Elucidating the structure and function of a membrane-active plant protein domain using in silico mutagenesis

The Solanum tuberosum (common potato) plant specific insert (StPSI) is an antimicrobial protein domain that exhibits membrane-disrupting and membrane-fusing activity upon dimerization at acidic pH, activity proposed to involve electrostatic attraction and membrane anchoring mediated by specific positively-charged and conserved tryptophan residues, respectively. This study is the first to employ an in silico mutagenesis approach to clarify the structure-function relationship of a plant specific insert (PSI), where ten rationally-mutated StPSI variants were investigated using all-atom and coarse-grained molecular dynamics. The tryptophan (W) residue at position 18 (W18) of wild-type StPSI was predicted to confer structural flexibility to the dimer and mediate a partial separation of the assembled monomers upon bilayer contact, while residues including W77 and the lysine (K) residue at position 83 (K83) were predicted to stabilize secondary structure and influence association with the model membrane. Mechanisms predicted to influence StPSI-membrane association included the partial separation of assembled monomers on the bilayer surface, formation of a specific salt bridge, and membrane anchoring of hinge 2 residues. The findings suggested that the structure-function relationship of StPSI involved several mechanisms that may each be modulated by specific key residues, insights that may support efforts to develop PSI with tailored membrane association for novel applications in plant biotechnology and crop protection.