This family of serine protease inhibitors belong to MEROPS inhibitor family I19, clan IW. They inhibit chymotrpsin, a peptidase belong to the S1 family .
They were first isolated from Locusta migratoria (migratory locust). These were HI, LMCI-1 (PMP-D2) and LMCI-2 (PMP-C) [1472051, 1740125, 10696590]; five additional members SGPI-1 to 5 were identified in Schistocerca gregaria (desert locust) [9475173, 11856311], and a heterodimeric serine protease inhibitor (pacifastin) was isolated from the hemolymph of Pacifastacus leniusculus (crayfish) .
Pacifastin is a 155-kDa composed of two covalently linked subunits, which are separately encoded. The heavy chain of pacifastin (105 kDa) is related to transferrins, containing three transferrin lobes, two of which seem to
be active for iron binding . A number of the members of the transferrin family are also serine peptidases belong to MEROPS peptidase family S60 . The light chain of pacifastin (44 kDa) is the proteinase inhibitory subunit, and has nine cysteine-rich inhibitory domains that are homologous to each other. The locust inhibitors share a conserved array of six cysteine residues with the pacifastin light chain. The structure of members of this family reveal that they are comprised of a triple-stranded antiparallel beta-sheet connected by three disulphide bridges .
The biological function(s) of the locust inhibitors is (are) not fully understood. LMCI-1 and LMCI-2 were shown to inhibit the endogenous proteolytic activating cascade of prophenoloxidase . Expression analysis shows that the genes encoding the SGPI precursors are differentially expressed in a time-, stage- and hormone-dependent manner.