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Soluble secretory proteins with a signal peptide reach the extracellular space through the endoplasmic reticulum-Golgi conventional pathway. During translation, the signal peptide is recognised by the signal recognition particle and results in a co-translational translocation to the endoplasmic reticulum to continue the secretory pathway. However, soluble secretory proteins lacking a signal peptide are also abundant, and several unconventional (endoplasmic reticulum/Golgi independent) pathways have been proposed and some demonstrated. This work describes new features of the secretion signal called Nβ, originally identified in NaTrxh, a plant extracellular thioredoxin, that does not possess an orthodox signal peptide. We provide evidence that other proteins, including thioredoxins type h , with similar sequences are also signal peptide-lacking secretory proteins. To be a secretion signal, positions 5, 8 and 9 must contain neutral residues in plant proteins–a negative residue in position 8 is suggested in animal proteins–to maintain the Nβ motif negatively charged and a hydrophilic profile. Moreover, our results suggest that the NaTrxh translocation to the endoplasmic reticulum occurs as a post-translational event. Finally, the Nβ motif sequence at the N- or C-terminus could be a feature that may help to predict protein localisation, mainly in plant and animal proteins.

The marine-facultative Aspergillus sp. MEXU 27854, isolated from the Caleta Bay in Acapulco, Guerrero, Mexico, has provided an interesting diversity of secondary metabolites, including a series of rare dioxomorpholines, peptides, and butyrolactones. Here, we report on the genomic data, which consists of 11 contigs (N50~3.95 Mb) with a ~30.75 Mb total length of assembly. Genome annotation resulted in the prediction of 10,822 putative genes. Functional annotation was accomplished by BLAST searching protein sequences with different public databases. Of the predicted genes, 75% were assigned gene ontology terms. From the 67 BGCs identified, ~60% belong to the NRPS and NRPS-like classes. Putative BGCs for the dioxomorpholines and other metabolites were predicted by extensive genome mining. In addition, metabolomic molecular networking analysis allowed the annotation of all isolated compounds and revealed the biosynthetic potential of this fungus. This work represents the first report of whole-genome sequencing and annotation from a marine-facultative fungal strain isolated from Mexico.

Soluble secretory proteins with a signal peptide reach the extracellular space through the endoplasmic reticulum-Golgi conventional pathway. During translation, the signal peptide is recognised by the signal recognition particle and results in a co-translational translocation to the endoplasmic reticulum to continue the secretory pathway. However, soluble secretory proteins lacking a signal peptide are also abundant, and several unconventional (endoplasmic reticulum/Golgi independent) pathways have been proposed and some demonstrated. This work describes new features of the secretion signal called N[beta], originally identified in NaTrxh, a plant extracellular thioredoxin, that does not possess an orthodox signal peptide. We provide evidence that other proteins, including thioredoxins type h, with similar sequences are also signal peptide-lacking secretory proteins. To be a secretion signal, positions 5, 8 and 9 must contain neutral residues in plant proteins-a negative residue in position 8 is suggested in animal proteins-to maintain the N[beta] motif negatively charged and a hydrophilic profile. Moreover, our results suggest that the NaTrxh translocation to the endoplasmic reticulum occurs as a post-translational event. Finally, the N[beta] motif sequence at the N- or C-terminus could be a feature that may help to predict protein localisation, mainly in plant and animal proteins.

Background NaTrxh, a thioredoxin type h , shows differential expression between self-incompatible and self-compatible Nicotiana species. NaTrxh interacts in vitro with S-RNase and co-localizes with it in the extracellular matrix of the stylar transmitting tissue. NaTrxh contains N- and C-terminal extensions, a feature shared by thioredoxin h proteins of subgroup 2. To ascertain the function of these extensions in NaTrxh secretion and protein-protein interaction, we performed a deletion analysis on NaTrxh and fused the resulting variants to GFP. Results We found an internal domain in the N-terminal extension, called Nβ, that is essential for NaTrxh secretion but is not hydrophobic, a canonical feature of a signal peptide. The lack of hydrophobicity as well as the location of the secretion signal within the NaTrxh primary structure, suggest an unorthodox secretion route for NaTrxh. Notably, we found that the fusion protein NaTrxh-GFP(KDEL) is retained in the endoplasmic reticulum and that treatment of NaTrxh-GFP-expressing cells with Brefeldin A leads to its retention in the Golgi, which indicates that NaTrxh uses, to some extent, the endoplasmic reticulum and Golgi apparatus for secretion. Furthermore, we found that Nβ contributes to NaTrxh tertiary structure stabilization and that the C-terminus functions in the protein-protein interaction with S-RNase. Conclusions The extensions contained in NaTrxh sequence have specific functions on the protein. While the C-terminus directly participates in protein-protein interaction, particularly on its interaction with S-RNase in vitro ; the N-terminal extension contains two structurally different motifs: Nα and Nβ. Nβ, the inner domain (Ala-17 to Pro-27), is essential and enough to target NaTrxh towards the apoplast. Interestingly, when it was fused to GFP, this protein was also found in the cell wall of the onion cells. Although the biochemical features of the N-terminus suggested a non-classical secretion pathway, our results provided evidence that NaTrxh at least uses the endoplasmic reticulum, Golgi apparatus and also vesicles for secretion. Therefore, the Nβ domain sequence is suggested to be a novel signal peptide.

Soluble secretory proteins with a signal peptide reach the extracellular space through the endoplasmic reticulum-Golgi conventional pathway. During translation, the signal peptide is recognised by the secretory recognition particle and results in a co-translational translocation to the endoplasmic reticulum to continue the secretory pathway. However, soluble secretory proteins lacking a signal peptide are also abundant, and several unconventional (endoplasmic reticulum/Golgi independent) pathways have been proposed and some demonstrated. This work describes new features of the secretion signal called Nβ, originally identified in NaTrxh, a plant extracellular thioredoxin, that does not possess an orthodox signal peptide. We provide evidence that other proteins, including thioredoxins type h, with similar sequences are also signal peptide-lacking secretory proteins. To be a secretion signal, positions 5, 8 and 9 must contain neutral residues –a negative residue in position 9 in animal proteins– to maintain the Nβ motif negatively charged and a hydrophilic profile. Moreover, our results suggest that the NaTrxh translocation to the endoplasmic reticulum occurs as a post-translational event. Finally, the Nβ motif sequence at the N- or C-terminus could be a feature that may help to predict protein localisation, mainly in plant and animal proteins.