Proteasomal degradation of unassembled mutant type I collagen pro-alpha 1(I) chains

We have previously shown that type I procollagen pro-alpha 1(I) chains from an osteogenesis imperfecta patient (OI26) with a frameshift mutation resul ting in a truncated C-propeptide, have impaired assembly, and are degraded by an endoplasmic reticulum-associated pathway (Lamande, S. R., Chessler, S . D., Golub, S. B., Byers, P. H., Chan, D., Cole, W. G., Sillence, D. O. an d Bateman, J. F. (1995) J. Biol. Chem, 270, 8642-8649). To further explore the degradation of procollagen chains with mutant C-propeptides, mouse Mov1 3 cells, which produce no endogenous pro-alpha 1(I), were stably transfecte d with a pro-alpha 1(I) expression construct containing a frameshift mutati on that predicts the synthesis of a protein 85 residues longer than normal. Despite high levels of mutant mRNA in transfected Mov13 cells, only minute amounts of mutant pro-alpha 1(I) could be detected indicating that the maj ority of the mutant pro-alpha 1(I) chains synthesized are targeted for rapi d intracellular degradation. Degradation was not prevented by brefeldin A, monensin, or NH4Cl, agents that interfere with intracellular transport or l ysosomal function. However, mutant pro-alpha 1(I) chains in both transfecte d Mov13 cells and OI26 cells were protected from proteolysis by specific pr oteasome inhibitors. Together these data demonstrate for the first time tha t procollagen chains containing C-propeptide mutations that impair assembly are degraded by the cytoplasmic proteasome complex, and that the previousl y identified endoplasmic reticulum-associated degradation of mutant pro-alp ha 1(I) in OI26 is mediated by proteasomes.