Metal ions restore the proteolytic resistance of denatured conglutin gamma, a lupin seed glycoprotein, by promoting its refolding

Basic 7S globulin in plants

Soybean seed basic 7S globulin (Bg7S)-like proteins are found in many plant species. Bg7S was originally thought to be a major seed storage protein but was later found to be multifunctional, with stress response, antibacterial activity, hormone receptor-like activity. Moreover, functional differences between Bg7S proteins from legumes and other plants have been revealed. In non-leguminous plants, Bg7S molecules inhibit the invasion of pathogenic microorganisms. However, although leguminous plants have a peptide called leg-insulin that can bind to Bg7S, non-leguminous plants do not have leginsulin. Bg7S in leguminous plants and other plants may have evolved in functionally different directions. Several homologs of Bg7S in plants are reported, but there is no homolog of this protein in peas, suggesting that the pea evolution might have followed a different route when compared to other leguminous plants. Although the functions of Bg7S are well documented in plants, recent studies suggest that this protein is also important in controlling blood glucose level, blood pressure and plasma cholesterol level, and cancer cell antiproliferative actions.

Insight into thermally induced structural changes of lupin seed γ-conglutin

A multidimensional analysis aimed to determine the thermal impact on γ-conglutin at the two oligomeric states was carried out. A wide range of biophysical and bioinformatic methods allowed to get insight into a thermal unfolding mechanism. The determined midpoint transition temperature (T_m) values were remarkably different, being 56.5 °C and 71.1 °C for γ-conglutin monomer and hexamer, respectively. The unfolding pattern for hexamer molecules included aggregation/precipitation, while monomers tended to form soluble aggregates after heat exposure. Interestingly, differences in the aromatic amino acid residues movements indicate that during thermal treatment of γ-conglutin hexamer red-shift occurred contrary to the monomer in the case of which blue-shift was noted. The obtained results provide an essential contribution to expand our knowledge about the molecular characterization of this intriguing lupin seed protein.

Post-translational cleavage pattern of Lupinus angustifolius γ-conglutin

BACKGROUND

Protein post-translational modifications are a key element for the functional diversity of the proteome. The modifications generally refer to the addition of functional groups to certain proteins; however, proteolytic cleavage is also one of the relevant events during protein maturation. γ-Conglutin is a unique protein fraction present in lupin seeds that is marked by numerous unusual properties. This protein fraction undergoes very complex post-translational maturation. Unfortunately, the precise mechanism of γ-conglutin post-translational processing is not yet fully understood.

RESULTS

Two independent methods were used to study γ-conglutin post-translational cleavage processing. Edman N-terminal sequencing indicates that the signal peptide is processed at Tyr34, while α- and β-subunit cleavage takes place between Ser295 and Ser296. High-resolution mass spectrometry revealed a great diversity of N-terminal sequences of γ-conglutin α-subunit. However, most abundant peptides also began from Tyr34. Mass spectrometric analyses additionally confirmed the subunit cleavage position between two serine residues.

CONCLUSIONS

The results indicate that the proteolytic processing of γ-conglutin signal peptide is not precise. On the other hand, the post-translational cleavage between α- and β-subunits of γ-conglutin is very conserved. Interestingly, the results also indicate that proteolytic processing leading to the formation of two subunits of γ-conglutin is incomplete, leaving a certain amount of the protein in an uncut form. © 2018 Society of Chemical Industry.

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.

Structure of γ-conglutin: insight into the quaternary structure of 7S basic globulins from legumes

γ-Conglutin from lupin seeds is an unusual 7S basic globulin protein. It is capable of reducing glycaemia in mammals, but the structural basis of this activity is not known. γ-Conglutin shares a high level of structural homology with glycoside hydrolase inhibitor proteins, although it lacks any kind of inhibitory activity against plant cell-wall degradation enzymes. In addition, γ-conglutin displays a less pronounced structural similarity to pepsin-like aspartic proteases, but it is proteolytically dysfunctional. Only one structural study of a legume 7S basic globulin, that isolated from soybean, has been reported to date. The quaternary assembly of soybean 7S basic globulin (Bg7S) is arranged as a cruciform-shaped tetramer comprised of two superposed dimers. Here, the crystal structure of γ-conglutin isolated from Lupinus angustifolius seeds (LangC) is presented. The polypeptide chain of LangC is post-translationally cleaved into α and β subunits but retains its covalent integrity owing to a disulfide bridge. The protomers of LangC undergo an intricate quaternary assembly, resulting in a ring-like hexamer with noncrystallographic D3 symmetry. The twofold-related dimers are similar to those in Bg7S but their assembly is different as a consequence of mutations in a β-strand that is involved in intermolecular β-sheet formation in γ-conglutin. Structural elucidation of γ-conglutin will help to explain its physiological role, especially in the evolutionary context, and will guide further research into the hypoglycaemic activity of this protein in humans, with potential consequences for novel antidiabetic therapies.