Special Biochemistry Seminar
Protein secretion is essential for animal development and cell viability. In eukaryotic cells, membrane and secretory proteins can reach the cell exterior via either a signal sequence-dependent conventional protein secretory mechanism or several unconventional paths independent of the endoplasmic reticulum (ER)-Golgi axis. Conventional protein secretion moves nascent polypeptides into the ER via a protein-conducting channel, which can be jammed if prolonged ribosome pausing or translation stalling occurs. How cells resolve clogged translocon and degrade partially translocated products is unclear.
We showed that translocon clogging at the ER triggers ribosome UFMylation, a post-translational modification that attaches ubiquitin-fold modifier 1 (UFM1) to the 60s ribosomal protein RPL26, which initiates a translocation-associated quality control (TAQC) mechanism. Genome-wide CRISPR knockout screens identified TAQC players and revealed an uncharacterized membrane protein named SAYSD1 as a key TAQC promoter. SAYSD1 associates with the clogged translocon and recognizes both ribosome and UFM1, engaging a stalled nascent chain to ensure its transport via the TRAPP complex to lysosomes for degradation. Like UFM1 deficiency, SAYSD1 depletion causes the accumulation of translocation-stalled proteins at the ER and triggers ER stress. Disrupting UFM1- and SAYSD1-dependent TAQC in Drosophila leads to intracellular accumulation of translocation-stalled collagens, defective collagen deposition, abnormal basement membranes, and reduced stress tolerance. Thus, ribosome UFMylation and SAYSD1 act together at the site of clogged translocon to safeguard ER homeostasis. Intriguingly, in addition to RPL26, the UFMylation system can directly target misfolded proteins in the cytoplasm.
Using α-Synuclein (α-Syn) as a mode substrate, we showed that mutating the UFMylation sites in α-Syn or genetically ablating the UFMylation system mitigates α-Syn secretion via an ER-independent, unconventional protein secretion (UcPS) pathway. Importantly, UFM1 is co-secreted with α-Syn, and the serum UFM1 level correlates with that of α-Syn. Thus, our studies have established distinct roles for UFM1 in conventional and UcPS, revealing potential therapeutic targets for human diseases associated with genetic mutations in the UFMylation system.