Abstract
Quinoin is a type 1 ribosome-inactivating protein (RIP) we previously isolated from the seeds of pseudocereal quinoa (Chenopodium quinoa) and is known as a functional food for its beneficial effects on human health. As the presence of RIPs in edible plants could be potentially risky, here we further characterised biochemically the protein (complete amino acid sequence, homologies/differences with other RIPs and three-dimensional homology modeling) and explored its possible defensive role against pathogens. Quinoin consists of 254 amino acid residues, without cysteinyl residues. As demonstrated by similarities and homology modeling, quinoin preserves the amino acid residues of the active site (Tyr75, Tyr122, Glu177, Arg180, Phe181 and Trp206; quinoin numbering) and the RIP-fold characteristic of RIPs. The polypeptide chain of quinoin contains two N-glycosylation sites at Asn115 and Asp231, the second of which appears to be linked to sugars. Moreover, by comparative MALDI-TOF tryptic peptide mapping, two differently glycosylated forms of quinoin, named pre-quinoin-1 and pre-quinoin-2 (~0.11 mg/100 g and ~0.85 mg/100 g of seeds, respectively) were characterised. Finally, quinoin possesses: (i) strong antiviral activity, both in vitro and in vivo towards Tobacco Necrosis Virus (TNV); (ii) a growth inhibition effect on the bacterial pathogens of plants; and (iii) a slight antifungal effect against two Cryphonectria parasitica strains.
Highlights
Ribosome-inactivating proteins (RIPs) are a group of specific toxins that inhibit protein synthesis [1]. This inhibition is a consequence of ribosome injury due to the enzymatic activity of ribosome-inactivating protein (RIP), which makes ribosomes unable to interact with transcription factors such as the elongation factor 2 (EF-2, in eukaryotes) or the elongation factor G (EF-G, in prokaryotes), blocking the translocation processes [2]
Quinoin is a type 1 RIP found in C. quinoa seeds and exhibits toxicity against different cell lines in vitro
Several studies have reported that some consumers are affected by innate immune responses or an insurgence of allergies or intolerances [46,47]. In this context, increased information on this type 1 RIP is of interest to verify the real impact of this toxin in a food that is highly requested today
Summary
Ribosome-inactivating proteins (RIPs) are a group of specific toxins that inhibit protein synthesis [1]. This inhibition is a consequence of ribosome injury due to the enzymatic activity of RIPs, which makes ribosomes unable to interact with transcription factors such as the elongation factor 2 (EF-2, in eukaryotes) or the elongation factor G (EF-G, in prokaryotes), blocking the translocation processes [2]. The specific enzymatic action of RIPs promotes cell death by apoptosis through the activation of caspases and caspase-like activities as well as the poly(ADP-ribose) polymerase cleavage [4]. RIPs are found in a large number of plants (flowering plants or Angiosperms) [6] and are distributed in different plant tissues (seeds, roots, leaves and bark) [5], novel findings have highlighted their presence in fungi [7] and metazoan [8], while analogous enzymatic activities (the Shiga-like toxin) have been investigated for a long time in bacteria [9]
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