Protein C Nagoya, an elongated variant of the human protein C, is retained and degraded within the cells in which it is produced (Yamamoto et al, J Clin Invest 90:2439, 1992). To determine the subcellular localization of the protein C Nagoya, the recombinant protein C bearing this mutation was expressed in Chinese hamster ovary (CHO) cells. The mutant protein C was not secreted from the cells and remained susceptible to endo-beta-N-acetylglucosaminidase H (endo H). Immunoelectron microscopy indicated that protein C Nagoya was retained in the endoplasmic reticulum (ER), whereas wild-type protein C was observed in both the ER and the Golgi apparatus. Metabolic radiolabeling with [35S] methionine in combination with chemical cross-linking showed that the protein C Nagoya existed in the ER as a complex with 78-kD glucose-regulated protein (GRP78) and 94-kD glucose-regulated protein (GRP94). Because both GRP78 and GRP94 associate to a far lesser degree with wild-type protein C than with protein C Nagoya, our data suggest that both stress proteins function as molecular chaperones and work in concert with the folding and assembly of protein C. These findings extend our understanding the molecular pathogenesis of protein C deficiency.