The rapid identification of novel plant metabolites and assignments of newly discovered substances to natural product classes present the main bottlenecks to defining plant specialized phenotypes. Although mass spectrometry provides powerful support for metabolite discovery by measuring molecular masses, ambiguities in elemental formulas often fail to reveal the biosynthetic origins of specialized metabolites detected using liquid chromatography-mass spectrometry. A promising approach for mining liquid chromatography-mass spectrometry metabolite profiling data for specific metabolite classes is achieved by calculating relative mass defects (RMDs) from molecular and fragment ions. This strategy enabled the rapid recognition of an extensive range of terpenoid metabolites in complex plant tissue extracts and is independent of retention time, abundance, and elemental formula. Using RMD filtering and tandem mass spectrometry data analysis, 24 novel elemental formulas corresponding to glycosylated sesquiterpenoid metabolites were identified in extracts of the wild tomato Solanum habrochaites LA1777 trichomes. Extensive isomerism was revealed by ultra-high-performance liquid chromatography, leading to evidence of more than 200 distinct sesquiterpenoid metabolites. RMD filtering led to the recognition of the presence of glycosides of two unusual sesquiterpenoid cores that bear limited similarity to known sesquiterpenes in the genus Solanum. In addition, RMD filtering is readily applied to existing metabolomics databases and correctly classified the annotated terpenoid metabolites in the public metabolome database for Catharanthus roseus.Plant metabolic networks generate amazing chemical diversity, but our understanding of the genetic factors responsible for plant chemistry remains primitive. The discovery and identification of metabolites has posed the greatest bottleneck in recent efforts to exploit metabolomics to address questions about the basis for biosynthetic diversity in the plant kingdom (Ji et al., 2009;Zhou et al., 2012). Since the specialized metabolism of nonmodel plants is taxonomically restricted, metabolite databases offer a poor representation of plant chemical diversity, and de novo recognition and discovery of metabolite chemistry is necessary. A common strategy for metabolite discovery has often started with the generation of tandem mass spectrometry (MS/MS) spectra, usually beginning with the most abundant metabolites, and uses characteristic fragment ions to assign metabolites to a particular class of compounds. Flavonoid identification from MS/MS spectra is often successful because most flavonoids yield MS/MS fragment ions characteristic of their flavonoid cores (Ma et al., 1997;Li et al., 2013). However, when MS/MS spectra fail to display classcharacteristic fragment ions, the recognition of a metabolite's structural class is less obvious.Specialized plant metabolites are often grouped as polyphenolic, terpenoid, alkaloid, polyketide, or fatty acid metabolites based upon the biosynthesis of their...

Influenza virus is a Class I enveloped virus which is initially endocytosed into a host respiratory epithelial cell. Subsequent reduction of the pH to the 5–6 range triggers a structural change of the viral hemagglutinin II (HA2) protein, fusion of the viral and endosomal membranes, and release of the viral nucleocapsid into the cytoplasm. HA2 contains fusion peptide (FP), soluble ectodomain (SE), transmembrane (TM), and intraviral domains with respective lengths of ~25, ~160, ~25, and ~10 residues. The present work provides a straightforward protocol for producing and purifying mg quantities of full-length HA2 from expression in bacteria. Biophysical and structural comparisons are made between full-length HA2 and shorter constructs including SHA2 ≡ SE, FHA2 ≡ FP + SE, and SHA2-TM ≡ SE + TM constructs. The constructs are helical in detergent at pH 7.4 and the dominant trimer species. The proteins are highly thermostable in decylmaltoside detergent with Tm > 90 °C for HA2 with stabilization provided by the SE, FP, and TM domains. The proteins are likely in a trimer-of-hairpins structure, the final protein state during fusion. All constructs induce fusion of negatively-charged vesicles at pH 5.0 with much less fusion at pH 7.4. Attractive protein/vesicle electrostatics play a role in fusion, as the proteins are positively-charged at pH 5.0 and negatively-charged at pH 7.4 and the pH-dependence of fusion is reversed for positively-charged vesicles. Comparison of fusion between constructs supports significant contributions to fusion from the SE and the FP with little effect from the TM.

No abstract

The influenza virus hemagglutinin (HA) protein has HA1 and HA2 subunits, which form an initial complex. HA1's bind host cell sialic acids which triggers endocytosis, HA1/HA2 separation, and HA2-mediated fusion between virus and endosome membranes. We report hydrogen-deuterium exchange-mass spectrometry(HDX-MS) on the HA2 subunit without HA1. HA2 contains fusion peptide(FP), soluble ectodomain(SE), transmembrane domain(TM) and endodomain. FP is a monomer by itself, while SE is a trimer-of-hairpins that includes an interior bundle of residue 38-105 helices, turns, and 154-178 strands packed antiparallel to the bundle. FP and TM extend from the same side of the SE hairpin, and fusion models often depict a FP/TM complex with membrane traversal of both domains that is important for membrane pore expansion. The HDX-MS data of the present study do not support this complex, and instead support independent FP and TM with respective membrane interfacial and traversal locations. The data also show low aqueous exposure of the 22-38 segment, consistent with retention of the 23-35 antiparallel β sheet observed in the initial HA1/HA2 complex. We propose the β sheet as a semi-rigid connector between FP and SE that enables close membrane apposition prior to fusion. The I173E mutant exhibits greater exchange for 22-69 and 150-191, consistent with dissociation of SE C-terminal strands from interior N-helices. Similar trends are observed for G1E mutant, as well as lower exchange for G1E FP. Fusion is highly-impaired with either mutant, which correlates with reduced membrane apposition, and for G1E, FP binding to SE rather than target membrane.