The maize E3 ligase ZmCER9 specifically targets activated NLRs for degradation

Abstract

The maize protein Rp1-D21, an autoactive derivative of the Rp1-D nucleotide-binding leucine rich repeat (NLR) disease resistance protein, triggers a spontaneous hypersensitive cell death defense response (HR). ZmCER9, a member of a class of E3 ligases involved in the endoplasmic reticulum associated degradation (ERAD) protein quality control system, is a suppressor of the Rp21-D21 HR phenotype. ZmCER9, an active E3 ligase, localizes to the ER, consistent with a role in ERAD. It physically interacts with and mediates the proteasome-dependent degradation of both Rp1-D21 and another autoactive Rp1-D derivate but neither interacts with nor affects the protein levels of three non-autoactive Rp1-D derivatives. ZmCER9 also suppresses the activity and directs the degradation of autoactive NLRs from Arabidopsis and barley. We describe a novel, and possibly general, mechanism that specifically degrades activated NLRs to suppress the defense response after activation.