut brush border membrane vesicles (BBMV) of susceptible larvae [69-71]. Toxin binding to membrane is a two-stage process involving reversible and irreversible steps, with irreversible binding generally associated with membrane insertion [69,70,72,73]. 

M. Ekobu. An artificial diet for screening Bacillus thuringiensis Cry proteins against sweet potato weevils, Cylas puncticollis and Cylas brunneus. MSc thesis, Faculty of Agriculture, Makerere University, 2006, p. 1-99. 

As mentioned, the 8-endotoxin (Cry protein) that makes up most of the conspicuous crystals is the main insecticidal component of B. thuringiensis. At sporulation, the majority of Bt strains produce crystalline inclusions that contain this insecticidal 8-endotoxin. The crystals account for 20% or more of the total bacterial protein at sporulation and may contain one or several endotoxins, which differ in activity. Many Bt toxin genes and genes for some... 

Engineering of Cry proteins to create better pesticides is possible. A mutant of CrylB resulted in a threefold increase of toxicity against the mosquito, perhaps by removal of a site sensitive to proteolytic instability. More often, increased receptor binding causes the increased efficiency of mutant types. 

Bee larval tests with Bt Cry proteins have been carried out in-house by biotech companies (S. Sims, personal communication), but there are no published reports of such work. Laboratory-based methods for rearing bee larvae have been established [48] and these could be adapted for use with Bt and other pest-resistance proteins, especially for the later larval instars which are known to ingest pollen as well as the glandular secretions of adult bees. 

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