Peroxin-dependent targeting of a lipid-droplet-destined membrane protein to ER subdomains

Lipid droplets (LDs) are endoplasmic reticulum (ER)-derived lipid storage organelles uniquely encapsulated by phospholipid monolayers. LD membrane proteins are embedded into the monolayer in a monotopic hairpin topology and are therefore likely to have requirements for their biogenesis distinct from those inserting as bitopic and polytopic proteins into phospholipid bilayers. UBXD8 belongs to a subfamily of hairpin proteins that localize to both the ER and LDs, and are initially inserted into the cytoplasmic leaflet of the ER bilayer before partitioning to the LD monolayer. The molecular machinery responsible for inserting hairpin proteins into membranes, however, is unknown. Here, we report that newly synthesized UBXD8 is post-translationally inserted into discrete ER subdomains by a mechanism requiring cytosolic PEX19 and membrane-integrated PEX3, proteins hitherto exclusively implicated in peroxisome biogenesis. Farnesylation of PEX19 uncouples ER/LD and peroxisome targeting, expanding the function of this peroxin to an ER-targeting pathway and suggesting a coordinated biogenesis of LDs and peroxisomes. Kopito and Schrul show that the peroxisome proteins PEX19 and PEX3 mediate the correct insertion of the lipid droplet protein UBXD8 into ER subdomains.