Esr proteins are secreted by the cells of the embryo surrounding region

Three highly homologous Esr genes are expressed specifically in the embryo surrounding legion at the micropylar end of the maize endosperm. The proteins belong to a family of small hydrophilic proteins that share a conserved motive with CIv3, the ligand of the receptor-like kinase CIv1. In this study, co-localization of Esr proteins with their mRNAs in the embryo surrounding region was shown with polyclonal antibodies recognizing all three Esr proteins. On a subcellular level the secretion of Esr proteins and their association with the cell wall was shown independently by cell fractionation, immunohistochemistry and transient expression of Gfp fusion proteins. Furthermore, a possible interaction of Esr proteins with a 35 kDa protein present in the lower half of maize kernels was suggested by in vitro affinity chromatography. Therefore Esr proteins share two characteristics with ligands of receptor-like kinases: they are released in the extracellular space and they have the capacity to form protein-protein interactions.