Selective proteasomal degradation from the Golgi apparatus membrane

ABSTRACT To maintain cellular homeostasis, protein quality control (PQC) machineries target damaged proteins for degradation. However, the PQC machinery, operating at the major protein sorting and processing site of mammalian cells, the Golgi, has remained largely elusive. Here we used a chemical biology tool to induce misfolding and ubiquitination of Golgi-localized model substrates. Our tool recruits a Cullin-RING ubiquitin ligase complex by exposure of a degron, recognized by the substrate receptor KLHDC2. Using this tool, we found that ubiquitinated Golgi membrane proteins are targeted for proteasomal degradation, a process that is critical to avoid Golgi fragmentation. This process is facilitated by the p97-unfoldase, which assembles with its adaptors UFD1-NPL4 and FAF2 at the Golgi. We further identified the rhomboid pseudo-protease RHBDD2 as an important contributor to Golgi homeostasis as it binds misfolded, ubiquitinated Golgi membrane proteins and regulates Golgi morphology. Our findings establish a molecular framework for Golgi-PQC in mammalian cells. The accumulation of misfolded Golgi proteins causes Golgi fragmentation. Ubiquitinated Golgi membrane proteins are degraded by the proteasome. The p97 unfoldase operates at the Golgi with its adaptors UFD1-NPL4 and FAF2. The rhomboid pseudo-protease RHBDD2 contributes to Golgi protein quality control.