COPII-CARGO INTERACTIONS DIRECT PROTEIN SORTING INTO ER-DERIVED TRANSPORT VESICLES

Vesicles coated with coat protein complex II (COPII) selectively trans port molecules (cargo) and vesicle fusion proteins from the endoplasmi c reticulum (ER) to the Golgi complex(1-3). We have investigated the r ole of coat proteins in cargo selection and recruitment. We isolated i ntegral membrane and soluble cargo proteins destined for transport fro m the ER in complexes formed in the presence of Sar1 and Sec23/24, a s ubset of the COPII components, and GTP or GMP-PNP. Vesicle fusion prot eins of the vSNARE family and Emp24, a member of a putative cargo carr ier family(4), were also found in COPII complexes. The inclusion of am ino-acid permease molecules into the complex depended on the presence of Shr3, a protein required for the permease to leave the ER3,5. Resid ent ER proteins Sec61, BiP (Kar2) and Shr3 were not included in the co mplexes, indicating that the COPII components bound specifically to ve sicle cargo. COPII-cargo complexes and putative cargo adaptor-cargo co mplexes were also isolated from COPII vesicles. Our results indicate t hat cargo packaging signals and soluble cargo adaptors are recognized by a recruitment complex comprising Sar1-GTP and Sec23/24.