Lysyl hydroxylase (LH) is a peripheral membrane protein in the lumen of the endoplasmic reticulum (ER) that catalyses hydroxylation of lysine residues in collagenous sequences. Previously, we have mapped its primary ER localization motif within a 40-amino acid segment at its C-terminus. Here, we have characterized this localization mechanism in more detail, and our results indicate that this segment confers ER residency in a KDEL-receptor-independent manner, and without any apparent recycling of the enzyme between the Golgi apparatus and the ER. In addition, we show that a rather long peptide region, rather than a specific peptide sequence per se, is required for efficient retention of a reporter protein in the ER. Accordingly, the minimal retention motif was found to require the last 32 C-terminal amino acids, and sequential substitution of all five charged residues within this critical segment interfered only marginally with the retention or association of the enzyme with the ER membranes. Moreover, our fold-recognition and structure-prediction analyses suggested that this critical peptide segment forms an extended loop within LH's iron-binding domain, and that this loop is exposed and readily accessible for binding. Collectively, our results define a novel retrieval-independent retention mechanism in the ER.

Abbreviations used: COP I, coatomer complex; DAOCS, deacetoxycephalosporin C synthase; ER, endoplasmic reticulum; IPNS, isopenicillin synthase; LH, lysyl hydroxylase; 2OG-FeII, iron-dependent 2-oxoglutarate dioxygenase; OST, oligosaccharyltransferase; PDI, protein disulphide isomerase; P-3-H, prolyl-3-hydroxylase.

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