Hydroxylation of lysyl residues is crucial for the unique glycosylation pattern 
found in collagens and for the mechanical strength of fully assembled 
extracellular collagen fibers. Hydroxylation is catalyzed in the lumen of the 
endoplasmic reticulum (ER) by a specific enzyme, lysyl hydroxylase (LH). The 
absence of the known ER-specific retrieval motifs in its primary structure and 
its association with the ER membranes in vivo have suggested that the enzyme is 
localized in the ER via a novel retention/retrieval mechanism. We have 
identified here a 40-amino acid C-terminal peptide segment of LH that is able to 
convert cathepsin D, normally a soluble lysosomal protease, into a 
membrane-associated protein. The same segment also markedly slows down the 
transport of the reporter protein from the ER into post-ER compartments, as 
assessed by our pulse-chase experiments. The retardation efficiency mediated by 
this C-terminal peptide segment is comparable with that of the intact LH but 
lower than that of the KDEL receptor-based retrieval mechanism. Within this 
40-amino acid segment, the first 25 amino acids appear to be the most crucial 
ones in terms of membrane association and ER localization, because the last 15 
C-terminal amino acids did not possess substantial retardation activity alone. 
Our findings thus define a short peptide segment very close to the extreme C 
terminus of LH as the only necessary determinant both for its membrane 
association and localization in the ER.