Abstract

Inside of a cell, snaking membranes comprising the ER synthesize up to 30% of the proteins encoded for by the genome. Many of these newly minted proteins only function after being exported from the ER to other cellular destinations. This occurs at sites distributed across the ER called ER exit sites (ERESs). In this talk, I present work characterizing ERE Ss and the secretory pathway using high resolution imaging technologies, including whole-cell volumetric FIB -SEM. Rather than vesicles alone, we find that the ER spawns an elaborate, interwoven tubular network of contiguous lipid bilayers (ER exit site) for protein export. This receptacle is capable of extending microns along microtubules while still connected to the ER by a thin neck. COPII localizes to this neck region and dynamically regulates cargo entry from the ER, while COPI acts more distally, escorting the detached, accelerating tubular entity on its way to joining the Golgi apparatus through microtubule-directed movement. Examining starved cells, where the secretory transport activity of ERE Ss switches to an autophagic role, we observe entire ERE Ss becoming encapsulated into lysosomes by FIB -SEM. We show this is mediated by ubiquitinated Sec31 and other machinery remodeling ERE Ss, triggering its engulfment by nearby lysosomes through an ESCRT -III dependent mechanism. This process could facilitate survival under starvation by facilitating bulk turnover of secretory pathway components.