The role of substrate-specific molecular chaperone in the trimerization of procollagen.

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HSP47 was originally identified as a collagen-specific stress protein located in the endoplasmic reticulum (ER). HSP47 binds to procollagen in the ER immediately after the nascent chain of procollagen enters the ER and dissociates from it in the cis-Golgi network. Binding affinity of HSP47 to various types of collagens including types I to V was revealed to be similar using BIAcore biosensor. The expression of HSP47 closely correlates with that of collagens including types I to IV in various cell lines and during the development of mouse embryos. Both HSP47 and types I and III collagens were also induced in some pathological conditions such as during the progression of liver fibrosis caused by the administration of carbon tetrachrolide into rats.We showed the results of the transfection of antisense RNA for HSP47 into BALBc/3T3 cells. We obtained several stable transfectants where the synthesis and accumulation of HSP47 were inhibited moderately and almost completely. The expression of procollagen was observed to be inhibited at levels of both protein synthesis and mRNA accumulation in the cells containing low level of HSP47. In addition to the inhibition of collagen synthesis, the secretion of procollagen was inhibited in these cells although the inhibition was not so evident because of the low level of collagen synthesis. Next, we tried to transfect the cDNA encoding alpha1 chain of type I collagen into the HSP47-antisense transfected cells. In this double transfectants, the level of HSP47 was low while the amount of procollagen alpha1 chain was comparable with that of control cells. In these cells, we found that procollagen was recovered in the detergent-insoluble fraction, indicating that HSP47 is involved in the solubility of alpha chains of pprocollagen in the ER.