Intracellular disposal of incompletely folded human alpha(1)-antitrypsin involves release from calnexin and post-translational trimming of asparagine-linked oligosaccharides

Protection of lung elastin fibers from proteolytic destruction is compromised by inefficient secretion of incompletely folded allelic variants of human alpha(1)-antitrypsin from hepatocytes, Pulse-chase radiolabeling with [S-35]methionine and sucrose gradient sedimentation and coimmunoprecipitation techniques were employed to investigate quality control of human alpha(1)-antitrypsin secretion from stably transfected mouse hepatoma cells, The secretion-incompetent variant null(Hong Kong) (Sifers, R, N,, Brashears-Macatee, S,, Kidd, V, J,, Muensch, H,, and Woo, S, L, C, (1988) J, Biol, Chem, 263, 7330-7335) cannot fold into a functional conformation and was quantitatively associated with the molecular chaperone calnexin following biosynthesis, Assembly with calnexin required cotranslational trimming of glucose from asparagine-linked oligosaccharides. Intracellular disposal of pulse-radiolabeled molecules coincided with their release from calnexin, Released monomers and intracellular disposal were nonexistent in cells chased with cycloheximide, an inhibitor of protein synthesis, Post-translational trimming of asparagine-linked oligosaccharides and intracellular disposal were abrogated by 1-deoxymannojirimycin, an inhibitor of alpha-mannosidase activity, without affecting the monomer population, The data are consistent with a recently proposed quality control model (Hammond, C,, Braakman, I., and Helenius, A. (1994) Proc, Natl, Acad, Sci, U, S, A, 91, 913-917) in which intracellular disposal requires dissociation from calnexin and post-translational trimming of mannose from asparagine-linked oligosaccharides.