Loss of Arabidopsis p24 function affects ERD2 traffic and Golgi structure and activates the unfolded protein response

Abstract

p24 proteins are key regulators of protein trafficking along the secretory pathway but very little is known about their functions in plants. A quadruple loss-of-function mutant affecting the p24 genes from the δ-1 subclass of the p24 delta subfamily (p24δ3δ4δ5δ6) showed alterations in the Golgi apparatus, suggesting that these p24 proteins play a role in the organization of the compartments of the early secretory pathway in Arabidopsis. Loss of p24δ-1 proteins also induced the accumulation of the K/HDEL receptor ERD2 at the Golgi apparatus and increased secretion of the ER chaperone BiP, an HDEL ligand, probably due to an inhibition of COPI-dependent Golgi-to-ER transport of ERD2 and thus retrieval of K/HDEL ligands. Although the p24δ3δ4δ5δ6 mutant showed enhanced sensitivity to salt stress, it did not show obvious phenotypic alterations under standard growth conditions. Interestingly, this mutant showed a constitutive activation of the unfolded protein response (UPR) and the up-regulation of the COPII subunit SEC31A, which may help the plant to cope with those transport defects in the absence of p24 proteins.