INTRACELLULAR ASSOCIATION BETWEEN UDP-GLUCOSE-GLYCOPROTEIN GLUCOSYLTRANSFERASE AND AN INCOMPLETELY FOLDED VARIANT OF ALPHA(1)-ANTITRYPSIN

Genetic variants of human alpha(1)-antitrypsin unable to fold into the native structural conformation are poorly secreted from hepatocytes. The molecular chaperone calnexin coimmunoprecipitates with secretion-i ncompetent variant null(song Kong) retained in stably transfected mous e hepatoma cells (Le, A., Steiner, J. L., Ferrell, G. A. Shaker, J. F. , and Sifers, R. N. (1994) J. Biol. Chem. 269, 7514-7519). Mobilizatio n of intracellular Ca2+ stores with metabolic poisons diminished inter action with calnexin and coincided with coimmuoprecipitation of a 150- kDa protein (p150), Mobilization of endoplasmic reticulum lumenal Ca2 with thapsigargin, an inhibitor of the microsomal Ca2+ ATPase, gave a similar result, Coimmunoprecipitation of p150 was specifically disrup ted in response to incubation of the cell lysate with exogenous CaCl2. Finally, in ECL Western blotting, p150 was recognized by polyclonal a ntiserum against UDP-glucose:glycoprotein glucosyltransferase that lik ely functions in glycoprotein folding and quality control (Sousa, M. C ., Ferrero-Garcia, M. A., and Parodi, A. J. (1992) Biochemistry 31, 97 -105), The data are consistent with a model in which perturbation of e ndoplasmic reticulum Ca2+ results in a stable physical association bet ween unfolded human alpha(1)-antitrypsin and UDP-glucose:glycoprotein glucosyltransferase.