The crystal structure of Arabidopsis BON1 provides insights into the copine protein family

SUMMARY The Arabidopsis thaliana BON1 gene product is a member of the evolutionary conserved eukaryotic calcium‐dependent membrane‐binding protein family. The copine protein is composed of two C2 domains (C2A and C2B) followed by a vWA domain. The BON1 protein is localized on the plasma membrane, and is known to suppress the expression of immune receptor genes and to positively regulate stomatal closure. The first structure of this protein family has been determined to 2.5‐Å resolution and shows the structural features of the three conserved domains C2A, C2B and vWA. The structure reveals the third Ca2+‐binding region in C2A domain is longer than classical C2 domains and a novel Ca2+ binding site in the vWA domain. The structure of BON1 bound to Mn2+ is also presented. The binding of the C2 domains to phospholipid (PSF) has been modeled and provides an insight into the lipid‐binding mechanism of the copine proteins. Furthermore, the selectivity of the separate C2A and C2B domains and intact BON1 to bind to different phospholipids has been investigated, and we demonstrated that BON1 could mediate aggregation of liposomes in response to Ca2+. These studies have formed the basis of further investigations into the important role that the copine proteins play in vivo. Significance Statement BON1 is a calcium‐dependent membrane‐binding protein that plays an important role in the inhibition of programmed cell death and other cellular defense responses in plant. Here we report the first structure from the copine family and present the biochemical properties of BON1, which will help the rational design of compounds to treat plant infection and disease.