Fig. 1. Domain organization, processing and structure of pacifastin-related members (panels A, B and C). (A) Presumed processing of a hypothetical pacifastin-related precursors (PP) into its pacifastin-related inhibitors (PI). The signal peptide (arrow) is followed by multiple PLD-related domains (PLD) (white boxes) that can be cleaved off if a dibasic cleavage site is present (scissors). (B) The six conserved cystein residues (Cys) of a PLD-related inhibitor domain. The specific pattern of three disulfide bridges is depicted by double lines and the reactive bond (P1–P1′) is located between the last two cysteins and represented by an asterisk (*). (C) Two-dimensional schematic structure of a PLD-related domain from N-terminus (N) to C-terminal part (C). Double lines depict the three disulfide bridges. The three anti-parallel beta-sheets are represented by three arrows and the reactive bond (P1–P1′) is indicated by an asterisk (*).

Fig. 2. Schematic representation of PLD-related inhibitor domains: SGPI-1 (group I), panel A and SGPI-2 (group II), panel B. SGPI amino acid residues are represented as circles and numbered from the N- to the C-terminus. The three anti-parallel beta-sheets are pictured as grey arrows and the three disulfide bridges are shown as green lines. A red dashed line represents the inner core interaction between the two involved residues (red circles). The residue colored in red, is a key residue in the species selective interaction with trypsin-like enzymes. The two residues of the reactive bond (P1–P1′) are colored in yellow, while neighboring residues that are important for enzyme interaction, are numbered from P12 to P5′. Blue lines represent the sub domains of the inhibitor that interact with the enzyme. Blue arrows illustrate the conformational changes that occur upon binding of the inhibitor to its target enzyme. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

An overview of pacifastin-related precursors in different arthropod species according to the protein database, Uniprot (http://beta.uniprot.org/)

This table is a summary of the pacifastin-related inhibitor domains and their coding precursors that were found in the Uniprot database. The pacifastin members are categorized according to order and organism and for each precursor, a Uniprot accession number is given as well as the level (source) at which the pacifastin-related precursor has been identified (genome prediction, transcript level or protein level).

Alignment of locust pacifastin-related inhibitors to illustrate the differences in core type (II and I) and in (predicted) peptidase specificity according to the P1 residue

The conserved cystein residues are marked in yellow, while the P1 residue at position 30 is marked in blue. The locust pacifastin-related inhibitors are sorted by the type of core interaction: I (K10/W25) marked in pink and II (F10/A26) marked in green. Notice the presence of the residues (Y10/G26) in SGPI-4C and LMPI-9. Based on the similarity of these two residues with F and A, these peptides are also predicted core II type inhibitors. The inhibitors that show no correlation between core type and peptidase specificity are marked in grey.