Abstract
The Solanum tuberosum plant specific insert (StPSI) has a defensive role in potato plants, with the requirements of acidic pH and anionic lipids. The StPSI contains a set of three highly conserved disulfide bonds that bridge the protein's helical domains. Removal of these bonds leads to enhanced membrane interactions. This work examined the effects of their sequential removal, both individually and in combination, using all-atom molecular dynamics to elucidate the role of disulfide linkages in maintaining overall protein tertiary structure. The tertiary structure was found to remain stable at both acidic (active) and neutral (inactive) pH despite the removal of disulfide linkages. The findings include how the dimer structure is stabilized and the impact on secondary structure on a residue-basis as a function of disulfide bond removal. The StPSI possesses an extensive network of inter-monomer hydrophobic interactions and intra-monomer hydrogen bonds, which is likely the key to the stability of the StPSI by stabilizing local secondary structure and the tertiary saposin-fold, leading to a robust association between monomers, regardless of the disulfide bond state. Removal of disulfide bonds did not significantly impact secondary structure, nor lead to quaternary structural changes. Instead, disulfide bond removal induces regions of amino acids with relatively higher or lower variation in secondary structure, relative to when all the disulfide bonds are intact. Although disulfide bonds are not required to preserve overall secondary structure, they may have an important role in maintaining a less plastic structure within plant cells in order to regulate membrane affinity or targeting.
Highlights
Instead of possessing an active immune system, plants rely on a complex system of antimicrobial compounds to defend themselves from pathogens, many of which are protein-derived
Molecular dynamics simulations of potato saposin-like protein orange curves) bond is likewise stable, but at a lower separation–hovering at approximately 3.5 Å (Fig 2, panels D2, D4, D5)
When Cys37-Cys68 is removed alongside Cys6-Cys99 (Fig 2, panel D6), much larger changes are observed
Summary
Instead of possessing an active immune system, plants rely on a complex system of antimicrobial compounds to defend themselves from pathogens, many of which are protein-derived. One such class of compounds are aspartic proteases (APs). Many, but not all, APs contain an additional primarily helical segment called the plant specific insert (PSI), which. Data is hosted at the Federated Research Data Repository (FRDR, https://www.frdr-dfdr.ca), courtesy of the Canadian Association of Research Libraries (CARL) and Compute Canada
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