Multifunctionality of plant proteins

Plant seeds proteins (SP) are traditionally considered the nitrogen reserve that supports the seedling growth during the first steps of germination. This exclusive role has recently been ruled out and emerging findings indicated that several biological activities became manifests upon the proteolytic breakdown. Protein cleavage at germination is believed to occur through a regulated mechanism involving the selective breakdown of specific peptide bonds which, before the native conformation is critically altered to allow the complete hydrolysis, origins large polypeptide fragments. During germination, several preformed and de novo-synthesized peptidases act di concerto sequentially. Some of the transiently formed intermediate peptides have been shown to possess specific bioactivities. In legume and pseudo-cereal seeds, proteins represent from 20% to 45%. The majority of them are SP, globulins usually classified according to their sedimentation coefficients as 7S (vicilin-like) and 11S (legumin-like). SPs are synthesized during seed developing and deposited inside the cells in specific membrane-bound organelles, called protein bodies (PBs). Many bibliographic data suggest that vicilins have active role in plant defence processes. Few examples. A fragment of 45 amino acids generated during germination by proteolysis of Macadamia integrifolia vicilin has been shown to in vitro inhibit several plant pathogenic fungi. BLAD is a stable intermediary product of vicilin breakdown, which accumulate in cotyledons of Lupinus species during germination and shows defense and antibiotic properties. Unlike vicilins, no specific biological activities have been described for legumins or legumin-derived fragments. These polypeptides were found inside cotyledons and selectively secreted outside the seeds. From an applicative point of view some biological properties have already been ascribed to specific seed storage proteins, such as radical scavenger capacity, anti-inflammatory, anti-obesity, ACE-inhibitory, hypoglycaemic, α-amylase inhibitor, trypsin-inhibitor, hypocholesterolaemic and immuno-modulation activities; thought the involved molecular mechanisms are not completely elucidated. Bioactive cryptic peptides, originated from protein hydrolysis, have also been identified. These protein fragments, inactive in the integer parent protein and released by enzymatic hydrolysis, once produced act as modulators of many biological processes in living organisms. Indeed, it has been proved that some of them are able to pass the intestine barrier, enter the blood stream, arrive to the target site and, depending on their amino acids composition and structure, play different roles. We have shown that SP-derived peptides from quinoa and other crops are able to interact with as Toll-like receptors, activating an immune-response pathway including NF-kB, and modulate cell inflammatory responses. A suitable transfected Caco-2 cell model is currently used. These cells contain the plasmid pNiFty2-Luc, whose promoter couples nuclear factor (NF)-kB-binding sites and the luciferase reporter gene luc. The immune and inflammatory responses of cells incubated with the tested bioactive molecules and IL1β as control elicitor, can be evaluated by measuring the bioluminescence emitted from cellular extracts after cell lysis. IL1 Toll-like receptors are also present in plants. These include N protein from tobacco, which is required for resistance to the pathogen tobacco mosaic virus, and L6 from flax, which confers resistance to flax rust. Neither L6 nor N protein are transmembrane receptors but may interact with their ligands in the cytoplasm. RPP5 from Arabidopsis shares similar structural features with L6 and N proteins. By and large RPP5 may play a key role by eliciting the plant defence responses at the beginning of germination or boosting them when the plant is attacked by pathogens.