Complexation of two proteic insect inhibitors to the active site of chymotrypsin suggests decoupled roles for binding and selectivity

cDNA cloning of two different serine protease inhibitor precursors in the migratory locust, Locusta migratoria

Recently, a novel serine protease-inhibiting peptide family, designated as the 'pacifastin family', has been described in locusts and crayfish. All members of this family possess a characteristic cysteine-rich domain. The present study describes the cDNA cloning, sequencing and transcript distribution of two novel pacifastin-related peptide precursors in the migratory locust, Locusta migratoria. Only one of the encoded peptides (HI) was identified previously, whereas six others represent new members of the pacifastin family. Northern blot analysis showed that both precursor transcripts are present in adult locust fat body. These could not be detected in the midgut. Interestingly, an in silico data mining approach of the expressed sequence tags (EST) database revealed the existence of Manduca sexta and Bombyx mori cDNAs that display pronounced sequence similarities with these locust pacifastin-related transcripts.

Comparative structure analysis of proteinase inhibitors from the desert locust, Schistocerca gregaria

The solution structure of three small serine proteinase inhibitors, two natural and one engineered protein, SGCI (Schistocerca gregaria chymotrypsin inhibitor), SGCI[L30R, K31M] and SGTI (Schistocerca gregaria trypsin inhibitor), were determined by homonuclear NMR-spectroscopy. The molecules exhibit different specificities towards target proteinases, where SGCI is a good chymotrypsin inhibitor, its mutant is a potent trypsin inhibitor, and SGTI inhibits both proteinases weakly. Interestingly, SGTI is a much better inhibitor of insect proteinases than of the mammalian ones used in common assays. All three molecules have a similar fold composed from three antiparallel beta-pleated sheets with three disulfide bridges. The proteinase binding loop has a somewhat distinct geometry in all three peptides. Moreover, the stabilization of the structure is different in SGCI and SGTI. Proton-deuterium exchange experiments are indicative of a highly rigid core in SGTI but not in SGCI. We suggest that the observed structural properties play a significant role in the specificity of these inhibitors.