De novo Sequencing and Native Mass Spectrometry Reveals Hetero-Association of Dirigent Protein Homologs and Potential Interacting Proteins in Forsythia × intermedia

The discovery of dirigent proteins (DPs) and their functions in plant phenol biochemistry was made over two decades ago with Forsythia × intermedia. Stereo-selective, DP-guided, monolignol-derived radical coupling in vitro was then reported to afford the optically active lignan, (+)-pinoresinol from coniferyl alcohol, provided one-electron oxidase/oxidant capacity was present. It later became evident that DPs have several distinct sub-families. In vascular plants, DPs hypothetically function, along with other essential enzymes/proteins (e.g. oxidases), as part of lignin/lignan forming complexes (LFCs). Herein, we used an integrated bottom-up, top-down, and native mass spectrometry approach to detect potential interacting proteins in a DP-enriched solubilized protein fraction from Forsythia × intermedia, via adaptation of our initial report of DP solubilization and purification. Because this hybrid species lacks a published genome, de novo sequencing was performed using publicly available transcriptome and genomic data from closely related species. We detected and identified two new DP homologs, which appear to form hetero-trimers. Molecular dynamics simulations suggest that similar hetero-trimers were possible between Arabidopsis DP homologs with comparable sequence similarity. Other identified proteins in the DP-enriched preparation were putatively associated with DP function or the cell wall. Although their co-occurrence after extraction and chromatographic separation is suggestive for components of a protein complex in vivo, none were found to form stable complexes with DPs under the specific experimental conditions we have explored. Nevertheless, our integrated mass spectrometry method development helps prepare for future investigations directed to detect hypothetical LFCs and other related complexes isolated from plant biomass fractionation.