Wheat grain softness protein (Gsp1) is a puroindoline-like protein that displays a specific post-translational maturation and does not interact with lipids

• Purification and structural characterization of the major expressed Grain softness protein1, i.e. Gsp1b, of wheat endosperm.
• Post-translational processing of Gsp1b.
• Absence of interactions between Gsp1b and membrane lipids.

Compared to puroindolines a and b, grain softness proteins (Gsp1) remained up to now the less characterized members of this wheat specific seed protein family. Due to their low expression levels, their purification remains relatively arduous. We report in the present work on the purification of wheat Gsp, Gsp1b and its post-translational maturation. We showed that Gsp1b underwent different proteolytic cleavages both in the N and C-terminal extremities of the preproprotein. Especially, after the putative signal peptide, a 29 mer peptide is highlighted that is highly truncated in puroindolines (8–9 mer). We also showed that Gsp1b and puroindolines display similar secondary structure but Gsp1b does not interact in vitro with lipids. In addition, by using the iterative threading assembly refinement (I-TASSER) methods, we showed that Gsp1b three-dimensional structure prediction is however identical to both those of puroindolines and of the dicotyledon 2S storage proteins. In contrast with puroindolines, all these data make us question about the role of Gsp1 proteins in endosperm texture and the previously suggested function in plant defense lent to these proteins.