A study reveals how nutrient-deprived cells redirect protein transport stations to cellular recycling centers for breakdown.
New proteins destined for areas outside the cell are produced on the endoplasmic reticulum (ER), an internal, winding membrane. Tubular, grape-like outgrowths on the ER, known as ER exit sites, act as transport stations, gathering these newly created proteins and moving them forward in their cellular journey.
Recent discoveries have shown that these ER exit sites also assist in delivering cellular materials and misfolded proteins to lysosomes—organelles responsible for breaking down and recycling cellular components. Additionally, ER exit sites serve as platforms for virus replication, including for COVID-19. Questions have arisen, however, regarding how a single structure—the ER exit site—can manage such a variety of roles.
A study in Developmental Cell from researchers at HHMI’s Janelia Research Campus, under the leadership of Ya-Cheng Liao, utilized super-resolution live-cell imaging and volume electron microscopy to observe the impact of nutrient stress on ER exit sites. Findings reveal that nutrient stress activates a cascade of molecules directing ER exit sites to lysosomes, where they are broken down—an innovative pathway likely used by cells to release amino acids required for internal protein synthesis.
The study showed that, under nutrient scarcity, ER exit sites are transported to specific lysosomes, which then ingest these sites. This process begins when nutrient-deprived cells trigger calcium release from lysosomes, prompting an enzyme called ALG2 to bind to ER exit sites via COPII, a structure attached to the neck connecting the ER to its exit site.
This ALG2-COPII connection initiates ubiquitination, a process critical for protein degradation. A lysosomal protein, which gathers cellular material for breakdown, recognizes the ubiquitin created through ubiquitination, guiding the ER exit site to the lysosome for destruction.
Upon reaching the lysosome, ALG2, attached to one side of the ER exit site, binds on the other side to a protein called ALIX. ALIX interacts with ESCRT, a protein complex located on the lysosome’s surface that plays a role in ingestion. This interaction gradually brings the ER exit site and lysosome closer together until the ER exit site is fully engulfed and digested by the lysosome.
In addition to observing this process in live cells, the research team reconstituted it within an artificial system, confirming the coordinated function of each component.
This study outlines a unique cellular pathway for managing stress, offering insights that may advance understanding of cellular aging. The findings could also provide further knowledge on roles involving ER exit sites, such as a specialized method by which viruses exit cells via lysosomes, potentially guiding new therapeutic developments.
https://doi.org/10.1016/j.devcel.2024.03.027
Summary
Endoplasmic reticulum exit sites (ERESs) are tubular outgrowths of endoplasmic reticulum that serve as the earliest station for protein sorting and export into the secretory pathway. How these structures respond to different cellular conditions remains unclear. Here, we report that ERESs undergo lysosome-dependent microautophagy when Ca2+ is released by lysosomes in response to nutrient stressors such as mTOR inhibition or amino acid starvation in mammalian cells. Targeting and uptake of ERESs into lysosomes were observed by super-resolution live-cell imaging and focus ion beam scanning electron microscopy (FIB-SEM). The mechanism was ESCRT dependent and required ubiquitinated SEC31, ALG2, and ALIX, with a knockout of ALG2 or function-blocking mutations of ALIX preventing engulfment of ERESs by lysosomes. In vitro, reconstitution of the pathway was possible using lysosomal lipid-mimicking giant unilamellar vesicles and purified recombinant components. Together, these findings demonstrate a pathway of lysosome-dependent ERES microautophagy mediated by COPII, ALG2, and ESCRTS induced by nutrient stress.
