Prior to secretion, procollagen molecules are correctly folded to triple helices in the endoplasmic reticulum (ER). HSP47 specifically associates with procollagen in the ER during its folding and/or modification processes and is thought to function as a collagen-specific molecular chaperone (Nagata, K. (1996) Trends Biochem. Sci. 21, 23-26). However, structural requirements for substrate recognition and regulation of the binding have not yet been elucidated. Here, we show that a typical collagen model sequence, (Pro-Pro-Gly) n , possesses sufficient structural information required for recognition by HSP47. A structure-activity relationship study using synthetic analogs of (Pro-Pro-Gly) n has revealed the requirements in both chain length and primary structure for the interaction. The substrate recognition of HSP47 has also been shown to be similar but distinct from that of prolyl 4-hydroxylase, an ER resident enzyme. Further, it has shown that the interaction of HSP47 with the substrate peptides is abolished by prolyl 4-hydroxylation of the second Pro residues in Pro-ProGly triplets and that the fully prolyl 4-hydroxylated peptide, (Pro-Hyp-Gly) n , does not interact with HSP47. We thus have proposed a model in which HSP47 dissociates from procollagen during the process of prolyl 4-hydroxylation in the ER.Collagen is the most abundant protein in vertebrate bodies and is folded as procollagen in the endoplasmic reticulum (ER). 1 The folding of procollagen in cells is unique, as is the final structure itself. In the folding of type I collagen, for instance, three ␣ chains (two ␣1(I) and one ␣2(I)) first associate at C-terminal propeptides, and then approximately 340 repeats of the Xaa-Yaa-Gly triplet form the 300-nm length triple helix (1). In the triple helix, three left-handed polyproline type II (PP-II) helices are supercoiled (2). The helix-forming process is believed to be coupled with prolyl 4-hydroxylation at Yaa positions (3). HSP47 transiently interacts with procollagen molecules in the ER in the process of folding and/or modification (4, 5). HSP47 is not secreted with procollagen from the cells, because it possesses the ER retention signal, Arg-Asp-Glu-Leu, at the carboxyl end (5, 6). The expression of HSP47 is well correlated with that of various types of collagen in cultured cells, in various tissues during development, and in pathophysiological conditions (7-12). Thus HSP47 is assumed to be a collagen-specific molecular chaperone (13). However, the binding sites or motif in the procollagen molecule have not been well elucidated. The molecular basis of HSP47 function, including the regulation of the substrate association and dissociation, also remains an enigma.Here, we studied substrate recognition of HSP47 using the recombinant chaperone protein and synthetic collagen model peptides and found that a very simple collagen model peptide mimics the native substrate of HSP47. The difference in substrate recognition between HSP47 and prolyl 4-hydroxylase (P4-H) was discussed in both primary and secondary s...