Targeting of protein ERGIC-53 to the ER/ERGIC/cis-Golgi recycling pathway

Our data indicate that pre-medial-Golgi targeting of ERGIC-53 depends to a large extent on the retrieval capacity of the di-lysine signal in its cytoplasmic domain, even though an RSQQE determinant and two COOH-terminal phenylalanines are required for correct localization within the ER-ERGIC-cis-Golgi recycling pathway. However, the cytoplasmic domain alone is not sufficient to target a monomeric reporter protein, L4T4C53, identically to endogenous ERGIC-53. A comparison of the acquisition of endo H resistance between monomeric L4T4C53 K(-4)stop and oligomeric L53T4C53 (K-4)stop, both lacking the KKFF ER-retrieval signal, is most consistent with the idea that the RSQQE determinant is reduced or nonfunctional in the monomeric background. But how could oligomerization influence the RSQQE determinant? Two of the four cysteine residues, which could potentially link ERGIC-53 oligomers by disulfide bridges, are located within a 15-amino acid distance from the transmembrane domain. Provided these cysteines covalently link the ERGIC-53 oligomers in the immediate vicinity of the membrane, this could actually lead to a close apposition of the RSQQE determinants on the cytoplasmic side of the membrane. It is conceivable that such an apposition is required for the RSQQE determinant to be functional, a situation not mimicked in the monomeric reporter protein.