Plant Pin-II Family Proteinase Inhibitors: Structural and Functional Diversity

Potato inhibitor II (Pin-II) proteins are plant serine proteinase inhibitors (PIs) that occur in various plant species. Pin-II PIs are characterized by inhibitory repeat domains (IRDs) which form their functional units and show extensive sequence variation. Various studies have been conducted to gauge the occurrence (spatial and temporal) of Pin-II PIs and their appearance in response to biotic stresses mainly herbivore attack. Many hypotheses have been proposed to justify the mechanism which has led to the evolution of Pin-II PIs as well as the selection pressure in force. In spite of ample diversity, these molecules are highlighted by their conserved features with respect to their gene and protein sequence and structure. The structural studies highlight the crucial role of conserved residues in stabilizing the reactive loops and the three-dimensional conformation of Pin-II PIs. The remarkable flexibility of reactive loops allows their binding to a wide range of proteinases (either endogenous or from the pest). Apart from defense, Pin-II PIs have also been speculated to have a significant role in endogenous functions, namely regulation of proteolysis, macromolecular trafficking, programmed cell death and consequently aid the plant growth and development in respective tissue. There have been attempts to test these candidates for their potential in insect control. In vitro experiments and insect bioassays at laboratory scale have given encouraging results and led to the field experiments in order to develop transgenic plants fortified with Pin-II PIs. This has also helped judging the fitness costs that usually transgenic plants have to pay in return of incorporating the foreign trait. _____________________________________________________________________________________________________________