Structural and Functional Features of a Wheat Germin-Like Protein that Inhibits Trypsin

The wheat leaf apoplast contains a protein that inhibits trypsin and belongs to the family of germin-like proteins called germin-like protease inhibitor (GLPI). Since it was first described in our laboratory, the objective of this study was to find out if GLPI is a new germin-like protein and to identify the molecular site responsible for its inhibitory action. Amino acid sequence fragments of GLPI have been determined using mass spectrometry and used to synthesize complementary DNA by reverse transcription PCR. This has allowed recovery of the amino acid sequence of the mature form of GLPI, which is indistinguishable from barley GLP and having pyrophosphatase/phosphodiesterase activity. Using chemical modifiers of amino acids, the unique Arg of GLPI is found to be necessary for preserving its protease inhibition activity. Furthermore, structural homology modeling has allowed prediction that Arg is located along the GLPI surface, which could aid in its activity on proteases. Given that GLPI acts as a superoxide dismutase and as pyrophosphatase/phosphodiesterase, it is deemed to be a multifunctional protein.