ENDOPLASMIC-RETICULUM QUALITY-CONTROL OF ASIALOGLYCOPROTEIN RECEPTOR H2A INVOLVES A DETERMINANT FOR RETENTION AND NOT RETRIEVAL

The human asialoglycoprotein receptor H2a subunit contains a charged p entapeptide, EGHRG, in its ectodomain that is the only sequence absent from the H2b alternatively spliced variant, H2b exits the endoplasmic reticulum (ER) even when singly expressed, whereas H2a gives rise to a cleaved soluble secreted ectodomain fragment; uncleaved membrane-bou nd H2a molecules are completely retained and degraded in the ER, We ha ve inserted the H2a pentapeptide into the sequence of the H1 subunit ( H1i5), which caused complete ER retention but, unexpectedly, no degrad ation, This suggests that the pentapeptide is a determinant for ER ret ention not colocalizing in H2a with the determinant for degradation, T he state of sugar chain processing and the ER localization of H1i5, wh ich was unchanged at 15 degrees C or after treatment with nocodazole, indicate ER retention and not retrieval from the cis-Golgi or the inte rmediate compartment. H1i5 folded similarly to H1, and both associated to calnexin. However, whereas H1 dissociated with a half time of 45 m in, H1i5 remained bound to the chaperone for prolonged periods, The co rrect global folding of H2a and H1i5 and of other normal precursors an d unassembled proteins and the true ER retention, and not exit and ret rieval, suggest a difference in their quality control mechanism compar ed with that of misfolded proteins, which does involve retrieval, Howe ver, both pathways may involve calnexin.