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SARS-CoV-2 enters cells via ACE-2, which binds the spike protein with moderate affinity. Despite a constant background mutational rate, the virus must retain binding with ACE2 for infectivity, providing a conserved constraint for SARS-CoV-2 inhibitors. To prevent mutational escape of SARS-CoV-2 and to prepare for future related coronavirus outbreaks, we engineered a de novo trimeric ACE2 (T-ACE2) protein scaffold that binds the trimeric spike protein with extremely high affinity (KD < 1 pM), while retaining ACE2 native sequence. T-ACE2 potently inhibits all tested pseudotyped viruses including SARS-CoV-2, SARS-CoV, eight naturally occurring SARS-CoV-2 mutants, two SARSr-CoVs as well as authentic SARS-CoV-2. The cryo-EM structure reveals that T-ACE2 can induce the transit of spike protein to “three-up” RBD conformation upon binding. T-ACE2 thus represents a promising class of broadly neutralizing proteins against SARS-CoVs and mutants.

Shewanella baltica , as one of the dominant spoilers in seafoods where they encounter acidic environments during spoilage, can synthesize putrescine from ornithine and cause food spoilage as well as health problems. Here, the ornithine decarboxylation (ODC) pathway composed of ornithine decarboxylases SpeC and SpeF and an ornithine-putrescine transporter PotE were identified in S. baltica by database searches and further by molecular biology operations, and SpeC functioned as an auxiliary adjusting component of ODC system. Ornithine and putrescine were found to promote putrescine accumulation through up-regulating the expression of speF and potE rather than speC . In addition, increased putrescine biosynthesis and alkalization of cytoplasm was detected at acidic pH especially at pH 6.0 compared to neutral pH. Particularly, the maximum up-regulation of ODC genes and the optimum decarboxylation activity of SpeF were detected at acidic pH around 6.0. It’s concluded that the ODC pathway plays dual roles in cytoplasmic acid counteraction and putrescine production of S. baltica . This study contributes to our understanding of the spoilage mechanism of spoilers in the food system, and provides a novel target for seafoods preservation.

The production of biogenic amines (BAs) set considerable obstacles in seafood preservation. However, the characteristic BAs and their roles in the spoilage profiles in large yellow croaker were still poorly understood. The BAs contents, free amino acids (FAAs) contents, aerobic bacterial count, sensory evaluation and several characteristic physicochemical indexes were monitored during storage at 4°C. It's forcefully uncovered that the putrescine and cadaverine were the main BAs and the sum value of them could be used as a novel quality indicator instead of BAI. The reasonable limitation for unaccepted corruption was suggested be around 60 mg/kg. The regression model (r > 0.8, P < 0.05) at 4°C indicated a significant correlation between the main BAs and total volatile basic nitrogen (TVB-N), protein hydrolysis index (P.I.%) as well as pH value, offering a new interpretation form for the physiochemical properties of BAs in aquatic products. Additionally, the evolution of FAAs show the predominant role of lysine, proline, histidine, glycine, and alanine during the whole storage and it can be concluded that BAs biosynthesis in large yellow croaker were moderately related with their precursor substance. This work provides a preliminary insight into the origin of BAs in large yellow croaker.Keywords: biogenic amine, free amino acid, large yellow croaker, quality indicator, regression model Practical Application: It was proposed that the sum value of putrescine and cadaverine which were the main BAs can be used as a novel quality indicator instead of BAI in large yellow croaker. And the regression models of BAs and other physiochemical indexes provided a new perspective for comprehending the spoilage profiles involving BAs biosynthesis. The study may potentially contribute to improving the evaluation of seafood freshness and provide new target site for seafood preservation.

Quorum sensing (QS) is a cell density-dependent signaling system responsible for various physiological activities in bacteria. We initially investigated the relation between a QS system and biogenic amine (BA) production in Shewanella baltica, the specific spoilage organism of refrigerated large yellow croaker (Pseudosciaene crocea). In addition, the inhibition effects of anthocyanins from blueberry and purple sweet potato against QS signals and putrescine production were explored. Two kinds of diketopiperazines, cyclo-(l-Pro-l-Leu) and cyclo-(l-Pro-l-Pro), and two kinds of BAs, putrescine and cadaverine, were detected in the culture extract of S. baltica cultivated in sterile large yellow croaker juice, wherein putrescine presented significantly positive correlations with cyclo-(l-Pro-l-Leu) and cyclo-(l-Pro-l-Pro). In addition, anthocyanins at subminimum inhibitory concentration inhibited the production of diketopiperazines and putrescine in S. baltica 23, a strain with strong putrescine production. Furthermore, a transcriptional analysis showed that anthocyanins suppressed the expression of the odc gene in S. baltica, the gene responsible for the production of putrescine from decarboxylation of ornithine. These results established a correlation of the main BA putrescine with the QS system in S. baltica and revealed that anthocyanins could be developed as new QS inhibitors and seafood preservative candidates. HIGHLIGHTS