ER exit sites mediated by the COPII adaptor sec24D selectively recruit lipid raft-preferring proteins for rapid ER export.

The determinants of sub-cellular trafficking for many membrane proteins are poorly understood. Lipid-driven membrane nanodomains known as lipid rafts have been widely implicated in post-Golgi traffic, but their involvement in protein sorting in the endoplasmic reticulum has not been widely considered. To assess the role of membrane domains in the early secretory pathway, we use the Retention Using Selective Hooks system to synchronize and quantitatively assess trafficking rates and destinations of model proteins with tunable raft affinities. We find that raft-preferring constructs exit the ER faster than raft-excluded and have distinct preferences for ER exit sites marked by specific isoforms of sec24 cargo adaptors. Namely, raft-excluded cargo localizes to sec24A-positive sites while raft-preferring cargo localizes to sec24D ERES, dependent on p24-family cargo adapters TMED2/10. Finally, sec24D, but not sec24A, ERES accumulate a fluorescent cholesterol analog. These observations suggest that association with raft-like domains affects protein export from the ER.