The importomer is a peroxisomal membrane protein translocase

Abstract

Three sites of membrane protein biogenesis (the endoplasmic reticulum, mitochondria and chloroplasts) receive unfolded substrates from organelle-specific protein targeting factors, then integrate them using separate translocation channels. Peroxisomes also receive membrane proteins from known targeting factors, but whether a separate translocase is needed for integration remains unknown. Here, using a novel genetic screening strategy, we reveal that the importomer-known for matrix protein import-integrates the peroxisomal membrane protein Pex14. In importomer mutants, Pex14 is arrested in a pre-integrated state on the peroxisome surface. To undergo integration, a Pex14 translocation signal binds the importomers substrate receptor Pex5 at a distinct site from matrix proteins. En bloc translocation of an engineered protein complex with Pex14s luminal region argues that integration occurs without substrate unfolding. Our work shows that the handling of membrane protein targeting and integration by discrete machineries is a fundamental principle shared by diverse membrane protein biogenesis pathways.