C. Egas et al., The saposin-like domain of the plant aspartic proteinase precursor is a potent inducer of vesicle leakage, J BIOL CHEM, 275(49), 2000, pp. 38190-38196
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.