Rice OsERG3 encodes an unusual small C2-domain protein containing a Ca2+-binding module but lacking phospholipid-binding properties

Abstract

Background: The C2 domain is a Ca2+/phospholipid-binding motif found in many proteins involved in signal transduction or membrane trafficking. OsERG3 is a homolog of OsERG1, a gene encoding a small C2-domain protein in rice. Methods: OsERG3 Ca2+-binding and phospholipid-binding assays were carried out using 3H-labeled phospholipid liposomes and a 45Ca2+ overlay assay, respectively. Cytosolic expression of OsERG3 was investigated by Western blot analysis and the OsERG3::smGFP transient expression assay. Results: OsERG3 transcript levels were greatly enhanced by treatment with a fungal elicitor and Ca2+-ionophore. OsERG3 protein proved unable to interact with phospholipids regardless of the presence or absence of Ca2+ ions. Nonetheless, OsERG3 displayed calcium-binding activity in an in vitro45Ca2+-binding assay, a property not observed with OsERG1. The cytosolic location of OsERG3 was not altered by the presence of fungal elicitor or Ca2+-ionophore. Conclusions: OsERG3 encodes a small C2-domain protein consisting of a single C2 domain. OsERG3 binds Ca2+ ions but not phospholipids. OsERG3 is a cytosolic soluble protein. The OsERG3 gene may play a role in signaling pathway involving Ca2+ ions. General significance: The data demonstrate that OsERG3 is an unusual small C2-domain protein containing a Ca2+- binding module but lacking phospholipid-binding properties.