Analysis of the Dual Function of the Exocyst Complex Subunit Exo70 as a Spatiotemporal Mediator of Secretion and Autophagy

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The secretory pathway is responsible for the delivery of proteins and lipids to the plasma membrane. Changes in the rate and spatial distribution of secretory activity are inherent to normal development, but also enable adaptation to stresses resulting from temperature changes or infection. High secretory activity triggered by immune responses, or cellular stress caused by heat, affect speed, fidelity and capacity of protein biogenesis via the secretory pathway, as well as degradation systems, often resulting in proteome imbalances (i.e. loss of protein homeostasis) . The exocyst is a conserved hetero-octameric complex mediating early tethering of post Golgi vesicles to the PM during exocytosis. Its Exo70 subunit plays a critical role as a spatiotemporal regulator by mediating numerous protein and lipid interactions. Previous studies and our preliminary data show that Exo70 subunits display typical traits such as plasma membrane localization and transit through trans-Golgi network. The Exo70 family in plants has expanded into many homologues (23 in Arabidopsis thaliana) and several studies support a specialization in the transport of specific protein cargoes to the plasma membrane. In addition, we and others, have shown that Exo70s can be diverted into the vacuole by autophagy. Our preliminary data shows that vacuolar degradation can be induced in response to proteotoxic stress or high secretory activity and is driven by two ATG8-interacting motifs (AIMs) which are conserved in the majority of Exo70 homologues. Interaction was also observed between the yeast ATG8 and Exo70, indicating evolutionary conservation across kingdoms. We therefore hypothesize that Exo70 plays a dual role in both the secretory pathway, as well as in autophagy to dampen secretory activity and thus restore cellular homeostasis. To date, the molecular understanding of the exocyst function in the secretory pathway remains challenging, and our knowledge regarding the intersection points between cellular pathways to safeguard and restore protein homeostasis is fragmentary. In this project we aim to elucidate the dual function of the Exo70 subunit by i) analysing the molecular mechanisms dictating plasma membrane binding specificities and ii) the role of its degradation by autophagy in maintaining protein homeostasis.