Identification in lupin seed of a serine-endopeptidase activity cleaving between twin arginine pairs and causing limited proteolysis of seed storage proteins

Structural and functional insights into the basic globulin 7S of soybean seeds by using trypsin as a molecular probe

The basic 7S globulin (Bg7S) is one of the major globulins of soybean seeds. Despite its dual subunit composition and oligomeric assembly, Bg7S has a compact 3D structure (PDB: 3AUP) which is stabilized by a network of inter- and intra-chain disulphide bridges. Bg7S shares several structural elements with a number of homologous proteins from other seeds, whose function is still uncertain. In this work, Bg7S native conformation was probed by using the proteolytic enzyme trypsin. In spite of the presence of many arginine and lysine residues, the protein resulted extremely recalcitrant to in vitro enzymatic cleavage. Indeed, only two scissile bonds located near the C- and N-termini of the large and small subunits, respectively, were cleaved. The partially cleaved products were stable even at prolonged incubation times. Although the generated small peptide fragments were not covalently bound to the remnant of the main chains, they were held in place, as assessed by denaturing and non-denaturing chromatographic approaches. Moreover, both the already observed pH-dependent association/dissociation behaviour of the protein and its insulin binding capacity were preserved both at neutral and acidic pH values. These results are in line with the growing view that the degradation of seed proteins, either storage and non-storage, may be a controlled process related to specific functionalities.

Proteolysis of the peanut allergen Ara h 1 by an endogenous aspartic protease

The 7S and 11S globulins of peanuts are subjected to proteolysis two days after seed imbibition, with Ara h 1 and the arachin acidic chains being among the first storage proteins to be mobilized. Proteolytic activity was greatest at pH 2.6-3 and is inhibited by pepstatin A, characteristic of an aspartic protease. This activity persists in seedling cotyledons up to at least 8 days after imbibition. In vitro proteolysis of Ara h 1 at pH 2.6 by extracts of cotyledons from seedlings harvested 24 h after seed imbibition generates newly appearing bands on SDS-PAGE. Partial sequences of Ara h 1 that were obtained through LC-MS/MS analysis of in-gel trypsin digests of those bands, combined with information on fragment size, suggest that proteolysis begins in the region that links the two cupin domains to produce two 33/34 kD fragments, each one encompassing an intact cupin domain. The later appearance of two 18 and 10/11 kD fragments can be explained by proteolysis within an exposed site in the cupin domains of each of the 33/34 kD fragments. The same or similar proteolytic activity was observed in developing seeds, but Ara h 1 remains intact through seed maturation. This is partly explained by the observation that acidification of the protein storage vacuoles, demonstrated by vacuolar accumulation of acridine orange that was dissipated by a membrane-permeable base, occurs only after germination. These findings suggest a method for use of the seed aspartic protease in reducing peanut allergy due to Ara h 1.

Proteolytic cleavage at twin arginine residues affects structural and functional transitions of lupin seed 11S storage globulin

The 11S storage globulin of white lupin seeds binds to a metal affinity chromatography matrix. Two unusual stretches of contiguous histidine residues, reminiscent of the multiple histidines forming metal binding motifs, at the C-terminal end of 11S globulin acidic chains were hypothesized as candidate elements responsible for the binding capacity. To prove this, the protein was incubated with a lupin seed endopeptidase previously shown to cleave at twin arginine motifs, recurrent in the sequence region of interest. Upon incubation with this enzyme, the loss of metal binding capacity paralleled that of the anti-his-tag reactive polypeptides. The recovered small proteolytic fragment was analyzed by mass spectrometry and N-terminal sequencing and found to correspond to the 24-mer region cleaved off at twin arginine residues and containing the natural his-tag-like region. Similarly, when lupin seeds were germinated for a few days, the his-tag containing 11S globulin chain was converted to a form devoid of such region, suggesting that this mechanism is a part of the natural degradatory process of the protein. The hypothesis that the ordered and controlled dismantling of storage proteins may generate peptide fragments with potential functional roles in plant ontogenesis is presented and discussed.