ER export of ERGIC-53 is controlled by cooperation of targeting determinants in all three of its domains

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Abstract
Selective export of proteins from the endoplasmic reticulum (ER) requires transport signals that have not been fully characterized. Here, we provide the first complete map of ER export determinants of a type I membrane protein, ERGIC-53, that cycles in the early secretory pathway. ER export requires a phenylalanine motif at the C-terminus, known to mediate coat protein II (COPII) interaction, that is assisted by a glutamine in the cytoplasmic domain. Disulfide bond-stabilized oligomerization is also required. Efficient hexamerization depends on the presence of a polar and two aromatic residues in the transmembrane domain (TMD). Oligomerization becomes independent on disulfide bonds when TMD hydrophobicity is increased. ER export is also influenced by TMD length, 21 amino acids being most efficient. When transferred to a signal-less construct, the established targeting motifs reconstitute full transport activity. The results suggest an ER-export mechanism in which transmembrane and luminal determinants mediate oligomerization required for efficient recruitment of ERGIC-53 into budding vesicles via the C-terminal COPII-binding phenylalanine motif.