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

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.