The naturally occurring nullHong Kong variant of human alpha 1-antitrypsin is truncated at its carboxyl terminus, and is retained and degraded in a pre-Golgi compartment of stably transfected murine hepatoma cells (Le, A., Graham, K. S., and Sifers, R. N. (1990) J. Biol. Chem. 265, 14001-14007). Long-term metabolic radiolabeling with [35S]methionine or [32P]orthophosphate in combination with low stringency immunoprecipitation of the nullHong Kong variant has resulted in the co-precipitation of a radiolabeled 90-kDa protein designated p90. Several criteria, including mobility in SDS-polyacrylamide gel electrophoresis, absence of asparagine-linked oligosaccharides, and immunoreactivity with peptide-specific antiserum, have indicated that co-precipitating p90 is identical to calnexin, a calcium-binding phosphoprotein of the endoplasmic reticulum membrane (Wada, I. W., Rindress, P. H., Ou, W.-J., Doherty, J. J., Louvard, D., Bell, A. W., Dignard, D., Thomas, D. Y., and Bergeron, J. J. M. (1991) J. Biol. Chem. 266, 19599-19610). Finally, results from co-immunoprecipitation analyses and velocity sedimentation experiments have verified that approximately 30% of the retained nullHong Kong variant polypeptides are associated with calnexin in a 1:1 molar ratio and can be dissociated with either deoxycholate or chelation of calcium ions at 37 degrees C. Overall, these findings may extend our current understanding of the molecular pathogenesis of serum alpha 1-antitrypsin deficiency.