Mechanistic insights into the first Lygus-active β-pore forming protein

The cotton pests Lygus hesperus and Lygus lineolaris can be controlled by expressing Cry51Aa2.834_16 in cotton. Insecticidal activity of pore-forming proteins is generally associated with damage to the midgut epithelium due to pores, and their biological specificity results from a set of key determinants including proteolytic activation and receptor binding. We conducted mechanistic studies to gain insight into how the first Lygus-active β-pore forming protein variant functions. Biophysical characterization revealed that the full-length Cry51Aa2.834_16 was a stable dimer in solution, and when exposed to Lygus saliva or to trypsin, the protein underwent proteolytic cleavage at the C-terminus of each of the subunits, resulting in dissociation of the dimer to two separate monomers. The monomer showed tight binding to a specific protein in Lygus brush border membranes, and also formed a membrane-associated oligomeric complex both in vitro and in vivo. Chemically cross-linking the β-hairpin to the Cry51Aa2.834_16 body rendered the protein inactive, but still competent to compete for binding sites with the native protein in vivo. Our study suggests that disassociation of the Cry51Aa2.834_16 dimer into monomeric units with unoccupied head-region and sterically unhindered β-hairpin is required for brush border membrane binding, oligomerization, and the subsequent steps leading to insect mortality.

Single nucleotide deletion of cqm1 gene results in the development of resistance to Bacillus sphaericus in Culex quinquefasciatus

The entomopathogen Bacillus sphaericus is one of the most effective biolarvicides used to control the Culex species of mosquito. The appearance of resistance in mosquitoes to this bacterium, however, remains a threat to its continuous use in integrated mosquito control programs. Previous work showed that the resistance to B. sphaericus in Culex colonies was associated with the absence of the 60-kDa binary toxin receptor (Cpm1/Cqm1), an alpha-glucosidase present in the larval midgut microvilli. In this work, we studied the molecular basis of the resistance developed by Culex quinquefasciatus to B. sphaericus C3-41. The cqm1 genes were cloned from susceptible (CqSL) and resistant (CqRL/C3-41) colonies, respectively. The sequence of the cDNA and genomic DNA derived from CqRL/C3-41 colony differed from that of CqSL one by a one-nucleotide deletion which resulted in a premature stop codon, leading to production of a truncated protein. Recombinant Cqm1S from the CqSL colony expressed in Escherichia coli specifically bound to the Bin toxin and had α-glucosidase activity, whereas the Cqm1R from the CqRL/C3-41 colony, with a deletion of three quarters of the receptor's C-terminal lost its α-glucosidase activity and could not bind to the binary toxin. Immunoblotting experiments showed that Cqm1 was undetectable in CqRL/C3-41 larvae, although the gene was correctly transcribed. Thus, the cqm1R represents a new allele in C. quinquefasciatus that confers resistance to B. sphaericus.

Biological functions of sphingomyelins

Sphingomyelin (SM) is a dominant sphingolipid in membranes of mammalian cells and this lipid class is specifically enriched in the plasma membrane, the endocytic recycling compartment, and the trans Golgi network. The distribution of SM and cholesterol among cellular compartments correlate. Sphingolipids have extensive hydrogen-bonding capabilities which together with their saturated nature facilitate the formation of sphingolipid and SM-enriched lateral domains in membranes. Cholesterol prefers to interact with SMs and this interaction has many important functional consequences. In this review, the synthesis, regulation, and intracellular distribution of SMs are discussed. The many direct roles played by membrane SM in various cellular functions and processes will also be discussed. These include involvement in the regulation of endocytosis and receptor-mediated ligand uptake, in ion channel and G-protein coupled receptor function, in protein sorting, and functioning as receptor molecules for various bacterial toxins, and for non-bacterial pore-forming toxins. SM is also an important constituent of the eye lens membrane, and is believed to participate in the regulation of various nuclear functions. SM is an independent risk factor in the development of cardiovascular disease, and new studies have shed light on possible mechanism behind its role in atherogenesis.