Plant defensins as a model to study the thermostability of surface hydrophobic clusters: The Pisum sativum defensin 2 (Psd2)

Plant defensins (PDs) display a CS alpha beta-fold that lacks a canonical hydrophobic core. They display almost all hydrophobic residues on the protein surface. The exposed hydrophobic residues form surface clusters stabilized by the vicinity of hydrophilic residues and the hydration shell. Here, we used Psd2 as a model to study the formation and stabilization of these local foldons named surface hydrophobic clusters (SHC). We characterized the temperature dependence of 15N CPMG relaxation dispersion profiles to describe the complex dynamics of Psd2 and indirectly study the thermodynamics of SHCs in PDs. We show a correlation between residues undergoing conformational exchange and the SHCs. Chemical shift changes between the native ground state and the first thermally accessible excited state enabled us to map the major conformational changes in Psd2 conformational equilibrium. The observation of a cold-driven excited state revealed that SHCs are stabilized by hydrophobic contacts, which are exposed at low temperatures, leading to a favorable decrease in enthalpy compensated by an unfavorable entropy reduction. At higher temperatures, we detected another excited conformer that may play a role in membrane-specific interaction, as previously described for other defensins.