Phosphorylation of the exocyst subunit Exo70B2 contributes to the regulation of its function

Abstract

The exocyst is a conserved hetero-octameric complex that mediates early tethering of post-Golgi vesicles during exocytosis. Its Exo70 subunit functions as a spatiotemporal regulator by mediating numerous interactions with proteins and lipids. However, a molecular understanding of the exocyst functions remains challenging. Exo70B2 localized to dynamic foci at the plasma membrane and transited through Brefeldin A (BFA)-sensitive compartments, indicating that it participates in conventional secretion. Conversely, treatment with the immunogenic peptide flg22 or the salicylic acid (SA) defence hormone analogue Benzothiadiazole (BTH), induced Exo70B2 transport into the vacuole where it colocalized with autophagic markers AUTOPHAGY-RELATED PROTEIN 8 (ATG8) and NEIGHBOR OF BRCA1 GENE 1 (NBR1). According with its role in immunity, we discovered that Exo70B2 interacts with and is phosphorylated by the MITOGEN-ACTIVATED PROTEIN KINASE 3 (MPK3). Mimicking phosphorylation inhibited Exo70B2 localization at sites of active secretion. By contrast, lines expressing phosphonull variants displayed higher Effector-Triggered Immunity and were hypersensitive to BTH, conditions known to induce the secretory pathway. Our results suggest a molecular mechanism by which phosphorylation of Exo70B2 regulates interaction with the plasma membrane, and couples the secretory pathway with cellular signalling.