The saposin-like domain of the plant aspartic proteinase precursor is a potent inducer of vesicle leakage

A unique feature of plant aspartic proteinase precursors is the presence of an internal domain, known as plant-specific insert, whose function is not completely understood. The three-dimensional structure of the plant-specifi c insert resembles that of saposin-like proteins, a group of lipid-binding proteins involved in a variety of physiological processes. Here we show tha t recombinant plant-specific insert is able to interact with phospholipid v esicles and to induce leakage of their contents in a pH- and lipid-dependen t manner. The leakage activity is higher at pH 4.5 and requires the presenc e of acidic phospholipids such as phosphatidylserine. To determine whether the same effect could be observed when the plant-specific insert is part of the precursor form, procardosin A and a mutant form lacking this specific domain were produced and characterized. Procardosin A displays a similar ac tivity profile, whereas the mutant without the plant-specific insert shows only residual activity. These findings indicate that the plant-specific ins ert domain of plant aspartic proteinases mediates an interaction of their p recursors with phospholipid membranes and induces membrane permeabilization . It is therefore possible that the plant-specific insert, alone or in conj unction with the proteolytic activity of plant aspartic proteinases, may fu nction either as a defensive weapon against pathogens or in late autolysis of plant cells.