Distinct roles for the cytoplasmic tail sequences of Emp24p and Erv25p in transport between the endoplasmic reticulum and Golgi complex

Heteromeric complexes of p24 proteins cycle between early compartments of the secretory pathway and are required for efficient protein sorting. Here we investigated the role of cytoplasmically exposed tail sequences on two p24 proteins, Emp24p and Erv25p, in directing their movement and subcellular location in yeast. Studies on a series of deletion and chimeric Emp24p-Erv25p proteins indicated that the tail sequences impart distinct functional properties that were partially redundant but not entirely interchangeable. Export of an Emp24p-Erv25p complex from the endoplasmic reticulum (ER) did not depend on two other associated p24 proteins, Erp1 and Erp2p. To examine interactions between the Emp24p and Erv25p tail sequences with the COPI and COPII coat proteins, binding experiments with immobilized tail peptides and coat proteins were performed. The Emp24p and Erv25p tail sequences bound the Sec13p/Sec31p subunit of the COPII coat (K-d similar to 100 mum), and binding depended on a pair of aromatic residues found in both tail sequences. COPI subunits also bound to these Emp24p and Erv25p peptides; however, the Erv25p tail sequence, which contains a dilysine motif, bound COPI more efficiently. These results suggest that both the Emp24p and Erv25p cytoplasmic sequences contain a di-aromatic motif that binds subunits of the COPII coat and promotes export from the ER. The Erv25p tail sequence binds COPI and is responsible for returning this complex to the ER.